Category Rocketing into the Future

THE LAST ROCKET FIGHTER

At the end of the war Russia captured from Germany three Me 163B interceptors and seven Me 163S glider trainers. These were tested in the large TsAGI wind tunnels as well as in gliding flight but a lack of the necessary propellants and unfamiliarity with the Walter engine prevented powered flights. The TsAGI analyses confirmed that the aerodynamics of the Me 163 were very suitable for high-speed flight. The Russians were of course even more interested in the Komet’s advanced successor, the Me 263, having captured some equipment and documentation along with technical staff who had worked on the project. The brewing Cold War and the threat of Western bombers renewed Soviet interest in fast, high-altitude rocket interceptors. Already in February 1946 the Lavochkin OKB was ordered to develop such an interceptor to detect and destroy enemy bombers. It had to be able to reach an altitude of 18 km (59,000 feet), achieve a top speed of 1,100 km per hour (680 miles per hour) in level flight at 5 km (16,000 feet), and be able fly at maximum thrust for six minutes. The plane was also to be capable of attacking at any time of the day and in all weather conditions, which meant that it had to be equipped with radar. Its armament was to be six 83-mm TRS-82 rockets. And to make the challenge even more difficult, the military wanted flight testing to start on 1 May 1947.

By the end of 1946 Lavochkin had finished the design for ‘Aircraft 162’: an all­metal plane powered by the familiar Dushkin RD-2M-3V, with a pressurized cockpit and air-to-air rocket launching tubes integrated in the fuselage. It was calculated that the rocket thrust would enable the plane to reach an altitude of 18 km (59,000 feet) in 2.5 minutes. Inspired by captured German research, the wings of the Lavochkin 162 were extremely innovative: not only were they swept, they were swept forward. Such wings provide most of the favorable high-speed characteristics of swept-back wings and offer the benefit of making the aircraft less sensitive to stall. When a plane with swept-back wings stalls (i. e. when the air can no longer correctly follow the contours of the wing because the angle of attack is too high) the air usually starts to let go at the root of the wing, and this effect quickly travels along the length of the wing to its tip causing the whole wing to rapidly lose lift and making the plane uncontrollable. When a forward-swept wing stalls, the disruption at the root cannot easily progress to the wingtips and ailerons. Giving a pilot much better control at high angles of attack makes his plane more maneuverable, which is very welcome on a fighter aircraft. But forward-swept wings were extremely difficult to build using standard 1940s aircraft materials since they require to be extraordinarily stiff. Otherwise, when the wingtips bend up or down at high speeds the airflow can rip the wings off (on a swept-back wing the airflow tends to push a twisting wingtip back to the horizontal). Because of this, along with the many unknown aerodynamic characteristics of swept wings, the La-162 was soon redesigned with well-understood straight wings.

A full-scale mockup was built but the chief designer, Semyon Lavochkin himself, soon began to express doubts about pursuing the concept. The plane would have a phenomenal rate of climb but its flying time and range would be extremely short. He believed further development of jet engines would result in better interceptor fighter designs. He was also concerned about the nitric acid propellant, which had proved to be very corrosive and unsafe in experiments during the war with RD-1 engines fitted in La-7 and La-120 aircraft. And there was the issue of the high operating pressures in the various pipes in the engine, which made it prone to leaks that resulted in a low reliability and heavy maintenance. Another issue was that the turbopump of the RD – 2M-3V would have not only to feed propellants to the combustion chamber but also drive an electric generator, which was calculated to be too weak to provide sufficient electrical power for the onboard radar. Furthermore, it was soon found that the high aerodynamic drag of the new straight wings would prevent the rocket engine from driving the plane to the required transonic flight speeds. All this, in combination with the troublesome development of the designated radar system, led Lavochkin to call a halt to work on the 162.

However, there was another Russian design bureau still working on a pure rocket – powered interceptor. The Mikoyan and Gurevich (MiG) design bureau had set out to develop a local version of the German Me 263 rocket fighter called the MiG 1-270. The Soviet military pushed their aircraft designers to copy as many as possible of the advanced German design features to get a head start in the new jet age. But Russian rocket engines were based on very different propellants than the Walter engine, and designers strongly resisted a radical switch from their own known and proven rocket technology. In addition, the MiG engineers and TsAGI aerodynamicists were not yet completely familiar with the characteristics and peculiarities of swept and delta­shaped wings and therefore preferred to employ conventional straight ones (the same was done in the US for the X-l experimental rocket plane, for the same reason).

Planform of the MiG 1-270.

MiG 1-270.

