Category Rocketing into the Future

MIXED UP; COMBINED JET/ROCKET INTERCEPTORS

Of course, as some German and Russian projects had explored during the war, there was a way to combine the fantastic rate of climb and high-altitude performance of a rocket and the endurance and range of a jet in a single aircraft.

After the failed attempts to create mixed-power ramjet/rocket aircraft during the war, the Russians did not resume their interest until the mid-1950s. The MiG 19SU (also known as the SM-50) was a MiG 19S equipped with a U-19 rocket boost unit based on a 29,000 Newton S3-20M engine running on nitric acid and kerosene. This was specifically intended for intercepting the elusive Lockheed U-2 reconnaissance aircraft. This CIA spy plane with its extremely long wings could fly at an altitude of 21 km (70,000 feet), beyond the reach of normal fighter aircraft. The rocket booster was a self-contained unit that required only electrical and mechanical attachment to the aircraft. It could be carried on the belly of a MiG 19S which, apart from some structural reinforcements and the interfaces, was otherwise a standard version of this jet fighter. Ground clearance was narrow with the booster under the fuselage, so the aircraft needed a smooth runway and the rotation immediately prior to taking off had to be shallow. From November 1957 to February 1958 five prototypes made a total of 44 rocket propelled flights that demonstrated the aircraft could reach a maximum altitude of 24 km (79,000 feet); an impressive improvement of the normal ceiling at 17.5 km (57,400 feet). This altitude was reached using a ‘standard intercept’ profile in which the MiG used its jet engine to get to 14 km (46,000 feet) and then ignited its rocket engine to make a quick jump further into the thin air. However, the total time to reach its peak altitude was 12 minutes, which was not particularly spectacular, and it could not remain there for any length of time. The wings and control surfaces were never designed to operate in rarefied air, and above 20 km (66,000 feet) it was about as maneuverable as a brick and difficult to point in the right direction for firing its guns and missiles (which in those days had to be aimed much more accurately than their modern, highly autonomous descendants). In addition, the aircraft was prone to stalling at high altitude and the lack of oxygen tended to make its turbojet flame out. This meant that even under the control of an exceptionally skilled pilot, the aircraft could only make a running jump up at a U-2 in the hope of firing a few shots in the general direction of the target at the top of its parabolic trajectory, before it tumbled down again. In short, the chance of a successful intercept was remote. A number of MiG 19P fighters which had the ‘Gorizont-1’ (Horizon) radio datalink for automatic guidance to a target by ground control were also modified for the U-19 booster but these MiG 19PUs were not very effective either. By then surface-to-air missiles were capable of shooting down a U-2; as was shown on 1 May 1960 with the downing of Gary Powers by an S-75 (SA-2) missile. (A second missile struck a high-flying MiG which was also attempting to intercept this particular U-2.) After this missile success, the MiG 19SU and MiG 19PU aircraft were retired.

In 1954 MiG also initiated the development of a mixed-power, short-range point defense interceptor variant of the Ye-2 swept-wing jet prototype fighter they were in the process of creating (the delta-winged version was the MiG 21, a famous aircraft type that became a huge success after its introduction in 1959). The new aircraft was powered by a Tumansky AM-9Ye turbojet that delivered a thrust of 37,000 Newton with afterburner and a Dushkin S-155 rocket motor whose maximum thrust was also

37,0 Newton. The S-155 ran on (non-hypergolic) nitric acid and kerosene, which meant it could draw fuel from the same kerosene supply as the turbojet engine. This plane was named the Ye-50 and three prototypes were built. The standard Ye-2 was already capable of flying at Mach 2 but the rocket engine was to increase its rate of climb as well as its maximum altitude and maximum speed. The S-155 engine and the tanks for the hydrogen peroxide running the propellant pumps were fitted in the tail, with the rocket nozzle located just above the jet engine exhaust.

The first prototype, the Ye-50/1, flew on jet power on 9 January 1956 under the control of test pilot Y. G. Mukhin, and first used its rocket engine on 8 June that same year. It made a total of 18 successful flights, three of which used combined jet and rocket power. On 14 July the Ye-50/1 made a routine checkout flight after its jet had been replaced but the new engine failed and the plane crashed. The pilot was unhurt but the plane was badly damaged and detailed inspection showed that it was beyond repair. Test flights could not resume until a second prototype of a slightly improved configuration was built. This Ye-50/2 aircraft was under construction from 18 July to 7 December 1956, and on 3 January 1957 made its maiden flight with test pilot V. P. Vasin. No fewer than 25 flights were carried out during the first half of the year, and the plane attained a maximum altitude of 25.6 km (84,000 feet) on 17 June with the help of rocket thrust; considerably higher than the normal ceiling of the standard Ye-2/MiG 21 of about 19 km (62,000 feet). On that same flight the Ye – 50/2 also reached a maximum speed of 2,460 km per hour (1,530 miles per hour), or Mach 2.33, which was about one-third of a Mach number faster than when flown under jet power alone. The Ye-50/2 was joined by the Ye-50/3, which featured a longer forward fuselage in order to increase the internal fuel load. This made its first flight in April 1957. After nine successful flights disaster struck on 8 August when the rocket engine exploded and the plane fell out of the sky. Unfortunately the ejection seat failed and test pilot, N. A. Korovin, was killed when the plane hit the ground. This curtailed the Ye-50 test flights.

In the meantime MiG’s Gorky factory had been ordered to build a batch of 20 pre-production aircraft in preparation for operational deployment as the MiG 23U (not to be confused with the trainer version of the MiG 23 ‘Flogger’ jet which had the same designation but was a totally different aircraft developed a decade later). None were ever built, however, because the Dushkin OKB which built the S-155 rocket motors had in the meanwhile closed down. In any case, the Air Force had seemingly lost its enthusiasm for mixed-power interceptors, at least in part because of the difficulties associated with operating and maintaining the rocket engine. Furthermore, the sheer vastness of the Soviet Union meant that a great number of aircraft would have been needed even to defend a limited number of high-value targets.

In parallel with the MiG Ye-50, a mixed-power version of the Yakovlev Yak-27 twin-engine fighter was under development. This Yak-27V first flew in May 1957 and operated the standard two RD-9Ye turbojets, each with an afterburning thrust of 38,000 Newton, in combination with an S-155 engine mounted in the tail. With this rocket boost the prototype managed to zoom to 25 km (16,000 feet) during its 2- year test program before closure of the Dushkin OKB and diminishing support from the Air Force curtailed further development.

It appears that in addition to Dushkin, the Kosberg rocket design bureau had been working on rocket engines based on oxygen and kerosene propellants for both the MiG Ye-50 and the Yak-27V but it is not clear whether these were ever flown.

These two planes proved to be the Soviet Union’s final attempts at a mixed-power fighter interceptor, because the concept was soon rendered obsolete by improve­ments in jet engine technology.

The US also experimented with mixed propulsion to satisfy a requirement issued in 1945 for a short-range interceptor. Operating in the immediate vicinity of its base this new aircraft was to provide the last line of defense against incoming high-speed bombers, and the specifications emphasized high speed, high flight ceiling and rapid climb. Bell proposed a weaponized, delta-wing version of its X-l supersonic research rocket plane. Republic offered its AP-31 mixed-power aircraft with inversely tapered wings (wider at the tip than at the root in order to avoid wingtip stalls at low speeds) and the capability to vary the angle of attack of the wing with respect to the fuselage in flight (for a high angle of attack and thus high lift during take-off and landing, and a low angle of attack and low drag for high-speed flight). Northrop proposed the XP-79Z, which would be similar to the aforementioned jet-powered XP-79B except that it would be based on a rocket motor (something other than the complex Rotojet that had been planned for the original XP-79). Convair offered a swept-wing aircraft with a very unusual ‘ducted rocket’ powerplant. This was basically a ramjet with several small rocket motors inside the combustion chamber. As well as delivering their own thrust, the rockets would act as igniters for the ramjet. Another four rocket engines, each with a thrust of 5,300 Newton, would be fitted outside the rear fuselage to get the plane into the air and to sufficient speed to enable the ramjet to function. A small Westinghouse J-30 jet engine would provide electrical and hydraulic power, and fly the aircraft back to base once it ran out of rocket and ramjet propellant (it was later proposed that this jet be replaced by a midget racing car engine for supplying power only). As it was to take off using a jettisonable dolly, the plane would be fitted with a lightweight undercarriage for landing only.

In April 1946 Convair was announced to have won the contest by Headquarters Air Material Command of the Air Force and the interceptor project was designated XP-92. Wind tunnel tests soon showed that instead of swept wings the aircraft would behave better with delta wings similar to those developed by Alexander Lippisch for the Me 163 (after Germany’s defeat Lippisch had been relocated to the US as part of Operation Paperclip). The pilot would operate the controls in the prone position in a cockpit that was mounted on the nose of the aircraft like a spike projecting from the circular ramjet air intake, and which could be completely ejected in an emergency. The four external rockets would now each have a thrust of 27,000 Newton and there would be a total of sixteen 220 Newton rocket engines inside the ramjet.

The design emerging was not particularly appealing to the eye: a tubular vehicle with a conical nose and a large triangular fin and delta-wings that made it look like a small dart. There is a saying in aircraft design which asserts “if it looks good, it flies good”. The XP-92 Dart did not look particularly trustworthy. The cockpit position in particular would severely limit the pilot’s view, making it impossible to check his ‘six o’clock’ (i. e. directly aft). In addition the whole propulsion concept proved to be overly comphcated, with its large numbers and types of engines. Just one prototype was built with a conventional cockpit and only a jet engine. It was used primarily for research into delta-wing aerodynamics. In that role it did become the first US delta­wing aircraft (and the first of many delta-winged Convair fighter designs). When the test flights ended in late 1953 the mixed-propulsion interceptor concept was already considered outdated by the USAF.

Although Republic did not win the contest it was awarded a contract to build an experimental mixed-power interceptor based on its AP-31 design. This became the XF-91 Thunderceptor, a heavily modified version of the F-84F Thunderstreak with a liquid propellant rocket engine added for fast climb and high-speed interception. The original idea was to use a Curtiss-Wright XLR27 four-chamber rocket motor with a total thrust of 60,000 Newton but due to development problems a Reaction Motors XLR1 l-RM-9 with about half the power was mounted instead (the first XF – 91 flew with empty XLR27 housing pods and holes for two nozzles above and two below its jet exhaust). The XLR11 was the first US rocket engine developed specifically for use in aircraft, and was also used in the X-l and D-558-2 high-speed research aircraft described in the next chapter. The compact rocket engine was housed in a pod under the jet tail pipe and had four individual thrust chambers that burned ethyl alcohol and liquid oxygen. Each chamber had a fixed, non-throttleable thrust of 7,000 Newton but the total thrust of the cluster could be regulated by turning individual chambers on or off. It was fairly reliable.

The first XF-91 Thunderceptor [US Air Force],

The XF-91’s four-chamber XLR-11 rocket engine was placed directly under its jet engine’s exhaust pipe [US Air Force].

The two XF-91 prototypes retained the originally proposed and unusual inversely tapered wing, with the angle of attack relative to the fuselage being variable in flight between minus 2 and plus 6 degrees. The innovative wing provided excellent control at supersonic as well as at very low speeds, reducing the tendency of the tips to stall before the rest of the wing at low speeds (which can lead to a loss of control). The problem with limited space to house the main undercarriage was solved with a highly original arrangement of narrow twin wheels in a bicycle arrangement which retracted outwards into the broad wingtips. Otherwise the design was fairly conventional, with a cross-shaped tail and a regular cockpit with an ejection seat. The armament planned for its eventual operational service consisted of various types of cannon and air-to-air rockets. The prototypes were built and tested at Edwards Air Force Base,

California. The first flight was on 9 May 1949 with Republic’s chief test pilot Carl Bellinger at the controls, and it was powered only by the turbojet engine. It was 30 months before the rocket engine was delivered and fitted. On its maiden flight using dual-power on 9 December 1952, the Thunderceptor flown by Russel ‘Rusty’ Roth became the first US fighter (as it was intended to become an operational military combat aircraft) to fly faster than the speed of sound in level flight. It achieved Mach 1.07 powered by the jet engine with afterburner and two of the four rocket chambers. The aircraft later reached a maximum speed of 1,812 km per hour (1,126 miles per hour, Mach 1.71) on the combined thrust of the 24,000 Newton jet engine with afterburner and 27,000 Newton from the four rockets, which was very good for the time. With the XLR27 rocket engine it would probably have been able to reach Mach 2. It could climb to an altitude of 14.5 km (47,500 feet) in just 2.5 minutes and its maximum altitude was 16.8 km (55,000 feet). But the rocket equipment and propellant took so much space that the Thunderceptor could carry fuel to run the jet engine without afterburner for a mere 25 minutes, and even for that it had to be equipped with two large drop tanks with extra jet and rocket propellant. This would have restricted the plane to a point defense role.

The second XF-91 prototype was similar to its predecessor, although it was later retrofitted with a V-shaped ‘butterfly’ tail. It suffered a serious engine failure during take-off in the summer of 1951 with Bellinger at the controls, while famous test pilot Chuck Yeager flew chase in an F-86 Sabre (as described later in this book, in 1947 Yeager became the first man to break the sound barrier in the X-l rocket plane). In Yeager, an Autobiography, Carl Bellinger describes what happened: “Chuck radioed, ‘Man, you won’t believe what’s coming out of your engine.’ A moment later I got a fire warning light. ‘Christ,’ I said, T think I’m on fire.’ He replied, ‘Old buddy, I hate to tell you, but a piece of molten engine just shot out your exhaust, and you’d better do something quick.’ ” Bellinger was still too low to use his ejection seat and black smoke was filling the cockpit, bhnding him. Assisted by Yeager’s calm instructions Bellinger managed to land the Thunderceptor with its rear fuselage burning violently. He scrambled out and ran for his life. Only 90 seconds had elapsed between Yeager’s warning and the emergency landing. In spite of the serious damage, the aircraft was repaired and equipped with a V-shaped tail for further experiments.

Together the two XF-91s completed 192 test flights over the course of five years, but no more were built. As with the Soviet combined-propulsion interceptors, the XF-91 suffered from its short range and very limited total flight time, with the result that it was soon rendered obsolete by a new generation of powerful, more versatile turbojet-only fighters. The second prototype was scrapped but the first one (minus engines) is on display at the National Museum of the US Air Force near Dayton, Ohio.

Around the same time as the new mixed-propulsion aircraft, there were also plans to equip existing jet fighters with auxiliary rocket engines for increased performance for short periods. The Aerojet LR63-AJ-1 that burned white fuming nitric acid and JP-4 jet fuel (a 50-50 blend of kerosene and gasoline) and could produce a thrust of

23,0 Newton was under development for the F-84 Thunderstreak but during the Korean War the Air Force decided to use it in the F-86 Sabre because the early-type

Sabres were being outperformed by the Soviet MiG 15s in North Korean service and it was hoped that the rocket engine would enable the Sabre to counter the MiG threat. Versions of the engine were tried on an F-84G in 1952 and on an F-84F in 1954, but by the time the LR63 had been modified for the F-86 and flight tests on a prototype started in 1956 the Korean War was long over. Neither the mixed – propulsion version of the F-86 Sabre nor that of the F-84 Thunderstreak became operational.

The US Navy still saw some use for carrier-based mixed-power interceptors. In 1955 they ordered six North American FJ-4 Fury fighters with a Rocketdyne AR-1 rocket engine mounted above the standard jet engine exhaust pipe. The AR-1 ran on hydrogen peroxide and standard JP-4 jet fuel, taking the latter from the same tanks as fed the jet engine. The rocket gave the Fury an additional 22,000 Newton of thrust for short periods, sufficient to reach a speed of Mach 1.41 and a maximum altitude of 21.6 km (71,000 feet); considerably better than the normal limits of the standard FJ-4 at just over Mach 1 and 14.3 km (46,800 feet). Development of the rocket – boosted Fury, called the FJ-4F, became a top priority for a while after the US got into a bit of a panic concerning intelligence estimates for the high-altitude capabilities of the new Soviet Tupolev Tu-16 ‘Badger’ and the Myasishchev Mya-4 ‘Bison’ bombers. They urgently needed a naval high-altitude interceptor to thwart the new threat. The first two FJ-4Fs entered flight testing in April 1957 at the Naval Air Test Center, Patuxent River, Maryland, and achieved the aforementioned performance. Between them they made 103 flights, accumulating a total of 3.5 hours of rocket operation. However, the other four designated FJ-4s were never converted into mixed-power interceptors and, unsurprisingly, the FJ-4F concept was soon abandoned. Apart from the obsolescence of the rocket propelled interceptor idea, the risks involved with storing and handling hydrogen peroxide on warships put an end to the development.

Around the same time, in 1957 the Vought company was thinking about boosting their F8U Crusader naval fighter with a rocket engine. The idea was to fit a Reaction Motors XLF-40 rocket engine above the standard jet tail pipe in order to increase the rate of climb for high-altitude intercepts. The XLF-40 burned jet fuel and hydrogen peroxide and delivered 35,600 Newton of thrust. Unfortunately an engine exploded during a ground test and killed two Reaction Motors workers, prompting the engine company to withdraw from the project. Vought continued development for a while with the intention of switching to a Rocketdyne XLR54 rocket engine with a thrust of 26,000 Newton. Dummy engines were installed in two Crusaders to verify that the rocket engines would fit, but no real engines were ever installed. Vought had also planned to equip its new XF8U-3 Crusader III with a Rocketdyne AR-1 engine (as on the FJ-4F) but this was canceled together with the entire Crusader III project.

In the early 1960s the Navy introduced the North American Vigilante supersonic carrier-based bomber. The company planned to fit this aircraft with an auxiliary jet fuel/hydrogen peroxide rocket engine but this never happened. Neither was a mixed – propulsion interceptor version equipped with a Rocketdyne XLR46-NA-2 engine that they tried to sell to the Air Force as the Retaliator.

Back in Europe, France had entered the Second World War with obsolete aircraft that were quickly defeated by the Luftwaffe, and during the war there were no further developments. Afterwards, however, the country was determined to become a strong military power again and worked indigenously on advanced military aircraft. A large number of innovative test planes were built and flown during the late 1940s and the 1950s. In 1944 the government had created SEPR (Societe d’Etude de la Propulsion par Reaction) in order to develop its own rocket engine capabilities. The French do not appear to have had any interest in a pure rocket fighter like the Me 163 with its inherently Umited range and endurance. They were more interested in aircraft with combined rocket and turbojet propulsion. Studies for a rocket propelled interceptor with auxiliary jet engines began in 1948 at the aircraft manufacturer SNCASO (Societe Nationale des Constructions Aeronautiques du Sud-Ouest), commonly referred to as Sud-Ouest. The general idea was to use both jet and rocket engines for a rapid climb and high-altitude intercept, and jet engines alone for the return to base. The rocket engine was to be based on the SEPR design successfully employed by the Matra M04 missile and its successors. However, an engine on a missile has to work only once and for a short time; a rocket motor for a manned aircraft must be more reliable, reusable and maintainable. For operational readiness of the aircraft and engine, the rocket propellant would have to be readily storable (exit liquid oxygen with its extreme cooling requirements), fairly insensitive to changes in temperature and to impurities in the tanks (exit hydrogen peroxide) and easy to produce and handle (again exit hydrogen peroxide). Somewhat surprisingly, it was decided to use nitric acid as oxidizer even although the corrosion of the engine and aircraft structure must have been apparent. (As we have seen, this problem made the Russians eventually discard nitric acid as a propellant for rocket interceptor aircraft.) The fuel was ‘furaline’, a mixture of 41% furfuryl-alcohol, 41% xylidene (dimethylaminobenzene) and 18% methanol; also a rather surprising choice since it was fairly difficult to produce, especially in comparison with the low-grade kerosene used in the Russian rocket engines for aircraft. An advantage over kerosene was that furaline is hypergolic with nitric acid, eliminating the need for an ignition system.

As the concept of rocket thrust (let alone mixed-propulsion) on a manned aircraft was totally new to France, it was decided first to build a relatively simple proof-of – concept design before embarking on the expensive and risky development of a high – performance aircraft. Therefore one of the existing SNCASO SO-6020 ‘Espadon’ (Swordfish) jet prototypes was fitted with a SEPR 25 engine that ran on nitric acid and furaline fed from tanks on the wingtips, and renamed the SO-6025 Espadon. The light alloy engine was slung under the rear fuselage and produced a modest thrust of 1,500 Newton. It was regeneratively cooled by a flow of nitric acid running around the chamber and nozzle before being injected into and burned in the combustion chamber. The pumps supplying the propellants were powered by the aircraft’s jet engine, the method pioneered by BMW during the war in their 003R combined jet/ rocket engine. The modified Espadon was intended to demonstrate the safety and reliability of the rocket engine and the capabilities of the mixed-propulsion concept. The first ground test of the plane fitted with the rocket engine was in May 1951 but

The Espadon under rocket power.

the first flight did not occur until 10 June 1952. A total of 76 rocket propelled flights had been made by July 1955, with the rocket-equipped Espadon becoming the first European aircraft to reach Mach 1 in level flight on 15 December 1953. An improved engine, the SEPR 251, was installed in another Espadon and flown 13 times between March 1953 and October 1954.

Based on the successful trials with the two Espadons, the French Air Force invited proposals from French aircraft manufacturers to design a Ughtweight interceptor with jet engines, rocket engines, or both, which would be able to achieve Mach 1.3, have a high climb rate and be able to operate from rough airfields.

Nord Aviation proposed the Nord 5000 ‘Harpon’ (Harpoon), which was initially conceived as a rocket propelled interceptor with sharp swept-back canard stabilizers and double-delta wings (in which the sweep angle of the leading edge changes at a certain point, as on the Space Shuttle). Morane-Saulnier proposed the MS-1000, of which not much is known. Neither was selected for further development. The Nord 5000 never got further than a wooden scale model.

SNCASO won a contract with designer Lucien Servanty’s SO-9000 Trident. This fast looking, bullet-shaped supersonic interceptor had a rocket engine fitted in its tail, augmented by a pair of small Turbomeca Marbore II jet engines with 4,000 Newton of thrust each for a fuel-economic return flight after the rapid climb and intercept on combined propulsion. Its wings were straight but relatively thin to minimize drag at high speeds, and had an overall span of 7.57 meters (24.8 feet). The light jet engines were on the wing tips. This kept the fuselage aerodynamically clean but required the wings to be especially strong and stiff, and hence rather heavy. The SEPR 481 rocket engine had three combustion chambers and these could be ignited and extinguished individually to give it a throttling capability in three steps of 15,000

SO-9000 Trident [SNCASO].

Newton and an impressive maximum thrust at sea level of 45,000 Newton. At an altitude of 11 km (36,000 feet) the engine even produced 52,000 Newton owing to the lower external air pressure trying to push the rocket exhaust back in. The gas generator driving the turbopumps ran on the same propellants as the combustion chambers, meaning that the rocket engine did not rely on power from the jet engines; it was an independent unit. One complication was that a mixture of water and methanol had to be injected into the gas generator to reduce the gas temperatures and prevent the turbine blades of the turbopump from melting.

To enable the pilot to escape from a stricken plane at high speed, the entire nose section of the Trident could be jettisoned; a system which offered the pilot a lot of protection but was a much heavier solution than an ejection seat (a similar system had already been used in the Heinkel 176 rocket plane and was also incorporated in the Douglas D-558-1 Skystreak, D-558-2 Skyrocket and Bell X-2 research aircraft, and similar concepts would later be suggested as possible methods for astronauts to escape from malfunctioning shuttles and spaceplanes). The control system was well suited to transonic and supersonic flight speeds, as all three tail surfaces (the vertical and two horizontal stabilizers) were all-moving, rather than small separate rudders and elevators. This prevented the controls from locking up due to Mach shock waves forming at the control hinge lines (the aforementioned dangerous ‘shock stall’). At low speeds normal ailerons on the wings were used, but at high speeds these would be locked in order to prevent shocks forming and in this configuration roll would be controlled by differential use of the horizontal tail surfaces (which would thus act as elevons). The tricycle undercarriage was retracted into the fuselage. The fully-loaded weight was 5,055 kg (11,140 pounds); more than the original 4,000 kg requirement but still relatively low for an interceptor.

SNCASO built two Trident prototypes. The rocket engine was not fitted for the initial test flights, as the engineers first wanted to determine how the plane handled at low speeds under jet power alone. The first jet-only prototype took off from Melun – Villaroche on its maiden flight on 2 March 1953 with test pilot Jacques Guignard at the controls. After completing a number of other successful flights the prototype was revealed to the public at the famous international air show at Le Bourget near Paris in June. Guignard also took the second Trident (still lacking a rocket engine) for its first flight on 1 September 1953 but owing to trouble with the relatively weak jet engines he was unable to gain sufficient height after take-off and hit an electricity pylon. The plane struck the ground so violently that the escape module broke off. Guignard was badly injured but survived. To preclude further take-off accidents the one remaining Trident was refitted with two licence-built early versions of the British Armstrong Siddeley Viper jet engine that had almost twice the thrust of the previous Marbore II engine. Test pilot Charles Goujon resumed the test flights on 16 January 1954. Later in the autumn the plane and the rocket engine were deemed to be ready and the first rocket propelled flight using one chamber of the SEPR 481 rocket engine was made on 9 September. Over the next 2 years the Trident made a total of 24 rocket propelled flights to explore its performance. It was capable of exceeding Mach 1 on jet power alone when pushed into a shallow dive but with the SEPR 481 it could achieve a top speed of Mach 1.63 and an altitude of nearly 15 km (52,000 feet). After the flight test program was terminated in April 1956 the first and only surviving Trident prototype was retired to the Le Bourget Musee de l’Air et de l’Espace museum. It is still there, suspended from the ceiling flying at Mach 0 but nevertheless still looking fast and beautiful in its bare metal color scheme with French national markings. The wingtip-mounted jet engines do give it a decidedly 1950s’ look, though.

The success of the Trident test program prompted the French Air Force to place an order for an improved version to serve as an operational mixed-power

SO-9000 Trident in the museum at Le Bourget [Mikael Restoux].

SO-9050 Trident II [SNCASO],

interceptor. Designated the SO-9050 Trident II, this plane was equipped with the new SEPR 631 two-chamber rocket engine with each chamber delivering 15,000 Newton of thrust (although less than the three-chamber engine this was a more propellant-economical compromise between climb rate and endurance). The fully – laden take-off weight was about 5,900 kg (13,000 pounds) and the wingspan was 6.95 meters (22.8 feet). The Trident II was still fitted with the Viper engines of British origin but the detachable cockpit concept of the SO-9000 was abandoned in favor of an ejection seat. In order to be able to mount air-to-air missiles under the fuselage the undercarriage legs were lengthened.

Flight testing of the first Trident II began on 19 July 1955 powered by the jet engines only. The first rocket powered flight was on 21 December. The second plane was damaged beyond repair shortly thereafter when both jet engines flamed out soon after take-off and test pilot Guignard had to make an emergency crash landing. This time he was only slightly injured. A third prototype, equipped to be flown without a pilot, controlled from the ground by radio link, was used as a replacement for the lost aircraft (the remote control feature was never actually used). The first prototype was used to explore the highest performance of the design while the third was for tests at low and medium speeds. The first Trident II disintegrated in mid-air during a dive on 21 May 1957 killing pilot Charles Goujon. The real cause for this disaster was never discovered but an explosion due to propellant leakages and structural failure induced by the abnormal aerodynamic loads were high on the list. However, confidence remained high that the Trident II would enter operational service. The magazine Flight reported that at the Le Bourget air show, which opened only 3 days after the crash, “a fascinating large-scale model shows a hypothetical Trident II base with camouflaged dispersals, refueling and servicing points, missile protection and other modem conveniences”. The surviving prototype was quickly brought up to the standard of its predecessors so that high-performance test flights could continue. It eventually reached a top speed of Mach 1.95 and a record altitude of 24 km (79,000 feet), both very impressive for the time. Jacques Guignard set two time-to-altitude records using this aircraft: 2 minutes and 36 seconds to 15 km (49,000 feet) altitude, and also 3 minutes and 17 seconds to 18 km (59,000 feet).

Despite losing two aircraft during the test campaign (not unusual for experimental planes in those days), in June 1956 the government placed a pre­production order for the operational Trident III. This had a Turbomeca Gabizo jet engine at the tip of each wing, each developing 11,000 Newton of thrust on its afterburner. The rocket main engine was the SEPR 632, which was very similar to the 631 except for using ‘tonka’ fuel (an approximately 50-50 mix of triethylamine and xylidine which is hypergolic with the nitric acid oxidizer) rather than furaline, which was difficult to produce. The new aircraft was to be equipped with a fire control radar and Matra R 511 air-to-air missiles. However, the first three production aircraft had the 631 engine and they did not carry armament. These early planes were tested until October 1958, with Captain Jean Pierre Rozier reaching an altitude of 26 km (85,000 feet), although this was not registered as a record because the flight was not supervised by the FAI (Federation Aeronautique Internationale), the international society responsible for logging world aviation records. The top speed set with a Trident III was Mach 1.95 at an altitude of 22.1 km (72,500 feet). But the whole program had been canceled in April 1958 with only four aircraft completed, another two almost finished, and four more in various stages of construction. Unfortunately all of the Trident IIs and Ills were scrapped, leaving us not a single example of this impressive product of the rebirth of French aeronautical prowess. This is all the more lamentable because the Trident III set an official FAI altitude record for a completely self-propelled rocket plane of 24,217 meters (79,452 feet) on 2 May 1958, achieved by pilot Roger Carpentier in response to the project’s cancellation a week earlier. The Trident III actually had the potential to push to 30 km (100,000 feet). Other experimental rocket aircraft had already gone higher than that but, unlike the Trident III, they were air-launched by carrier planes and therefore could not claim official FAI records for self-propelled flight.

As the Russians and Americans had found out, mixed-propulsion planes based on small jet engines and big rocket engines had much too little range and endurance for practical military use. At full rocket power the Trident III could stay in the air only for about 4 minutes! Like other air forces, by the end of the 1950s the French shifted their interest away from short-range interceptors to multi-purpose long-range fighter aircraft. In the future, interceptors would be powered by large jet engines and exploit rocket power only for extra boost. Following this philosophy, aircraft manufacturer SNCASE (Societe Nationale des Constructions Aeronautiques du Sud-Est, not to be confused with SNCASO, although in 1957 they merged to form Sud Aviation) won a contract to develop the SE-212 Durandal, a delta-wing mixed-propulsion interceptor named for a mythical medieval French sword. Developed by a team headed by Pierre Satre, it was primarily powered by a SNECMA Atar 10IF jet engine which delivered a thrust of 43,000 Newton with afterburner, and had a single-chamber 7,500 Newton SEPR 65 auxiliary rocket engine. With a total weight of 6,700 kg (15,000 pounds) it

The second Durandal at the 1957 Paris Air Show.

was a bit heavier than the Trident II and III. The Durandal was intended to be armed with a single AA-20 air-to-air missile carried under the centerline or two 30­mm cannon. As with the Espadon, the pumps of the rocket engine were driven by the jet engine through an auxiliary gearbox. The rocket engine ran on the hypergolic combination of nitric acid and triethylamine-xylidine (TX).