As a result the 1-270 essentially became an Me 263 with a Russian engine, a more conventional wing and tail design and a longer fuselage. The tail was different in that it had a horizontal stabilizer high on the vertical fin in a T-arrangement to diminish aerodynamical interference from the main wings. But the new plane had an ejection seat, a first for Soviet fighter aircraft and essential for bailing out at transonic speeds. In this respect it was better than its German predecessors. The rocket engine was the RD-2M-3V (described earlier) with two combustion chambers. It gave the 1-270 less thrust than the original HWK 109-509C of the Me 263. While the main combustion chamber of the German engine provided 20,000 Newton its counterpart on the RD – 2M-3V could yield only 11,000 Newton. The smaller ‘cruise flight’ chambers of the two engines were comparable at about 4,000 Newton. Even so, the combined thrust of both RD-2M-3V chambers was still expected to give the plane a maximum speed close to supersonic and make it possible to reach an altitude of 17 km (56,000 feet) in 3.2 minutes. For ease of maintenance and replacement of the engine, the fuselage could be split into two sections. Power came from a generator connected to a small propeller on the nose (like in the Me 163B) in addition to a generator cleverly connected to the turbopump of the rocket engine. The cockpit was pressurized, and armor plate in the forward structure and an armored windscreen would protect the pilot from enemy gun fire. Like the Me 263, the MiG 1-270 had a tricycle undercarriage. The two main wheels retracted into the fuselage since there was no room for them in the thin wings. As a result these wheels had a very narrow separation, making the plane wobbly on the ground and difficult to land on a rough field or in the presence of a cross wind. The maximum take-off weight was 4,100 kg (9,100 pounds), of which 2,100 kg (4,700 pounds) was propellant. It was armed with two 23-mm NS-23 cannon and eight RS-82 solid propellant air-to-air rockets, and it was meant to defend large industrial installations as well as military bases. The threat was anticipated to be American B-29/B-50 Superfortress bombers and the new B-36 Peacemaker that could fly at altitudes up to 15 km (48,000 feet).

Two prototypes were built, Zh-1 and Zh-2. The first was intended for gliding tests and was fitted with a ballast mass instead of an engine. Pilots trained for the gliding flights on a Yak-9 fighter used as a glider, loaded with lead weights to reproduce the weight and balance of the 1-270. Both the Yak-9 trainer and the glider version of the 1-270 were towed into the air by a Tu-2 bomber. The first glide test of the 1-270 Zh-1 occurred on 3 February 1947 with test pilot V. N Yuganov at the controls. Until July of that year the Zh-1 remained connected to the tow airplane for the entire flight, but for the second phase of the unpowered tests it was released at altitudes of 5 to 7 km (3 to 4.5 miles) to ghde home and land. It reached a maximum speed of 600 km per hour (370 miles per hour) during these unpowered tests. In early 1947 the Air Force put pressure on MiG’s chief designer, Artem Mikoyan, to get the 1-270 ready for a powered flight demonstration during the annual air display in Moscow on 18 August. The gliding flight test phase was curtailed and an RD-2M-3Y engine installed in the second prototype, Zh-2, in May. Unfortunately the smaller combustion chamber blew up in ground tests on 16 July and damaged the tail section. The aircraft was repaired but could not be readied in time for the air display. An aircraft exploding during that international showcase would not make a good impression on the Soviet leadership and the rest of the word!

On 26 August, well after the show, the Zh-2 made its first two powered taxi runs and a short hop with A. K. Pakhomov at the controls. Its first (and unfortunately also last) flight was on 2 September and it lasted just 7 minutes. Pakhomov successfully climbed to an altitude of 3 km (10,000 feet), then initiated a gliding descent back but widely overshot the planned landing point and crashed beyond the airfield perimeter. He was unhurt but the aircraft was damaged beyond repair. Fortunately by then the first prototype had been upgraded by replacing the dummy engine with a real one, and powered tests resumed using the Zh-1 with V. N. Yuganov at the controls. A taxi run was made on 29 September. On the first flight on 4 October it took off under the roaring combined thrust of the two combustion chambers. The main chamber was shut down 130.5 seconds into the flight at an altitude of 4,450 meters (14,600 feet). The plane reached a maximum speed of 615 km per hour (382 miles per hour) at an altitude of 2,900 meters (9,510 feet) operating on the smaller combustion chamber only. As he glided back Yuganov found that the undercarriage would not extend so he made a belly landing that was so soft that the plane was barely damaged. The poor reliability of the RD-2M-3V rocket engine however continued to slow down the test flight phase when its largest combustion chamber exploded during a ground test and blew off a large part of the tail. The Zh-1 was repaired but was not ready for its third powered flight until January 1948. By then, however, a new problem had shown up: after each flight the engine had to be rinsed with water to remove the dangerous and corrosive nitric acid propellant, but during the freezing Russian winter this was impossible without also turning the motor into a block of ice. There was no alternative to postponing further testing until March.

In the meantime, the Aviation Technology Committee of the Air Force carried out a reassessment of the 1-270’s potential as an operational interceptor, with shatteringly negative conclusions. A major complaint was that the engine couldn’t be restarted in flight without the high risk of an explosion caused by nitric acid accumulating in the combustion chamber between extinction and reignition. The corrosive and dangerous nature of the propellant was another major issue: the I – 270’s parts and particularly its oxidizer tank, as well as the technicians working on the plane, were constantly under attack by corrosive nitric acid vapors. Personnel dealing with the engine had to wear bulky protective suits that made work on the plane arduous. The acid tank required labor-intensive removal, checks and replacement every 2 months. Special materials and acid-resistant coatings (as many as nine layers for the most critical areas) were used but corrosion occurred anyway. The difficulty in landing an unpowered 1-270 (as shown during its first powered flight) and the impossibility of rinsing the engine with water during the winter were also Usted as severe operational limitations. It is clear that the Air Force had lost its enthusiasm for the design.

By then the idea of an interceptor powered only by a rocket engine was rapidly becoming outdated. The turbojet-propelled MiG 15 had already flown in December 1947, would soon enter service, and would satisfy most of the requirements initially set for the 1-270. The MiG 15bis version had a nearly supersonic maximum speed of 1,075 km per hour (668 miles per hour) and a flight ceiling of 15.5 km (50,900 feet). Furthermore, it had a maximum range of 1,200 km (750 miles), which could even be extended to 1,980 km (1,230 miles) using externally carried drop tanks. That made it much more useful than the 1-270, which was basically only capable of a brief sortie to attack enemy aircraft that came within several tens of kilometers of its base. The much longer powered-flight endurance of the jet also meant it could attack bombers multiple times. It would even have time to dogfight with enemy fighters, something that was becoming an important requirement. Whereas in powered flight the earlier Me 163B could easily leave behind the enemy propeller fighters that it encountered, the 1-270 would have had to engage new Western jet fighters which could match it in horizontal flight. The only advantage the MiG 1-270 still had over the MiG 15 and other early jets was its rate of climb. The MiG 15 could reach an altitude of 15 km (49,000 feet) in about 5 minutes, but the 1-270 could get there in 3 minutes (with its more powerful engine the Me 263 would have done it in 2 minutes). But this single advantage didn’t outweigh all the weak points of the 1-270 and the strong points of the MiG 15.

Moreover, the development of surface-to-air guided missiles was advancing at a great rate. The S-25 (NATO codename SA-1 Guild) and the infamous S-75 (SA-2 Guideline) entered service during the 1950s and could reach and destroy high-flying intruders even more rapidly than a rocket propelled interceptor because they had no pilot for whose survival acceleration levels had to be kept to a reasonable maximum. Between them the MiG 15 turbojet and the S-25 and S-75 missiles rendered the 1-270 obsolete, with the inevitable abandonment of the idea of a manned rocket interceptor. The MiG design bureau focused its efforts on jet interceptors and fighters as the new generation of military aircraft.

The pure rocket fighter was briefly revived in the late 1940s at the OKB-2 design bureau, where a team led by A. Ya. Bereznyak (designer of the wartime BI rocket plane) and the German Siegfried Gunther jointly worked on a multi-chamber rocket interceptor. During the war Gunther had been a leading designer at the Heinkel company, and after failing to find a job in the US or the UK had offered his services to the Soviets. Gunther and Bereznyak worked on different but similar versions of their supersonic rocket aircraft indicated by project numbers 466,468 and 470. These were all delta-winged designs powered by a rocket engine with four combustion chambers installed in the tail. The maximum total thrust of this engine was to be

82,0 Newton at sea level increasing to 96,000 Newton at an altitude of 20 km (66,000 feet). The plane had no horizontal stabilizers (typical of delta-winged aircraft) but had an enormous central vertical fin as well as two smaller fins under the wings. It would take off with a jettisonable dolly or a sled driven by solid propellant boosters, and land on skids. The pilot would have a pressurized, armored cockpit equipped with an ejection seat. A radar in the nose would help him to find his target, which he would engage using either four 23-mm cannon or two canisters with six unguided missiles each. A strike version of the aircraft carrying four high – explosive bombs was also considered as an option. The 470 interceptor was envisaged to fly at a top speed of 1,910 km per hour (1,190 miles per hour) above 11.5 km (38,000 feet) altitude, equivalent to Mach 1.8, and to climb to 20 km (66,000 feet) in 2 minutes and 14 seconds. But none of these aircraft designs made it off the drawing board. Inevitable competition with surface-to-air missiles and jet fighter designs promising much larger range meant that the entire project was judged to be obsolete by June 1951. OKB-2 was disbanded and its personnel transferred to other activities. Gunter returned to West Germany in 1954 and joined Heinkel AG when that company reopened in 1955. He never spoke about his work in the USSR, and neither did the Russians, but it seems that he had worked on various other advanced Soviet aircraft apart from the 466/468/470. In 1950 Ernst Heinkel said of his former employee, “I am convinced that Giinter worked on those Soviet designs that today have become a problem for the Western World.”