Development of the SEPR 65 started in December 1953 and its first ground test was performed on 4 November 1954. The initial Durandal prototype took to the sky on 20 April 1956 and its first rocket propelled flight was on 19 December that year. The second prototype first flew on 30 March 1957 and it employed rocket power for the first time in April 1957. During its test program the Durandal reached a speed of 1,444 km per hour (898 miles per hour, Mach 1.36) on jet power alone and 1,667 km per hour (1,036 miles per hour, Mach 1.57) with rocket thrust, both at an altitude of about 12 km (39,000 feet). The two aircraft made a total of 45 rocket flights before the program was terminated in May 1957. The main reason for the cancellation was the fact that carrying a single missile it would have only one chance to hit its target and would then be unable to defend itself. This gave it little advantage over a guided surface-to-air missile, and so rendered it of very limited use to the French Air Force. Parts of the first Durandal are in storage at the Musee de l’Air et de l’Espace at Le Bourget, near Paris. It was intended to fit a derivative of the SEPR 65 engine, the SEPR 651, to the small Nord 1405 Gerfaut II delta-wing experimental aircraft but this was never done.

Aircraft company Dassault also developed a small mixed-propulsion interceptor, the delta-winged MD-550 – Mystere Delta 550. It had no horizontal stabilizers but an exceptionally large vertical stabilizer. It was to have two Turbomeca Gabizo engines side-by-side for jet propulsion and a single-chamber SEPR 66 rocket engine located between and beneath them. The rocket had a gas generator driving the turbopumps and burned nitric acid and furaline to deliver a thrust of 15,000 Newton. After the development started in September 1953 the first SEPR 66 ground tests took place on 10 January 1955. Over a year later, on 19 January 1956, the engine made its first test flight on the Mystere IV B05, a ‘flying test stand’ based on the Dassault Mystere IV fighter-bomber. On the tenth rocket flight an explosion of the combustion chamber due to an ignition delay severely damaged the tail of that aircraft. Nevertheless, the pilot managed to land. The problem was quickly traced to the very low temperature at the high altitude at which the plane was flying, and rapidly resolved. At the end of this testing the Mystere IV B05 had made 55 rocket flights and proven that the concept worked. A prototype of the MD-550 (lacking a rocket engine) was built and flown for the first time in June 1955 on jet power provided by two temporary Viper engines (which were, however, never replaced with the intended Gabizo turbojets). This aircraft was then modified, equipped with a smaller tail fin, afterburners, and a SEPR 66, and named the Mirage I. Pilot Gerard Muselli ignited the SEPR 66 for the first time in flight on 17 December 1956, pushing the Mirage I up to an altitude of 12 km (39,000 feet) and a speed of Mach 1.6. Previously, the maximum speed attained on jet power alone had been Mach 1.2 so the rocket thrust added a considerable 0.4 Mach. But the project was terminated in late January 1957 with the aircraft having made only five rocket flights. Like the SNCASE Durandal (canceled 4 months later) the Mirage I had very limited military use because it could only carry a single air-to-air missile.

In reaction to the Air Force’s shift in interest to multi-purpose fighters with a long range and multiple armament, Dassault designed the Mirage II. This was a bigger aircraft powered by two Gabizo engines and a SEPR 661, which was a dual-chamber version of the SEPR 66 with the same 15,000 Newton maximum thrust but the option of a 7,500 Newton intermediate thrust by using just one chamber. The SEPR 661 was only ever fired on a ground test bench and the single Mirage II prototype only flew on jet power, but this aircraft design formed the basis for the subsequent, much more successful mixed-power Mirage III.

The Mirage III was a very slender delta-wing interceptor based on the innovative ‘area ruling’ concept which dictates that to limit aerodynamic drag, changes to the frontal cross section of an aircraft should be made as gradual as possible. This in turn means that near the wings, the fuselage must become thinner in order to maintain a constant cross section with respect to the nose and tail. This gave rise to the famous ‘wasp waist’ (or ‘Cola Cola’ or ‘Marilyn Monroe’) configuration that can be seen on many supersonic fighters. With its maximum take-off weight of 13,500 kg (29,700 pounds) it was much larger and heavier than the Tridents and Durandal. The Mirage III was optimized as a high-altitude interceptor, emphasizing rate of climb and speed rather than maneuverability and low-speed handling. Its primary propulsion was a single SNECMA Atar 101G.1 turbojet which delivered a thrust of 44,000 Newton with afterburner. The pumps of its SEPR 84 rocket engine were powered by the jet engine, feeding the motor with nitric acid and TX. The combustion chamber had a double wall through which nitric acid flowed en route to the chamber, thereby cooling the engine. Another smart feature was that the high – pressure kerosene also served as a hydraulic fluid for operating various valves. Even although it had only one chamber, the SEPR 84 could provide a maximum of 15,000 Newton as well as an intermediate level of 7,500 Newton of thrust. The development of the new rocket engine started at the end of 1956, and it was first ignited in the air on 12 July 1957 on the Mirage III 001 prototype piloted by Roland Glavany. This plane flew a total of 30 missions, during which Glavany managed to reach Mach 1.8 in level flight at an altitude of 12 km (39,000 feet).

The success of the prototype led to a pre-production order for ten Mirage IIIAs, which were intended to be multipurpose fighters rather than merely interceptors. The Mirage IIIA was longer than the prototype, had a larger wing, and was fitted with the powerful Atar 9B turbojet which could dehver a thrust of 58,900 Newton with afterburner. The Mirage IIIA 02 prototype was initially fitted with the first improved pre-production SEPR 840 engine and had sufficient propellant for an 80 second burst of thrust. The aircraft made its first rocket propelled flight on 18 February 1958, and it performed a total of 15 rocket flights with the SEPR 840. It went on to make an impressive 444 rocket propelled test flights with the operational SEPR 841 version of the engine. On 24 October 1958 Glavany nudged the Mirage IIIA 01 prototype past Mach 2 in level flight with the help of a SEPR 840, becoming the first European pilot to do so. On 25 January 1960 a rocket boost enabled the aircraft to reach an altitude of 25.5 km (83,700 feet). The Mirage IIIA design suffered from corrosion of the nitric acid tank and damage to the underside of the fuselage caused by divergence of the rocket exhaust at high altitude (where the lower air pressure permits the exhaust to expand into a wide plume). However, these problems were solved by changing the material of the acid tank and beefing up the fuselage thermal protection.

The first major production version, the Mirage IIIC, had its maiden flight in 1960 and entered operational service in December the following year (the IIIB was a two- seat trainer and had no provision for a rocket engine). It was armed with two 30-mm cannon and had five attachment points for missiles, bombs or extra fuel tanks. The SEPR 841 with its propellant tanks could be added as a separate package to the aft fuselage. If the rocket motor was not needed for the mission, it could be left vacant or replaced by an additional jet fuel tank. The Mirage IIIC remained in use with the French Air Force until 1970, making 1,505 rocket propelled flights involving 2,064 ignitions of the SEPR 841. The rocket engine could give a very powerful kick which allowed the aircraft to accelerate from Mach 1 to Mach 2 in only 90 seconds; on jet power alone this would take about 4 minutes. In terms of added engine weight, this boost came fairly cheaply. The Atar 9B-3 turbojet had a weight of 1,460 kg (3,210 pounds) and on afterburner could deliver 58,900 Newton (equivalent to 5,890 kg) of thrust; a thrust to weight ratio of 4. The SEPR 841 had a weight of only 158 kg (348 pounds) and delivered 15,000 Newton (equivalent to 1,500 kg) of thrust: a thrust to weight ratio of 9.5; more than double that of the jet engine. Operating the rocket engine from the cockpit was also very simple: the instrumentation was limited to an ignition switch, a handle for thrust level selection, a control light to indicate that the engine was operating and another light to indicate a malfunction.

The ensuing Mirage HIE, which had various improvements to better suit the role of ground attack aircraft, could be fitted with a SEPR 844 engine in which the TX

Ground testing of an SEPR 844 on a Swiss Mirage IIIS [F +W Emmen].

fuel of the 841 was replaced by kerosene (the HID was the two-seat trainer version of the HIE). The main benefit of this was that the rocket engine could get its fuel from the same supply as the jet engine. But because this new propellant combination was not hypergolic, a small tank of TX still had to be carried to ignite the engine (the tank was sufficient for two in-flight starts). The rocket engine could operate for a

A SEPR 844 on a Swiss Mirage IIIS operating in flight [F +W Emmen],

total of 2 minutes and could be stopped and reignited in flight. The SEPR 844 could deliver a thrust of 12,600 Newton at sea level and 15,600 Newton at an altitude of 20 km (66,000 feet) for a specific impulse of 220 seconds. The Mirage HIE was also built under licence in Switzerland by F + W Emmen as the Mirage IIIS for the Swiss Air Force, which opted to equip its aircraft with SEPR 844 rocket engines.

The Mirage III went on to become the most successful operational mixed-power fighter aircraft ever by logging over 20,000 rocket propelled flights. Despite the use of nitric acid, a spent rocket pack could be made ready for a new mission in less than 15 minutes. This demonstrated that rocket engines could be used by an operational fast – reaction interceptor aircraft. Acid corrosion problems such as experienced by the Russians were avoided by adding a corrosion inhibiter to the nitric acid propellant. A total of 164 SEPR 841 and 111 SEPR 844 engines were built and each could be used for an average of 35 flights. SEPR rocket packs were used on Mirage III fighters by the air forces of no fewer than six countries: France, Libya, Pakistan, South Africa, Spain and Switzerland. This made the SEPR 84 family the most built, most reused, and most operational man-rated (i. e. intended for use on piloted aircraft) series of rocket engines ever. Nevertheless, improved jet engines, accurate long-distance air-to – air missiles, and ground radar networks capable of providing advanced warning of airspace intruders meant that the auxiliary rocket engine was no longer needed, and subsequent French fighter aircraft did not have an add-on rocket capability (although some could be fitted with RATO boosters for take-off). France ended the operational service of the SEPR 844 in 1984 but the Swiss continued to use them until 1996.

Hence the French interceptor philosophy had moved from the concept of a rocket plane with auxiliary jet engines, to a turbojet aircraft with auxiliary rocket power, to a pure jet fighter without any provision for in-flight rocket propulsion. It meant the end for rocket propelled military aircraft in France.

The United Kingdom, although at the forefront of turbojet engine technology, had not done much on rocket and rocket plane development during the war. Based on the knowledge and equipment they managed to capture in Germany in 1945, and assisted by several German rocket scientists, Britain rapidly caught up on rocketry. The initial focus was on the development of jettisonable RATO units, most notably the Lizzie liquid oxygen/petrol engine for the Vickers Wellington bomber, a modified version of the German RI-202 for the Avro Lancastrian (a passenger aircraft based on the Lancaster bomber), and the ‘cold’, compressed-air-fed hydrogen peroxide/ calcium permanganate Sprite engine for the de Havilland Comet jet airliner.

The Sprite, developed in-house by the aircraft company, was meant to help the airliner take off from the many ‘hot and high’ airports in the British Empire where the air density, and therefore the aircraft’s lift at take-off, were diminished by high temperatures and high elevations. The two Sprites on the Comet could each provide a 22,000 Newton boost for 16 seconds. Flight tests on the aircraft prototype started in May 1951 but the system was never used for real operations; injecting water into the combustion chambers of the jet engines proved to be a simpler and cheaper method of gaining the necessary extra boost.

An important innovation for an upgraded Sprite was the use of permanent silver plated nickel gauze packs as the catalyst, replacing the expendable liquid potassium permanganate solution used in the Walter engines as well as the early version of the Sprite. This type of catalyst, which was not consumed and hence did not need to be replenished for each flight, would be incorporated in all following British hydrogen peroxide rocket engines. The Sprite later became the Super Sprite, in which kerosene was injected into the combustion chamber and then spontaneously ignited due to the high temperature of the steam and oxygen gas developed by the decomposition of hydrogen peroxide (which the British named High Test Peroxide, HTP). At 19,000 Newton the engine’s thrust was actually less than that of its predecessor, but it had an increased bum time of 40 seconds. The Super Sprite was used operationally by the Vickers Valiant bombers of the Royal Air Force in the early 1950s.

In early 1951 the estimated capabilities of near-future Soviet long-range bombers led the British to believe they would need to be able to intercept bombers flying at extreme altitudes as well as at speeds up to Mach 2. The limitations of the available early warning radar system, in combination with the expected large scale of an air attack, meant that the United Kingdom required a large force of high-performance interceptor aircraft with very fast climb rates and high service ceilings. However, the existing British fighters were nowhere near capable of thwarting the perceived Soviet threat. Based on their experience with RATO rocket engines, the British made plans to use rocket engines for new high-speed, high-altitude manned interceptors.

Initially there was also some interest in pure rocket propelled interceptors, with a government specification leading to the Short P. D.7, Bristol 178 and Hawker P.1089 concepts. The Short P. D.7 design had large delta wings that provided room for four large internal kerosene tanks, while the fuselage housed the additional liquid oxygen needed for the Screamer rocket engine (more on this engine later). The high-subsonic airplane was to have been able to reach an altitude of 9 km (30,000 feet) in less then 3 minutes from the moment that its wheels started to roll over the runway. But once at the altitude of enemy bombers the aircraft would have hardly any propellant left, making it difficult to engage anything. This very limited range meant that the rocket interceptors would have to be stationed as close as possible to their potential targets and in very large numbers, which translated into a large number of individual bases along the coast. Like the French, the British quickly found range and endurance of a pure rocket fighter too limited to be of much use, and hence regarded rocket power on aircraft as being practical only if combined with turbojet thrust. They therefore asked their industries to propose concepts for interceptors using both jet and rocket engines. In addition, they wanted competitive industries to integrate rocket engines based on different propellants in their designs, in order to preclude becoming stuck in dead-end developments.

For the Royal Navy, the Hawker company proposed a new version of their Sea Hawk carrier-based jet fighter augmented with an Armstrong Siddeley Snarler rocket engine running on liquid oxygen and water-methanol (i. e. methyl-alcohol – water) and producing a thrust of 9,000 Newton for an increased rate of climb. The engine was installed in the P.1040 prototype of the Sea Hawk, which was fairly straightforward because the double exhausts of the single Rolls Royce Nene turbojet engine were positioned at the sides of the fuselage, thus leaving the tail conveniently free for the rocket. The fuselage was reinforced to deal with the extra rocket thrust, the Snarler’s turbopumps were connected to the 23,000 Newton jet engine via a gearbox, and the jet fuel tank capacity was decreased in order to make room for the new liquid oxygen and water-methanol tanks. There was enough propellant to run the rocket engine for 2 minutes and 45 seconds at full power. The aircraft was subsequently redesignated P.1072.

Design of the Hawker P.1072.

The plane’s Snarler engine was first ignited during a flight on 20 November 1950 by Hawker’s chief test pilot Trevor Wade. The August 1954 edition of the magazine Flight described it as follows: “The day was dull and overcast, and the P.1072 could be heard only distantly through dense cloud. Those on Bitteswell airfield were quite ignorant of whether Wade had fired the rocket or not, and were still waiting expectantly when the fighter appeared low down at full power. Passing overhead, Wade did a smart roll; all had gone well.” Six rocket propelled flights were made in total, three piloted by Wade and three by Neville Duke. The rocket engine was only fired during a steep climb in order to preclude the plane developing so much speed as to enter the transonic regime, for which its aerodynamics were inappropriate. With rocket thrust the plane could (theoretically) reach an altitude of 15 km (50,000 feet) within 3.5 minutes of its wheels starting to roll over the runway. This translated into an impressive rate of climb of over 73 meters per second (240 feet per second). But this performance could never be verified because the absence of a pressurized cabin meant that the pilot could not fly higher than 12 km (40,000 feet). Nevertheless the experimental prototype did establish that the concept of an auxiliary rocket engine worked. The test campaign also established that despite the cryogenic liquid oxygen the aircraft could be kept fully fueled for extended times without the engine’s valves freezing up (although the oxygen would boil off and so require constant topping up). Interestingly, the aforementioned Flight article does not comment on the fact that the Snarler engine exploded when Duke attempted a restart during the sixth flight on 19 January 1951; presumably that was still secret information at the time. This incident is believed to have been caused by a small quantity of fuel left in the system after the previous firing of the engine, which then instantly exploded upon reignition. Luckily the damage was relatively small, and the pilot landed the plane safely. The P.1072’s tail section was repaired but by then the Navy had decided that using afterburners on their jet engines was an effective and less complicated means of increasing thrust on fighter aircraft. The rocket propelled Sea Hawk was thus canceled. The P.1072 never flew again, and unfortunately it does not exist anymore.

The Avro company responded to the government’s request for a mixed-power interceptor with a proposal for an aircraft based on the Armstrong Siddeley Viper 2 turbojet which produced a sea level thrust of 7,000 Newton, in combination with an Armstrong Siddeley successor to the Snarler rocket engine with the equally inspired name Screamer. The Screamer burned liquid oxygen and standard aviation kerosene, produced a maximum thrust of 35,000 Newton, and could be restarted in flight. The Snarler had used water-methanol as fuel because of its good cooling characteristics. The Screamer could run on the same kerosene fuel as an aircraft’s jet engine, which meant it did not require separate fuel tanks, but the disadvantage was that kerosene is a much less effective coolant than water-methanol. Hence the Screamer depended on a water-cooled combustion chamber and nozzle. In comparison to an engine based on hydrogen peroxide the Snarler’s and especially the Screamer’s propellant was less costly and safer, but the liquid oxygen had the disadvantage of requiring very cold storage. Unlike the Snarler, the Screamer engine was required to run independently of the jet engines, which could therefore be turned off. It had its own gas generator in which a small amount of liquid oxygen and kerosene were burned to provide gas to drive the turbopumps to feed propellant to the rocket engine’s combustion chamber (water was injected into the gas generator’s exhaust to cool the very hot gas in order to prevent it damaging the turbopump assembly).

The resulting Avro 720 aircraft was a tailless delta-wing design somewhat similar to the later French Mirage. It was to be primarily constructed of metal honeycomb sandwich, a light but strong material (although difficult to repair) now widely used for spacecraft. It was intended to arm it with a pair of de Havilland Blue Jay (later renamed Firestreak) passive infrared air-to-air missiles carried on underwing pylons. The Type 720 was meant for the Royal Air Force but a derivative called the Type 728 would satisfy the specific requirements of the Royal Navy (it would be equipped with the much more powerful Gyron Junior jet engine instead of the Viper, and have a strengthened undercarriage for safe operation from aircraft carriers). Thanks to the low drag, exceptionally low weight and good altitude performance of the Screamer, both versions were to be able to reach Mach 2, be able to climb to 12 km (40,000 feet) in only 1 minute 50 seconds and have a maximum altitude of 18 km (60,000 feet). Armed with two heat seeking missiles under the wings the 720/728 was expected to be fully capable of intercepting high – performance bombers and blowing them out of the sky. A contract was issued for the construction of two prototypes and a structural model for ground tests. In 1956 the first aircraft was almost finished when the entire project, including the development of the Screamer, was canceled. The military had somewhat belatedly come to the conclusion that using liquid oxygen in an interceptor aircraft was not such a good idea after all. Oxygen has to be kept at the very low temperature of minus 183 degrees Celsius (minus 279 degrees Fahrenheit) in order to keep it liquid. Even with extensive insulation around the aircraft’s internal tanks it was expected that onboard oxygen would boil off at a rate of 4% per hour. So either it must be stored in special tanks and pumped into the aircraft shortly before flight, or if it is stored in tanks in the plane it must be topped off. As already determined by the Germans in 1939 when they reviewed von Braun’s early design for a vertical take­off rocket interceptor, this is a major problem for a quick-reaction interceptor that must always be ready for a rapid take-off.

Despite the cancellation of the Avro 720/728, a scheduled test of the Screamer on a Gloster F8 Meteor jet fighter continued. The rocket engine was to be placed on the underside of the aircraft, along with a liquid oxygen tank that would be jettisoned if the pilot ever had to make an emergency belly landing. The rocket’s fuel and water tanks were inside the aircraft’s fuselage. Ground firing tests were performed, but the full modification of the Meteor was not finished and its Screamer was never used in flight.

With Avro also out of the competition, Saunders Roe was the only company left working on a true, newly-designed mixed-power interceptor. Their SR.53 developed under the leadership of chief designer Maurice Brennan was a single-seat delta-wing aircraft which combined the de Havilland Spectre rocket with an auxiliary turbojet. The rocket engine would be used for rapid ascent and attack, while the jet engine mounted above it would enable the aircraft to make a low-power return to base. Like the Avro Type 720, the SR.53 would only be armed with two de Havilland Blue Jay

Design of the Saunders Roe SR.53.

(Firestreak) missiles, in this case mounted on the wingtips. The SR.53 was equipped with a T-tail so that the horizontal stabilizers were kept out of the wake of the wings. These stabilizers were of the all-moving type, thus avoiding the shock-stall problems of conventional elevators. Both the wings and the stabilizers were kept thin in order to limit high-speed drag and had a thickness ratio of only 6%. The take-off weight of 8,400 kg (18,400 pounds) included 5,000 kg (11,000 pounds) of propellant.

The de Havilland Spectre rocket engine was specifically developed for a mixed- propulsion interceptor and was heavily based on wartime German research by the Walter company. Hydrogen peroxide was decomposed by being passed through the silver plated nickel gauze catalyst and the resulting hot oxygen used to bum standard aviation kerosene. Cooling of the combustion chamber was achieved by running the hydrogen peroxide through a double wall. Pumps driven by the SR.53’s small jet engine fed the propellants to the Spectre’s combustion chamber from tanks that were pressurized with air. This pressurization served to suppress cavitation (the forming of bubbles due to local low pressure) in the pumps. Static firings of the rocket engine began in 1952 and demonstrated that it could be throttled from 8.9 to a maximum of

36,0 Newton. The Spectre was flown on a modified Canberra jet bomber test bed (described later) in 1957 before it was deemed ready for incorporation in the SR.53. The auxiliary jet engine was the Armstrong Siddeley Viper 8, which could produce a maximum thmst of 7,300 Newton. This also powered the onboard systems. It was not equipped with an afterburner, and in fact did not provide enough power to enable the plane to fly around after an overshoot on landing (as with the German wartime Komet, the pilot was committed to his approach). The rocket and jet engine received their kerosene from separate tanks but if necessary the turbojet could draw upon the rocket engine’s kerosene wing tanks. The combined power of both engines gave the SR. 53 a top speed of Mach 2.2 and a maximum altitude of 20.4 km (67,000 feet). From standing on the runway, it could climb to an altitude of 15 km (50,000 feet) in just 2 minutes and 12 seconds with a very impressive climb rate of 270 meters per second (890 feet per second). However, it had sufficient propellant on board only for 7.5 minutes of full rocket thrust.

The US backed the development financially with $1.5 million (equivalent to $12 million in 2010) and the UK Ministry of Defence signed a contract with the company in May 1953 to build three SR.53 prototypes, although the third one was canceled in January 1954 to cut costs. The Ministry of Defence had expected the first SR.53 to fly in mid-1955 but problems with the Spectre and its propellant supply system (one engine even exploded in a ground test) delayed delivery of the first SR.53 prototype to January 1957. With registration number XD 145, this machine was sent by water (the factory was on the Isle of Wight) and by road to the Ministry’s aircraft testing site at Boscombe Down for ground test firings of its Spectre engine. The aircraft took to the skies under rocket power for the first time on 16 May 1957 with test pilot John Booth at the controls. It was found to behave as expected with good flight characteristics in general, although the somewhat lower than advertised performance of the Viper engine in combination with an increase in aircraft weight meant that its performance fell short of the specification.

A potential problem with the aircraft’s recovery from a so-called ‘deep stall’ was discovered when a Gloster Javelin jet fighter, which had a similar configuration with

The SR.53 at the Famborough air show in 1957.

delta wings and T-shaped tail, crashed during a test flight because at extreme angles of attack the turbulent wake of the stalled wings blanketed the horizontal stabilizer, rendering its elevators ineffective and thereby making a recovery impossible (as this involved using the elevators to push the nose down). To prevent this from happening to the SR.53 a set of four small solid propellant rockets were installed in the rocket engine’s cowhng in the tail: if the pilot ever discovered himself in a deep stall, he could ignite these motors to pitch the aircraft back down (the motors were units that the Royal Marines used to shoot hooks and lines up for cliff assaults). There was also an issue with the Viper flaming out when the SR.53 was pushed over after a steep climb, resulting in a short duration of zero G (the principle on which aircraft for 0-G weightlessness-simulating research are based) that disrupted the flow of propellants. More important, however, was a serious design flaw that was discovered during the test flight campaign: the rear airframe structure suffered badly from the high level of acoustic energy (i. e. sound vibrations) generated by the powerful Spectre engine. The loads were not sufficient to immediately damage the aircraft but prolonged firing of the engine rapidly weakened the structure. This is called ‘fatigue’ and is a well known issue in aircraft design and maintenance (the problem is similar to how a spoon weakens when you bend it several times until it eventually breaks due to the growth of micro-fractures in the metal). Fatigue on the rear structure of the aircraft delayed the development program even further.

Meanwhile the Royal Air Force had come to the conclusion that it really required something better than the SR.53, which was hampered by its weak turbojet and very limited range and endurance. The small Viper could only be used to fly back to base, as it did not even provide enough power for the SR.53 to take off without additional rocket thrust. Saunders Roe responded with the SR. 177, an improved design in which the 7,300 Newton Viper jet engine was replaced by the more powerful de Havilland Gyron Junior that could deliver a thrust of 32,000 Newton in normal use and almost twice as much with afterburner. This engine was the first British turbojet specifically designed for sustained running at supersonic speeds, enabling the SR. 177 to fly faster than Mach 1 for relatively long periods if assisted by rocket thrust. In addition, a new version of the Spectre engine was to be installed. The Spectre 5 delivered the same maximum thrust as its predecessor in the SR.53 but could be throttled over a wider range down to just 10% of its maximum power, enabling it to be run in combination with the jet engine for prolonged low supersonic flight. The airframe was enlarged to accept the big Gyron Junior, and the positions of the jet and rocket engines were reversed with respect to the SR.53 (i. e. the turbojet was now located below the rocket engine). It also had a larger nose to accommodate the intended new radar system that would assist the pilot to find his target. Like the SR.53, the SR.177 had a pair of Blue Jay (Firestreak) missiles at its wingtips but these could be replaced with pods, each containing 24 unguided air-to-air rockets. An auxiliary power unit with a hydrogen-peroxide steam turbine was installed to power the onboard systems in the event that the turbojet flamed out at high altitude. All in all, the take-off weight of the SR.177 was one-and-a-half times that of the SR.53. Although the general layout was similar to its predecessor (including the 6% thickness ratio of the wings and tailplane) the SR.177 was virtually a new design. The

large Gyron Junior required a lot more air than the Viper, so the sleek lines of the SR.53 with its modest air inlets just behind the cockpit canopy were sacrificed for a huge chin-mounted intake. Of course, the benefits outweighed aesthetics: the new jet engine would considerably improve endurance, enabling the SR. 177 to cruise for a long time on jet power alone. This time could be extended by fitting jet propellant drop tanks under the wings and also by aerial refueling of kerosene from tanker aircraft. The limited supply of rocket propellant could thus be conserved for making a dash once a target was identified. The combined rocket and jet thrust were expected to give the SR.177 a top speed of Mach 2.35 at 12 km (40,000 feet) and Mach 2.75 at 21 km (70,000 feet). For a short-warning interception mission it would be able to achieve Mach 1.6 at 18 km (60,000 feet) in just 4 minutes, but this would consume its entire propellant load and require a gliding return. The powerful jet engine was also expected to enable the SR.177 to be effective in light ground attack and photographic reconnaissance roles, for which the rocket engine would be removed.

In July 1956 funding was secured for the construction of 27 SR.177s, with the first prototype expected to fly by April 1958. It was a large project employing some 400 Saunders Roe and subcontractor engineers on design work alone. Flight testing with the SR.53 was planned to continue, but only to assist in the development of its bigger sister.

However, as elsewhere, changes in military requirements and the evolution of the interceptor concept from a pure rocket plane, to a mixed-power aircraft with a small auxiliary jet engine (the SR.53) and to a mixed-power design with a large jet engine were inevitably leading to the next step of discarding the rocket engine entirely. The death knell for the SR.177 as a Royal Air Force interceptor was the infamous 1957 Defence White Paper in which Minister of Defence, Duncan Sandys, expressed the belief that all manned fighter aircraft would soon be rendered obsolete by guided missiles. Only the development and production of the English Electric Lightning jet fighter would be continued. For the short term this pure jet interceptor was deemed sufficient, because it had become clear that the new Soviet bombers were nowhere near as fast and high-flying as had been feared earlier. No more-capable jet or rocket interceptors would be required, because in the longer term the defense of the country could be safely turned over to surface-to-air missiles. The SR.177 fell victim to what would prove to have been a disastrously erroneous policy that all but killed the UK’s independent military aviation industry. The fact that new, manned fighter aircraft are still being developed and produced today is testimony to how overoptimistic Sandys was about the capabilities of missile systems.

Saunders Roe still tried to sell the SR.177 design to the Royal Navy, as well as to likely foreign customers. The Germans were very interested in its rapid interception capabilities, since their border with the Warsaw Pact meant the Luftwaffe had little time to react to intruding aircraft. When in August 1957 the British decided against the Royal Navy version of the SR.177 the writing was on the wall. Development did continue for a while longer with the British Ministry of Supply agreeing to fund the construction of five of the planned six prototypes in anticipation of sales to Germany. However, the Germans decided not to take financial risks and ordered the

Artistic impression of an SR. 177 in Luftwaffe service.

Lockheed F-104G Starfighter instead, as did many other European NATO countries. This was a pure jet which combined the roles of ground-attack fighter, photo-reconnaissance and interceptor into a single all-weather aircraft. It was later discovered that Lockheed had paid out millions of dollars in ‘Sales Incentives’ (i. e. bribes) to several European pohticians to buy their aircraft and secure what was called the ‘Deal of the Century’. Whatever the reason for the Germans’ change of mind, in hindsight their decision to buy a multipurpose jet fighter instead of a mixed – power interceptor and its derived ground-attack and reconnaissance variants was sensible. The Starfighter was a highly successful, albeit rather dangerous, fighter that remained in operational service with the Luftwaffe until 1987.

With no support left the SR. 177 project was canceled just before Christmas 1957, with the design being 90% complete and the production of various equipment for the first six aircraft already well under way. No complete prototype had yet been made other than a full-scale wooden mockup (unfortunately no longer in existence) which was used for assessing the cockpit layout and the proper fitting of the various pipes, cables and control systems (nowadays this is done by using 3-dimensional computer models, which can be updated and improved much faster than a mockup).

The SR. 53 test flight campaign continued, with the first prototype being joined by the second aircraft with registration number XD 151. This made its first flight on 8 December 1957. Unfortunately it crashed on 5 June 1958 shortly after taking off for its 12th flight. The exact cause was never established but it seems the aircraft lacked the power to climb because its rocket engine had shut down just as the plane left the ground. It struck a pylon at the end of the runway and violently exploded, killing test pilot John Booth. The test program resumed using the first prototype. Saunders Roe proposed turning XD 145 into a true test bed for new rocket engines, including air­launching it (from the back of a Valiant bomber) rather like the experimental X-15 rocket aircraft in the US, but the Ministry of Defence felt the aircraft was not suitable for such a program and halted funding. The decision was not unreasonable, because the SR.53’s aluminum structure would limit its maximum speed to around Mach 2 to avoid structural failure by overheating. Also its air intakes and its still relatively thick wings would probably have caused problems at speeds far over Mach 2. Moreover the aircraft lacked reaction control thrusters for attitude control at extreme altitudes (although these could probably have been retrofitted, as was done to the Starfighter-derived NF-104A described in the next chapter). In October 1958 the flight campaign ended and the only remaining SR.53 was transferred to the Royal Air Force Museum storage facility at Henlow. It remained there until 1978 when it was donated to the Brize Norton Aviation Society, which restored the aircraft for static display. It can now be seen in the Royal Air Force Museum at Cosford. The two SR.53 prototypes together accumulated a total flight time of 22 hours and 20 minutes over 56 flights by three pilots.

During the late 1950s the Spectre rocket engine was also intended to be used in the Italian ‘Leone’ (Lion), which was an attempt by aircraft manufacturer Aerfer to develop their Sagittario 2 supersonic (in a dive) jet into a competitive interceptor, but the project was abandoned before a single prototype could be built.

Saunders Roe had also investigated a third mixed-power interceptor, the rather large P.187. This would have been powered by two Gyron turbojets with afterburners and four Spectre 5 rocket motors, sufficient to enable it to attack targets at Mach 2 within a range of 420 km (260 miles) and up to an altitude of 18 km (60,000 feet). It would have taken 3 minutes and 50 seconds to climb to 12 km (40,000 feet), which was a relatively slow rate of 52 meters per second (171 feet per second). It was to be equipped with an advanced radar and four air-to-air missiles. The propellant required for this performance would have made its take-off weight almost four times that of the SR. 177! The aircraft would have been crewed by a pilot and a radar operator. It featured a sliding nose for improved viewing during landing, an idea that was later adopted by the Concorde supersonic airliner. The P.187 project never left the drawing board.

The cancellation of the SR.53 and SR.177 meant the end of the development of true mixed-power interceptors in the UK but there were also several projects focused on fitting auxiliary rocket engines into existing military jets.

One idea was to put a rocket on the English Electric Lightning jet interceptor, which was still under development when this option was considered. Like on the Mirage III the rocket system would be designed as a self-contained pack that could easily be fitted on the belly of the aircraft if needed. The rocket would considerably increase the maximum altitude of the Lightning, enabling the plane to attack high­flying intruders. The Napier Double Scorpion was developed for this purpose. It had two thrust chambers which burned kerosene in hot oxygen gas that was made by the decomposition of hydrogen peroxide with catalyst packs of metal gauzes and pellets. The engine had a turbopump powered by hydrogen peroxide that was decomposed by a separate, smaller catalyst pack. The total thrust was almost 36,000 Newton, and each combustion chamber could be individually throttled in steps of 0 to 50 to 100% thrust. To test the engine, it was installed on an English Electric Canberra jet bomber. The rocket system, complete with engines, tanks and plumbing, formed an integrated unit that could be carried in the standard bomb bay of the Canberra, thus minimizing the required modifications. To prevent the Canberra from starting to pitch due to the additional rocket thrust from its belly, the nozzles were angled slightly upward so as to aim the thrust through the aircraft’s center of mass. Test flights in early 1956 with Canberra number WK163 started off with a single Scorpion thrust chamber, in which configuration it was even flown at the Paris air show that year. Flights with several versions of the Double Scorpion followed. The system was found to be very reliable and to require relatively little maintenance. A complete overhaul was required after a total firing time of 1 hour (similar to the SEPR 84 series of the Mirage III) but since the Scorpion could only be operated for several minutes per flight, due to the limited propellant supply on board, this was not needed very often. The aircraft successfully demonstrated the Double Scorpion at the Farnborough air shows of 1956, 1957 and 1958, as well as taking part in the 1958 Battle of Britain memorial show.

The Canberra was a subsonic aircraft, and the rocket engine could not be used to increase its maximum speed because the aerodynamics were unsuitable for transonic flight. But it could boost the aircraft’s maximum flight altitude, as it was intended to do for the Lightning interceptor. On 28 August 1957 the rocket enabled WK163 to capture the world altitude record with a flight up to 21.4 km (70,310 feet) under the control of pilot Michael Randrup and observer Walter Shirley. It was the third record attempt: on the first flight the plane had not flown high enough, while on the second the official barograph with which they were to measure the altitude malfunctioned. Experimental rocket planes had flown higher in the US but they were dropped from carrier aircraft rather than taking off on their own power and thus their achievements were not recognized by the FAI as official records. A red scorpion and details of the achievement were painted on WK163’s nose. Now retired after a long career as a flying test bed for all manner of new equipment, this aircraft has been preserved by Classic Aircraft Projects at Coventry Airport and still flies (without rocket power) at air shows in the UK.

Apart from testing engines, there was actually an operational job for which rocket-enhanced Canberra’s proved to be ideal. In the 1950s the British performed atomic bomb tests as part of the development of their own nuclear deterrent. The explosions shot radioactive particles high into the atmosphere and the bomb developers needed samples for study. Because the tests with WK 163 had shown that the rocket engine improved the standard service ceiling of 15 km (49,000 feet) by about 30%, a pair of Royal Air Force B6 Canberras were fitted with Double Scorpions to fly high-altitude missions over Australia and the Pacific and collect samples from test explosions.

One of these aircraft, registration number WT207, is loaned to Napiers to conduct further flight testing of their Double Scorpion propulsion unit. On 9 April 1958 pilot John de Salis and navigator Patrick Lowe are flying a test, climbing from 13.7 km to 18.3 km (45,000 to 60,000 feet) in just under 4 minutes by combined jet and rocket

power (a very modest climb rate, but the mixed-power Canberra was meant simply to reach high altitudes, not rapid rates of climb and high speeds). After shutting down the Double Scorpion, the aircraft descends to 17 km (56,000 feet). At that point de Salis ignites the rocket engine once again in order to burn off the residual hydrogen peroxide to ensure that none of this dangerous Uquid remains in the tank for landing. However, upon ignition the crew immediately notices a rumbling noise and buffeting of the aircraft, and a warning light for the rear bomb bay flips on: the rocket engine is on fire! Before they can start the carbon dioxide fire extinguisher in the bomb bay an explosion blows apart the central section of the plane, ripping both wings and the tail off. Mist appears in the crew cabin due to rapid depressurization. Lowe jettisons the hatch above his seat and ejects first, then de Salis shoots out through the clear canopy above the cockpit. They set a record for the highest ejection seat escape ever, which is only broken in 1966 when the crew of an SR-71 Blackbird escapes at an altitude of 24 km (80,000 feet). The likely cause of the loss of WT207 was a leak in the engine which allowed kerosene and HTP (hydrogen peroxide) to mix and ignite outside the combustion chamber. After modifications to the Scorpion the flight testing resumed, but not for long because the British decided that there was after all little need for an auxiliary rocket pack for the Lightning interceptor (which was already demonstrating a rather phenomenal rate of climb and speed for its day). The Scorpion engine was canceled, and with it all rocket development work at Napier ceased. It also meant the end of the Canberra flights using that company’s rocket engine after a total of 500 rocket propelled flights.

Another engine that was mated with the Canberra was the 36,000 Newton thrust de Havilland Spectre, which also used hydrogen peroxide and kerosene. It was fitted in a Canberra and first ignited in the air in January 1957. The test flights showed that the propellant delivery system required further development work. The Spectre was removed from the aircraft and subsequently fitted on another Canberra. This aircraft was never used for nuclear sampling missions but the test flights cleared the engine for use on the aforementioned SR.53 mixed-power interceptor.

In the mid-1960s the South African Air Force was interested in purchasing the subsonic Blackburn Buccaneer, which was in service in the British Royal Navy as an attack aircraft. The Buccaneer was meant to fly very low in order to avoid detection by radar. However, due to their ‘hot and high’ conditions, the South Africans wanted the aircraft to be equipped with rocket engines for extra thrust during take­off. This resulted in the Buccaneer S.50 version with two retractable Bristol Siddeley BS.605 rocket engines fitted just behind the bomb bay. Together these could give 30 seconds of thrust at 36,000 Newton to supplement the 180,000 Newton from the twin jet engines running on afterburner. As with the Super Sprite and Scorpion, the BS.605 used kerosene and hydrogen peroxide as propellants. The modifications required to the standard Buccaneer design were modest. South Africa bought 16 of the planes, which flew with 24 Squadron from 1965 until 1991. Ironically, the rockets turned out not to be necessary and were eventually removed from all operational Buccaneers. The roar of the twin BS.605 rocket engines was only unleashed four times, during air show displays.

The number of mixed-power interceptors developed in the 1950s and early 1960s is astonishing, especially considering that the results in terms of operationally useful aircraft were fairly disappointing. In those years, Cold War tensions produced large aeronautical development budgets. Remaining at the forefront of technology meant a parallel investigation of every possible means of gaining military advantage. In the end the mixed-propulsion interceptor proved to be a dead end, since improvements in turbojet technology, advanced radars for early detection of airspace intruders, and the growing military desire for multi-purpose fighter aircraft, combined to render placing rocket engines on aircraft redundant. The added performance simply was not worth the disadvantages of shorter range and endurance, greater aircraft weight, and added complexity. And the role of the interceptor itself became much less important during the 1950s as it became possible to use surface-to-air missiles to shoot down high-flying bombers. Moreover, the Soviet Union was developing ballistic missiles to deliver nuclear warheads, and these could not be intercepted by any manned aircraft. Fast, low-flying bombers carrying nuclear weapons remained a serious threat, but rocket propelled interceptors were ill-equipped to do anything about these.

There was still a role for high-altitude, high-speed reconnaissance aircraft (‘spy planes’) but they required endurance to be able to penetrate far into enemy territory and cover an adequate area. Advanced jet aircraft, specifically the Lockheed SR-71 Blackbird, soon filled that role. The SR-71 could not fly quite as fast and as high as some of the latest experimental rocket planes, but it could undertake missions lasting hours rather than minutes.

The mixed-power high altitude aircraft did come back one more time in 1963 in the form of the NF-104A AeroSpace Trainer, a heavily modified F-104A Starfighter with a rocket engine on its tail that was developed to train pilots in flying at extreme altitudes (of which more later).

Had mixed-powered interceptors remained of interest to the military, then their evolution would probably have eventually led to advanced systems combining rocket and jet functionalities into a single engine. Such engines, which operate as turbojets at low speeds and altitudes, ramjets at high speeds, and rockets at high altitudes, are currently seen as the key to making fully reusable single-stage spaceplanes possible. But the demise of the mixed-power interceptor sent jet engines and rocket engines their separate ways, with jet engines powering aircraft and rocket engines powering expendable missiles, space launchers and a few specialized high-speed, high-altitude experimental aircraft.

SILBERVOGEL

The basic, modern rocket spaceplane concept was bom in the 1930s. It involved a hypersonic rocket aircraft launched by a rail with the assistance of a rocket sled, and capable of flying almost around the globe by using a suborbital trajectory in which it repeatedly bounced off the upper layers of the atmosphere, like a flat stone skipping over the surface of a pond. This ‘rocket glider’ idea was way ahead of its time, and in fact is still ahead of our time. The ‘SilbervogeP (German for ‘Silverbird’) was the brainchild of Austrian-German aerospace engineer Eugen Sanger. He first published

M. van Pelt, Rocketing into the Future: The History and Technology of Rocket Planes, Springer Praxis Books, DOI 10.1007/978-1-4614-3200-5 8, © Springer Science+Business Media New York 2012

the idea in his 1933 book Raketenflugtechnik (Rocketflight Technology). In fact, this work was an elaborated version of his original plan for a doctoral dissertation, but his university had advised him to find a more classical topic and he subsequently got his degree for a study of aircraft wings. Rocket pioneer Hermann Oberth had shown that a rocket plane could only hope to achieve a long range by starting with a very steep ascent, leveling off at high altitude in order to use the remaining propellant to attain its maximum speed in the thin air, and then glide back down for the remainder of its flight. Sanger’s calculations led him to the same conclusions. His initial rocket plane design involved an aircraft shaped like an elongated bullet with large but thin double-wedged wings (like those of the X-2 subsequently built in the US), a small single tail fin and two small horizontal stabilizers. It bore little resemblance to the blunt-shaped biplanes of that time.

Together with Austrian-German mathematician and physicist Irene Bredt, who he would marry in 1951, Sanger further iterated the design during the 1930s and early 1940s. Initially they intended their plane to transport passengers at hypersonic speeds all over the globe but he also tried to pitch the idea as an intercontinental bomber to the Austrian military in 1933. Not surprisingly, in an age where transonic jet airliners were still science fiction the proposal was rejected, primarily because of the risk of the rocket engine exploding.

The German Luftwaffe, however, did see value in a ‘RaketenBomber’ (Rocket Bomber), as well as the basic rocket engine development and testing Sanger was already conducting at the University of Vienna. Not wanting to be outdone by the Wehrmacht (Army) and its rocket development by von Braun’s team, the Luftwaffe in 1936 invited Sanger to establish a secret rocket propulsion research institute in Trauen, Germany. It was named “aircraft test site Trauen” to hide its true nature, and Sanger was even given a fake job at the University of Braunschweig so that people wouldn’t wonder where he was working. Like Wemher von Braun, Sanger funded his work via the military as this was the only source available in Germany during the war. Under his leadership, the Trauen site quickly became an impressive laboratory with a wind tunnel, a rail launch track test rig, and a large rocket engine test stand. Sophisticated experimental rocket engines that ran on fuel oil and liquid oxygen were developed and tested, including components for a 1,000,000 Newton engine intended to propel the Silbervogel. An innovative feature of Sanger’s engine designs was the cooling of the combustion chamber by circulating propellant around it. This cooling strategy, sometimes called the ‘Sanger-Bredt design’, was actually implemented in early rocket engines such as that of the A4/V2, and in most liquid propellant rocket engines since then (including, as we have seen, those used to propel aircraft). For the engine of the Silbervogel, Sanger proposed to use a water loop to cool the massive nozzle and combustion chamber because the propellants he intended to use (fuel oil and liquid oxygen) were poor coolants. The water would be turned into superheated steam by the high temperatures in the engine and then be fed into a turbine to run the propellant pumps and the water coolant pump. After the turbine, the water would be condensed and pumped back into the cooling loop. But having three types of liquids (fuel, oxidizer and coolant) and all the associated pipes, valves and tanks made it a relatively complicated engine.

Although the Nazi government was interested in the Silbervogel as a possible ‘Amerika Bomber’, for which a number of more traditional aircraft designs were also developed, it was clear that the concept was much too advanced for the technologies available in the 1940s and that the bomber would never be ready in time to affect the war. Even Sanger himself predicted that it would take at least 20 years to make his machine operationally useful. The institute at Trauen was closed down in 1942 when the military gave priority to the VI pulsejet bomb and A4/V2 rocket developments at Peenemiinde, because these would be able to be introduced sooner and hence make a greater contribution to the war effort.

Sanger and Bredt transferred to the DFS institute in order to continue their studies and experiments on ramjet technology. But the Silbervogel never attracted sufficient support and funding, and they ended up spending most of the war developing basic rocket engine technology and a design for a ramjet fighter that never got to fly either. They nevertheless always kept thinking about their favorite concept in private and in 1944 summarized all their design and experiment work in a secret report titled ‘Uber einen Raketenantrieb fur Fernbomber’, subsequently translated and published in the US as ‘A Rocket Drive For Long Range Bombers’. The concept they finally arrived at was a 100,000 kg (220,000 pound) rocket plane that would be 28 meters long with a wingspan of 15 meters. It featured a pressurized cockpit for a single pilot near the nose, a central bomb bay, and the main rocket engine as well as two smaller auxiliary engines in the tail. The rest of the fuselage would mostly consist of two parallel rows of tanks containing a total of 90,000 kg (200,000 pounds) of liquid oxygen and fuel oil. Two short wedge-shaped wings with swept-back leading edges and a horizontal tailplane that had a small fin on each tip give the Silbervogel a conventional-looking rocket plane shape. However, the design incorporated the innovative idea of having the fuselage itself generate lift through its ‘flat iron’ shape, hence acting as a partial lifting body.

A powerful rocket sled would be used to help the Silbervogel take off from a long rail track. The sled’s rocket engines would not need to be very efficient, since they would only operate for a brief time and would not leave the track with the plane. For the sled, trading efficiency and thus propellant weight for more brute power in order to get the Silbervogel going would involve little penalty. Sanger proposed to use von Braun’s A4/V2 rocket engines. The starting speed given by the innovative rail launch system resulted in a smaller, lighter spaceplane requiring a less powerful engine and smaller wings in comparison to a similar vehicle that had to take off on its own (at higher speeds the same amount of lift can be provided with smaller wings). The rail system also meant that the Silbervogel would only need a light undercarriage for its glide landing with empty tanks, thereby further reducing its weight. Furthermore, the rail sled would make detailed knowledge of transonic flight behavior unnecessary, as it would push the plane through this mysterious region of aerodynamics whilst it was still firmly connected to the track (as mentioned above, transonic aerodynamics were not properly understood until well after the war). The rail launch system would limit the possible launch azimuths, but Sanger suggested that it could be used with either end as the starting point. As Germany’s enemies were primarily to the west (the US) and the east (the USSR), a single launch track

Illustration of the Silbervogel in a translated version of the original 1944 report.

aligned east-west would probably have been adequate. The spaceplane would be able to alter its flight direction after leaving the track, but the larger the maneuver the more costly it would be in terms of energy and thus range. The aircraft, the rocket sled, the track and the other infrastructure on the ground would all be reusable.

The 1944 Silbervogel design was to use a 6,000,000 Newton thrust rocket sled to accelerate to a speed of 1,800 km per hour (1,100 miles per hour) on a rail track 3 km (2 mile) long. As this would occur in only 11 seconds it would expose the plane and pilot to a substantial but certainly manageable acceleration of 4.6 G. Leaving the first stage sled behind (which would have brakes to rapidly decelerate to avoid flying off the track) the Silbervogel would climb unpowered to an altitude of 1.7 km (5,600 feet) while turning into the intended direction of flight, then continue gliding up at an angle of 30 degrees. At an altitude of 3.7 km (12,000 feet), 25 seconds after take-off, the onboard 1,000,000 Newton rocket engine would fire and in about 8 minutes push the aircraft up to an altitude of 150 km (500,000 feet). Leveling off, the Silbervogel would then continue to accelerate to a speed of 22,000 km per hour (14,000 miles per hour). At the end of its powered run the aircraft would have consumed almost all of its propellant (90% of its take-off weight) and under the constant thrust of its rocket engine reached its peak acceleration of 10 G, which is about the tolerance limit for a trained pilot. In the climbing phase of the flight the G would build up gradually, as gravity would partially counteract the rocket’s thrust, but in level flight the increase from 7 G to 10 G would occur in a mere 20 seconds. Nevertheless, the pilot would sit upright with his back to the rocket engine so that his heart would not have to pump

eled drive plin*

Illustration of the Silbervogel rail launch system in a translated version of the original

1944 report.

blood into his brain against the direction of the acceleration. Knowledge gained from Luftwaffe acceleration experiments on human volunteers and drugged primates led Sanger to believe that such acceleration would not cause a well-trained pilot seated in this position to black out.

The ultimately attained speed would be some 6,000 km per hour (3,800 miles per hour) short of that required to enter orbit, so after reaching its highest altitude the Silbervogel would gradually descend into the stratosphere. However, at an altitude of 40 km (130,000 feet) the increasing air density would generate enough lift to cause the plane to “bounce” back up to about 125 km (410,000 feet) altitude. Part airplane, part satellite, the Silbervogel would repeat this profile a number of times, with each successive bounce getting shallower and covering less distance horizontally owing to the continuous loss of speed resulting from the aerodynamic drag. The aircraft would heat up each time it hit the atmosphere but have time to cool down (by radiating the heat away) during each hop back into space.

The pilot would not have much window space to admire the view because to keep the external shape smooth, and probably also the difficulty of producing windows for extreme temperatures, Sanger foresaw the pilot being provided with side view slits and “optical aids” (presumably periscopes). However, for landing, he noted, “a kind of detachable windshield can be used, since then the pressurization of the cabin and maintenance of the bullet-shape are unimportant”. Exactly what this would have involved is not entirely clear, but it seems to suggest the pilot was to open the cockpit and stick his head out when approaching the airfield; a rather awkward and archaic way to land a hypersonic spaceplane.

Sanger’s baseline hypersonic bomber, equipped with a rocket engine capable of a specific impulse of 300 seconds, would have dropped a rather small bomb of 300 kg (650 pounds) on New York, then continued its unpowered hop-fly over the continent to land in Japanese-held territory in the Pacific. The Silverbird could only lay a small explosive egg, but the psychological and hence political impact of a German bomb of any size dropped on US soil would have been substantial (as was the first US raid on Japan). With a more efficient engine and/or closer targets, the bomb load could have been increased to several metric tons. After landing as a glider (like the Space Shuttle Orbiter) the aircraft would be re-launched back to Germany, dropping another bomb on the way. From point to point, the range would be a spectacular 19,000 to

24,0 km (12,000 to 15,000 miles). Sanger proposed that it would be possible to stretch this distance even further and enable a landing back at the launch site if the efficiency of the rocket engine could be increased. One possibility to achieve this would be to add metal particles to the fuel in order to increase the exhaust temperature, improving the specific impulse to 400 seconds and so either producing higher thrust with the same propellant consumption or running the engines at the same thrust for longer without increasing the tank volume. The basic idea was sound, because metal particles were later added to solid propellants to increase the performance of boosters such as those used by the Space Shuttle. However, for liquid propellant engines it turned out to be impractical since particles can only be kept well mixed with liquid propellants if they are turned into a gel, and doing that leads to all manner of complications.

The feasibility of bouncing off the atmosphere was inadvertently demonstrated in 1962 by Neil Armstrong when flying the X-15. He came down from a high altitude with an overly steep angle of attack (i. e. with the aircraft’s nose too far up), skipped back up and as a consequence severely overshot his planned landing site at Edwards. In fact, he flew all the way to Los Angeles. However, he managed to make a gliding turn with a huge radius and thus fly back to base. Although he bounced only once, he proved the extended range of a Silbervogel-type trajectory. Crewed space capsules returning to Earth can use similar ‘skip re-entry’ trajectories, typically bouncing only once. The Apollo capsules were capable of flying such a ‘skipping’ trajectory, but it was never an operational necessity. However, the Soviet unmanned Zond prototype lunar mission capsules did demonstrate it successfully during several flights.

A postwar analysis of the aerothermodynamics of the Silbervogel discovered that Sanger and Bredt had made an error in the heat load calculations: during the first re­entry into the atmosphere the structure of the spaceplane would have become much hotter than they predicted and consequently would have needed additional protection in the form of heat shield material which would have made the vehicle much heavier than the original design, leaving little room in terms of weight for people or bombs.

After the war Sanger and Bredt were recruited by the French government to work on missiles and ramjets. In 1949 Sanger founded the Federation Astronautique, and in 1951 he was made the first president of the International Astronautical Federation that hosts the annual International Astronautical Congress. Meanwhile in the Soviet Union, Stalin had become fascinated by the Silbervogel concept, which had come to his attention through three captured copies of Sanger and Bredt’s highly secret 1944 report. Stalin saw it as a possible means of attacking the United States because the advent of nuclear weapons meant that the puny bomb load of Sanger’s design could now yield a very powerful punch. In 1947 Stalin sent his son Vasily together with aviation experts Serov and Tokayev to either win over or kidnap Sanger and Bredt and bring them to Russia to further develop their concept. However, Vasily overly enjoyed his playboy life in Paris and Tokayev defected to the British and blew the operation, so nothing came of it. By then, however, the design bureau NII-1 NKAP, established by Mstislav Keldysh in 1946 specifically to work on a Soviet version of the Silbervogel, had further elaborated on the idea presented in the captured report. By 1947 they had concluded that with the rocket engines and structures which either already existed or were likely to become available in the near future, some 95% of the initial weight of the spaceplane would have to be propellant; Sanger and Bredt had calculated ‘only’ 90%. This would leave very little weight for the structure, tanks, engines, and other essential equipment. The poor ratio between take-off weight and structural weight is a well known issue for any spaceplane concept, even today, and particularly so for a pure rocket propelled vehicle.

The Russians decided that ramjets using atmospheric oxygen instead of internal oxidizer would make the design more feasible: the weight limitation of the vehicle without propellant (i. e. the dry weight limit) would increase from 5 to 22%, while the lower maximum speed in comparison to the Silbervogel would still give the plane an intercontinental range of 12,000 km (7,500 miles). This so-called ‘Keldysh Bomber’ would have a take-off weight of about 100,000 kg (220,000 pounds). It would start with an 11 second run along a track that was 3 km (2 mile) long, pushed by a rocket sled powered by five or six liquid oxygen/kerosene RKDS-100 rocket engines with a combined thrust of 6,000,000 Newton. The bomber would climb to an altitude of 20 km (66,000 feet) and accelerate to Mach 3 using a single RKDS-100 and a pair of wingtip-mounted kerosene ramjets. The ramjets would then flame out due to a lack of oxygen and possibly be jettisoned to shed their weight. Next, the rocket engine would push the vehicle to a speed of 18,000 km per hour (11,000 miles per hour) and up into space, after which there would be a series of atmospheric skips just like the Silbervogel. It was estimated that developments in rocket and aircraft technologies would make it possible to start serious development of the design in the mid-1950s. But by then the concept of the intercontinental ballistic missile, which was easier and cheaper to develop and did not require a far-away landing site, had made the Keldysh Bomber obsolete.

Walter Domberger, the former head of A4/V2 development in Germany, tried to interest the US military in the Silbervogel concept when he joined Bell Aircraft in 1950, carefully calling it the ‘Antipodal Bomber’ rather than its wartime moniker of the ‘Amerika Bomber’ (‘antipodal’ meaning two sites exactly opposite each other on the Earth’s surface; more or less the Silbervogel’s maximum flight distance). But the sheer technical complexity of advanced materials, hypersonic aerothermodynamics and guidance accuracy, together with the doubtful military value of such an aircraft, meant that a real project never materialized. Nevertheless Dornberger’s lobbying did eventually lead to the X-15 rocket plane, the X-20 Dyna-Soar concept and ultimately the Space Shuttle. (The Shuttle is not a true rocket plane and it doesn’t use a skip re­entry trajectory but it does resemble Sanger’s design in that it has a flat base, straight fuselage sides, a rounded top, and a series of massively powerful rocket engines in the back.) The X-15 and the Shuttle incorporated several features that Sanger had not foreseen but whose inclusion would have increased both the weight and complexity of the Silbervogel. One example is a reaction control system to control the spaceplane outside the atmosphere and to properly orientate it for re-entry. Another is the need for precise guidance control sensors and electronics. Although one X-15 pilot managed to fly a manual re-entry when his guidance control system failed, for a safe re-entry both the X-15 and the Shuttle required guidance computers that were not available in the early 1950s. Such assistance would certainly have been required for the complicated bouncing trajectory of the Silbervogel.

As a passenger transporter, the original role Sanger intended for his rocket plane, the Silbervogel would have been much too complex and expensive because of its huge propellant consumption, its elaborate take-off and landing infrastructure, and its payload mass and volume allowing only a few passengers. These considerations still plague recent concepts for hypersonic airliners such as the US National Aerospace Plane (NASP) of the late 1980s. The fact that even the less complex and more economical Concorde never reached its financial break-even point makes it difficult to see how a hypersonic passenger transport vehicle could ever be a financial success.

Sanger returned to Germany in 1954 and three years later became director of the newly-created Institute for the Physics of Jet Propulsion in Stuttgart. However, when in 1961 the German government found out that Sanger was secretly making trips to Egypt, presumably to assist a group of rogue German engineers to develop missiles that could be used to attack Israel, they forced him to resign. The loss of his position, and scant prospect that his Silbervogel would be built during his lifetime, sent him into a state of depression and his health rapidly deteriorated. Sanger was unofficially rehabilitated when it was realized he had only acted as a consultant for an Egyptian meteorological sounding rocket project and had lectured at the university of Cairo. He then continued his work by assisting with a new spaceplane project at the revived Junkers company. Based on decades of research, he proposed a 200,000 kg (450,000 pound) reusable, two-stage vehicle consisting of a delta-wing carrier rocket plane and a smaller delta-wing orbital spaceplane. A steam-rocket sled was to provide a starting velocity of about 900 km per hour (560 miles per hour) at the end of a track 3 km (2 mile) long. The steam rocket would be relatively simple, using only heated water, but not very efficient in terms of performance. However, as with the original Silbervogel’s sled, that would not be particularly important for the overall spaceplane system’s performance. Its simplicity would save development effort and cost, whilst making it reusable would be relatively straightforward due to the easy – to-handle and cheap propellant (water) and the absence of extremely hot exhaust gases to eat away at the nozzle.

The orbital vehicle, called ‘HORUS’ (Hypersonic ORbital Upper Stage), would separate from the carrier at an altitude of 60 km (200,000 feet), then proceed into a 300 km (980,000 feet) orbit while the carrier plane returned to Earth. Having fulfilled its mission, HORUS would de-orbit and glide back to Earth very much like the Space Shuttle Orbiter. Both planes would be powered by similar rocket engines, three on the carrier and one on the orbital plane, each engine delivering 2,000,000

Newton of thrust at sea level and burning the powerful combination of liquid oxygen and liquid hydrogen. This is another similarity with the later Space Shuttle, whose three main engines used those propellants and each had a sea-level thrust of

1,800,0 Newton. In fact, the entire concept has similarities to some of the Space Shuttle designs of the early 1970s. Designated the Junkers RT-8, this reusable launcher was to be able to place a 3,000 kg (7,000 pound) payload into low orbit. An improved version would later use a single-stage spaceplane with an integrated rocket-ramjet engine.

In October 1963 Sanger also accepted a professorship at the Technical University of Berlin where in February 1964, aged 58, he died while giving a lecture. Shortly before his death he wrote that he believed the US and the USSR would direct their full technological capacity towards an ‘Aerospace Transporter’ similar to the RT-8 as soon as the ‘Moon Race’ ended and that “there is therefore at the moment a unique but only a short-lived opportunity for Europe, with its great intellectual and material resources, to become active in a sector of spaceflight in which the major space powers have not yet achieved an insuperable lead”. His last spaceplane design, which after his death evolved into the ‘Sanger-Г RT-8-02, would take off vertically without the need for a launch sled and rail, but it never left the drawing board. Junkers was unable to secure enough support from the government since the ambitious, expensive and economically risky project was deemed to be a step too far.