The British also initiated a military rocket aircraft project just after the Second World War. The Fairey company planned a new delta-winged research aircraft called the Delta 1, which they initially envisaged as taking off vertically from a very short and steep ramp. It was to lead to an operational interceptor that would use a similar mode of take-off. The company was granted a contract in July 1946 to develop an unmanned rocket aircraft to test the concept. Simply called the Fairey VTO (Vertical Take-Off) it had an Armstrong Siddeley Beta rocket engine which was derived from the Walter HWK 109-509 ‘hot’ engine and used the same propellants. However, the Beta had two combustion chambers and nozzles each developing a thrust of 4,000 Newton, and these could be independently swiveled, one side-to-side and the other vertically so that, together, they could steer the aircraft (by what would now be called thrust vector control) at low velocities. This was important, because when the VTO left the ramp it would not be flying fast enough for its aerodynamic control surfaces to have sufficient ‘grip’. Two 3,000 Newton solid propellant boosters were added for take-off. The starting method and propulsion were so similar to the German wartime Natter that the Fairey VTO soon gained the nickname ‘Son of Natter’.

Tests started in 1949 from a ship moored in Cardigan Bay in Wales, then in 1951 resumed at the vast Woomera Rocket Range in Australia. There were stabilization

The unmanned Fairey VTO [Fairey Aviation Company].

problems early on because the autopilot, which was derived from that of the German A4/V2 rocket, had to be carefully adjusted to accommodate the completely different aerodynamics. When this was finished the take-off launches and ensuing flights were satisfactory. About 40 of these expendable test vehicles were built and launched, but it was eventually decided that the manned Fairey Delta 1 should be a jet-powered research vehicle that would take off in a conventional manner from a runway. As in the USSR, unmanned surface-to-air missiles had already taken over the role intended for the VTO rocket interceptor.

BUSINESS CASE

In spite of the multiplicity of studies and technology developments, government and industry funding for spaceplanes and other types of reusable launch vehicles remains modest at best: space agencies and the launch vehicle companies always seem to opt for a conventional, expendable rocket as their next generation launcher; spaceplanes are perpetually the next-next generation, with the result that for the last 50 years their full development initiation has always been 20 years in the future. This isn’t due to a lack of concepts, because in addition to the spaceplane proposals described in this chapter there are literally hundreds of ideas and designs at a variety of levels of maturity and realism. David Ashford of Bristol Spaceplanes reckons that the lack of progress on orbital spaceplanes can be attributed to an entrenched mind-set of the world’s space agencies and the vested interests of launcher industries, leading them to continue to pursue improvements and cost-reductions for their expendable launch vehicles rather than to replace these with something better. But it seems to me that really the same issues that killed the high-profile spaceplane concepts of the 1980s are still the root of the problem: uncertain economic benefits, very high development costs and great technical and financial risk; a lethal mix for any project that is not driven by a strong military or political agenda such as the Manhattan atomic bomb development of the 1940s and the Apollo lunar program of the 1960s respectively.

Reusable launchers are more expensive to develop than expendable ones, since in addition to the difficulty of developing something that can go into orbit a spaceplane must also be designed to come back, which involves re-entry into the atmosphere, a descent phase and a soft landing. Furthermore, using such vehicles requires a large infrastructure involving a long runway, facilities for vehicle and engine maintenance, (cryogenic) propellant production factories and storage tanks, and logistical systems to manage the distribution of spare parts. Large aircraft like the Airbus 380 and large launchers like Ariane 5 typically cost $10 billion to develop and it is hard to imagine how a spaceplane that combines the functions of both these vehicles is going to cost less. In fact, it is easier to see how it would cost significantly more. For instance, the reusable Venture Star SSTO abandoned in 2001 due to the expensive problems with its X-33 precursor was expected to cost close to $35 billion to make operational, and its price would certainly have increased if the additional developments to overcome the X-33 problems were carried out.

As regards the recurring costs (i. e. the costs which are imposed for every flight), a reusable launcher requires inspection and maintenance prior to each mission and, as experience with the Space Shuttle showed, this will be more complicated and hence more expensive than for a conventional aircraft. Furthermore, it is currently expected that due to the high strains involved in launch and re-entry in combination with the need to keep the vehicle’s structure extremely light, spaceplanes will be able to make at most several hundred flights before they must be scrapped. Replacement rates and costs will therefore be higher than for airliners, where a single aircraft may undertake over 10,000 flights before having to be withdrawn from service. The operations costs for reusable systems therefore run the risk of turning out to be actually greater than for expendable rockets. The high development, infrastructure and maintenance costs mean that operating reusable launchers can only translate into attractive launch prices if they perform many flights per year. It is just like with commercial airlines, which keep their planes in the air for as many hours as possible in order to keep their costs down. A rapid turn-around is required to limit the size and hence the buy-cost of the vehicle fleet. Yet to be able to make many flights there must be a large number of customers who require many more payloads to be launched than is currently the case: today there are about 70 launches per year, although some carry multiple satellites; in contrast, on any normal day there are close to 30,000 airliners in the skies above the USA alone. But the launch market will only significantly increase in size (with space tourists and new satellite applications which are currently prohibitively expensive) if launch prices fall by a factor of 10 or so, which in turn requires efficient reusable systems with low maintenance overheads. It is a difficult Catch-22 situation: launches should become cheaper when the market is sufficiently large, but the market cannot dramatically increase until launch prices drop significantly. How the launch market will grow as a function of launch price reductions is debated heavily, and seems to be driven more by opinions than by hard statistical data.