Like Max Valier, but unlike most other rocket and spaceflight enthusiasts of the time, Sanger was a real rocket plane pioneer: he fully intended his rocket engines to propel rocket aircraft rather than ballistic rockets such as the A4/V2. Sanger was certainly a genius but, Uke Wernher von Braun, his willingness to work for the Nazi military in order to move his dream machines closer to reahty, even if it meant they would be used to bomb innocent civilians, has made him a rather controversial figure (his 1944 report contains a map of New York displaying the expected efficiency of a bombing campaign by a fleet of Silbervogel planes). Irene Bredt worked for the institute in Stuttgart until 1962 and survived her husband by 19 years. In 1970 she was honored with the Hermann Oberth Gold Medal for her scientific contributions.

Sanger and Bredt’s ideas have certainly influenced rocket plane design since the end of the war: the idea for the Space Shuttle can be traced back (via Dyna-Soar and the Bell ‘Antipodal Bomber’) to their revolutionary design. In his 1963 book, which was published in 1965 in English as Space Flight; Countdown for the Future, Sanger cites all of the reasons why, in his opinion, “aerospace planes” hold the future of spaceflight instead of expendable missiles: lower costs per flight, the ability to flight- test each type of production vehicle, the possible use of airfields in densely populated countries, easy self-transportation of the vehicle from the production plant to launch site, and the ability to use them for both orbital missions and long-distance flights. These are still the principal reasons for pursuing spaceplanes although nowadays the need for (and the advantages of) such vehicles seem less obvious. Certainly nothing resembling the Silbervogel has yet been built. In the same book he writes, “military aerospace planes will be used as reconnaissance planes, fighter planes against Earth satellites and extraterrestrial space stations, satellite inspections planes etc” and also “we can possibly count space fighter planes among the most important of all space

weapons”. At that time work on the X-20 Dyna-Soar was still ongoing, but after its cancellation work on such military orbital vehicles all but ceased (with the exception of the Space Shuttle, which was partially based on military requirements). However, considering the current renewed interest by the US Air Force in hypersonic vehicles Uke the X-51 and the Falcon Hypersonic Technology Vehicle, as well as automated orbital shuttles like the X-37, Sanger may well have been right on this after all.

ZERO-LENGTH LAUNCH

The use of Rocket Assisted Take-Off (RATO) boosters became very common shortly after the Second World War because the early jet engines delivered relatively low thrust when the aircraft was moving slowly. Heavy jet aircraft in particular needed a bit of help to get going. But soon the take-off thrust of jet engines had increased to the point that RATO was only required for heavy cargo aircraft using short runways or airfields in hot places and at high elevations (they are still in use today, but only in very limited circumstances).

A different idea for mixing rocket and jet propulsion that was investigated was to use a powerful rocket booster to shoot an airplane straight into the sky, dispensing with the need for a runway. With this ultimate stretch of the RATO concept, fighter aircraft could be launched anywhere and anytime, even from a truck trailer!

For Cold War military planners the vulnerability of airfields and their concentration of aircraft was always a major issue, especially for the early, underpowered jets that could not take off from rough fields and required especially long runways. This concern led to the need for Vertical Take-Off and Landing (VTOL) aircraft: jet fighters that (as with helicopters) would not need much more than a clearing in a forest to operate from. Most of the VTOL fighter concepts proved to be impractical but the developments ultimately led to aircraft like the swivel-nozzle Harrier, which can take off and land virtually anywhere whilst operating as a conventional jet fighter when in the air.

During the 1950s, blasting jet fighters into the air using rockets was a simpler way of liberating aircraft from their dependence on runways for take-off (although the returning aircraft would normally still need a prepared airfield for landing). The idea had already been pioneered by the British ship-launched, rocket-catapulted Hurricat fighter of the Second World War, but the new jet fighters were much heavier than the old Hurricane propeller fighter and required much larger boosters to get airborne: so large, in fact, that they were impossible to fit into the airframe and (like RATO units) had to be attached externally and jettisoned immediately after use. This had several benefits though. One was that the heavy rocket equipment did not need to be taken with the aircraft during its entire mission. Another was that not much modification to existing aircraft would be needed to accommodate the external rocket boosters. Also of benefit was that because the rockets would be jettisoned soon after take-off, they would not need to be especially light and efficient: relatively simple solid propellant boosters similar to those used in surface-to-air missiles would suffice. This concept became known as Zero-Length Launch (ZEL).

In the early 1950s the US Air Force began a program called ‘Zero Length Launch, Mat Landing’ (ZELMAL) in which a Republic F-84G Thunderjet fighter was to be shot into the air using a large solid propellant rocket booster. (The F-84 was selected because it was sufficiently light that it could be launched by already available rocket boosters.) To solve the problem of the need for a landing strip upon return, the idea was for the fighter to be equipped with a hook to snag an arresting cable suspended close to the ground in order to come to a quick stop, rather like on an aircraft carrier, except that instead of rolling to a halt the aircraft (without lowering its undercarriage) would smack down onto an inflatable mattress measuring 25 x 245 meters (80 x 800 feet) and 1 meter (3 feet) thick. An additional perceived benefit of this technique was that ZELMAL aircraft would not need an undercarriage and so would be lighter than comparable conventional fighter aircraft.

The Glenn L. Martin Company was selected to manage the development of the system, with the Goodyear Tire & Rubber Company making the air-filled mat. Tests started at Edwards Air Force Base, California, on 15 December 1953 with a pilotless F-84G being launched from a trailer normally used to fire Matador cruise missiles (it seems that the same type of rocket booster was employed). As planned, the aircraft was lobbed into the air and then crashed onto the hard desert floor. The next test less than a month later, on 5 January 1954, was equally promising. It was manned by test pilot Robert Turner and the G-levels that he experienced during the launch were no worse than during a conventional catapult launch from an aircraft carrier although he accidentally jerked one hand and throttled the engine back, almost stalling it. Turner made another flight on 28 January. Both flights went surprisingly well, and surviving movie footage (which is on the Internet) shows a very smooth operation with a fluent acceleration and a clean separation of the booster. This indicated that rocket-boosted take-offs were a feasible operational military possibility. In both tests Turner landed conventionally. Landing on the mat would prove much more problematic. The first time the rubber mat was inflated after being transported on a couple of trailer trucks, it was found to leak so badly that parts of it had to be sent back to the manufacturer. The first mat landing on 2 June 1954 became a fiasco when the aircraft’s arresting hook tore the mat wide open and caused a very hard landing. The plane was damaged beyond repair and Turner was rendered inactive for months due to back injuries. Two more mat landings were conducted but the sudden impact on the mat remained too hazardous. Test pilot George Rodney suffered a neck injury from his mat landing: “We tied ourselves into the seat real well, so we wouldn’t pitch forward into the control column and the instrument panel, but unfortunately your head, it goes into a big arc and comes down on your chest.” After 28 rocket launches ZELMAL was terminated.

The sudden lift-off of a ZEL launch must have been rather strange for the pilots: they were sitting in the cockpit of a familiar aircraft but rather than seeing a runway in front of them they were looking up into the sky at a steep angle. And instead of the reassuring, slowly growing push of the jet engine there was a sudden explosion of power hurtling them into the air. It is a bit like sitting in your own car but with an additional dragster racing car’s engine in the boot.

Despite the termination of ZELMAL the idea of launching a fighter with a rocket booster was still believed valid. The Air Force initiated a new program in 1957 that dispensed with the mat landing and so was simply named ZEL. It was to involve the launch of a nuclear-armed strike aircraft from a truck trailer which, since it could be hidden anywhere, would be hard for the enemy to destroy during a first strike. After dropping his bomb, the pilot of the undercarriage-less aircraft would simply bail out over friendly territory. To be able to carry a heavy atomic bomb into Soviet territory the aircraft would have to be much larger than the F-84. The selected F-100 Super Sabre was about twice as heavy as the F-84, so a new rocket booster was developed by Rocketdyne. This solid propellant rocket could deliver a thrust of almost 578,000 Newton for 4 seconds and accelerate the F-100 at about four G. It was affixed under the aircraft’s rear fuselage, at a slight angle so that its thrust was aimed through the center of gravity and would thus not cause any rotation. At burnout, the plane would be flying at 450 km per hour (280 miles per hour) at an altitude of 120 meters (400 feet). Preliminary tests were started with a so-called ‘iron bird’, a structure of steel and concrete that simulated the weight and the mass distribution of the F-100. These tests showed that if the booster were not precisely aligned with respect to the center of gravity of the entire contraption it could perform some very impressive backward summersaults. But this was soon fixed, and the earlier problem of the pilots’ hand on the throttle moving backwards due to the acceleration was remedied by introducing a fold-out handle which the pilot could slip his hand into.

The first manned launch of an F-100 at Edwards Air Force Base on 26 March 1958 went perfectly. Test pilot A1 Blackburn said he found the flight “better than any ride you can find at Disneyland”. On his second launch, however, the rocket did not separate, even when he tried to shake it free using wild maneuvers. He had to use his ejection seat and let the plane crash in the desert because it was impossible to land it with the big booster attached. The investigation showed the attachment bolts had not sheared off as they were intended to. Thereafter explosive charges were provided that could blow the boosters off on command. Another 14 successful flights were made by October. A sign on the trailer claimed it to be the ‘World’s Shortest Runway’. The tests were not kept secret: footage of one of these launches was used in the Steve Canyon television series. There was even a public demonstration of the system, with the pilot showing off by performing a slow roll immediately after booster separation. The technical feasibility of the ZEL concept was proven, but its operational role was not so clear.

First of all there were the practical as well as safety and security issues relating to driving a fighter with a nuclear weapon on a truck through dense forests, over narrow roads and through tunnels. Critics said it would be better to launch ZEL aircraft from fixed, protected positions. To test this idea, a few launches were performed out of a hardened shelter at Holloman Air Force Base in New Mexico, the last of which took place on 26 August 1959. But launching from fixed positions denied the flexibility and elusiveness of the mobile system. A more serious threat to the project was that the idea of sending nuclear-armed fighter aircraft into the Soviet

An F-100 ZEL with its impressive rocket booster [US Air Force].

Union was rapidly being made obsolete by the increasing reliability and accuracy of unmanned ballistic missiles. Although 148 F-lOOs were modified to enable ZEL launches, the program went nowhere.

The concept of a ZEL nuclear-armed strike fighter was picked up once more, this time by the German Luftwaffe in 1963. Working with Lockheed, they organized rocket-launch experiments using an F-104G Starfighter at Edwards Air Force Base. The Rocketdyne booster could push the Starfighter to a speed of 500 km per hour (310 miles per hour) in just 8 seconds. Lockheed test pilot Ed Brown, who performed the flights, was very impressed: “All I did was push the rocket booster button and sit back. The plane was on its own for the first few seconds and then I took over. I was surprised at the smoothness, even smoother than a steam catapult launch from an aircraft carrier.” The successful experiments were followed up by further tests at the German Air Force base at Lechfield. But this project was also canceled for the same reasons as the F-100 ZEL and because the tests using the expendable rocket boosters were quite expensive. In addition, the much more practical YTOL Hawker Siddeley GR. 1 Harrier was by then under development. This made its first flight in December 1967. It could not only take off vertically without rocket booster assistance, it could also land vertically. A German F-104G equipped with a rocket booster and a dummy nuclear missile is on display at the Luftwaffe museum in Berlin-Gatow in Germany.

In France there was a proposal for a ZEL version of the mixed-power Durandal in which the aircraft would be launched from a mobile trailer using a cluster of sohd propellant rockets attached at an angle on the tail, but this was not developed into a real system.

Soon after the US started to experiment with rocket launched F-84Gs, the Soviets initiated a very similar project using their MiG 19 (which, as described earlier, was also converted into a high-altitude mixed-power interceptor around this same time). Rather than launching nuclear-armed strike aircraft, the Soviets intended their rocket-launched interceptor to play a role similar to that they had envisaged for their earher rocket propelled aircraft, namely a fast-reaction point-defense interceptor. Launching these from truck trailers with large rocket boosters would make it possible to station them at remote locations or in battle areas where there were no suitable runways. The MiG design bureau prepared a proposal for a trailer-launch system for the MiG-19. This was given the go-ahead in 1955. MiG came up with a modified version of the MiG-19S, designated the SM-30, which had a reinforced structure to handle the high rocket thrust, the ventral fin was replaced by two new fins straddling the rocket booster, and a special headrest to protect the pilot from whiplash at the onset of the sudden acceleration.

Like the US Air Force and the Luftwaffe, the Soviets used a large, jettisonable

A standard MiG 19 at the Letecke museum near Prague, Czech Republic [Michel van Pelt],

solid propellant rocket booster which was mounted on the rear fuselage and pointed slightly downwards. The PRD-22 booster had sufficient thrust to shoot the 8,000 kg (17,000 pound) plane into the sky. The SM-30 could be transported on a large trailer-truck combination but had to be placed on another type of trailer for the launch. The plane was connected to this trailer by bolts that would shear and release the aircraft upon ignition of the booster. When launched from soft surfaces, such as from within a forest, soldiers would be required to dig a trench behind the booster to prevent the powerful exhaust from creating a fountain of loose dirt that would be visible to the enemy from far away. The first test launch was performed using a remote-controlled unmanned airplane in the autumn of 1956. The launch went smoothly (just like the Americans had experienced) but the trailer was wrecked, thus proving that it needed to be fitted with a blast shield for protection. On 13 April 1957 test pilot Georgi M. Shiyanov made the first manned flight, which was a big success. He had trained with a special launch simulator catapult (even successfully enduring an excessive 18 G acceleration during one test when a technician made an error arming the catapult). Several more SM-30 launches were performed, all of which were successful (film of the launches is on the Internet). However, landing the heavy fighter on rough and small landing strips like those envisioned to be available near forward battle areas was not so easy; getting the plane to stop before it ran out of runway was very tricky using the standard MiG-19 drag parachute and brakes. An arresting-cable system (Uke on an aircraft carrier) was therefore tried out.

In the end, however, the SM-30 project was terminated because of the difficulty of driving the large, heavy aircraft across the countryside and because an airplane which does not require a runway for take-off but does require one for landing is not all that useful. Just as the F-100 and F-104G ZEL were rendered obsolete by the introduction of long-range nuclear missiles, the SM-30 turned out to be less effective for forward air defense than the new mobile battlefield surface-to-air missiles.

FICTION

While on the subject of future spaceplanes let us have a look at how these were (and still are) depicted in fiction. The existence of spaceplanes as an efficient means of transportation between Earth and space is generally taken for granted in science fiction, much like airliners are used in the real world. Apart from ideal aircraft-like operations, one thing that most of the spaceplanes in science fiction books, movies and television series appear to have in common is an amazingly efficient propulsion system because most of the volume is available for passengers and cargo rather than being taken up by bulky propellant tanks.

Take for instance the Orion III ‘Pan Am Space Clipper’ of the famous movie 2001, A Space Odyssey of 1968. We are shown a spaceplane that operates much like an airliner, even with stewardesses. No take-off is shown in the movie but it appears that the Orion is a single stage vehicle with double-delta wings (similar to those of the Space Shuttle, which had not yet been designed when the movie was produced). Windows cover only a short stretch of the middle of the fuselage, which makes sense for a spaceplane requiring large volumes of propellant. Still, as a relatively small single-stage-to-orbit spaceplane (judging from the size of the windows) 200Гs Orion must have fantastically efficient engines; jet engines, if the series of holes that can be seen on the leading edges of the plane’s wings and in front of the engine module are air intakes. However, having intakes on the wing leading edges would be rather poor engineering: tapping off the air before it has a chance to flow over the wings would seriously reduce lift. Nevertheless, Orion is probably the most believable spaceplane ever depicted in any block-buster movie.

The most famous space fighters of the movies, the Star Wars’ X-wing and Battle Galactica’s Viper, have wings and very apparent air intakes but spend most of their time in the vacuum of space. Even so, they seem to require only very small amounts of propellant as there are no large tanks to be seen. This does not seem to make sense: if you can fly and maneuver in space for hours without any external airflow, why bother with air intakes and wings for the brief periods in an oxygen-rich atmosphere? Moreover, these vehicles seem to maneuver in space as if they were flying through air, banking into nice round turns, flying in loops etc. In space there are of course no aerodynamic lift and drag forces, making such maneuvers only possible with the help of a serious set of reaction control thrusters which aren’t apparent in these fictional designs. Moreover, such airplane-like actions are rather useless in space (but not in a movie, as it certainly looks more exciting than real orbital mechanics).

Fireball XL5, a spaceplane shown in the British 1962-1963 television series of the same name also lacks apparent propellant tank volume, but according to the series it is powered by a “nutomic reactor” that enables it to have a range of many lightyears. Although the Fireball is a fictional vehicle without any regard for the limitations of real spaceflight, it is interesting to note that it takes off using a rocket powered sled and a mile-long launch rail that culminates in a 40 degree “sky ramp”, whereupon the spacecraft uses its own propulsion to ascend into space; very similar to the take­off mode of Sanger’s Silverbird.

In reality, even although serious work on orbital spaceplanes has been underway since before the Second World War, they still only fly in the realms of fiction. Like most of the hardware depicted in 2001, A Space Odyssey, nothing like an Orion III was available in the real year 2001; nor in 2011 for that matter. Only the videophone used the movie is actually better in today’s reality. In general, science fiction often overlooks the complexity of spaceplane transportation, and especially the limitations of propulsion technology. Of course, with engines working on fantasy and built out of unobtainium, anything is possible. What the spaceplanes of the movies do show us, however, is the ultimate goal of the real-world launch business: airline-like flights into space.

Breaking the barrier

“You’ve never been lost until you’ve been lost at Mach 3.” – Paul F. Crickmore

(test pilot)

Until about the start of the Second World War the strange phenomena which develop at transonic speeds were academic, since the propeller aircraft of the day did not fly that fast. But starting in 1937 mysterious accidents began to occur at high speeds. An experimental early version of what was to become Germany’s most potent fighter of the war, the Messerschmitt Bf 109, disintegrated as its pilot lost control in a fast dive. Pretty soon other new, high performance military airplanes were running into similar difficulties. For these fast, propeller-driven fighters the airflow over the wings could achieve Mach 1 in a dive, making air compressibility a real rather than a theoretical issue.

In the US, the aeronautics community was rudely awakened to the realities of this unknown flight regime in November 1941 when Lockheed test pilot Ralph Virden was unable to pull his new P-38 fighter out of a high-speed dive and crashed (due to the problem of ‘Mach tuck’ described in a previous chapter). It became clear that any propeller fighter pilot who inadvertently pushed his fast plane into a steep dive was risking his life. Aggravating this problem was that bombers were flying ever higher, which meant that in order to reach their prey interceptors had to venture into the thin, cold air of the stratosphere in which the speed of sound was lower, and thus issues of air compressibihty occurred at slower flight speeds than they did when flying nearer the ground.

A really thorough understanding of high-speed aerodynamics was initially not necessary, because measures to prevent control problems focused on limiting the dive speed and temporarily disrupting the airflow to prevent shock waves from forming on the wings and controls. Due to the limitations of propellers and piston engines, it was accepted that conventional aircraft would never be able to fly faster than sound.

Then jet and rocket aircraft appeared. These were quickly realized to require the potential to fly at Mach 1 or even faster for extended durations, so a real understanding of transonic and supersonic aerodynamics rapidly became a ‘hot’ issue that promised real military advantages. The name ‘sound barrier’ had been coined by a journalist in 1935 when the British aerodynamicist W. F. Hilton explained to him the high-speed experiments he was conducting. In the course of the conversation Hilton showed the newsman a plot of airfoil drag, explaining: “See how the resistance of a wing shoots up like a barrier against higher speed as we approach the speed of sound.” The next morning, it was incorrectly referred to in the newspaper as “the sound barrier”. The name caught on because the issues which conventional high-speed aircraft invariably encountered on reaching transonic speeds gave the impression that the magic Mach number was indeed a barrier that would need to be overcome.

It not only represented a physical barrier, but also a psychological one: there were many skeptics who said supersonic flight was impossible because aerodynamic drag increased exponentially until a veritable wall of air emerged. They pointed to the loss of the second prototype of the de Havilland DH 108 Swallow on 27 September 1946. This high-speed jet disintegrated while diving at Mach 0.9, killing pilot Geoffrey de Havilland Jr., and crashing into the Thames Estuary. Shock stall had pitched the nose downwards, and the resulting extreme aerodynamic loads on the aircraft cracked the main spar and rapidly folded the wings backwards. Others, however, noted that rifle bullets could fly at supersonic speeds, so the sound barrier was not an impenetrable wall. Indeed, during the war it had been realized that streamlined bullet shapes were ideal for supersonic speeds. This is how rockets such as the German А4/ V2 got their familiar shape. The V2 achieved Mach 4 as it fell from the sky towards London. In fact, because it fell faster than the speed of sound there was no audible warning of the imminent danger until the impact reduced whole blocks of houses to rubble; only afterwards did the sound arrive. But aircraft need wings, rudders, ailerons and other devices to develop lift and facilitate control, making their aerodynamics much more complicated than those of bullets and rockets. The traditional tool for gathering aerodynamic data and developing new aircraft and wing shapes was the wind tunnel. However, the technology available at that time did not permit accurate and reliable measurement of airflow conditions at transonic speeds: the aircraft models placed in the wind tunnels would generate shock waves in the high-speed air flowing around them, and these in turn would reverberate and reflect across the test section of the tunnel. As a result, there was a lot of interference and the measurements of the model did not correlate to the real world in which aircraft flew in the open air rather than in an enclosed tunnel. Also, you can scale an aircraft but you cannot scale the air, so air flowing around a small-scale model does not necessarily behave in the same way as air flowing around a real airplane.

The least understood area was from about Mach 0.75 to 1.25, the transonic regime where the airflow would be unstable and evolve quickly, and for which no accurate aerodynamic drag measurements and theoretical models were available. It was called the ‘transonic gap’; the aerodynamicists nightmare equivalent to the ‘sound barrier’ so dreaded by pilots. The aerodynamic drag is especially high in this range of speeds, peaking at just below Mach 1. However, it actually diminishes considerably at higher supersonic speeds (which is why modern aircraft either fly well below or well above Mach 1, spending as little time and fuel as possible at transonic speeds). Drag occurs at transonic speeds for two reasons: firstly as a result of the build-up of shock waves where the airflow reaches Mach 1 (typically over the wings), and also because the air behind the shock waves often separates from the wing and creates a high-drag wake. At even higher speeds the shock waves move to the trailing edge of the wing and the drag-inducing air-separation diminishes and finally vanishes, leaving only the shock waves.

All major military powers realized that if their aircraft were to remain state-of- the-art and competitive, then transonic aerodynamics was an area that really needed to be explored and mastered. Specialized and heavily instrumented research aircraft would be needed, speeding through the real atmosphere rather than a wind tunnel. In effect these airplanes were to be flying laboratories. In the US, work was started on the Bell X-l, the first of the famous X-plane series and the first aircraft to break the dreaded sound barrier. The Russians initiated their transonic research using the captured German DFS 346, but soon moved on to designing their own aircraft.

The UK started development of the Miles M.52 research aircraft in 1943. It was to be powered by an advanced turbojet (because the British had considerable experience on such engines, and little on rockets). The jet’s fuel economy meant it would be able to take off using its own power. The M.52 might have become the first plane ever to exceed Mach 1 if the secret project hadn’t been canceled by the new government in early 1946, weeks before completion of the first prototype for subsonic testing. Apart from dramatic government budget cutbacks, one reason for the cancellation was that, based on captured German research, it was feared that the M.52’s razor sharp but straight wings were unfit for high-speeds and that swept-back wings were a must for supersonic flight. The Miles engineers had thought about a delta wing for the M.52 during the war, but discarded it as being too experimental for their short-term project. However, the Bell X-l did not have swept wings either, and both aircraft used all-moving horizontal stabilizers to preclude shock-stall problems (interestingly, the Bell engineers got the idea for the special tailplane from the M.52 team during a visit to Miles Aircraft in 1944). Had the British continued their ambitious project, they could have beaten the US in breaking the sound barrier: in 1970 a review by jet engine manufacturer Rolls Royce concluded that the M.52 would probably have been able to fly at supersonic speeds in level flight.

A 30%-scale radio-controlled model of the original M.52 design powered by an Armstrong Siddeley Beta rocket engine and launched from a de Havilland Mosquito did reach a speed of Mach 1.38 on 10 October 1948. This was quite an achievement, but by then the manned X-l had stolen the show with its record-braking Mach 1- plus flight over the desert of California about a year earlier. In spite of the UK’s prowess in aeronautical design and records, there never would be a British counterpart to the X-plane series of the Unites States.

The development of specialized rocket aircraft purely to reach extreme speeds and altitudes went in parallel with that of the rocket/mixed-power operational interceptor. However, where the interceptors needed only to go as high as the maximum altitude that enemy bombers could achieve, there were no limits for the experimental aircraft: they were meant to provide information on entirely new areas of aerodynamics and aircraft design, and their designers and pilots kept on coaxing ever more impressive performance from them. Rocket engines proved to be very appropriate for propelling research aircraft up to extreme speeds and altitudes, since

endurance was not of great importance. Rocket engines were light and relatively simple compared to jet engines of similar thrust, and because they did not need air intakes this made it much easier to design airframes suitable for supersonic flight. From the 1940s through to the late 1960s the rocket propelled X-planes achieved velocities and altitudes unrivaled by contemporary jet aircraft, with some of their pilots gaining ‘astronaut wings’. The rocket powered interceptor turned out to be a dead end but the early rocket research aircraft led to the Space Shuttle and the current designs for future spaceplanes. New experimental rocket planes are still being developed, although when they fly they are often unmanned.

If flying mixed-propulsion interceptor prototypes was a risky business, then the pilots of early experimental research rocket aircraft had a truly dangerous job. These aircraft were, by definition, going beyond the known boundaries of velocity, altitude and aerodynamics; what pilots refer to as “pushing the envelope”. Such planes had to incorporate new, often hardly tested, technology such as experimental wing designs, powerful rocket engines and innovative control systems. Unsurprisingly, whilst being tested several of these experimental aircraft crashed, blew up, or were ripped apart by aerodynamic forces. There were no accurate computer simulations and knowledge databases to warn of design errors, incorrect assumptions and unexpected situations that are nowadays resolved long before a new airplane makes its first test flight. In fact, the research aircraft of the 1940s, 1950s and 1960s were providing the data required to set up such models, and they had to obtain it the hard way. Modern aerodynamic design tools still depend on the experience gained in those years.

In addition, the means of escaping from a plane heading for disaster were much more limited than for today’s test pilots, who have sophisticated avionics on board to tell them what is happening to their aircraft, and reliable ejection seats which permit a bail-out at any speed and altitude. In a recent interview for NOVA Online, Chuck Yeager, the first man to break the sound barrier in the X-l, summarized the test pilot philosophy of time as follows: “Duty above all else. See, if you have no control over the outcome of something, forget it. I learned that in combat, you know… you know somebody’s going to get killed, you just hope it isn’t you. But you’ve got a mission to fly and you fly. And the same way with the X-l. When I was assigned to the X-l and was flying it I gave no thought to the outcome of whether the airplane would blow up or something would happen to me. It wasn’t my job to think about that. It was my job to do the flying.” The urgency of Cold War developments, as well as an acceptance of loss of life ingrained into pilots and aircraft developers during the Second World War, meant high risks were taken and many test pilots perished as their new aircraft succumbed to some overlooked detail in the design.

SPACEPLANE REVIVAL

By the mid-1980s it had become clear that a spaceplane based on rocket propulsion only was very difficult to achieve, and that introducing airbreathing engines for at least part of the flight would be necessary. Whilst a single-stage orbital rocket plane must consist of at least 90% propellant, for an efficient airbreathing spaceplane this drops to less than 70%. In other words, for the same payload and propellant weight the structure weight of an airbreathing spaceplane can be more than tripled to over 30% instead of 10% of the overall weight. The dramatic relaxation of this constraint just might make a single-stage spaceplane possible, although it would require a propulsion system significantly more complex than a pure rocket design.

In Germany in 1985, Messerschmitt-Bolkow-Blohm (MBB, with which Junkers had been merged in the meantime) revived the idea for a two-stage, horizontal take­off and landing (HTHL) spaceplane. They named the new concept the ‘Sanger-IF, and its goal was to lower the launch price of satellites and other cargo by a factor of three to ten. As with the Sanger-I it would involve a large, delta-winged hypersonic carrier aircraft and a smaller orbital vehicle, but the carrier would take off from a regular runway and instead of rocket engines it would use airbreathing turboramjets with liquid hydrogen as fuel. Since no oxidizer would be carried, this would result in great mass and volume savings. However it required five very complicated engines that would work as turbojets at speeds up to Mach 3.5 and then as ramjets at higher velocities (the Lockheed SR-71 Blackbird had turboramjet engines but the Sanger-II carrier would be required to fly much faster than the SR-71’s top speed of Mach 3.2). It would be 84 meters (275 feet) long and have a wingspan of 41 meters (135 feet). After taking off from a runway in Europe the carrier aircraft would fly at Mach 4 to

Model of the Sanger-II concept [МВВ].

the appropriate latitude for its intended orbit (typically south of Europe, closer to the equator) at an altitude of 25 km (82,000 feet) in order not to pollute the critical ozone layer of the atmosphere. Next it would accelerate to Mach 7 and fly up to an altitude of 31 km (100,000 feet) to release its upper stage, which would use its own rocket engine (burning liquid oxygen and liquid hydrogen) to continue into orbit while the carrier glided back to base. In a modified form the first stage would also be able to function as a hypersonic airhner capable of flying 230 passengers at a top speed of Mach 4.4 for a distance of 11,000 km (6,500 miles); e. g. Frankfurt to Cape Town in under 3 hours. The upper stage would either be a non-reusable, unmanned ‘Cargus’ with up to 15,000 kg (33,000 pounds) of payload, or a HORUS reusable shuttle with a two-person crew and either 36 passengers or 3,000 kg (7,000 pounds) of cargo. The maximum lift-off weight of the Sanger-II would be 350,000 kg (770,000 pounds).

The German government funded a concept study in order to refine the design, as well as a technology development program that led to ground runs of Europe’s first turboramjet engine at MBB in 1991. However, it was concluded in 1994 that full development would be much too costly and the operational launch cost savings in comparison to those of expendable launchers too uncertain, so the entire project was canceled. All that seems to be left are a large scale model of the spaceplane and a laboratory-sized ramjet demonstrator, both of which are on display in the German Technik Museum Speyer (close to the jet-engined Buran shuttle).

The Sanger-II project was part of a kind of spaceplane revival that began in the mid-1980s and ended abruptly in the mid-1990s. This renewed interest was prompted by the rationale that something new was required in order to cut the costs of access to space in comparison with uneconomical expendable rockets and the Space Shuttle, and also that the technology necessary for this was now within reach. In parallel with Germany’s Sanger-II the US devoted a lot of effort to the aforementioned NASP, the British worked on their HOTOL spaceplane, and France, Japan and Russia were all independently working on reusable aircraft-like launch vehicles. It appeared that the days of the expendable launch vehicle were finally numbered.

Star-Rakers loading cargo at an airport [Rockwell International].