In addition, current spaceplane designs are only capable of reaching low orbits so expendable rocket stages would still be required to boost satellites into higher orbits. And of course these ‘kick stages’ eat up payload volume and weight. Most current spaceplane concepts would not be able to place today’s telecommunication satellites into geosynchronous orbit, this being the most profitable part of the non­government satellite launch market. Spaceplanes therefore need a large new market in low orbit, something that space tourism could provide if the flights were sufficiently affordable and safe; failing that, they will have to rely upon the increased use of small satellites intended to work in low orbit. Whether new markets would be sufficient to justify the development of a reusable launch vehicle is the $10 billion – plus question that is very difficult to answer right now. Even in the mature and well understood airline market, aircraft companies are generally betting the farm when engaging in the development of large new aircraft like the Airbus 380 and the Boeing 787 Dreamliner. You can imagine what the risks will be in trying to enter a relatively new, poorly understood market like that of future space launches with projects having costs on such a scale. In addition, spaceplanes face competition from smart low-cost expendable launchers, especially at low flight rates. For instance, Reaction Engines estimates that at a flight rate of 70 missions per year, a single flight of their Skylon spaceplane would cost in the order of $30 to 40 million. Therefore in terms of cost per kilogram into orbit this means Skylon might be beaten by the SpaceX company’s Falcon Heavy expendable launcher whose development budget was much lower than that for Skylon.

An additional problem is the very long time to bring a complicated vehicle like a spaceplane into operational service: the Airbus 380 took about 13 years to develop, the Ariane 5 rocket about 12 years, and the F-22 fighter aircraft around 20 years. Any aircraft which incorporates as many new technologies as an airbreathing spaceplane will take at least two decades to advance from conceptual design to fully operational system, and that is not counting the additional time to develop and fly any sub-scale pathfinder test vehicles.

It is therefore not difficult to appreciate why spaceplane projects find it very hard to attract private investors; they are generally not interested in high-risk ventures that might deliver some unknowable return on investment after several decades. To limit the risks it seems to make sense to first build one or more smaller, less complex, less expensive demonstrators before committing to the development of a fully operational spaceplane. This philosophy appears to have been adopted in the US, where several hypersonic and scramjet test vehicles are currently being developed and flown. But while the investments are still substantial, such prototypes typically do not have any commercial use. Governments often finance at least the development and prototype phase; indeed this is how most expendable launch vehicle developments started, and is how the Concorde came into being. Government organizations could certainly play an important role in the develop­ment of the basic technology, such high-temperature materials and scramjets, just as in the early days of aviation the basic airfoil shapes were developed by NACA (the forerunner of NASA) and subsequently employed in almost all aircraft, even today. Apart from purely economic reasons the development of new strategic technology, the generation of high-quality jobs, the guaranteeing of national and especially military access to space, and indeed national prestige, can all serve as stimuli for governments to invest in high-risk technological projects such as spaceplanes. But new expendable launchers can also satisfy many of these desires at potentially lower costs and risks.

If it is difficult to close the business case for orbital spaceplanes, is it possible that a suborbital vehicle may make more sense? A smart concept that was investigated as part of the FESTIP study of the mid-1990s was that of a ‘Suborbital Hopper’ that involves a reusable vehicle which releases its payload into space at a speed just short of that required for orbit. A small rocket stage then gives the cargo the final kick to enter orbit. Such a launcher saves huge amounts of propellant by not having to boost its own weight into orbit, and is less constrained by the need to keep structure weight to the absolute minimum. In addition, such a vehicle may find profitable markets in rapid point-to-point transportation of people and time-sensitive cargo all across the world, space tourism, and undertaking short-duration microgravity and high- altitude experiments that need more time than can be provided by sounding rockets (point-to-point transportation may account for only a small fraction of the commercial aviation industry but because that industry is enormous it might still be far bigger than any short-term space launch market). In a mihtary role it could act as a strategic bomber, uncatchable spy plane, rapid-reaction satellite launcher and rapid intervention vehicle capable of delivering special forces anywhere in the world within 2 hours. This last application is currently being studied by the US Marines under the name SUSTAIN, for ‘Small Unit Space Transport And INsertion’. In fact, most work currently done in the field of hypersonic flight and propulsion is primarily for military purposes and is not intended to make space available to civilian travel and commerce. A suborbital spaceplane able to (almost) fly around the globe once would be something in between the short-range suborbital rocket planes described in the next chapter and a fully orbital spaceplane: a lower-cost, lower-risk project paving the way for a truly orbital spaceplane both in terms of technology and market development.