Even as the Space Shuttle was being developed, in the US in the 1970s there were many studies of a possible successor in the form of a single-stage-to-orbit reusable launch vehicle. The intrinsic weight issues led to the conclusion that complicated tri­propellant rocket engines would very likely have to be created for rocket propelled spaceplanes. These would initially use low-energy but high-density kerosene as fuel to generate high thrust for take-off with a limited tank volume, and then low-density but high-energy liquid hydrogen to efficiently accelerate up to orbital velocity. Sled- launch systems such as Sanger envisioned were also seen as a potential solution; for instance the Reusable Aerodynamic Space Vehicle single-stage spaceplane proposed by Boeing, which envisaged using two Space Shuttle Main Engines for propulsion. Rockwell International offered an alternative concept called the ‘Star-Raker’, a delta-winged HTHL SSTO with ten (!) “supersonic-turbofan/air-turbo-exchanger/ ramjet” engines, three large rocket motors and an undercarriage that would be jettisoned and recovered by parachute. The company issued colorful illustrations showing several Star-Rakers at a commercial airport with their hinged noses open to load cargo, thus emphasizing airline-like operations.

The various studies by NASA, the US Air Force and contracted industries in the 1970s led to a classified military program called Copper Canyon (as with most secret military programs the meaningless name was intended to mask what it was about) that ran between 1982 and 1985, and out of which the US National Aerospace Plane (NASP) emerged as the less classified follow-on announced by President Reagan in his State of the Union of 1986. NASP was to lead to an air-breathing scramjet HTHL spaceplane prototype designated the X-30, operational derivatives of which would be able to function as a single-stage-to-orbit launch vehicle, a hypersonic airliner called the Orient Express (somewhat similar to the dual use the Germans had in mind for the first stage of their Sanger-II), or a military Mach 12 reconnaissance plane and/or strategic bomber offering the response speed of a ballistic missile and the flexibility, accuracy and ‘recallability’ of a bomber. (RecallabiUty means that you can change your mind about blowing some place to bits when the bomber is already on its way, something impossible with a ballistic missile.) Early artistic impressions showed an elegant Concorde-like design which looked as sleek and fast as it was intended to become.

The program was jointly run by NASA and the Department of Defense. In 1990 Rockwell International became the prime contractor for its development. By then the spaceplane had grown considerably in weight and size in comparison to the original design of 1984. Having lost its resemblance to the Concorde, it now had a wedge-shaped aerodynamic configuration called a ‘waverider’ (essentially a hypersonic surfboard) with most of the lift being generated by a shock wave compressing the air below the plane. This shock wave, created by the forward fuselage, would also compress the air before it entered the engines, effectively

NASP as NASA imagined it in 1990 [NASA].

supplying the scramjets with more oxygen. The aft fuselage formed a gigantic integrated nozzle for expanding the scramjet’s exhaust. There were small wings to trim the aircraft and provide control. Its overall configuration was ideal for efficient high-speed flight but gave poor lift at low speeds and in particular for taking off.

Much new technology was required, including a lightweight, composite-material hydrogen tank and advanced computer programs for modeling the airflow around the aircraft and through the engines. The hydrogen fuel would require to be carried in the form of a slush (liquid and ice mixture) to limit the volume of the propellant tanks and therefore the size, and most importantly weight, of the aircraft’s structure (a kilogram of hydrogen ice has a lower volume than a kilogram of liquid hydrogen). Heat-resisting carbon materials would be needed for the aerodynamic surfaces that would endure temperatures over 1,700 degrees Celsius (3,000 degrees Fahrenheit) during hypersonic ascent and atmosphere re-entry, and titanium aluminide panels for most of the fuselage. A major hurdle was the development of the scramjet in which liquid hydrogen would be injected into the combustion chamber and be ignited by the hot compressed air rushing in at hypersonic speed. The exhaust would primarily consist of water vapor and be environmentally friendly but the decision to use hydrogen as fuel was mainly driven by the need for high performance and high efficiency.

The NASP design incorporated the clever idea of using the atmospheric heating of the vehicle to increase the thrust of its scramjet engines: by circulating hydrogen fuel through the plane’s skin to warm it up prior to injection into the engine the energy generated by atmospheric drag was effectively added to the thrust of the scramjet. At the same time the cryogenic hydrogen flow would cool the aircraft. It was initially believed that this revolutionary scramjet propulsion and temperature control system would make it possible for NASP to reach Mach 25 in the high atmosphere; enough for it to achieve orbit without the need for additional rocket engines. However, as the development progressed it became clear that the maximum speed attainable would be about Mach 17, as at higher speeds the weight of the active thermal control system would exceed that which would be required to add conventional rocket engines and propellant. Hence for NASP to be used as a single-stage-to-orbit spaceplane it would need additional rocket propulsion. In fact, rocket propulsion would be required also to get NASP up to Mach 3 for the scramjets to take over. And rocket propulsion would be required to perform the deorbit maneuver at the end of an orbital mission. To keep the vehicle sufficiently light, it was planned to use a new type of rocket engine that, apart from onboard fuel, would be fed with air liquefied during atmospheric flight by a combination of ram-compression and cooling by liquid hydrogen; an airbreathing rocket engine known as LACE (‘Liquefied Air Cycle Engine’). Updated requirements stated that the X-30 must carry a crew of two and that, although it was an experimental vehicle, it must also be able to deliver a small payload into orbit. Hence rather than a demonstrator, the X – 30 was to be a semi-operational vehicle. It was supposed to be able to fly a mission every 72 hours (compared to a maximum of one per month for the Space Shuttle) and require only 100 workers for its operations. The ensuing fully operational launch vehicle derivative was expected to cut the cost per kg of payload by a factor of ten.

Meanwhile the estimated cost of the full development into an operational vehicle had increased far beyond the originally projected $3.3 billion for a relatively modest technology demonstrator. By the early 1990s the projected cost for the demonstrator was $17 billion and the fully operational launch vehicle would require another $10 to $20 billion (in 2011 dollars those numbers would be close to 27 and 16 to 32 billion, respectively). It was also expected that another two decades of development would be required to master all the relevant technological issues prior to building a working prototype. Moreover, the argument that NASP would enable airplane-like operations that would result in low costs per flight and rapid turn-around times sounded awfully similar to those predicted but never achieved for the Space Shuttle. The use of liquid hydrogen alone would mean a complete departure from conventional airport storage and distribution facilities, essentially ruling out the use of normal airfields because it would be prohibitively expensive to equip a sufficiently large network of them with the required production, storage and handling facilities. The severe cost increase and schedule stretch, uncertain operational benefits, and the necessity to comply with too many (civil space, commercial airline and military) requirements eroded support for NASP below the critical level. In addition, the collapse of the Soviet Union severely reduced the push in the US for ambitious technical and military programs. Inevitably, it was terminated in 1993.

NASP was initially superseded by the Hypersonic Systems Technology Program (HySTP) in which NASA and the Department of Defense continued with technology development on a less ambitious scale, but when the Air Force withdrew in 1995 the development of a US spaceplane pretty much expired.

During the winter of 1993-1994 the Air Force’s Phillips Laboratory conducted a six-week study of an interesting alternative called ‘Black Horse’ which called for a single-stage rocket propelled spaceplane using hydrogen peroxide and kerosene. This non-cryogenic propellant combination is better suited than cryogenic fluids such as liquid hydrogen and liquid oxygen for rapid-reaction launches for military purposes, and furthermore has a much higher density leading to smaller tanks and therefore an overall smaller, lighter aircraft. But it lacks the specific impulse required for a single­stage spaceplane. To compensate for this lack of performance (and made practicable by the use of storable fluids) once the Black Horse was airborne it would rendezvous with a large subsonic tanker aircraft (a converted airliner) for aerial refueling prior to continuing into orbit. The spaceplane would effectively start its flight not from the ground at zero speed but from 13 km (43,000 feet) and Mach 0.85 (a more advanced system could involve a newly developed supersonic tanker able to do the refueling at a much higher speed). As a pure rocket plane the structure weight constraints for this Aerial Propellant Transfer (APT) vehicle where nevertheless severe. And although aerial refueling is standard procedure for agile military jet fighters, doing it with an

84,0 kg (185,000 pound) rocket plane would be a different matter entirely. In early 1994 Martin Marietta investigated a near-term suborbital X-plane with turbofans and liquid oxygen/kerosene rocket engines to demonstrate the APT concept. Flying up to about half orbital speed, Mach 12, this aircraft could fire an expendable upper stage to insert a satellite into low orbit, or use a Sanger-type atmospheric bounce trajectory to fly for long distances or drop bombs. Being about half the size of the Black Horse design it was called ‘Black Colt’. Several people who worked on these studies went on to establish the Pioneer Rocketplane company in 1996 to develop a commercial satellite launcher based on ATP called ‘Pathfinder’. However, they quickly found out that a switch from hydrogen peroxide to liquid oxygen as oxidizer was required to turn Black Colt into an effective launch vehicle. In the late-1990s, just as the size and complexity of the vehicle increased, the intended market of launching constellations of communications satellites evaporated due to develop­ments in less costly terrestrial mobile phone networks. Without a strong demand, the Pathfinder could no longer be pursued.

The British HOTOL (short for ‘Horizontal Take-Off and Landing’) was a concept for an unmanned, single-stage-to-orbit spaceplane able to use ordinary runways. It would require the development of a novel propulsion system that combined turbojet, ramjet and rocket engine elements. As with the Sanger-II and X-30/NASP, it would use hydrogen as fuel and draw upon atmospheric oxygen as much as possible in order to save the weight and volume of oxidizer that the vehicle would otherwise be required to carry.

In 1982 rocket engineer Alan Bond came up with the idea for a new type of engine combining airbreathing and rocket propulsion that he thought would enable a spaceplane to consist of only one single vehicle (like NASP). Around the same time Robert Parkinson of British Aerospace (BAe) was conducting a study of a reusable launch vehicle. Bond’s engine design, the rights for which had by then been bought by Rolls Royce, was combined with BAe’s launch vehicle concept and in 1985 the BAe/Rolls Royce HOTOL project was bom. It became an official national program in 1986 when the government decided to fund a 24 month proof-of-concept study led by Parkinson and John Scott of Rolls Royce.

The design that emerged looked somewhat like a torpedo with small delta wings at the rear, a single moveable fin up near the nose, and air intakes at the aft-bottom fuselage. It would be powered by four of the Rolls Royce RB545 Swallow engines, which in the atmosphere would use liquid hydrogen to pre-cool the hot air entering the engine and thereby make unusually high compression possible. The air initially entering the engine would have a temperature of about 1,000 degrees Celsius (1,800 degrees Fahrenheit) because it would arrive at high speed and then be slowed down almost to a standstill, resulting in almost all kinetic energy being converted into heat. Subsequent compression in the engine for efficient combustion and thrust generation would increase the air temperature even further, so starting off with the hot air would quickly lead to unacceptably high temperatures in the engine. However, using pre-cooling the compression would start with air which had been chilled to minus 130 degrees Celsius (minus 200 degrees Fahrenheit), yielding less extreme temperatures after high compression. In addition, cooling the air would avoid the need for heavy, high-temperature materials in the compressor section of the engine. Some of the now relatively warm hydrogen coming out of the air pre-cooler system would be used to drive the engine’s hydrogen turbopump while the rest would be burned together with the cooled air in the turbojet section of the engine at relatively low speeds and in the ramjet section at supersonic speeds. At an altitude of 26 km

The British HOTOL concept [BAe].

(85,000 feet) and flying at Mach 5 the engines would be switched to pure rocket mode to bum hydrogen with onboard liquid oxygen instead of air. Unlike NASP, there would not be any scramjet. The return flight would be completely unpowered.

From the very start, HOTOL was to be able to fly into space and back completely automatically, without a crew and with limited contact with ground control stations. Any astronaut passengers would merely be payload in a special container that would fit into the cargo bay located in the middle of the fuselage.

The development of HOTOL soon came up against a typical spaceplane issue that must have also plagued NASP and Sanger-II: the large movement of the center of gravity as the flight progressed. Most of the propellant was housed in the forward and center fuselage whilst the heavy and fixed weight of the propulsion system was all necessarily placed at the aft end of the vehicle. This meant that the plane’s center of gravity moved significantly aft as propellant was consumed. At the same time the large range of Mach numbers meant the aerodynamic center of air pressure moved significantly during the flight: it being aft while accelerating during the ascent, and forward while slowing down upon return. Another complicating factor was that the plane would take off with virtually full tanks but would later approach the airport and land with empty tanks and very little payload, so the center of gravity would thus be completely different at the same relatively low velocities at the start and the end of a mission. Balancing the centers of gravity and pressure became very tricky for the HOTOL lay-out, forcing the designers to modify the fuselage and to locate the wings far aft in order that their lift could counteract the weight of the engines and keep the plane stable at all speeds and propellant loads. But the new configuration that solved the balancing problems required a reduction of the payload. This was compensated by abandoning the conventional aircraft take-off in favor of a rocket propelled trolley (resulting in a similar take-off approach to that Eugen Sanger had in mind for his Silbervogel). This was a serious departure from the originally envisaged aircraft-like operations capability and significantly lowered the commer­cial attractiveness of the project. And when the design team nevertheless found that they had to resort to all manner of untried, experimental materials and structure technologies to maintain a decent payload at a reasonable flight price this caused the government to withdraw its support in 1988, and shortly thereafter Rolls Royce also pulled out. In an effort to save the project in 1990 BAe approached the Soviets to study launching a HOTOL-derived vehicle off the back of their Antonov An-225 (as planned for their MAKS concept). This so-called ‘Interim HOTOL’ would also abandon the complex RB545 engines and instead use conventional rocket engines, possibly of Russian pedigree. Thus HOTOL would become a pure rocket plane launched by a subsonic carrier first stage that would release it at an altitude of 9 km (30,000 feet) and a starting velocity of about Mach 0.7. Antonov also studied the possibility of fitting the An-225 with two additional jet engines (turning it into an An-325) to carry a larger version of the vehicle. But neither the UK government nor the European Space Agency expressed any serious interest and the disintegrating USSR could ill afford to participate, so in 1992 the project was canceled.

Whereas Sanger-II was to be a jet-engined hypersonic carrier aircraft for a rocket propelled upper stage, and NASP was initially conceived to be an airbreathing-only hypersonic spaceplane, the original HOTOL was to fly itself into space using rocket motors and would therefore have been a real rocket plane in that respect (albeit with additional jet propulsion). On the other hand it was to fly unmanned, making it more like a missile than a plane capable of being controlled by a pilot at will. For these spaceplanes the distinction between a jet aircraft, a pre-programmed launcher and a rocket plane becomes rather ambiguous.

The Soviets also had their own aerospaceplane projects. From the mid-1970s to the late 1980s the Myasishchev Experimental Design Bureau worked on the MG-19, a rather megalomaniac concept involving a triangular lifting body with a 500,000 kg take-off weight and a 40,000 kg payload to low orbit. It would use turbojets and then scramjets to get to Mach 16, and subsequently rocket propulsion with hydrogen fuel superheated by a nuclear reactor to achieve orbit. The complexity of the vehicle, the dangers involved in flying a nuclear reactor in a hypersonic aircraft, and the priorities of the Buran space shuttle project meant that by the 1990s this project was gone. The idea of using nuclear power in a spaceplane was not new (since the aforementioned Martin Astroplane with its nuclear magnetohydrodynamic engines had already been proposed in the US in 1961) but it is surprising that Myasishchev still considered it to be a sane solution as late as the 1980s.

The USSR’s specific response to NASP was the Tupolev Tu-2000, a long-range heavy bomber and single-stage-to-orbit vehicle whose turbojet/scramjet/rocket engines used liquid hydrogen and liquid oxygen. Development of this spaceplane started in 1986, aiming for an initial experimental two-person design called the Tu – 2000A that would be capable of reaching Mach 6. After the collapse of the Soviet Union work continued in Russia with two tests of an experimental, sub-scale scramjet at subsonic and supersonic speeds up to Mach 6 using S-200 tactical missiles in 1991 and 1992. However, in 1992 the project was suspended owing to a lack of funds. From the few concept images available it appears that the Tu-2000 would have looked very similar to NASP.

In Japan some spaceplane development was ongoing during the late 1980s and early 1990s under the designation ‘Japanese Single Stage To Orbit’ (JSSTO). Four LACE propulsion units were to drive the vehicle to Mach 5, then six scramjets would accelerate it to Mach 12. Further acceleration to orbital speed would also be achieved using the LACE engines, but now being above the ‘sensible’ atmosphere the engines would be fed air that was liquefied and stored in tanks earlier in the ascent, a process called ACE (‘Air Collection Engine’). Mitsubishi Heavy Industries tested a sub-scale scramjet in a hypersonic wind tunnel in 1994 but the work does not seem to have progressed much further than that.

In a research effort named STS (Space Transportation System) 2000, Aerospatiale in France in the late 1980s and early 1990s investigated a single-stage ramjet/rocket spaceplane that looked similar to the Concorde, as well as a Sanger-II-like concept in which a ramjet/rocket aircraft carried a rocket propelled second stage spaceplane that would be separated at Mach 6. Another French company, Dassault Aviation, worked on a Mach 7 scramjet aircraft that would air-launch an expendable Ariane 5 second stage carrying a Hermes-derived shuttle. This was called STAR-H, for ‘Systeme de Transport spatial Aerobie Reutilisable – Horizontal’ (i. e. airbreathing reusable space transportation system for horizontal take-off and landing). These French concepts were not particularly new, all resembling (aforementioned) ideas from the mid-1960s such as the European Space Transporter, the Dassault Aerospace Transporter and the various two-staged spaceplanes studied by the British Aircraft Corporation.

Of all the spaceplane projects of the 1980s, NASP, a single-stage-to-orbit vehicle with (at least initially) only airbreathing propulsion, appears to have been the most ambitious and most complex. Nevertheless, despite the large amount of technology development that was carried out, not much of this is in the public record; the project was so advanced that even today many of its details remain secret. The Sanger-II was theoretically the least complex by using two stages instead of the more constraining single-stage-to-orbit approach and incorporating airbreathing and rocket propulsion in two separate vehicles. Sanger-II’s first stage would not fly fast enough to require exotic thermal protection materials and scramjets; conventional titanium panels (with additional carbon layers at the hot-spots) and relatively simple ramjets would suffice. During re-entry, the vehicle, being mostly large and now empty tanks, would have a significantly lower density than the Space Shuttle Orbiter (hence a larger surface area in comparison to its weight) and so would encounter relatively benign temperatures. In contrast to NASP it would have been able to put substantial payloads into orbit and operate as a real launch vehicle, potentially beating everybody else to the market. However in the end all of the spaceplane concepts proved to be too far ahead of their time: scramjet and combined propulsion technology was not yet sufficiently developed and the computer programs for simulating airflow and combustion at high speeds and temperatures were not mature enough. An article on NASP in the magazine Flight International in October 1987 had already included an ominous warning: “The odd thing is that the excitement is based almost entirely on theoretical research and small-scale laboratory work. Nobody has run a Mach 25 scramjet continuously for more than a few seconds, and no powered atmospheric vehicle has attained anything like the speeds envisioned for NASP.” Nevertheless, in March 1992 Popular Mechanics enthused “Space Race 2000 is on”, anticipating one of the international spaceplane contenders developing a real vehicle by the turn of the century. But by the mid-1990s all the spaceplane projects had more or less gone: the ideal, airliner-like spaceplane as described in the Introduction of this book was (and still is) a long way off.

Part of the issue is that whereas for a conventional aircraft the design can be split into elements like fuselage, wings and engines, and each can (to some extent) follow an independent development track, a spaceplane requires a fully integrated approach in which any small change of one item can have dramatic implications for the rest of the design. For HOTOL for instance, it turned out that for each additional kilogram of inert weight that the vehicle design gained (say, an additional piece of electronics or thermal insulation) some 25 kg of additional propellant would be needed to restore the vehicle’s performance; and additional tank and structure mass would be required to house this extra propellant, which itself further increased the overall weight. It was a vicious circle. Another parameter with a dangerously strong growth factor was the specific impulse of the rocket propulsion, where a change of just 1 % would impose a 4% change in the gross lift-off weight. Therefore a slight increase in rocket engine performance would pay off but over-optimism concerning the engine’s performance could make the vehicle considerably heavier, larger and more expensive. Accepting a slight reduction in speed to compensate for weight growth is possible in the design of normal aircraft, but a no-go for a spaceplane that needs to achieve a fixed minimum velocity if it is to enter orbit. As only 3% of HOTOL’s take-off weight would consist of payload, it would not require many design changes to consume all of this weight allocation for useful cargo and thus render the vehicle completely useless. The Concorde had a 5% payload but its design was considerably less sensitive to changes because it flew much slower than HOTOL and had more scope to trade-off between payload, speed and range; flying slightly slower than originally envisaged does not a ruin an aircraft design, but for a spaceplane it means the difference between achieving orbit or not.

The very high sensitivity of its performance to the input assumptions makes the design of a single-stage-to-orbit vehicle particularly difficult, and obliges engineers to pay very close attention to the fine detail. It also requires that a wide range of new technologies be advanced to maturity before the spaceplane design can be finalized, because any small discrepancy in expected weight or performance can have major consequences for the entire vehicle design. All this, plus the need to make the vehicle easy to maintain and operate, makes clear just how daunting the task of developing a spaceplane is.

BACHEM’S BUDGET INTERCEPTOR

Another German rocket interceptor that posed an extreme risk to its own pilots was the Bachem Ba 349. This was really a surface-to-air anti-aircraft missile with a pilot on board to compensate for the absence of sufficiently advanced electronics, and was designed in response to a request by the Luftwaffe’s Emergency Fighter Program in early 1944 for a ‘Verschleissflugzeug’ (literally ‘Wear-and-tear Aircraft’, meaning a short-life aircraft). The situation had changed dramatically since the start of the war. Gone were ‘Blitzkrieg’ (Lightning War) and aerial supremacy for the Luftwaffe. The war had become one of attrition, and Germany, its heartland now under attack, was unable to compete with the industrial might of the US. The Luftwaffe no longer ruled the skies over the occupied countries, and was rapidly losing control of the airspace above Germany. Some radical defense system was needed to stop the Allied bombers from destroying the German industries, cities and transportation infrastructure. This new weapon had to be introduced as soon as possible, not require much scarce raw materials such as airplane-grade metals, and be easy to operate by pilots with little training (since by then most of the experienced pilots had either been killed or were prisoners of war). Moreover, the new aircraft would preferably not need runways or other vulnerable ground facilities that could be easily found and destroyed by enemy bombers.

Outsider Erich Bachem, who had left Fieseler in order to set up a small company to manufacture parts for aircraft, devised a truly radical design which fitted the need. His ‘Projekt BP 20’ was a rocket interceptor to be made mostly using cheap wooden parts, glued and screwed together. Its configuration was extremely simple, having a tube-shaped fuselage, two straight stubby wings and a T-shaped tail. Roll control was by differential use of the rudders on the lower and upper vertical fin (moving one to the left and the other to the right) so the wings required no moving parts. It would use few ‘war-essential’ materials reserved for other aircraft and be easy to build in volume by semi-skilled labor: a BP 20 could be constructed in only about 1,000 man­hours. Moreover, owing to the design’s simplicity most of the parts could be made in small woodworking shops distributed throughout Germany without interfering with aircraft production in factories which were already at peak capacity. Because of its distributed nature, production would also be relatively safe from sudden destruction by enemy bombers.

The BP 20 would be launched straight up while being controlled from the ground, and the pilot would bail out at the end of the flight. That way the pilots would not require training in the most difficult aspects of flying, which are take-off and landing. Bachem proposed to build large numbers of launch platforms around key industrial targets so that his interceptors would deter Allied bombers sufficiently for them to leave those targets alone. The RLM’s response was less than enthusiastic so Bachem showed his plan to Heinrich Himmler, chief of the security forces. Himmler, eager to increase his influence in the Third Reich, agreed to fund the development and placed an order for 150 machines paid with SS funds. Himmler, of course, intended that the interceptors be flown by SS men. By saving Germany from the “bomber menace” he would increase his own power in the Nazi government. Not wanting to be left out, the RLM later placed an order for another 50 interceptors for the Luftwaffe. The highly classified project was assigned top priority and the operational vehicle became the Ba 349, code-name ‘Natter’ (Viper).

Layout of the Bachem Natter.

If flying the Me 163 was a pretty brave thing to do, getting into a Natter should be considered truly heroic. Or rather, suicidal. The flight would start with the wooden contraption standing upright against a 15 meter (50 feet) tall, open-structure launch tower (or even just a modified telephone pole) with three vertical tracks engaging the wingtips and the edge of the ventral fin. The pilot would climb in, lie on his back in a seat angled at 90 degrees, and wait for the bombers to come within range. The entire gantry could turn around its vertical axis to enable the Natter to be oriented correctly depending on the direction the bombers were coming from. On command, the ground crew would take cover and the pilot would test the flight controls. Then he would start the central Walter HWK 109-509A engine (similar to that of the Me 163B) and verify its functionality. Next he would ignite the four solid propellant Schmidding rocket boosters, two strapped to each side. These delivered a total thrust of 47,000 Newton and because the vehicle weighed only 2,200 kg (4,850 pounds) they could readily lift it off the ground. Riding the tower, which provided initial stabilization, the machine would soon build up enough speed for the aerodynamic surfaces to keep it flying straight. The flight controls would remain locked in the neutral position until the solid boosters burned out, some 10 seconds into the flight. The initial acceleration was nearly 2 G, meaning twice as strong as the acceleration of gravity (i. e. the Natter would accelerate upwards about twice as fast as it would fall, because the total thrust was twice its total weight at lift-off). Hence at that moment the pilot would feel three times his normal weight, a combination of one G force of gravity plus the two G of the acceleration. When the strap-on boosters burned out they would be jettisoned by explosive bolts, the flight controls would unlock, and the autopilot would begin to steer to its assigned target using guidance commands received from the ground by radio. The vehicle would continue to climb under power of the Walter engine alone, accelerating at a rate of about 0.7 G to a maximum velocity of 700 to 800 km per hour (435 to 500 miles per hour). It would be guided towards a position in front of the bombers at an altitude of 9 km (30,000 feet), and only then would the pilot take over for the final phase of the attack. During the automatic phase of the flight the pilot was required to hold on to hand grips to prevent him from accidentally pulling on the control column while under the force of acceleration.

Closing in on his victim, the pilot would jettison the plastic nose cone to reveal a battery of missiles in front of the cockpit: either 24 R4M ‘Orkan’ (Hurricane) missiles of 3.5 kg (8.0 pounds) each, or 33 Henschel Hs 217 ‘Fohn’ (Warm Wind) rockets of 2.6 kg (6 pounds) each. Pulling the trigger would fire all of the unguided anti-aircraft missiles at the target in a single devastating salvo. Since the Natter would have little time for the attack, it made more sense to pack all its firepower into a single punch than to use conventional cannon more suitable for repeated precision strikes. Shortly after the missiles were fired, the engine would exhaust its propellant and the pilot would quickly glide at high speed down to an altitude of about 3 km (10,000 ft), where he would jettison the entire nose section, release his safety harness and fold the control column forward, an action that would release a braking parachute from the rear of the vehicle. The pilot would be thrown out by the sudden deceleration and subsequently deploy his own parachute. The tail with the engine, the most precious part of the aircraft, would land under the braking parachute to be refurbished and used again; only the nose section would be lost.

Development commenced with scientists of the Technical University of Aachen calculating the Natter’s aerodynamics using a large analog computer. The Deutsche Versuchsanstalt fur Luftfahrt (DVL) in Braunschweig then tested models in a wind tunnel. The results showed that the Natter would behave “satisfactorily” up to speeds of about Mach 0.95. In the meantime the solid propellant boosters were tested at the Bachem-Werke factory in Waldsee. Due to the high priority given by the SS to the development, less than 4 months after Bachem made his initial design sketches three full sized Natter prototypes were completed.

On 3 November 1944 the Ml (first) prototype, which took off from a trolley, was towed to an altitude of 3 km (10,000 feet) by a Heinkel He 111 bomber. Pilot Erich Klockner made various tests of the control and stability of the machine, discovering it to be rather sensitive and to react pretty violently to small movements of the stick. Due to the plane’s bucking and shaking he could not finish the test flight as planned with a realistic drop of the forward section, bail-out and retrieval of the tail section, so he decided simply to eject the cabin roof and jump out. The roof got caught at the hinge and did not fall away entirely, but Klockner managed to bail out from the side and landed safely by parachute. The Heinkel tow plane tried to land with the pilotless prototype, but without active control and lacking an undercarriage it hit the ground hard, tumbled end over end and was destroyed. Klockner went on to make several flights with the М3, which was fitted with a fixed undercarriage, each time bailing out successfully while the tow plane landed with the complete Natter behind it. The Natter design clearly did fly, but the tests showed that the dynamics of the tow cable and the aerodynamic influence of the fixed undercarriage had adverse effects on its stabihty. A free glide without the undercarriage would be required to unambiguously determine the Natter’s flight characteristics. Klockner refused, convinced that more tests and modifications were needed before such a flight could be made. The SS did not approve of his criticism and he was kicked out of the project.

The SS found another guinea pig, and on 14 February 1945 Luftwaffe test pilot Hans Ziibert took off in M8 towed by an He 111 bomber. Like the Ml, the M8 took off from a jettisoned trolley and once in the air was as aerodynamically ‘clean’ as the envisioned operational Natter. At an altitude of 5.5 km (18,000 feet) Ziibert released himself from the tow plane and pushed the plane into a steep (75 degree) nose dive to gain speed, then leveled off to execute a series of flight control tests. He found that the plane handled well at the attained maximum speed of about 600 km per hour (370 miles per hour) and that its stall velocity was about 200 km per hour (125 miles per hour). As per the test plan he then tried to deploy the braking chute, only to find that the release lever was stuck. Next he tried to eject the nose by activating the explosive bolts that held it to the rest of the vehicle but again nothing happened. It was time to get out: Ziibert ejected the cabin hood, put the plane into a steep climb to bleed off speed and struggled to escape. When just 1,200 meters (3,900 feet) above the ground and flying at 300 km per hour (190 miles per hour) the Natter went into a spiral dive. Ziibert hung on for three full turns, then dived over the left wing, passing beneath the tail unit. He landed safely by parachute but the M8 smashed into the ground at high speed. Of this flight Ziibert reported: “My general impression of the machine is very good and I can describe its flying characteristics as very benign.” The reason for the braking parachute failing to release was that a screw was blocking the release lever. The screw had been inserted in order to prevent inadvertent deployment of the chute on the ground, and the technicians had forgotten to remove it just before flight.

In parallel with these gliding flights a series of unmanned vehicle launches tested the rocket boosters, the Walter motor and the autopilot. Beginning on 18 December 1944, fifteen of these were shot into the sky from the Lager Heuberg military training area near Stetten am kalten Markt. These tests led to the abandonment of the plan to reuse the Walter engine, because it invariably got damaged by the rather hard impact of the parachute landing. The tests also proved that the thrust of the boosters varied by as much as 50% either way, making the stability of the vehicle very unpredictable during its initial boosted ascent.

Preparations on the launch pad for the flight of an unmanned test version of the Natter.

Launch of an unmanned Natter prototype.

By early 1945 Germany had all but lost the power to defend its own airspace and, driven by desperation, the SS ordered a manned Natter launch even though many of the unmanned test flights had ended in crashes and explosions. The SS found a pilot willing to risk his Ufe in Lothar Sieber, a Luftwaffe Second Lieutenant who had been demoted to the rank of Private after drinking alcohol on duty, and who hoped that his flight would restore his previous rank. On 1 March Sieber got into Ba 349A1 number M23 at the Lager Heuberg military area. The booster rockets were ignited and the wooden contraption raced along its launch rail and shot straight up, thereby making Sieber the first person to be launched vertically from the ground by rocket power. He did not have much time to enjoy this accomplishment though, because after climbing about 100 meters (330 feet) the machine suddenly turned over on its back and lost its cockpit cover. Since this cover included the headrest it is likely that the acceleration snapped Sieber’s head back against the fuselage, at least rendering him unconscious and perhaps even snapping his neck. The Natter reached a peak altitude of about 450 meter (1,500 feet) some 15 seconds into the flight, then it dived essentially vertically into the ground. Whether the cover had not been installed correctly or had simply not been sufficiently closed was never discovered. Sieber was posthumously promoted to First Lieutenant. Development and testing continued

US soldiers with a Natter.

using unmanned prototypes but it was clear that the Natter would not be ready for service before the Allies overran Germany. And of course its use would be limited owing to the scarcity of the special propellants required for the Walter engine. Thus no Natter was ever launched against an Allied bomber. After the war, US forces found many Natters in various states of completion. One of those that they took back to America for evaluation is currently awaiting restoration at the National Air and Space Museum’s Steven F. Udvar-Hazy Center close to Washington Dulles International Airport.

A high-fidelity Natter replica is on display at the Fantasy of Flight Museum in Florida. Looking at this crude wooden tube with its small wings, you cannot imagine anyone volunteering to pilot it, to be shot up at close to 800 km per hour (500 miles per hour) to attack a fleet of heavily armed bombers. The Planes of Fame Air

Museum’s Chino location in California has a full scale model that was built by hobbyist George Lucas (not to be confused with the creator of Star Wars), who also made the full-scale model of the Me 163B that is on display in the same museum. In Europe a Ba 349A replica is displayed in the Deutsches Museum in Munich, Germany, featuring realistic colors and markings of one of the unmanned test aircraft. On its horizontal stabilizers there is an authentic-looking text promising a reward to anyone who finds the tail section and returns it to the military, evidently because some units were expected to drift out of the test area while descending on their parachute. Another German museum, the Stettener museum located near the place where the Natters made their test flights, shows a replica of the one flown by Sieber. It is depicted standing against the launch tower with mannequins of Bachem and Sieber in front of it in discussion about the upcoming flight. At the time of writing, a group in Speyer, Germany, is also building a detailed replica of a Natter according to the original plans. This is intended to be flightworthy except for the rocket propulsion, but it is not actually meant ever to take to the sky. Eventually the team hopes to build a small series of ten aircraft.

The Natter was basically a guided surface-to-air missile with the pilot acting as its control system; a job that was soon taken over by electronics in the new air defense missiles that became operational after the war. These were smaller and lighter than a human, did not require training, and were cheap enough not to require retrieval after use. The Ba 349 remains unique as the first, and last, piloted air defense missile. It is interesting to note that the Natter bears similarities to the Space Shuttle in that both were crewed, vertically launched, liquid propellant rocket planes that had jettisonable solid propellant boosters for take-off, with the liquid propellant motors being started and checked (and stopped, if needed) prior to igniting the boosters, at

Natter replica in the Fantasy of Flight Museum in Florida [Rogier Schonenborg].

which moment there was no way to prevent the vehicle from leaving the ground. In 1952 Bachem noted, “within a few months we had to track down, go through and solve numerous problems associated with vertical take-off, problems which the designers of future spacecraft will also have to look at”, and also, “The attainment of great goals is not possible by a single leap, it is the result of an arduous climb up a steep ladder, step by step! Perhaps through our labor we have constructed one rung on that ladder!” Although true, these words appear to impart too much philosophical meaning to what was basically a last-ditch, near-suicidal weapon system.

Erich Bachem remained in Germany after the war and resumed his innocent pre­war business of designing and building camping trailers and motor homes. He joined the business of a neighbor, Erwin Hymer, whose company is still manufacturing recreational vehicles and camping trailers in the same buildings in which Bachem’s rocket interceptors were made. One of Hymer’s popular lines of camping trailers is called ‘Eriba’, Bachem’s nickname as a student derived from ERIch BAchem. You can thus still ride a Bachem, not at transonic speeds but certainly at less risk to your life.

CHASING THE DEMON IN THE AIR

The US Army Air Force and NACA (National Advisory Committee for Aeronautics; forerunner to NASA) in 1944 initiated a program called X-l (originally it was XS-1 for ‘Experimental Sonic One’ but the ‘S’ was dropped early on). Its purpose was the development and use of a rocket research aircraft specifically in order to investigate the mysterious transonic region of speed, determine whether there was such a thing as the sound barrier and, if there were not, pass beyond Mach 1. Initially NACA had expected to use an advanced turbojet-powered aircraft which would take off under its own power (just like the British M.52) and, in a very scientific way over a series of flights, study transonic phenomena at different subsonic speeds just short of Mach 1 (because the initial design was not expected to be capable of exceeding the speed of sound). But the Army Air Force was in a hurry to find out whether the sound barrier was a myth, and they pushed for a simpler design based on existing technology that would soon be able to reach and hopefully even surpass Mach 1. Based on previous experience with the Northrop XP-79, as well as early information about the Me 163 Komet, they were confident that a rocket propelled and air-launched, but otherwise fairly conventional aircraft would suffice. As the military was paying for the project, their views prevailed.

The Bell Aircraft Company was awarded a contract for three prototype aircraft in March 1945, just before the war in Europe ended. Consequently, when the X-l was designed the important German wartime discoveries about transonic flight were not yet available. As a result, the X-l had conventional wings rather than the swept-back wings of the revolutionary German type. But the wings were relatively thin, with a maximum thickness of only 10% of the chord (the width of the wing at any point). In comparison, the wing of the Me 163 varied in thickness between 14% at the root and 8% at the tip; for the DFS 346 the maximum thickness was 12%. But because their wings where swept the effective thickness with respect to the air flow was actually less (as explained in the description of the Me 163). Conventional straight wings for subsonic propeller fighter aircraft were generally thicker, with a typical ratio of 15%. The wings of the X-l were made especially strong to be able to handle the powerful shock waves that were expected in spite of their narrow width.

To compensate for the huge amount of aerodynamic drag, a powerful engine was needed. But at that time the US was not as advanced as the UK and Germany in turbojet technology and it did not yet have a jet engine that could provide sufficient thrust to push an aircraft beyond Mach 1. Also, problems were foreseen in ensuring a proper airflow into a jet engine during transonic flight. So as not to delay the project, the X-l designers opted to install a relatively simple, home-grown liquid propellant rocket engine. A liquid propellant rocket engine would also be much smaller than the giant jet engine of the M.52 and would not need air intakes, making integration with the aircraft (both in design and construction) less complicated, which would in turn enable the development to progress faster and with fewer surprises. For instance, not requiring an enormous air duct to pass right through the length of the fuselage meant the wings could be connected by a single spar, resulting a simple, sturdy design with a relatively low weight.

An important requirement was that the propellants be relatively safe and easy to handle, as well as available in large quantities. This excluded the nasty and difficult – to-produce hydrogen peroxide used in Germany, as well as the dangerous nitric acid favored by Russian rocket plane designers. The engine selected for the X-l was the

Reaction Motors XLR1 l-RM-3, which burned a fuel that consisted of a mixture of five parts ethyl alcohol to one part water, in combination with liquid oxygen. These propellants were non-toxic, did not spontaneously ignite on coming into contact, and gave reasonable performance. Moreover, unlike (for instance) gasoline, the alcohol-water fuel mixture could be used to cool the engine: the water improved the cooling capabilities for only a modest decrease in specific impulse. This early version of the XLR11 did not have turbopumps but relied on pressure from a nitrogen tank to drive the propellants into the combustion chambers. It was a pure American design and not based on any German technology, since that was not available when the engine was developed. The four combustion chambers of the XLR11 each produced a thrust of 6,700 Newton, and the engine could be throttled simply by varying the number of chambers ignited at any time. At full power the engine would consume the onboard supply of propellants in less than 3 minutes but this was expected to be sufficient for a short leap beyond Mach 1 if the aircraft were dropped from a carrier plane at high altitude (in contrast, the M.52 would have been able to fly under power for about 20 minutes). After its powered run, the X-l would glide back for landing.

The airframe was constructed from high-strength aluminum, with propellant tanks welded from steel (the patch of frost you can see on many of the rocket X- planes is caused by water vapor in the air freezing on the fuselage at the location of the frigid liquid oxygen tank). For the shape of the fuselage, the designers decided to model it after a 0.50 caliber gun bullet; a piece of hardware which was known to be able to fly faster than Mach 1 and whose shape was based on extensive earlier research on the aerodynamics of munitions. The X-l was basically a bullet with wings. It looks very stubby to us today, and also in comparison to the previously described German DFS 346 that was otherwise very similar in purpose and concept. In order to adhere to the bullet shape there was an unconventional cockpit with its window streamlined flush with the fuselage. Bailing out would have been terribly difficult, because the pilot did not have an ejection seat (a novel technology at that time) or an escape capsule (like the DFS 346 or M.52); he would have had to exit through a small hatch on the starboard side of the nose. It would have been quite a feat in a rapidly tumbling, disintegrating airplane that might be on fire. And even if the pilot were to make it through the hatch, he would have almost certainly struck either the sharp wing or the tail. Health and Safety did not really exist in those days.

In addition to these rather blunt aerodynamic design solutions, the X-l employed one sophisticated idea: an all-moveable horizontal tail plane (inspired by that of the British M.52 concept) set high on the vertical tail fin to avoid the turbulence from the wings. It was known that the elevon controls on conventional stabilizers generated strong shock waves at high speeds, making the airplane impossible to control in the all-important pitch direction and ultimately producing the infamous ‘Mach tuck’ that caused it to nose over into a terminal dive. But if the entire stabilizer is moved, not just a part of it, no shock wave forms on its surface and there is no elevon to become blocked; in other words, it allowed control of an aircraft at transonic and supersonic speeds. This was such a revolutionary discovery that the US hid it from

Bell X-l number 1 in flight [US Air Force].

the Soviets for as long as possible. During the Korean War the all-moving tail gave the US F-86 Sabre jet fighter a real advantage over the agile Soviet MiG-15, whose conventional tail had elevons which made it difficult to control at speeds approaching Mach 1. The all-moving horizontal stabilizer promptly became a standard feature on all supersonic aircraft, including the Russian successors to the MiG 15.

The X-l had good flight characteristics at transonic as well as lower speeds, both under rocket power and while gliding. Pilots found it a delight to fly, very agile with the handling characteristics of a fighter. It had a length of 9.5 meters (31 feet) and a wingspan of 7.0 meters (23 feet). Fully loaded with propellant it weighed 6,690 kg (14,750 pounds). Any propellant left after a powered flight was jettisoned in order to avoid landing with the hazardous liquids on board, and its dry weight was 3,107 kg (6,850 pounds)

Although originally designed for a conventional ground take-off, the X-l was air- launched from a high-altitude B-29 Superfortress bomber to maximize the use of its own propellant to accelerate to supersonic speed in the higher atmosphere, where both the aerodynamic drag and the speed of sound were significantly lower. At sea level a plane must exceed 1,225 km per hour (761 miles per hour) to surpass the speed of sound but at an altitude of 12 km (39,000 feet) Mach 1 is ‘only’ 1,062 km per hour (660 miles per hour). This meant the transonic and phenomena which the researchers were interested in would occur at slower, easier to attain speeds.

X-l number 3 being mated with its B-50 Superfortress carrier [US Air Force].

The X-l flight tests were to be undertaken at Edwards Air Force Base, at that time named Muroc Army Airfield, the famous test flight airfield out in the Mojave Desert of California. The base is next to Rogers Dry Lake, a large expanse of flat, hard salt that offers a natural runway. The desert also offers excellent year-round weather, as well as a vast, virtually uninhabited area with plenty of free airspace. All this made the base perfect for testing new high-speed and potentially dangerous rocket aircraft, especially if they were to remain secret.

By today’s aviation standards the X-l was a very risky aircraft. Apart from the rather dubious means of escape for the pilot, it also had no backup electrical system. During one flight, test pilot Chuck Yeager found himself in a powerless X-l due to a corroded battery just after being dropped from the carrier aircraft. He could neither ignite the engine nor open the propellant dump valves, since both required electrical power. Luckily, engineer Jack Ridley and Yeager had installed a manual system to get rid of the dangerous fluids just before that very flight, so he could still empty the tanks before landing; the X-l had not been designed to land safely with the weight of a full propellant load.

The original X-l aircraft, the X-l-1, made its first unpowered glide flight on 25 January 1946 over Florida’s Pinecastle Army Airfield, flown by Bell Aircraft chief test pilot Jack Woolams. The first powered flight was on 9 December 1946 at Muroc using the second X-l aircraft, with Bell test pilot Chalmers ‘Slick’ Goodlin (‘Slick’ being a flattering moniker in those days) at the controls. He also piloted the X-l-1 on its first powered flight on 11 April 1947. Two months later the Air Force, unhappy with Bell’s cautious and thus slow “pushing” of the flight envelope in terms of speed and altitude, terminated the flight test contract and took over. Captain Chuck

Chuck Yeager with his X-l [US Air Force].

Yeager, a veteran P-51 Mustang pilot of the Second World War, was selected to attempt to exceed the speed of sound in the X-l-1. After being assigned to the program, which was understood by all involved to be extremely dangerous, he was told by program head Colonel Boyd: “You know, we’ve got a problem. I wanted a pilot who had no dependents.” Yeager responded that he was married and had a Uttle boy, but that this would only make him more careful. This was judged sufficient explanation.

In October 1947, after several glide and powered flights, both pilot and aircraft are deemed ready to officially break the sound barrier. On the 14th, teams of technicians and engineers awaken early in order to prepare the small, bright orange X-l for flight and install it in the bomb bay of its B-29 carrier. Then the four-engined bomber takes off and chmbs to an altitude of 6 km (20,000 feet). At 10:26 a. m., the X-l-1, which Yeager has christened ‘Glamorous Glennis’ after his wife, is dropped at a horizontal speed of 400 km per hour (250 miles per hour). Yeager Ughts the four XLR11 rocket chambers one by one, rapidly climbing as he does so, and then he levels out at about 13.7 km (45,000 feet). Trailing an exhaust jet with shock diamonds (caused by shock waves in the supersonic gas flow) from the four rocket nozzles, the X-l approaches Mach 0.85. Entering the poorly understood transonic regime, Yeager momentarily shuts down two of the four rocket chambers, holding the plane at about Mach 0.95 to carefully test the controls. As on previous flights there is buffeting and shaking due to the invisible shock waves forming on the top surface of the wings, but apart from that the plane responds well to his steering inputs. It is time. At an altitude of 12 km (40,000 feet) he levels off, reignites the third rocket chamber and watches the needle move smoothly up the Mach meter.

Suddenly the buffeting disappears and the needle jumps off the scale (which only went up to Mach 1; apparently not everyone was so confident in the X-l’s supersonic capability). Yeager lets the X-l accelerate further, and for 20 seconds flies faster than Mach 1. At supersonic speed, a strong bow shock wave forms in the air ahead of the needle-like nose, but the flow over the wings has smoothed out and he discovers that the plane behaves rather well. Not only is the X-l able to survive surpassing the dreaded sound barrier, it is functional and controllable beyond Mach 1. Satisfied, Yeager shuts down the engine and glides back to land on the dry lake at Muroc.

The recorded peak flight speed was Mach 1.06 at an altitude of 13 km (43,000 feet), corresponding to an actual airspeed of about 1,130 km per hour (700 miles per hour). On his return to base, Yeager reported that the whole experience had been “a piece of cake”. It may be that he broke the sound barrier on the previous flight when the recorded top speed was Mach 0.997, as inaccuracies in the measurements might have masked a speed slightly over Mach 1. However, no sonic boom was heard on that occasion, whereas it was on the day the sound barrier was officially broken. The loud explosion-like noise scared several people on the ground into believing that the X-l had blown up; no one had ever heard a sonic boom before.

This first-ever officially recorded Mach 1-plus flight made Yeager a national hero and the quintessential test pilot of the new jet age. His 1985 autobiography, Yeager, was a multi-million-copy best seller, and he plays a prominent role in Tom Wolfe’s famous book The Right Stuff, as well as the eponymous movie (in which he has a cameo as the old fellow near the bar in Pancho’s Happy Bottom Riding Club). The introduction to the movie perfectly describes the X-l program: “There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, 750 miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it the sound barrier. Then they built a small plane, the X-l, to try and break the sound barrier.” If you desire a flavor of the rough world of the early jet and rocket plane test pilots and the first seven US astronauts, Wolfe’s book and the movie are indispensable. Some of the tales may seem fictional, inserted to spice up the story, but most of it is true. Bell test pilot ‘Slick’ Goodlin demanding a $150,000 bonus for attempting to break the sound barrier, then being replaced by Air Force Captain Yeager willing to do the job on his government salary of just over $200 a month is true. So is the famous incident in which Yeager breaks two ribs in a riding accident, says nothing to his superiors to avoid being replaced for the historic Mach 1 flight, and then gets his close friend and X-l engineer Captain Jack Ridley to furnish him a piece of a broom handle so that he can pull the lever to close the X-l’s door using his other hand; unfortunately, the historic piece of wood has been lost to history.

Breaking the sound barrier would have been a great publicity coup for the US Air Force, which had recently gained its independence from the Army, but the flight was kept secret in the interests of national security. Then in December the trade magazine Aviation Week (often referred to as ‘Aviation Leak1) unofficially broke the news. The Air Force did not confirm the story until March 1948, by which time Yeager and his colleagues were routinely flying the X-l up to Mach 1.45. The National Aeronautics Association voted that its 1947 Collier Trophy be shared by the main participants in the program: Larry Bell for Bell Aircraft, Captain Yeager for piloting the flights, and John Stack of NACA for scientific contributions. They received the 37-year-old prize from President Harry S. Truman at the White House. Yeager kept the prestigious trophy in his garage and used it for storing nuts and bolts.

The original X-l-1 ‘Glamorous Glennis’ became one of the most famous planes ever. Not only was it the first to fly faster than the speed of sound, it also attained the maximum speed of the entire X-l program: Mach 1.45. Furthermore, it was the only X-l to make a ground take-off (also with Yeager at the controls). On 8 August 1949, on the program’s 123rd flight, Air Force Major Frank K. ‘Pete’ Everest Jr., flew the X-l-1 to the new altitude record of 21,916 km (71,902 feet). Like all X-l records, it was unofficial, as according to FAI rules an aircraft must take off and land under its own power in order to be able to claim an official record (in 1961 this even prompted the Soviets to hide the fact that the world’s first spacefarer, Yuri Gagarin, had landed by parachute separately from his capsule). On the next flight, on 25 August, also with Everest on board, the X-l-1 suffered a cracked canopy and the cockpit lost pressure at an altitude of approximately 21 km (65,000 feet). Fortunately Everest was wearing a pressure suit that quickly inflated to prevent his blood from boiling in the thin air, making him the first pilot to have his life saved by such a suit. The X-l-1 was retired in May 1950 after a total of 82 flights (both gliding and powered) with ten different pilots. It was given a well-earned place in the Smithsonian Air and Space Museum alongside the Wright Flyer and Lindbergh’s Spirit of St. Louis, and it has recently been joined by a distant relative in the form of SpaceShipOne. Upon presenting the X-l to the museum, Air Force Chief of Staff General Hoyt Vandenberg said that the program “marked the end of the first great period of the air age, and the beginning of the second. In a few moments the subsonic period became history and the supersonic period was born.” The XLR11 engine that was used during Yeager’s historic flight is on display separately at the same museum. When I first saw both the aircraft and the engine I was surprised at how crude they appear by today’s standards, dramatically showing the fairly basic technology that was available to the X-l team in tackling the challenge. The Air Force Flight Test Center Museum at Edwards Air Force Base has an X-l replica.

Bell built three aircraft for the program: X-l-1 (serial number 46-062), X-l-2 (46­063) and X-l-3 (46-064). X-l-1 and X-l-3 were flown by the Air Force while X-l-2 was used by NACA, which had by then established a permanent presence at Edwards (initially NACA Muroc Flight Test Unit, it was renamed NACA High­Speed Flight Research Station in 1949 and then NACA High-Speed Flight Station in 1954. After the formation of NASA it became NASA Flight Research Center in 1959 and finally NASA Dryden Flight Research Center in 1976). In their original configuration, the three X-ls made a total of 157 flights between 1946 and 1951, of which 132 were under rocket power. They were flown by 18 different pilots but Yeager, with a total of 34 flights, was the most experienced X-l pilot of the program.

The X-l-2 was essentially identical to X-l-1, and made its first powered flight on 9 December 1946 with Bell test pilot Goodlin at the controls. By October 1951 it had

NACA X-l-2 [NASA],

completed 74 gliding and powered flights, flown with nine different pilots. Then it was rebuilt as the X-1E, one of the second generation of X-l planes.

The X-l-3 differed by having the turbopump-driven XLR11-RM-5 engine (in the XLR11-RM-3 of its predecessors high-pressure nitrogen fed the propellant into the combustion chambers). By using turbopumps, the pressures in the propellant supply lines could be kept relatively low, and metal fatigue problems diminished (concerns of which had resulted in the grounding of the X-l-2 after its 54th powered flight). The lower pressure also resulted in a considerable mass saving on the nitrogen tanks. On the other hand, the high level of complexity of the new turbopump system delayed production. When the aircraft was delivered to Muroc in April 1951 it was three years behind schedule. It gained the nickname ‘Queenie’ for being a Hangar Queen (an airplane that requires extraordinary preparation and maintenance time in the hanger). The X-l-3 made only one glide flight, and that was on 20 July 1951 with Bell test pilot Joe Cannon at the controls. Sadly, the aircraft was lost on 9 November whilst being de-fueled following a captive flight test mated to its B-50 carrier bomber (an improved form of the B-29). As Cannon pressurized the liquid oxygen tank a dull thud was heard, followed by a hissing sound as white vapor escaped from the X-l-3’s center section. Then a violent explosion engulfed the rocket plane and its carrier aircraft in yellow flames and black smoke. Both the X-l-3 and the B-50 were totally destroyed. Cannon managed to get out of the X-l-3, but spent nearly a year in hospital recovering from severe bums on his legs, arms and body. The X-l-3 was the first (but not the last) rocket X-plane to be lost due to a violent, mysterious explosion.

Bell X-1A [US Air Force],

To follow up on the success of the original X-l aircraft, Bell received a contract to build a second generation of X-l aircraft with the potential to fly at speeds exceeding Mach 2. These aircraft, the X-l A to X-1E, were powered by the turbopump XLR11- RM-5 engine that was also incorporated in the X-l-3. It had the same 27,000 Newton maximum thrust of the XLR11-RM-3 and was throttled by varying the number of active combustion chambers. The X-l A resembled the X-l, but had a bubble canopy and a stretched fuselage to carry more propellant for a longer powered flight. It was delivered to Edwards on 7 January 1953. The first ghde flight was made by Bell pilot Jean ‘Skip’ Ziegler, who went on to make five powered flights in it. Afterwards, the aircraft was handed over to the Air Force.

In parallel with the Air Force’s X-1A flights, NACA initiated its own high-speed research with the Douglas D-558-2 Skyrocket (more on this later). On 20 November 1953 Scott Crossfield achieved Mach 2.005 in this aircraft, beating the Air Force to the ‘magic number’ of Mach 2. The Air Force promptly initiates ‘Operation NACA Weep’ in which a series of ever-faster flights culminate on 12 December 1953 with Yeager boosting the X-l A to a new air speed record of Mach 2.44 at an altitude of

22.8 km (74,700 feet). Moreover, Yeager achieves this speed in level flight, whereas Crossfield had required to push his Skyrocket into a shallow dive in order to surpass Mach 2. However, Yeager’s elation is short lived, because soon after setting the new speed record his aircraft starts to yaw, and when he tries to compensate this causes it to suddenly pitch up violently. The aircraft enters an inverted flat spin from which Yeager is unable to recover. Bailing out is not possible at the high speed with which the aircraft is tumbling from the sky because it is not equipped with an ejection seat. Accelerations of up to 8 G throw him so violently around inside the cockpit that his helmet breaks the canopy. Only when the aircraft enters the denser atmosphere, at an altitude of 7.6 km (25,000 feet), is he able to restore control. He has literally fallen 15 km (50,000 feet). Unperturbed, Yeager glides back to Edwards and lands safely. Aerodynamidsts had predicted that such ‘inertia coupling’ might occur when flying at high speeds but the X-1A was the first to experience it. This is a very dangerous phenomenon in which the inertia of the aircraft fuselage overpowers the stabilizing aerodynamic forces on the wings and tail. Aircraft that have low roll inertia relative to their pitch and yaw inertia are especially susceptible to it. In practice, this means that planes having stubby wings and long fuselages, and in which the mass is spread over the length of the plane rather than being concentrated near its center of gravity, will probably have problems at high speeds. With its long, relatively slender fuselage, the heavy rocket engine in the tail, and its Mach 2 + flight speeds, the X-l A matched this profile. Pilots had up to then felt that with experience and a basic flight control system, any situation in the air could be handled. But at the extreme altitudes and speeds that the new research aircraft could attain, inertia coupling would require the development of much more sophisticated flight control systems.

An attempt to surpass Yeager’s record speed with the X-1A would be extremely dangerous and was never tried. However, flying the X-l A to higher altitudes was still possible. On 26 August 1954 USAF test pilot Major Arthur Murray set a new record of 27.56 km (90,440 feet). In September the aircraft was transferred to NACA High­Speed Flight Station, which returned it to Bell for the installation of an ejection seat; all of the Air Force’s high-speed and high-altitude flights had been done without the pilot having a quick and secure means of escape!

Bell X-l A in NACA service [NASA].

Joe Walker gets into the X-1A [NASA],

On 20 July 1955 NACA test pilot Joseph Walker made a familiarization flight in the modified aircraft. Then, on 8 August, as he is sitting in the cockpit preparing for another drop, there is an explosion in the engine compartment of the X-1A. Flames erupt from the propellant tanks and leave a trail in the B-29’s slipstream. In addition, the X-lA’s landing gear has been blown down into the extended position, making it impossible to land the carrier aircraft without the X-1A touching the runway first and likely breaking apart. Walker manages to get out of the rocket plane into the relative safety of the bomb bay, grabs a portable oxygen tank to breathe, and then returns to dump the rocket plane’s propellant in an effort to save both aircraft. But it is too late, and the B-29 jettisons its burning load. As the X-1A falls it suddenly pitches up and almost hits its carrier, then spirals down and smashes into the desert floor, exploding on impact. Walker and the B-29 crew return to base uninjured. The X-1A had performed a total of 29 flights (including aborts) by four pilots.

The second aircraft of the new series, the X-1B, was similar in configuration to the X-1A except for having slightly different wings (for its last three flights its wings were slightly lengthened). The Air Force used the X-1B for high-speed research from

The cockpit of the X-1B [National Museum of the US Air Force],

October 1954 to January 1955, whereupon it was turned over to NACA, whose pilots (Neil Armstrong amongst them) flew it to gather data on aerodynamic heating, a new field of study that became ever more important as aircraft speeds increased.

Aerodynamic heating occurs when the speed of the airflow approaches zero, most particularly in the strong shock waves at the leading edges of the wings and the nose of a supersonic aircraft, where much of the kinetic (movement) energy of the air is converted into heat that can transfer into the aircraft. At extreme speeds the heat can damage the structure of a plane, and even if the temperatures remain relatively low the cycles of heating and cooling that a plane goes through during each flight can still weaken its structure in the long term. Moreover, the aerodynamic heat can make life very uncomfortable for the pilot (and passengers) if no adequate cockpit or flight suit cooling system is installed. For instance, when the Concorde supersonic airliner was cruising at Mach 2.2 its nose reached 120 degrees Celsius (250 degrees Fahrenheit). When the Space Shuttle entered the atmosphere at Mach 25 on returning from orbit its nose reached a searing 1,650 degrees Celsius (3,000 degrees Fahrenheit). Special structural materials (such as the titanium alloy used on the SR – 71 capable of flying at Mach 3) and thermal protection materials (Uke on the Space Shuttle) were required to survive the heat at extreme flight speeds.

To be able to make detailed measurements of the temperatures on different areas on the X-1B, NACA installed 300 thermocouple heat sensors over its surface. During this test campaign the aircraft was also equipped with a prototype reaction control system comprising a series of small hydrogen peroxide rocket thrusters mounted on a wingtip, the aft fuselage, and the tail to provide better control at high altitudes where there is Uttle air for the aerodynamic control surfaces to work with. On the X-1B this system was purely experimental, as the maximum altitude was typically kept to about 18 km (60,000 feet) at which it could still rely on its standard aircraft control system; in fact, the X-1B reached its highest ever altitude of 19.8 km (65,000 feet) three years prior to the installation of the reaction control system. Subsequently a similar system was installed on the X-15, which could fly so high that it was essentially in a vacuum and unable to rely on rudders, ailerons and elevons alone. For the Mercury, Gemini and Apollo spacecraft of the 1960s, thrusters were the only means of controlling the attitude of the vehicle. The X-1B played a pioneering role in the development of such systems.

Moreover, midway through its flight test program the X-1B was equipped with an XLR11-RM-9 engine which had a novel low-tension electric spark igniter instead of the high-tension type of the earlier XLRlls. NACA flew the aircraft until January 1958, when it was decided to ground it owing to cracks in the propellant tanks. It had completed a total of 27 flights by eight Air Force and two NACA test pilots, all of which had been intended to be powered but some had ended up as glide flights due to problems with the rocket engine. In January 1959 the X-1B was given to the National Museum of the US Air Force at Wright-Patterson Air Force Base in Ohio, where it is still on display.

The X-1C was intended to test onboard weapons and munitions at high transonic and supersonic flight speeds, but while it was still under development operational jet fighters such as the F-86 Sabre and the F-100 Super Sabre were already shooting cannon and firing missiles while flying at such speeds, so the X-1C was canceled in the mockup stage.