The ‘Astroliner’ suborbital rocket plane launch system proposed by Kelly Space & Technology in the US in the 1990s was a similar concept, with the addition of a Boeing 747 serving as a first stage. The jetliner would tow the rocket plane to an altitude of 6 km (20,000 feet) and Mach 0.8. The Astroliner would separate and shoot up to 110 km (360,000 feet) in order to release an expendable upper stage through a nose door and place several metric tons of payload into a low orbit. The rocket plane itself would continue its suborbital trajectory, re-enter the atmosphere and land on a conventional runway. The Astroliner would have jet engines for tow – flight assist and powered final descent and landing, and three Russian RD-120 liquid kerosene/liquid oxygen rocket engines for the zoom into space. During 1997 and 1998 the company conducted tests of the tow-launch concept at Edwards using a modified F-106 Delta Dart jet fighter towed behind a large C-141 Starlifter transport aircraft. Apart from this, the project does not seem to have progressed much although the concept is still advertised on the company’s website.

Most current launchers are not exactly environmentally friendly because they burn large amounts of kerosene and rubber-like solid propellants on every flight. However, since the worldwide launch rate is very low their impact when compared to airplanes or cars is fairly negligible. Several modem rockets use liquid oxygen and hydrogen as propellants for at least some of their rocket stages, the combustion of which results in nothing more than water vapor. But what if spaceplanes are launching into orbit on a regular basis? The good news is that owing to the need for high performance, these vehicles will very probably also use hydrogen as fuel and burn it with oxygen drawn from the air during airbreathing flight phases and then with liquid oxygen for rocket propulsion. They would not emit any carbon dioxide or toxic gases. However, even water vapor may not be completely harmless when emitted at massive rates: at high altitudes it may linger for a long time, and it is not yet clear what the environmental impact would be. The water condensation trails left in the sky by high-flying jets have, for instance, already been shown to have a measurable effect on the amount of sunlight which reaches the ground. Moreover, liquid hydrogen is difficult to produce; it currently requires around 15 kilowatt- hours of energy per kilogram, so the source of the energy for making the fuel becomes very important. But that is not a particular spaceflight problem, it is part of the overall clean-energy issue.

Compensating for pollution by spaceplanes might be an increase in environment­monitoring satellites as a result of a fall in launch prices, data from which may well increase our understanding of weather and climate and result in the proper measures being taken to protect our world. In addition, astronauts generally return from space deeply impressed with the notion of how small the Earth really is and how thin the atmosphere appears from orbit. Flying more people into space may greatly increase awareness of the fragility of our planet. Finally, the heavy usage of hydrogen fuel by spaceplanes may boost the world’s hydrogen industry. Spaceplanes could very well become the first large-scale commercial users of liquid hydrogen, reducing hydrogen prices and stimulating the development of efficient production, transportation and storage technologies. The economy and practicality of clean, hydrogen-powered cars could be improved by this. At the very least, spaceplane operators could incorporate energy-efficient systems and renewable energy sources into their ground operations; a new industry has the advantage that it can adopt sustainability and environmental awareness right from the start. Of course, the fact that spaceplanes would be reusable should save much energy and materials for the production of vehicles in comparison to expendable rockets, the valuable structures and other equipment of which are lost when they burn up in the atmosphere or crash into the sea.

In spite of all the potential benefits, developing and possibly flying a spaceplane or any type of reusable launch vehicle is still an (economic and technical) adventure rather than an everyday routine. And, as stated in the quote that opened this chapter, that is inhibiting success.

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.

Joyriding a rocket plane

“Ah, but a man’s reach should exceed his grasp, or what’s a heaven for?” –

Robert Browning

On 4 October 2004 the (unofficial) airplane altitude record of 107.8 km (353,700 feet) established by the X-15 in 1963 was finally broken. Not by a new, large-budget government rocket aircraft but by the privately developed SpaceShipOne rocket plane which pilot Brian Binnie flew to an altitude of 112.0 km (367,400 feet). The project was entirely funded by a private sponsor and the vehicle was developed and flown by a small commercial aircraft company.

This revolutionary development in rocket planes and spaceflight had its roots in the X prize, a $10 million reward announced in 1996 for the first private enterprise to develop and launch a suborbital vehicle. The competition’s rules dictated it had to be capable of carrying three people to the ‘edge of space’, and this, in accordance with International Aeronautical Federation regulations, was defined as an altitude of 100 km (62 miles). An X prize vehicle would give its passengers a thrilling ride, enabling them to view the curvature of the Earth and enjoy several minutes of weightlessness in the same way as experienced by X-15 pilots. To prove its reusability, the X prize organization required the same vehicle to make a second flight within two weeks of the first launch with at most 10% of its dry weight being replaced. In addition to the pilot, it had to be capable of carrying two passengers, but the flights required to win the prize could be made by the pilot only.

The purpose of the prize (in 2004 renamed the ‘Ansari X Prize’ following a multi­million dollar donation from entrepreneurs Anousheh Ansari and Amir Ansari) was to encourage the development of suborbital space tourism and thus kick-start a non­governmental human spaceflight industry. It was modeled after the aviation prizes of the early twentieth century which tremendously boosted aviation, such as the Orteig Prize for crossing the Atlantic that was won by Charles Lindbergh and the Schneider Trophy that encouraged the development of extremely fast seaplanes (the heritage of which was evident in several fighter planes of the Second World War, most notably the Spitfire). Twenty-six teams from around the world declared their participation in the competition, with some intending to employ relatively simple rockets (one even a modern derivative of the A4/V2 design) launched from the ground or slung beneath stratospheric balloons, others choosing rocket powered spaceplanes, and others rather exotic concepts such as pulse-jet driven flying saucers. One group even imagined a do-it-yourself suborbital rocket plane which you would be able to assemble in your own garage and launch from the nearest airfield.

The big surprise, however, was Scaled Composites, the Californian company of famous aircraft designer Burt Rutan, which initially shied away from publicity but in April 2003 revealed a project that was far ahead of its competitors. Not only did the company have a good plan but also real hardware: a fully operational twin-engined turbojet high-altitude carrier plane called the White Knight, a mobile mission control center, a mobile propulsion test facility, and a prototype of the Space – ShipOne air-launched, three-seat rocket plane. The company was by then already known for its innovative small aircraft designs, among them the Voyager aircraft that in 1986 flew around the Earth in just over 9 days without refueling or landing (73% of its weight at take-off consisted of fuel, leading to design constraints somewhat similar to those faced by spaceplane designers).

SpaceShipOne is primarily built using composite materials, a signature of Scaled Composites’ designs, as indicated by the name of the company. The fuselage is bullet shaped, similar in appearance to the X-l. Its stubby wings have a slightly swept-back

SpaceShipOne in a glide flight [Scaled Composites, LLC].

SpaceShipOne carried under the White Knight aircraft [Scaled Composites, LLC].

leading edge, a straight trailing edge, and a vertical fin bearing a single horizontal stabilizer at each wingtip. The total length is 8.5 meters (28 feet), the wingspan is 8.2 meters (27 feet), and the total take-off weight is 2,900 kg (6,380 pounds). Its flight profile resembles that of the X-15 by involving an air-drop, boosted ascent, ballistic trajectory into space, re-entry and glide back to the ground. But it is intrinsically a much simpler aircraft, designed not for cutting-edge research flights but purely as a precursor for commercial tourism flights, and it benefits from an additional 40 years’ of developments in aerodynamics, materials and avionics (as well as the considerable experience of the X-15 program). Although its maximum speed is Mach 3 rather than the X-15’s Mach 6.7 and the mission does not call for extreme speed, it does call for extreme altitude. And whereas the X-15 could not survive a steep descent into the atmosphere and so had to fly a 40 degree ascent and descent trajectory over a horizontal distance of some 500 km (300 miles), SpaceShipOne flies up and down almost vertically so that its entire flight occurs within 40 km (25 miles) of its base. This greatly simplifies its operations by not requiring a large network of ground stations, chase planes and emergency landing sites.

SpaceShipOne is propelled by a single, revolutionary rocket motor which is a mix of a solid rocket booster and a liquid propellant motor. This SpaceDev SD010 hybrid motor uses a solid rubber-like HTPB (hydroxyl-terminated polybutadiene) grain as fuel, but in combination with liquid nitrous oxide (also known as laughing gas). The main benefit over a solid propellant booster is that this hybrid engine can be throttled and shut down at any moment by varying the amount of liquid oxidizer that enters the combustion chamber. Without the liquid oxidizer, it is totally safe from explosion during transport and handling. Furthermore these propellants have a higher specific impulse. The hybrid is also simpler than a liquid propellant rocket engine by having only one valve and redundant igniters. In contrast to the complex

SpaceShipOne flight profile [Scaled Composites, LLC].

SpaceShipOne shoots up under rocket power [Scaled Composites, LLC].

XLR99 engine of the X-15 the SD010 uses only non-toxic, easy-to-handle propellants and it has never failed to start. It has a maximum thrust of 75,000 Newton, a specific impulse of 250 seconds, and a maximum total burn duration of 87 seconds.

Prior to re-entering the atmosphere the plane’s two tail booms and the rear half of the wings fold upward on a hinge that runs the length of the wing. This ‘feathered’ position gives the aircraft a high-drag that allows a safe, stable “carefree, hands-off’ penetration of the atmosphere which greatly reduces aerodynamic and aerothermal loads. For this innovative solution Rutan was inspired by a badminton shuttlecock, which always orients itself correctly with the direction of flight. The cockpit has a spacecraft-like environmental control system and features many windows to provide a good view for the pilot and passengers (although no passengers were carried). The aircraft has three flight control systems: a direct manual control for subsonic speeds, an electric control system for supersonic speeds (where muscle power alone is unable to handle the aerodynamic forces), and a reaction control system for high altitudes. The thrusters emit non-toxic cold gas (there is no combustion involved). State-of- the-art instrumentation provides the pilot with the precise guidance information he needs to manually fly SpaceShipOne during the critical boost and re-entry phases. Flight test data is sent to a mission control center during each flight, where it is recorded for careful post-flight analyses.