The X-1D was to take over from the X-1B in testing aerodynamic heating. It had a slightly increased propellant capacity, a new turbopump which enabled the tanks and propellant feed lines to work at a lower pressure, and somewhat improved avionics (i. e. the onboard electrical and electronic equipment). On 24 July 1951 Bell test pilot Jean ‘Skip’ Ziegler made what would turn out to be the only successful flight of the X-1D. On being dropped by its B-50 carrier the aircraft made a 9 minute unpowered glide which ended with a very ungraceful landing due to the failure of the nose gear. The repaired aircraft was turned over to the Air Force, which assigned Lieutenant Colonel ‘Pete’ Everest as the primary pilot. On 22 August the X-1D took to the sky for its first powered flight, partly contained within the bomb bay of its B-50 carrier. But the mission had to be aborted owing to a loss of nitrogen pressure needed to feed the propellants into the turbopump of the rocket engine. Because it would be dangerous to land the B-50 with a fully loaded X-1D, Everest attempted to jettison the propellant. Unfortunately this triggered an explosion and a fire, and once again an X-l had to be jettisoned. Luckily no one was hurt. The explosions of the X-l-3 and the X-1D were finally traced to the use of leather gaskets in the oxygen propellant supply plumbing (which had likely also caused the loss of the X-l A). The leather had been impregnated with tricresyl phosphate (TCP), which firstly becomes unstable in the presence of pure oxygen and can then explode if subjected to a mechanical shock. It was one of the hard lessons learned during the X-l program.

After the loss of the X-l-3 and the X-1D (the crash of the X-1A would not occur until several years later) it was decided to upgrade the X-l-2 and redesignate it as the X-1E to continue the high-speed flight test campaign. It was christened ‘Little Joe’ in honor of its primary Air Force test pilot, Joe Walker. The most visible modifications included a protruding canopy, a rocket assisted ejection seat, and thinner wings with knife-sharp leading edges and a thickness ratio of 4% (better suited to supersonic flight). The surface of the plane was covered with hundreds of tiny sensors to register structural strain, temperatures and airflow pressures. The X-1E made its first glide flight on 15 December 1955 with Walker at the controls. He went on to make a total of 21 flights, attaining a maximum speed of Mach 2.21. NACA research pilot John McKay took Walker’s place in September 1958 and made five more flights, with a maximum attained speed of Mach 2.24. It was permanently grounded in November 1958 owing to structural cracks in the fuel tank wall, and now guards the entrance of NASA Dryden Flight Research Center.

Joe Walker with the X-1E [US Air Force].

The X-l program thus opened the door to supersonic flight, and its experimental results facilitated a new generation of military jets that could fly faster than the speed of sound. The various X-ls truly adhered to the Edwards Air Force Base motto of ‘Ad Inexplorata’ (Into the Unknown).

In friendly competition with the Air Force’s X-l program, the US Navy, working with NACA, initiated tests using its mixed-power Douglas D-558-2 Skyrocket. The Navy/NACA D-558 program pursued a more conservative approach to the problems of high-speed flight than did the USAF/NACA X-l. In contrast to the decision by the Air Force to go straight to supersonic rocket propelled planes, the Navy started with the transonic D-558-1 Skystreak jet-powered research aircraft. This was more in line with the careful scientific approach which NACA advocated. The D-558-1 had only just been able to surpass Mach 1 in a dive. By using rocket power in addition to a jet engine the D-558-2 was to explore the transonic and supersonic flight regimes and investigate the characteristics of swept-wings at speeds up to Mach 2. The Navy was also particularly interested in the strange phenomenon that made high-speed, swept-wing aircraft of that time pitch their nose upwards at low speeds during take-off and landing, as well as in tight turns. The original plan was to modify the fuselage of the D-558-1 to accommodate a combination of a rocket and a jet engine, but that soon proved impractical. The D-558-2 became a completely new design that had its wings swept at 35 degrees (its predecessor had straight wings) and its horizontal stabilizers at 40 degrees. The wings and the tail section would be fabricated from aluminum, but the fuselage would be primarily magnesium. For take-off, climbing and landing the Skyrocket would be powered by a Westinghouse J34-40 turbojet engine drawing its air through two side intakes on the forward fuselage and producing a thrust of 13,000 Newton. To attain high speeds, a four-chamber rocket engine with a total sea-level thrust of 27,000 Newton would be fitted. The Navy called this the LR8-RM-6 but it was basically the same XLR11 engine as used in the Bell X-l. The design called for a flush canopy similar to that of the X-l in order to obtain a sleek fuselage, but this would have so limited the pilot’s visibility that it was decided to use a normal raised cockpit with angled windows. The resulting increased profile area at the front of the aircraft had to be balanced by a slight increase in the height of the vertical stabilizer. Somewhat reminiscent of the German DFS 346 rocket aircraft, the pilot was housed inside a pressurized nose section that (as on the D-558-1) could be jettisoned in an emergency. The capsule would be decelerated by a small drag chute, and when it had achieved a suitable altitude and speed the pilot would bail out to land under his own parachute.

On 27 January 1947 the Navy issued a contract change order to formally drop the production of the planned final three D-558-1 jet aircraft and substitute instead three of the new D-558-2 Skyrockets.

The Douglas company invited its pilots to submit bids to fly the new rocket plane during the test program. However, at that time Yeager had not yet made his historic Mach 1 flight in the X-l and trying to break the sound barrier was still seen by most test pilots as a quick and easy way to “buy the farm” (i. e. die). Rather than ignore the offer, which would have been bad for their reputations, the pilots conspired to

NACA 144, the second Skyrocket [NASA].

submit exceptionally high bids that would surely not be accepted by the company. However, John F. Martin was away delivering an airplane for Douglas and unaware of the plot. He submitted a reasonable bid and was promptly accepted as the Skyrocket’s project pilot. On 4 February 1948 Martin took off from Muroc Army Airfield in the first aircraft (Bureau No. 37973; NACA 143) for the Skyrocket’s maiden flight. At that time this aircraft employed a jet engine and was configured only to take off from the ground. It was tested in this configuration by the company until 1951 then handed over to NACA, which kept it in storage until 1954 and then modified it by removing the jet engine, installing an LR8-RM-6 rocket engine, and configuring the aircraft for air-launch from the bomb bay of a P2B (the naval version of the B-29). However, it was subsequently only used for one mission: an air-drop familiarization flight on 17 September 1956 by NACA pilot John McKay. In total NACA 143 made 123 flights, mostly in order to validate wind-tunnel predictions of the Skyrocket’s performance. One interesting discovery was that the airplane actually experienced less drag above Mach 0.85 than the wind tunnels data indicated, thus highlighting the discrepancies between wind tunnel results and actual flight measurements that still existed at that time.

Skyrocket Bureau No. 37974 (NACA 144) had a much more interesting career. It also started out with a jet engine only, in which configuration NACA pilots Robert A. Champine and John H. Griffith flew it 21 times for subsonic airspeed calibrations and to investigate longitudinal and lateral stability and control. They encountered the expected pitch-up problems, which were often severe and occurred very suddenly. In 1950 Douglas replaced the turbojet with an LR8-RM-6 and modified the airframe to be carried by a P2B (B-29) bomber. The release at an altitude of about 9 km (30,000 feet) and the increased thrust compared to the turbojet enabled company pilot Bill Bridgeman to fly this aircraft up to a speed of Mach 1.88 on 7 August 1951, and on 15 August reach a maximum altitude of 24.2 km (79,494 feet) and set an unofficial world altitude record. Bridgeman flew the aircraft a total of seven times.

A Skyrocket being loaded into the bomb bay of its carrier aircraft [NASA].

NACA 144 being dropped from its carrier bomber [NASA],

During his supersonic flights he encountered a violent rolling motion due to lateral instability which was curiously weaker on his Mach 1.88 flight than on a Mach 1.85 flight that he made in June.

It was then turned over to NACA, which started its own series of research flights in September 1951 with legendary pilot Scott Crossfield. Over the next several years

Crossfield flew NACA 144 at total of 20 times, gathering data on longitudinal and lateral stability and control, aerodynamic loads and buffeting characteristics at speeds up to Mach 1.88. On 21 August 1953 Marine Lieutenant Colonel Marion Carl, flying for the Navy, set a new unofficial altitude record of 25.37 km (83,235 feet). NACA technicians then extended the rocket engine nozzle in order to prevent its exhaust gas from affecting the rudders at supersonic speeds and high altitudes (where the exhaust expands into an enormous plume). As explained later in this chapter, such additions also improve the efficiency of an engine at high altitudes; in the case of the D-558-2 increasing the thrust by 6.5% at 21 km (70,000 feet) altitude.

Meanwhile, people in the project where lobbying for the go-ahead from NACA to attempt to cross the Mach 2 boundary. They knew the Air Force was planning to try to fly faster than twice the speed of sound using the X-1A in celebration of the 50th anniversary of the first flight by the Wright brothers. The Navy and Scott Crossfield, who was a Naval officer prior to joining NACA as a civilian test pilot, were eager to claim this record. NACA preferred to focus on a steady scientific approach and leave record setting to others, but Crossfield convinced NACA director Hugh L. Dryden to consent to a Mach 2 flight attempt with the NACA 144 Skyrocket. Some years later Crossfield admitted, “It was something I wanted to do; particularly if I could needle Yeager about it.”

The NACA project team knew their aircraft would need to be pushed to the very limit of its capabilities. The extra thrust from the new nozzle extension would help, but more was required. Extremely frigid liquid oxygen was put into the oxidizer tank 8 hours before the flight to cold-soak the aircraft, because this would reduce fuel and oxidizer evaporation due to the aircraft’s own heat during the flight and thereby leave more propellant in the tanks for several more seconds of powered acceleration. To limit drag as much as possible they cleaned and thoroughly waxed the fuselage, even taping over every little seam in the aircraft’s surface. The heavy stainless steel propellant jettison tubes were replaced with aluminum ones. In addition, these were positioned into the rocket exhaust stream so that they would bum off once the engine was ignited and were no longer required, further reducing the aircraft’s weight and drag. Project engineer Herman O. Ankenbruck devised a flight plan to make the best use of the Skyrocket’s thrust and altitude capabilities. It was decided that Crossfield would fly to an altitude of approximately 22 km (72,000 feet) and then push over into a slight dive to gain a little help from gravity. Despite having the flu and a head cold, Crossfield made aviation history on 20 November 1953 by becoming the first man to fly faster than twice the speed of sound; although barely: his maximum speed was Mach 2.005, or 2,078 km per hour (1,291 miles per hour). But this record stood for a mere 3 weeks, when the X-1A flew considerably faster. No attempts were made to push the D-558-2 to higher speeds; it had reached the limits of its design and there was no way that it could hope to reclaim the speed record from the X-1A.

More flights were made by NACA 144 with NACA pilots Scott Crossfield, Joe Walker and John McKay gathering data on pressure distribution, stmctural loads and stmctural heating. It flew a total of 103 missions, including the program’s finale on 20 December 1956 when McKay took it up for data on dynamic stability and sound-pressure levels at transonic and supersonic speeds.

NACA 144 returning to Edwards, with an F-86 flying chase [NASA],

The third Skyrocket (Bureau No. 37975; NACA 145) could also be air-launched and was equipped with both an LR8-RM-6 rocket engine and a Westinghouse J34- 40 jet engine which had its exhaust pipe exiting the belly of the plane. Taking off under its own power on 24 June 1949 this aircraft became the first Skyrocket to exceed the speed of sound, thereby proving that the design was well suited to supersonic flight; pilot Eugene F. May noted that upon passing Mach 1, “the flight got glassy smooth, placid, quite the smoothest flying I had ever known”. By November 1950 NACA 145 had completed 21 flights by company pilots May and William Bridgeman, and then it was turned over to NACA. In September the following year pilots Scott Crossfield and Walter Jones began flying it to investigate the notorious pitch-up phenomenon. For this, the aircraft was flown with a variety of configurations involving extendable wing slats (long, narrow auxiliary airfoils), wing fences (long but low vertical fins that run over the wing) and leading edge chord (width) extensions. They found that whilst fences significantly aided in the recovery from sudden pitch-ups, leading edge chord extensions did not. This disproved wind tunnel tests which had indicated the contrary, and clearly demonstrated the need for full-scale tests on real aircraft. Wing slats, when in the fully open position, eliminated the pitch-up problem except in the speed range of Mach 0.8 to 0.85. The data obtained from these tests was extremely valuable when developing supersonic fighter aircraft. In June 1954 Crossfield began using NACA 145 to investigate the aircraft’s transonic behavior with external stores such as bombs and drop tanks (the bombs were empty dummies, as only their shape and position were relevant). Pilots McKay and Stanley Butchart completed NACA’s investigations on this, with McKay flying the last of NACA 145’s 87 missions on 28 August 1956.

Together the three Skyrockets flew a total of 313 missions, both taking off from the ground on jet power as well as being air-launched from a carrier. They gathered invaluable data on the stability and control of swept-wing aircraft at transonic and supersonic speeds. The data enabled a better correlation between wind tunnel results and flights by real aircraft in the open sky, making wind tunnel tests more useful in the design of high speed aircraft. Especially benefiting from the D-558’s research, as well from the X-l program, were the so-called ‘Century Series’ supersonic fighters: the F-100 Super Sabre, F-101 Voodoo, F-102 Delta Dagger, F-104 Starfighter, F – 105 Thunderchief and F-106 Delta Dart. The various makers of these magnificent aircraft all exploited the flight research done at Edwards, giving the US military an important edge over their Soviet counterparts.

NACA 143, the first Skyrocket, is on display at the Planes of Fame Museum in Chino, California. NACA 144, the first aircraft to fly at Mach 2, is hanging from the ceiling of the National Air and Space Museum in Washington D. C. NACA 145 can be found outside on the campus of Antelope Valley College in Lancaster, California, not far from Edwards.

In late 1944, as the design of the X-l was getting underway, it became clear to the US Army Air Force that supersonic aircraft would greatly benefit from swept wings like those pioneered in Germany. Bell thus responded to the Air Force request for a successor to the X-l with their Model 37D, which was essentially an X-l that had its wings swept back at 40 degrees. However, aerodynamic and structural analyses soon demonstrated that such an upgrade of the X-l design was not very practical, and the proposal was rejected. In September 1945, just after the Second World War ended, Bell came back with an entirely new and much bolder concept which they called the Model 52. Even although the X-l had yet to fly, the Bell engineers told the Air Force that their new aircraft would be able to get close to Mach 3 at altitudes above 30 km (100,000 feet). The Air Force was sold on the concept and named it the XS-2 (later shortened to the X-2). This revolutionary airplane had wings that were swept back at 40 degrees (as before) but now they were mounted to the fuselage with 3 degrees of dihedral and had a 10% thickness ratio (as explained earlier, swept wings can have a greater relative thickness than a straight wing for a given critical Mach number). The wings had a bi-convex profile (a double-wedged cross section which resembled an elongated diamond) that was expected to be particularly suitable for supersonic flight as already indicated by wind tunnel experiments performed in Italy in 1940 (also the canceled British Miles M.52 would have been equipped with bi-convex wings). Like on the X-l, the horizontal tailplane was all-moveable but an innovation was that the stabilizers had the same sweep as the wings.

Where the X-l series was to surpass the sound barrier, the X-2 was envisioned to best the ‘heat barrier’. The temperatures on its exterior were expected to reach about 240 degrees Celsius (460 degrees Fahrenheit) due to severe aerodynamic heating. To survive this, the wings and tail surfaces were made using heat resistant stainless steel and the fuselage was a high strength copper-nickel alloy called К-Monel. In order to maintain a comfortable temperature in the cockpit, a cooling system weighing 225 kg (496 pounds) was installed which, under normal conditions, was sufficient to keep a room containing 300 people nice and cool.

The X-2 would be air-dropped from a B-50 bomber and land without propellant on the dry lake near Muroc, so its landing gear comprised a deployable center-line skid, a small skid under each wing, and a short nose wheel which hardly protruded beyond the fuselage. (Its peculiar attitude on the ground gave the impression that the front carriage had collapsed.) It looked very much like a manned rocket, with a

The first X-2 with its B-50 carrier, chase planes and support crew [US Air Force].

rather small cockpit capsule right at the front, housed inside a sharp pointy nose. Just as on the D-558-2 Skyrocket, in an emergency the X-2’s entire pressurized nose assembly would be jettisoned and soon stabilized by a small parachute. The pilot would then have to manually open the canopy at a safe altitude and speed, and bail out. Although NACA was concerned about this system, the Air Force considered it an adequate means of escape at extreme flight speeds and altitudes and approved the design. It is another example of the more careful but slower NACA approach versus the bolder Air Force seeking faster progress in order to stay ahead in aviation (with respect to the Soviets certainly, and probably also in friendly competition with the Navy and NACA).

To propel the X-2 to Mach 3, it was equipped with an advanced Curtiss-Wright XLR25-CW-3 pump-fed dual-chamber rocket engine that ran on water-alcohol and Uquid oxygen and produced a total thrust of 66,700 Newton at sea level; about two- and-a-half times that of the XLR11 used by the X-l. The upper combustion chamber could produce a maximum of 22,200 Newton and the larger, lower chamber twice that. They could be run together or separately, and each could be throttled between 50 and 100% of its full thrust level (whereas the XLR11 could only be adjusted by varying the number of chambers ignited). With full propellant tanks the X-2 weighed 11,299 kg (24,910 pounds), and its landing weight with empty tanks was 5,613 kg (12,375 pounds); both of these weights where almost twice the corresponding figures for the X-l.

The Air Force ordered two X-2 Starbuster research aircraft (airframes 46-674 and 46-675) from Bell Aircraft for the initial flight test program. NACA would initially provide advice and support, and install data-gathering instrumentation, then later use the aircraft for its own test flight campaign.

The X-2 represented a major advance in technology over the X-l. In particular, the development of the XLR25 rocket engine delayed the program by several years and many issues concerning the structure of the aircraft and its flight control system had to be overcome. The planned revolutionary fly-by-wire system where the pilot’s control inputs would be interpreted by a computer and then translated into electrical signals to operate motors of the control surfaces was abandoned in 1952 because its technology was too immature. It was replaced by a conventional and much heavier hydraulic power-boosted system. This unfortunately also meant that the operation of the aircraft was completely up to the pilot, without any intervention from a computer to ensure that no maneuvers were made which would be dangerous at certain speeds and altitudes.

The Air Force purchased a Goodyear Electronic Digital Analyzer (GEDA) analog computer which NACA turned into a rudimentary X-2 flight simulator, the first ever computer simulator to be used in aviation. This machine, which could simultaneously handle the various complex interdependent mathematical equations that described the motions of the X-2, helped pilots to familiarize themselves with the aircraft and its expected handling characteristics. It also allowed detailed preparation and checking of flight plans before assignment to the real aircraft. In due course the measurements made during the actual flights helped to improve the simulator.

Consistent (although probably not intentionally) with the X-l speed indicator only going up to Mach 1, the X-2 cockpit had a meter limited to Mach 3 and an altimeter that only went to 100,000 feet (30.5 km), even though the plane was intended to (and did indeed) fly considerably faster and higher than that! In

The second X-2 with collapsed nose gear following the program’s first glide flight [US Air Force].

addition, the cockpit had a standard gyro system to indicate the plane’s attitude, which the pilots found to be so inaccurate as to be unusable.

Owing to the development problems it was early 1952 before Bell concluded the captive flight tests with the X-2 remaining mated to the B-50. The first glide flight on 27 June 1952 took place at Muroc (which by then was Edwards Air Force Base) with Bell test pilot Jean ‘Skip’ Ziegler at the controls. The plane used on the occasion was the second X-2 (46-675) because it had been decided to leave the first aircraft at the company so that it could be equipped with an XLR25 engine as soon as one became available. Unfortunately, at the end of its first glide flight the plane was damaged by a rough landing that collapsed its nose gear. While this repair was underway, a wider central skid was installed to make landing easier. When testing resumed in October 1952, both glide flights resulted in satisfactory landings.

With the glide tests finished, the plane was returned to Bell for modifications. As the first rocket engine delivered had not yet been installed in the first (untested) X-2, it was decided to put it in the already flown one. More captive flight tests were then performed to verify the proper operation of the new propulsion system (without any ignition) at high altitude. Sadly, Ziegler, a veteran of many flights in the X-l series, died on 12 May 1953 when this X-2 suddenly exploded during a captive flight over Lake Ontario while he was checking the aircraft’s liquid oxygen system. B-50 crew member Frank Wolko also died, but the bomber managed to jettison the burning X – 2 into the lake and land safely. The X-2 was never recovered and the B-50 had been damaged beyond repair. It was later found that the explosion was likely caused by the same inflammable leather gasket problem that caused the loss of the X-l-3 and X-1D, and possibly also the X-1A.

Once the remaining X-2 airframe 46-674 had been equipped with an XLR25 engine, the testing of this aircraft began with a series of glide flights. No problems were foreseen, since the glide landings with the second X-2 had been satisfactory after the wider skid was installed. The flight team was therefore surprised when 46- 474’s first flight ended in a very unstable landing in which the aircraft skidded sideways over the salt lakebed. After repairs, the next flight ended similarly. It appeared that the high position of the aircraft’s center of gravity on the ground due to the tall landing skid booms made it wobble upon touching down. The skid’s height was decreased, changing the plane’s 7-degree nose-down angle to 3 degrees. This did the trick. The aircraft made perfect landings from then on. Now the X-2 was finally ready for its powered maiden flight. The first attempt took place on 25 October 1955 but because of a nitrogen leak pilot ‘Pete’ Everest had to complete the mission as a glide flight. The second attempt was aborted while still attached to the carrier aircraft and ended in another captive flight. On 18 November everything finally worked. As planned, only the smaller of the two thrust chamber was ignited. The maximum speed attained was Mach 0.95. However, a small fire had broken out in the tail of the aircraft. Although this did not look very severe in the post-landing inspection it nevertheless meant several months of repair. Following several more aborted attempts, Everest completed a second powered flight on 24 March 1956, this time using only the larger thrust chamber. If anything, these early flights showed the X-2 to be a complex aircraft that was difficult to fly and to maintain. Due to these problems the development and flight test program was already three years behind schedule.

When both combustion chambers were used on 25 April they enabled the X-2 to fly supersonically for the first time: it reached a speed of Mach 1.40 and a maximum altitude of 15 km (50,000 feet). Everest performed three powered flights in May that pushed the X-2’s speed to Mach 2.53, making him the ‘Fastest Man Alive’. Another pilot, Air Force Captain Iven C. Kincheloe, made a supersonic flight on 25 May, but a malfunction obliged him to shut the engine down early.

In a rocket, the role of the nozzle is to correctly expand the hot exhaust from the high pressure inside the combustion chamber to a considerably lower pressure but a much higher speed. For maximum efficiency (i. e. specific impulse) the expelled gas should reach the same pressure as the ambient atmosphere at the end of the nozzle. Over-expansion (in which the exhaust reaches a pressure lower than that of the air) causes a loss of thrust; as indeed does under-expansion. The higher the altitude the lower the ambient air pressure, which means that at high altitudes the exhaust can be expanded further through a longer nozzle, enabling the same engine to deliver more thrust (at the cost of the maximum thrust at lower altitudes, where the exhaust will be over-expanded). In June 1956 the X-2 received an engine nozzle extension to give it more thrust at high altitudes where there is low aerodynamic drag, thus enabling it to fly faster. Everest made a supersonic checkout of the upgraded X-2 on 12 July 1956, and on the 23rd made his final flight in the aircraft to gather data on

An X-2 igniting its engine just after being dropped by its carrier B-50. [US Air Force].

aerodynamic heating. During this mission he reached a speed of Mach 2.87 at an altitude of 21 km (68,000 feet). Kincheloe then took over as project pilot and made a series of flights in an attempt to reach the aircraft’s greatest possible altitude. To achieve this, the X-2 had to make a powered ascent at an angle of 45 degrees. This was difficult to judge using the cockpit instrumentation owing to the inaccurate gyro system, so engineers simply drew a line on the windscreen with a red grease pencil: if Kincheloe kept this line parallel to the horizon while looking out to the side, he would be climbing at the required angle. After two aborted attempts he achieved the very respectable altitude of 26,750 km (87,750 feet) on 3 August 1956. On 7 September he shattered his own record by reaching a spectacular 38,466 km (126,200 feet) flying at Mach 1.7, which also marked the first time anyone had exceeded

100,0 feet altitude (corresponding to 30.5 km, but 100,000 is obviously more impressive as a ‘magic number’). Since at this altitude 99.6% of the atmosphere is below the aircraft, Kincheloe was named the ‘First of the Spacemen’. He later said that at the highest point, “Up sun the sky was blue-black in color and the sun appeared to be a very white spot. The sky conditions down sun, were even darker in color. This dark condition existed through the horizon where a dark gray band appeared very abruptly. This gray band lessened in intensity until eventually its appearance resembled that of a typical haze condition. Extremely clear visual observation of the ground within a 60 (degree) arc directly beneath the aircraft was noted.” As expected of a military test pilot, this report was factual and devoid of any emotional response. On three occasions Kincheloe tried to go higher, but each attempt ended in an abort. His altitude record (unofficial due to the use of a carrier plane) stood until the X-15 rocket plane surpassed it in August 1960.

The X-2 was scheduled to be transferred to NACA in mid-September, which was eager to start a series of missions to investigate aerodynamic heating and study the handling characteristics of the aircraft at extreme altitudes and speeds. However, the Air Force was keen to reach Mach 3, which was the next ‘magic number’ in aviation,

Captain Mel Apt in the X-2. [US Air Force].

and managed to get an extension and check out another of its pilots, Captain Milburn ‘Mel’ G. Apt. While Apt practiced missions on the GEDA simulator, representatives from the Air Force, NACA and Bell agreed on a flight plan. It was clear the mission would involve a lot of risk, as understanding of the dynamics of a Mach 3 airplane was fairly sketchy in the 1950s. In fact, the limited aerodynamic data gathered from wind tunnel experiments for the X-2 was only valid up to Mach 2.4; what happened beyond that could at that time only be discovered by practical “cut-and-try”.

On 27 September 1956 all was ready to attempt the record flight. Thanks to the grease pencil line on his cockpit window, Apt flew an almost perfect profile of speed and altitude as a predefined function of time and became the first person to fly faster than thrice the speed of sound. The maximum speed attained was an incredible Mach 3.196; equivalent to 3,369 km per hour (2,094 miles per hour). Sadly, the excitement was very short lived. As he turned back towards Edwards, Apt for some reason made too sharp a turn and lost control due to inertia coupling; the problem first suffered by Yeager in the X-1A in 1953 and which may well have been avoided if the intended fly-by-wire flight control system had been implemented in the X-2. After a series of violent combinations of roll, pitch and yaw the aircraft entered a relatively smooth subsonic inverted spin, but Apt could not get it under control. During his attempts he never unlocked the rudder, which had been manually secured prior to accelerating to supersonic speeds in order to avoid dangerous shock waves forming over the vertical stabilizer. We will never know whether unlocking the rudder would have helped to escape from the spin. Realizing that he would not be able to gain control of the plane, Apt separated his escape capsule. Unfortunately he did not manage to get out of the capsule before it slammed into the desert floor (the problem that NACA had warned of when the system was accepted by the Air Force). Ironically, the X-2, now without its cockpit, stabilized itself and continued to descend in a series of undulating glides followed by stalls, before hitting the ground and coming apart.

The most spectacular achievement of the X-2 was therefore also its last, and Apt’s death cast a shadow over the program. It was a highly experimental and dangerous machine, a fact that was downplayed at the time in order to ensure continuing public support. However, the X-2 program had accomplished much of what it had set out to do: identifying the peculiarities of high-altitude flight and speeds exceeding Mach 2. Unfortunately, some of the lessons were learned the hard way. It was now clear that the safe operation of aircraft at very high speeds would require more sophisticated control systems, in particular incorporating so-called ‘stability augmentation’ since at Mach 3 things happen very quickly and a pilot receives little warning before inertia coupling causes loss of control. In fact, X-2 pilots found that above Mach 2.5 the safest thing to do was not to do anything at all, as any small steering correction could give rise to dangerous instabilities. One simple measure implemented during the X-2 flights was the already mentioned mechanical locking of the rudder at supersonic speeds. Everest even had a metal grab bar installed at the top of the instrument panel, on which he would place both of his hands at extreme speeds in order to force himself not to move the stick (a very difficult task for a pilot used to always being in active control of his plane). The extremely successful (and

much better known) X-15 rocket plane program benefited greatly from both the good and the bad experiences of the X-2.

The dangerous nature of their X research aircraft was pretty much downplayed by both the Air Force and Bell Aircraft. The documentary movie Flight into the Future released by the Department of Defense in 1956 duly explained how important and challenging the research work at Edwards was, but it failed to say anything about the risks and accidents, of which there had already been many. It showed pilot Everest kissing his wife goodbye in the morning and going to work just as if he were going to spend his time at a desk. No mention was made of the considerable risks that he was undertaking on a regular basis, and that his wife was probably wondering whether he would survive to have dinner with her that evening. Many test pilots at Edwards died paving the way for the future of aviation, flying various experimental and prototype rocket planes and jet aircraft. The movie included a routine test firing of the rocket engine of the X-2 with personnel standing literally alongside the nozzle, which was a risky thing to do because rocket engine’s were still not all that reliable (as an engine explosion during a ground test of the X-15 would later emphasize).

Not much of the X-2 has survived. The one that was dropped in Lake Ontario was never recovered. The one that crash-landed by itself near Edwards was salvaged, and some thought was given to reassembling the aircraft to continue the test program but this was rejected and the remains were buried (apparently nobody remembers where on the vast base). Souvenir hunters occasionally find bits and pieces at the crash site. A replica of the X-2 was constructed for the 1989 television series Quantum Leap, and it is currently being restored for display at the Planes of Fame Museum in Chino, California.

The X-2 also made it onto the big screen, first in 1956 in the movie Toward the Unknown (apparently a translation of the Latin motto of Edwards Air Force Base). It is a story about a daring test pilot trying to redeem himself after having succumbed to torture while a prisoner of war, and also win back the love of a girl. Other than using actual X-2 footage, the story has little to do with the real flight program. In 2000 the entertaining movie Space Cowboys featured a plane which appears to be a (computer generated) two-seat version of the X-2. In the prologue one of the pilots manages to rip a wing off the aircraft during a flight in 1958, after which both occupants (played by Clint Eastwood and Tommy Lee Jones) employ ejection seats to save themselves. So much for historical accuracy!

MODERN CONCEPTS

Although the large spaceplane programs were canceled, some related developments did survive. Work on hypersonic scramjet propulsion in the US was continued in NASA’s Hyper-X program, and resulted in two test flights of the small X-43A. This unmanned experimental vehicle was launched from the nose of a Pegasus rocket that was itself dropped from a converted airliner, and set new records for an airbreathing vehicle by achieving Mach 9.6 and a scramjet burn of 10 seconds on its latest flight in November 2004. Scramjet development tests are continuing with the similar X-51, which flew for the first time in May 2010 and reached a speed of ‘only’ Mach 5 but a much longer powered-flight time of 200 seconds. The second flight, in June 2011, was unsuccessful due to a failure of the scramjet engine. Some people insist that the US military already has a highly secret orbital spaceplane in operation (such as the ‘Blackstar’ reported by Aviation Week) but the evidence is unconvincing.

Shortly after HOTOL foundered in 1988, members of the engine design team led by Alan Bond set up a new company (Reaction Engines Ltd) to continue to develop the HOTOL concept, focusing initially on an improved version of the RB545 engine called the SABRE (‘Synergistic Air Breathing Engine’) and in particular the crucial precooler section. The spaceplane concept currently being worked on is the Skylon presented in the Introduction of this book as a “perfect spaceplane” (it is named after a futuristic art structure included in the 1951 Festival of Britain, which the fuselage strongly resembles). The designers reckon they have fixed the flaws in the HOTOL design, in particular the stability problem due to the heavy engines in the aft part of spaceplane. The Skylon solution is to locate the engines in the middle of the vehicle, housing them in nacelles at the tips of the delta wings in the same way envisaged for the Keldysh Bomber in 1947. This prevents the center of gravity from moving aft as the propellant tanks are depleted. Moreover, since the engines are not fully integrated with the fuselage they can be tested separately from the remainder of the vehicle. The engine nacelles have a peculiar banana-shape because their air intakes have to point directly into the airflow, whereas the spaceplane’s wings and body must fly at an angle to create lift. Each engine will give a maximum thrust of

1,350,0 Newton in airbreathing mode, and 1,800,000 Newton in rocket mode.

According to the company their SABRE propulsion would make Skylon very safe and reliable, and enable it to take off without the rocket trolley that would have been necessary for HOTOL. But this meant Skylon would need a sturdy undercarriage as well as strong brakes to stop itself before the end of the runway if a problem were to occur just before take-off. It was decided to cool the brakes by water, which would boil away and dissipate the heat caused by the braking friction. The cooling water would be jettisoned following a successful take-off, thus reducing the weight of the undercarriage by several metric tons. At landing Skylon would be empty and hence fairly Ught, so the brakes would not need water cooling in order to be able to stop the plane without catching fire. Due to its aerodynamic characteristics upon re-entry, the vehicle would slow down at higher altitudes than the Space Shuttle Orbiter, keeping the skin of the vehicle significantly cooler, hence requiring only a durable reinforced ceramic for most of its skin. The turbulent airflow around the wings during re-entry

Model of the SABRE engine [Reaction Engines Limited],

Oxygen Payload Sabre Tank Container Engine

Cutaway of the Skylon concept [Adrian Mann & Reaction Engines Limited],

would, however, necessitate active cooling of some parts of the vehicle. Skylon is expected to be able to put 12,000 kg of payload into low orbit. Its take-off noise is expected to be acceptable for taking off from regular airports in populated areas but the runways would have to be extended to 5.6 km (3.5 miles) in length, of which the first 4 km (2.5 miles) would require to be stronger than usual to cope with the heavily laden Skylon rolling at high speed.

An independent review by the European Space Agency, which is also funding part of the technology development for Skylon, concluded in 2011 that the overall design “does not demonstrate any areas of implausibility”. Reaction Engines is confident that Skylon will soon reach a technical maturity sufficient to convince investors that it is a valid commercial opportunity which warrants funding to full development. The project’s cost estimates indicate that if a fleet of 90 vehicles were produced it would be possible to buy a Skylon for about $650 million, which is roughly comparable to a large jet airliner. Early customers would pay $30 to $40 million per flight but with more aircraft flying and an increasing total number of flights, the price should fall to around $10 milhon per launch. In comparison, a current Ariane 5 expendable rocket costs around $150 million per launch and puts less mass into a low orbit.

At the time of writing, Reaction Engines is doing tests on an experimental version of the precooler and plans to build a sub-scale version of the SABRE to demonstrate (on the ground) the complete engine’s airbreathing and rocket modes as well as the transition between these. Tests of the nacelle in which the SABRE is to be housed are to be performed using a Nacelle Test Vehicle (NTV). This is planned to be launched from the ground and use rocket engines to get up to Mach 5, at which speed internal ramjet combustion systems will simulate the operation of the air-breathing engine. The remainder of the nacelle test article will be as close as possible to the real thing, including the control systems and internal flow ducts. The NTV is also to get some data on shock interactions between the nacelle and the Skylon’s wing. According to Bond, “we could have a Skylon plane leaving Heathrow airport sometime during this century”.

In 1991 Bristol Spaceplanes, another small aerospace company in the UK, began working on rocket plane concepts. Their ‘Spacecab’ design involves a Concorde-like carrier aircraft that uses four ordinary turbojet engines to take off and accelerate to Mach 2 and two rocket engines to reach Mach 4. At that speed a small, delta-winged rocket propelled orbiter carrying two pilots and either six passengers or cargo would separate and climb into orbit. The company insists this is a very conservative design that does not require any new technology to be developed. A next-generation vehicle called ‘Spacebus’ would fit the carrier aircraft with newly developed turbo-ramjets to achieve Mach 4 and rocket engines for Mach 6. Its enlarged orbiter would have room for fifty passengers. David Ashford, the company’s managing director, has published his ideas in two popular science books: Your Spaceflight Manual – How You Could be a Tourist in Space Within Twenty Years (written with space tourism promoter Patrick Collins) and Spaceflight Revolution.

Private companies like Reaction Engines and Bristol Spaceplanes keep working on spaceplane technology, and the development of hypersonic and scramjet engines for military applications is strongly supported in the US (specifically in the Falcon program). Non-military government space agencies such as NASA and the European Space Agency have not completely given up on reusable launch vehicles either. The Future European Space Transportation Investigation Programme (FESTIP) study by ESA in 1994-1998 for instance, investigated many basic reusable launcher concepts, several of which were spaceplanes. In 2005 ESA also ran a small internal study (in which I participated as System Engineer) for a small rocket plane called Socrates. This was intended to fly at speeds up to Mach 12 and be operated for about 30 flights. It was specifically to investigate spaceplane operations and maintenance, such as how long it takes to replace rocket engines, how an onboard health monitoring system could help speed up maintenance, and how long thermal protection materials could last. A reusable rocket plane’s ability to make many test flights at relatively low cost should result in a higher reliability in comparison to modern expendable launchers which are usually deemed operational after only a single qualification flight but typically have a failure rate of 2 to 3% (meaning that two or three payloads per hundred are lost, in turn pushing up insurance costs).

India, a country that is making great strides in spacecraft and launcher technology, is investigating a concept known as AVATAR (for ‘Aerobic Vehicle for hypersonic Aerospace TrAnspoRtation’). This vehicle would take off using airbreathing turbo-ramjet engines and full tanks of fuel but none of the liquid oxygen that it would later need for its rocket propelled flight phase. Instead, during atmospheric flight separate ram air intakes collect air that is subsequently liquefied using liquid hydrogen-cooled heat exchangers; similar to the ACE principle of the JSSTO spaceplane concept. But unlike JSSTO, AVATAR involves another step in which the liquid oxygen in the air is mechanically extracted and stored in the previously empty oxidizer tanks so that they will be full by the time AVATAR requires to switch over to rocket propulsion. Although this is an extremely ambitious project, India is developing its capabilities at a rapid pace.

LAST STAND ROCKET PLANES

The need for a revolutionary air defense fighter led to many other German rocket interceptor designs near the end of the war, as well as an even more bewildering array of advanced propeller planes, jet fighters and intercontinental bombers that are beyond the scope of this book. Even limiting the focus to rocket airplanes results in a lengthy list of concepts.

Junkers came up with the EF127 ‘Walli’, a rather whimsical code name for what was to have been a relatively large, horizontal take-off rocket fighter equipped with a retractable tricycle undercarriage (i. e. one with wheels under the wings and the nose), a take-off weight of 2,960 kg (6,530 pounds), a wingspan of 6.7 meters (22 feet) and an armament comprising a pair of MG 151/20 20-mm cannon and a dozen air-to-air rockets. With two solid propellant take-off assist boosters each delivering a thrust of 10,000 Newton and an HWK 105-509C dual-chamber rocket engine it was predicted to be able to climb to an altitude of 10 km (6 miles) in a mere 75 seconds, which was significantly faster than either the Me 163B or the Me 263. Apart from the boosters it had many similarities with the Me 263, but had a more conventional shape with two straight wings and a tail with horizontal stabilizers.

Then there was the Arado Аг E381 minifighter, of which the first design had a length of just 4.7 meters (15 feet), a wingspan of 4.4 meters (14 feet) and a weight of only 1,200 kg (2,650 pounds). It was to have been carried to altitude slung under an Ar 234 jet bomber, a plane that was actually operational in August 1944. Because the E381 was so small and simple, heating and electrical power would be supplied by the host during the ascent. Communication between the E381 pilot and his colleague in the Ar 234 would be via a telephone link. The E381 was to be released at an altitude of about 1 km (0.6 miles) above the enemy bomber formation to execute its attack in a shallow dive at close to 900 km per hour (560 miles per hour), firing either a single MK 108 cannon or air-to-air missiles. The Walter HWK 109-509C dual-chamber engine would subsequently be ignited to climb for a second attack pass. At that point the E381 would dive and glide to the ground unpowered. It would land using a skid, similar to the Me 163. Then the little plane was designed to be easily dismantled into wing, fuselage and tail units that could be manually loaded onto a truck for return to base.

Due to the limited ground clearance of the E381 when suspended under its carrier plane whilst on the ground, its pilot would lie on his stomach in the cramped cockpit and view through a bubble window in the nose. This prone position would also help him to sustain the high G-forces when making sharp maneuvers at high speeds. Pilots in a conventional sitting position could experience tunnel vision, black-outs (loss of vision) and even loss of consciousness when sharply starting a steep ascent or a tight turn because the centrifugal force would push blood into the lower part of their body, depriving the brain of sufficient oxygen. Similarly, when pilots would suddenly push their planes into a dive the negative G-forces would push blood into the upper part of the body, causing a reddening of his vision known as ‘red-out’. These phenomena were much less trouble when the pilot was in a prone, horizontal position, since there was little vertical distance between his upper and lower body, making it easier for the heart to keep pumping sufficient blood to the brain. Also, the prone position led to a more aerodynamic fuselage shape with a smaller frontal area, reducing drag.

The second design of the E381 was slightly larger and had a weight of 1,270 kg (2,790 pounds). What appears to be the Аг E381 III had a length of 5.7 meters (19 feet), a wingspan of 5.1 meters (17 feet) and a weight of 1,500 kg (3,300 pounds). Instead of a single MK 108 cannon, the third version was armed with six RZ-65 air – to-air rockets that would be fired from the leading edges of the wings. But the E381 project never materialized. After the war, the prone pilot position was tested in the UK using a second, experimental cockpit attached to the nose of a Gloster Meteor F8 jet fighter. For safety, the conventional cockpit was also manned. The tests showed that the prone pilot could endure slightly higher G-levels, but this position proved to be very uncomfortable, to lead to vertigo (a sensation of motion when one is actually stationary) and to seriously limit the pilot’s rear view; the latter being a very serious disadvantage in a dogfight. Furthermore, it would be very difficult to devise a good ejection seat escape mechanism for this awkward position: ejecting forward was not an option because the plane would catch up with the pilot and collide with him. After 55 hours of flight testing, the idea was abandoned. The prone position soon became obsolete after the war anyway thanks to special anti-G suits with inflatable bladders in the trousers: when they experienced negative G-forces in a sitting position, these bladders would quickly inflate to firmly press against the abdomen and legs in order to restrict the draining of blood away from the brain.

Arado also proposed to develop the TEW 16/43-13, a low-wing rocket interceptor that had a wingspan of 8.9 meters (29 feet), a Walter HWK 109-509A engine, and an armament of two MG 151/20 20-mm cannon and two MK 108 30-mm cannon set in its nose. The wings and tailplane were moderately swept back, and the pilot would sit in a conventional upright position. The company also devised the TEW 16/43-15, a combined jet/rocket powered variant in which a jet engine was mounted on top of the (lengthened) fuselage.

In his 1929 book Wege zur Raumschiffahrt (Ways to Spaceflight) Hermann Oberth had contemplated a rocket aircraft built like a tank to fly into an enemy air fleet. It could then use conventional gunfire to destroy targeted planes but he suggested that collision would also be an effective means of attack. By 1943 this dramatic concept started to look like a valid idea to stop Allied bombers, leading Lippisch to suggest a ‘ram rocket’. This would be launched vertically using a liquid propellant engine and solid propellant boosters (like the Natter) and be equipped with a sharp steel nose to slice through a bomber without damaging the ram rocket (or its pilot). In November 1944 the Zeppelin works, famous for building giant airships, had the same idea and proposed the ‘Rammjager’ (Rammer Fighter). Taking off from a jettisonable tricycle carriage, their little plane would be towed aloft by a fighter aircraft and be released near the bombers. The pilot would ignite a single solid propellant Schmidding rocket (similar to the Natter boosters) to accelerate to 970 km per hour (600 miles per hour) and launch his 14 R4M rockets. He was then expected to make a second pass and actually ram his plane through the tail section of a bomber. It was calculated that the Rammjager would slice cleanly through without great loss of speed and stabihty, and survive the collision thanks to its armored cockpit and reinforced fuselage and wings. It would then glide back unpowered and land on a retractable skid. By virtue of not having a liquid propellant rocket engine, the Rammjager was even simpler than the Natter. After towed glide flights in January 1945 an order for sixteen pre-production prototypes was placed. However, US bombers destroyed the Zeppelin factory before their construction could commence.

Another rather ludicrous proposal by Zeppelin was for the ‘Fliegende Panzerfaust’ (Flying Armored Fist, or Flying Bazooka). This was to be an aircraft filled with high explosives to ram and blow up an enemy bomber; the pilot was supposed either to eject just before impact or sacrifice himself to ensure an accurate hit. If possible, the plane would also be equipped with a cannon to shoot down other bombers in advance of the final ramming attack. In line with the need for a very simple and cheap design, it would be powered by six solid propellant rocket motors only. The machine would be towed to altitude, for which it was equipped with a hooked nose connecting to the tail of a regular fighter aircraft. It would have been some 6 meters (20 feet) in length with a wingspan of 4.5 meters (15 feet). The project never got any further than a full-scale mockup that was presented to the SS in January 1945.

Then there was the DFS ‘Eber’ (Boar), a tiny rocket fighter that either an Fw 190 propeller fighter or an Me 262 jet fighter would tow using a long pole, then release some 300 meters (1,000 feet) above the bombers. The Eber would engage a target in a gliding dive utilizing either a single Mk 108 cannon or air-to-air rockets, and then ignite two solid propellant motors that would deliver a thrust of 15,000 Newton for 6 seconds to make a second attack in which it would ram a bomber. The prone pilot would be protected by an armored cockpit and use a special spring-dampened shding seat to soften the 100 G deceleration shock that would result from the collision with a large enemy plane. After the final attack, a parachute would pull the pilot and his seat from the cockpit. At sufficiently low speed, he would jettison the seat and land using his own parachute. The airplane itself was expendable.

The fact that the idea of ramming enemy planes was popular near the end of the war reflects a level of panic in German military planning and an increasing fanatical demand for pilots to sacrifice themselves in defense of their doomed motherland. The Sombold So 344 ‘Rammschussjager’ (Ram-shoot Fighter) was another approach to the ramming plane concept. Rather than ramming a bomber itself, this rocket aircraft would carry a 400 kg (880 pound) warhead on its nose and fire it into the middle of a bomber group using a solid rocket motor built into the bomb. The warhead was fitted with four stabilizing fins and a proximity fuse for automatic detonation when close to an enemy plane. Allied bombers flew in tight formations so that the onboard gunners could provide cover to each other’s planes and work together to bring down attacking fighter planes. The explosion of the warhead released by the So 344 was expected to destroy three of four bombers and open the formation sufficiently for other German fighters to engage individual bombers. The So 344 was expected to join this carnage employing a pair of machine guns.

The So 344 was to be powered by either a single Walter HWK 109-509 engine or a number of solid propellant rocket motors, have a wingspan of 5.7 meters (19 feet) and a take-off weight of 1,350 kg (2,980 pounds). Its cockpit was located well behind the wings, just in front of the vertical and horizontal stabilizers. It was designed to be carried into the air on top of another aircraft, to land employing a skid, and then be dismantled into two sections for transportation by truck back to its base.

Messerschmitt also produced designs for simple towed ramming rocket aircraft. The basic design for the Me P.1103 of July 1944 had its pilot lying prone inside an armored cylinder cockpit. After release from a conventional Bf 109 fighter or even an Me 262 jet fighter tow plane, it would attack at 700 km per hour (435 miles per hour) shooting a single MK 108 30-mm cannon before ramming one of the bombers. A parachute would then pull the pilot and his seat out of the aircraft. Another chute would soft-land the reusable assembly consisting of the armored cockpit, the cannon, and the floor-mounted Walter RI-202 ‘cold’ rocket engine (the same as in the RATO pods used on German bombers) for retrieval. The tail section would be lost, but that was to have been only a simple wooden empennage derived from the VI unmanned flying bomb. Another interesting fact about the Me P.1103 is that it was to have a rearward firing rocket launcher to defend itself from Allied fighters. A design for an alternative P.1103 version of September 1944 shows a sitting pilot looking through a conventional bubble canopy. The Me P.1104 was another Messerschmitt design for a simple towed, rocket propelled defensive interceptor. This one would have had an HWK-109-509A rocket engine (as on the Me 163B) and been equipped with a single MK 108 cannon. The total weight of the fully loaded plane was to be 2,540 kg (5,600 pounds), so its rocket engine should have provided a maximum speed of 800 km per hour (500 miles per hour) and a range of some 90 km (60 miles).

All these concepts for small and cheap rocket propelled fighters were consigned to the archives by the Luftwaffe, which was undoubtedly better news for the pilots who would have been required to fly these rickety death traps than it was for the crews of the Allied bombers. However, some other rocket fighter designs did progress a little further through the development process.

The Focke Wulf ‘Volksjager’ (the People’s Fighter; not to be confused with the Heinkel He 162 ‘Volksjager’ jet fighter) was based on the design for the Та 183 jet fighter. It was to be a defense interceptor with a similar role and mode of operation to the Me 163. It also looked a lot like the Komet, with a Walter HWK 109-509A2 and sharply swept wings mounted mid-fuselage which were to be mostly made of wood. The wingspan would be 4.8 meters (16 feet) and the total length a mere 5.3 meters (17 feet). It would take off from a jettisonable dolly, climb to the enemy bombers and attack using two MK 108 30-mm cannon located in the lower fuselage sides, then glide back unpowered and land on a retractable skid. Unlike the Me 163 (but like the Та 183) it had a horizontal stabihzer mounted on the vertical fin to form a ‘T’ shaped tail. With the addition of four solid propellant strap-on boosters, the Volksjager was intended to reach an altitude of 5.9 km (19,400 feet) in just 60 seconds, and 16.5 km (54,100 feet) in 100 seconds. This was a significantly faster climbing speed than the Komet, which did not have boosters. The maximum speed was expected to be about 1,000 km per hour (620 miles per hour). Three aircraft were under construction when the war ended but none ever flew. As was the case for the He 162 jet that was also called the ‘People’s Fighter’, the Focke Wulf was probably meant to be flown by relatively untrained pilots recruited from the Hitlerjugend, the Nazi youth movement. But the He 162 proved too difficult for amateurs to fly, and this would probably also have been the case for the Focke Wulf rocket interceptor.

Another Focke Wulf design was the Та 283, an airplane with a slender fuselage, wings swept back at 45 degrees, and two large ramjet engines. Since ramjets do not work at low speeds, for take-off the aircraft was to have a single Walter HWK 109- 509A rocket engine and about 30 seconds worth of rocket propellant. To prevent the big ramjets from disturbing the airflow over the wings, they were to be mounted on the sharply swept tailplane. The armament was envisaged to comprise two MK 108 cannon. When this concept was judged lacking in performance an improved design was proposed in which the fuselage intended for the Fw 252 jet fighter would have a small jet engine emplaced in the central fuselage, two ramjets on the tailplane and a Walter HWK 109-509A just beneath the tail boom. This design was so preliminary that it didn’t even have a proper designation, but after the war it came to be known as the Fw 252 ‘Super Lorin’ (after the inventor of the ramjet concept, Frenchman Rene Lorin).

Heinkel’s contribution to the mixed bag of radical rocket fighter concepts was the P.1077 ‘Julia’. As designed in August 1944, the wooden Julia would have been 6.98 meters (22.9 feet) long, had straight shoulder-mounted wings with a total span of 4.6 meters (15 feet) and a tail with twin vertical stabilizers. Its propulsion was to have been one Walter HWK 109-509C engine with separate combustion chambers for the initial climb and subsequent cruise, and two booster rockets mounted on either side of the fuselage. It would have had two MK 108 cannon housed in blisters on the sides of the forward fuselage. It was initially envisaged that the pilot would fly in a
prone position but an alternative design was prepared in which he would sit in a conventional manner. Like its Bachem Natter competitor, the Julia would have been launched vertically off the ground, but after the attack the pilot would have landed it on a retractable skid rather than bail out. In September 1944 the RLM ordered 20 Julia prototypes and then two weeks later demanded the production of 300 operational aircraft per month. Nothing came of this, of course. Only towing trials with a full-scale mockup were performed before it was destroyed when the Vienna woodworks was bombed, and the Schaffer company in Linz had time only to start two unpowered prototypes, neither of which was completed.

The DFS 346 of the German Institute for Sailplane Flight was not intended for combat, it was designed purely as an experimental research plane. Its chief designer, Felix Kracht, gave it wings swept back at 45 degrees, a streamlined fuselage with a prone pilot, and an HWK 109-509C rocket engine. It was to be taken to high altitude by a Domier Do 217 bomber and then air-launched, a novel concept which, it was hoped, would save the DFS 346 sufficient propellant (otherwise needed for take-off and ascent) to break the sound barrier. The altitude provided by the carrier plane also resulted in lower aerodynamic drag due to the thinner atmosphere and meant Mach 1 could be reached at a lower absolute flight speed since the speed of sound is lower in the colder high-altitude air. The pilot was in a pressurized section that formed a self-contained escape capsule which could be separated from the plane in an emergency. Stabilized by a small parachute, the capsule would fall to an altitude of about 3 km (10,000 feet) where the air pressure is safe, the Plexiglas nose section would separate and the pilot would slide out and land using his parachute. The unfinished DFS 346 prototype was captured by the Soviets at the end of the war, taken to Russia, rebuilt and actually flown (we will discuss this further in a later chapter).

The concept of air-launching was later adopted in the USA for the world’s first supersonic rocket plane, the post-war X-l. The highly swept wings of the DFS 346 were actually much more advanced than the conventional straight wings of the X-l and make the DFS 346 look more like a modern supersonic fighter than the famous

American rocket plane which relied upon brute rocket power and sturdy wings rather than sophisticated aerodynamics to blast through the sound barrier.

Felix Kracht also designed the DFS 228 rocket propelled reconnaissance aircraft, a fairly conventional sailplane design with long, slender but straight wings spanning 17.6 meters (57.7 feet). It was to have a dry weight of 1,350 kg (9,280 pounds) and a take-off weight of 4,210 kg (2,980 pounds). It would be air-launched from on top of a Do 217, be powered by a Walter rocket motor, and land using a belly-mounted skid. The pilot was to be prone in a pressurized escape capsule. The long wings were designed to enable the plane to achieve a cruise altitude of 24 km (80,000 feet), safe from interception by Allied fighters. Its maximum speed was estimated to be about 900 km per hour (560 miles per hour). The mission profile was for a powered climb followed by a slow, unpowered descending glide, then reigniting the rocket motor to climb back to altitude, and so on until the propellant was finished. The resulting saw­tooth flight pattern that would give the aircraft the relatively long range of 1,050 km (655 miles) also prompted its nickname of ‘Sagefish’ (Sawfish).

Walter designed a new rocket motor for the plane because the requirement for a very streamlined fuselage in combination with weight balance made it necessary to place the motor near the plane’s center of gravity, which required fitting a long thrust tube to the tail. The resulting motor was an elongated version of the single­chamber HWK 109-509A2 named the HWK 109-509D. The DFS built three prototypes: VI, V2 and V3. The DFS 228 VI had a conventional seat but it was shghtly inclined to the rear to accommodate the fuselage streamlining and to assist the pilot in handling high G-loads (a similar idea was introduced in the F-16 fighter jet several decades later). The VI made several unpowered glide flights after being carried to altitude on top of a Do 217, and was later fitted with the HWK 109-509D for ground tests. This prototype was captured by the Americans at the end of the war and in 1946 was sent to Britain for study, but it arrived in very bad condition and was apparently scrapped. The V2 had a cockpit with a prone pilot position and apparently made several glide flights before being damaged in a landing accident. The V3 was never finished. None of these aircraft ever flew under rocket power.

Rather than develop a completely new aircraft, in mid-July 1943 Arado proposed to produce a high-altitude reconnaissance rocket plane by modifying the Ar 234 jet, which had made its first flight in June. In the initial design the two jet engines were to be replaced by two HWK 109-509A rocket engine pods. The resulting Ar 234R ‘Hohenaufklarer’ (High-altitude Scout) would be able to take off by itself, ascend to an altitude of 16.5 km (54,000 feet) and photograph its targets while descending in a shallow glide. It would be equipped with the pressurized cockpit already created for the Ar 234C reconnaissance jet. A later design had a dual-chamber HKW 109-509C built into the tail instead of pods slung under the wings. This version was to be towed to an altitude of 8 km (26,000 feet) by a Heinkel He 177 bomber (a large, long-range bomber then under development) and upon being released it would boost itself up to 18 km (59,000 feet). The Ar 234R would be able to photograph areas some 250 km (155 miles) from its base, being towed 200 km (125 km) towards its target and flying a further 50 km (30 miles) on rocket power. The plane would subsequently glide over its targets, igniting the rocket engine at intervals to maintain sufficient altitude.

Freed from the need to mount engines under the wings, a special highly efficient wing was designed to give the aircraft a glide angle of 1 to 14 (in which it would lose 1 meter of altitude for each 14 meters of horizontal flight), sufficient to enable it to glide the entire 250 km (155 miles) back to base.

Another Ar 234 variant fitted with a rocket engine was actually under construction when the Allies captured the factory in which it was being assembled. It consisted of an Ar 234B fuselage equipped with a new concave-curved swept wing optimized for high-speed flight. It was powered by a pair of BMW 003R engines, each consisting of a BMW 003 turbojet with a BMW 718 liquid propellant rocket engine mounted on top. The fact that the pumps for the rocket propellant were powered by the jet engine resulted in a nicely compact assembly. The rocket engine could deliver an additional 10,000 Newton of thrust for 3 minutes during take-off and ascent. Unfortunately the prototype was scrapped.

The sole surviving Ar 234B can be seen in the National Air and Space Museum’s Steven F. Udvar-Hazy Center near Washington Dulles International Airport. It is the basic jet type with a pair of Walter RI-202 ‘cold’ RATO pods under its wings. The parachute pack intended to retrieve the pods following jettisoning soon after take-off can be clearly seen on these examples.

Another pragmatic design enhanced the standard Me 262 jet fighter (which had a jet engine under each wing) with an additional rocket engine. To test the principle an Me 262 was adapted to accommodate a redesigned HWK 109-509A2 motor with the combustion chamber installed in the tail; a portion of the lower part of the rudder was cut away to make room for the nozzle and the rocket exhaust. This Me 262 C-la ‘Heimatschiitzer Г (Home Defender I) prototype made its first rocket assisted take­off on 27 February 1945. The combined thrust from the two jets and the single rocket reduced the take-off run by at least 200 meters (660 feet) and pushed the plane to an altitude of 8 km (26,000 feet) in about 3 minutes (about half the time required by a standard Me 262 and similar to the Me 163 Komet’s performance). Major Heinz Bar later managed to intercept and shoot down an American P-47 Thunderbolt fighter in the rocket propelled test plane by climbing to about 9 km (30,000 feet) in a little over 3 minutes.

A similar concept was the Me 262 C-lb ‘Heimatschiitzer IF which was provided with two BMW 003R combined jet-rocket engines for boosted thrust. Only a single prototype was built and its only flight with the combined propulsion was made on 26 March 1945. This engine was also to have been mounted on an He 162 ‘Volksjager’ (People’s Fighter), the standard version of which had a single BMW 003 jet engine on its back. However, the creation of this He 162E prototype could not be achieved before the war ended.

Despite the success of the ‘Heimatschiitzer Г it was judged that the rocket engine added too much complexity to the jet fighter, making it more difficult to maintain. In addition, the tanks needed for the less propellant-economic rocket severely reduced the available space for fuel for the jet engines, which limited the range and flight duration of the fighter in comparison to a standard Me 262 (issues that would also plague the post-war mixed-power interceptor designs that we will discuss later). These considerations led to the design of the ‘Heimatschiitzer IIP which had a rocket engine bolted onto the belly of the aircraft and its propellant in drop tanks that would be jettisoned once they were empty so that the plane would not be burdened by their useless weight and aerodynamic drag. The advantages of this arrangement were that it required fewer modifications of the standard Me 262, and the modified Me 262 could take off without the rocket engine if it were not needed. Another plus was that maintenance of the rocket system could be separated from maintenance of the aircraft: a fighter would not be grounded by a malfunctioning rocket engine, but could be quickly fitted with a replacement engine while the faulty unit was taken to the repair shop (an idea which, after the war, would be applied to the French Mirage rocket-motor equipped fighters).

Walter designed a powerful ‘hot’ engine based on his HWK 109-509 series for the ‘Heimatschiitzer IV’. This HWK 109-509 S2 gave a thrust of 20,000 Newton and weighed just 140 kg (310 pounds). The engine was to be mounted on the belly of the plane, just behind the roots of the wings. Two jettisonable tanks would be carried externally on bomb attachment points under the nose, with flexible hoses delivering a total of 1,200 liters of T-Stoff to the Walter motor. The C-Stoff would be carried in the rearmost converted fuselage tank. This was designated the Me 262 C-3 and work on the prototypes started in January 1945. Tests on the Т-Stoff tanks apparently revealed a problem with feeding fuel when the drop tanks were mounted lower than the rocket motor. In April the Allies overran the factory at Jenbach before a single prototype could be completed.

Eugen Sanger’s design for the ‘Silbervogel’ (Silverbird) intercontinental bomber surely ranks as the most ambitious of Nazi Germany’s rocket plane concepts. It was the first, more or less realistic design for an aircraft capable of flying on the edge of space and it is therefore described in more detail in a following chapter.

Germany hoped that its superior rocket and jet airplane technology would counter the vast numbers of conventional planes that were being manufactured by Americans in their secure homeland. Given a bit more time, this might have worked at least for a while and greatly prolonged the war in Europe. The Luftwaffe might have been able to put some really revolutionary aircraft into the skies, and a German rocket aircraft would likely have been the first to exceed the speed of sound. However, by the time the ‘wonder weapon’ airplanes received proper support it was already too late, and in any case the developments were too scattered, rivalry between the military services too disruptive, and the constraints in terms of material and propellant availability too great. Some of the small interceptors relied on Arado jet carrier planes that were not themselves available, and those meant to be towed behind conventional bombers and fighters would have been vulnerable to enemy fighters during the first phase of their mission due to the low speed of the towing operation. Many of the RLM designs now seem to be more the results of desperation and wishful (albeit creative) thinking than the reality of military operations. Nevertheless many post-war airplane designs were strongly influenced by some of the more brilliant German developments, particularly the Me 163 and Me 263.