The only SpaceShipOne aircraft was registered as N328KF, with N the prefix for US-registered aircraft and 328KF chosen by Scaled Composites to stand for 328 К (for kilo, meaning thousand) feet, corresponding to the 100 km altitude goal (registry number N100KM was already taken).

The White Knight plane, SpaceShipOne’s carrier, is itself an innovative aircraft. It too is made mostly out of composite materials. It has two afterburning turbojets, thin wings that have a total span of 25 meters (82 feet) and two tail booms. Most of the cockpit, instrumentation and other internal equipment are identical to those installed on SpaceShipOne, enabling it to flight-qualify much of the equipment intended for SpaceShipOne, thereby sharing the development costs for the two aircraft. The White Knight could be used as a trainer aircraft for SpaceShipOne pilots. The high thrust from its turbojets with afterburners in combination with the low weight, as well as it enormous speed brakes for rapid deceleration meant that rocket plane pilot trainees could use the White Knight to rehearse SpaceShipOne’s boost flight, approach and landing very realistically.

On 21 June 2004 the White Knight took the diminutive SpaceShipOne with 62- year-old pilot Mike Melvill to an altitude of 14 km (46,000 feet). The spaceplane was dropped into a gliding flight, then fired its rocket motor for 76 seconds. Shortly after ignition of the rocket motor, wind shear suddenly made the aircraft roll 90 degrees to the left. Melvill attempted to correct it and unexpectedly rolled 90 degrees to the right. He then managed to level the plane again and proceed with the steep but still somewhat unstable powered boost to a maximum speed of Mach 2.9. During the rocket bum Melville reported a loud bang that was later reahzed to have been caused by the overheating and subsequent crumpling of a new aerodynamic fairing that had been fitted around the rocket nozzle. Fortunately the fairing’s collapse did not affect the flight. After burn-out of the engine the plane continued unpowered to an altitude in excess of 100 km (62 miles). This coasting phase and the following free-fall back to Earth lasted about 3.5 minutes, during which time Melvill opened a bag of M&Ms and watched them float weightlessly around the cockpit. At the highest point of the trajectory the vehicle’s speed was almost zero. Then it began to fall, accelerating to a maximum speed of Mach 2.9 (the same as its maximum speed going up, as potential energy converted back to kinetic energy). During the fall, the two tail booms and rear parts of the wings were put in a vertical position to achieve the high-drag configuration that facilitated a safe, stable penetration of the atmosphere. The thickening air then decelerated the vehicle, and subjected Melvill to a tolerable 5 G deceleration. The re-entry air temperatures remained less than 600 degrees Celsius (1,100 degrees Fahrenheit) owing to the large area of the underside of the aircraft and the relatively modest velocity. There was no need for heat shields or tiles because the hot re-entry phase was brief and the air at high altitude too tenuous to transfer a lot of heat; the skin of the aircraft remained much cooler than the surrounding air (the X-2 flew its ‘heat barrier’ research flights at similar speeds but at much lower altitudes, while the X-15 and orbital vehicles returning from space endure much higher temperatures as a result of their faster entry speeds). In fact, SpaceShipOne’s structure hardly contains any metal parts. At 17 km (57,000 feet) the wings and tail were repositioned and the aircraft reverted to a conventional glider for its descent to the runway in the Mojave Desert in California.

“It was a mind blowing experience, it really was; absolutely an awesome thing,” Melvill said after landing. With this flight he became the first private civilian to fly an aircraft into space, as well as the first person to leave the atmosphere in a non­government sponsored vehicle. (All rocket aircraft except the early, pre-war rocket – boosted gliders were developed under government contracts for military or research purposes.) Measured by the number of world newspapers that carried the story above the fold, the flight was the second largest news event of the year, being topped only by the capture of Saddam Hussein in Iraq.

Work on Scaled Composites’ suborbital spaceplane concept began right after the X Prize announcement in 1996 and the full development program was initiated in April 2001, hidden from the public and the competitors by the inhospitable Mojave Desert. To finance the project the company got a $30 million grant from Paul Allen, Microsoft cofounder and third-wealthiest person in America. Since the X Prize was $10 milhon, Allen could not expect to get a return on his investment any time soon but he was in it for the sense of adventure rather than for the money. The overall plan was to mature the concept, then sell improved vehicles to a space tourism company. “Spaceflight is not only for governments to do,” Allen said. “Clearly, there’s an enormous pent-up hunger to fly into space and not just dream about it.”

SpaceShipOne made its first captive flight on 20 May 2003 and shortly thereafter Rutan announced the project to the public. After a second captive flight there were seven successful glide drop tests before pilot Brian Binnie made the first powered flight on 17 December of the same year (deliberately marking the 100th anniversary of the first ever powered aircraft flight by the Wright brothers). A short burn of the rocket motor pushed the aircraft to Mach 1.2 and an altitude of 21 km (68,000 feet). The left main gear collapsed due to a roll oscillation upon landing but the damage was minor and Binnie was uninjured. After another glide test flight there was a series of progressively faster and higher flights, culminating in the one in June 2004 that put Mike Melvill into space. During the test program SpaceShipOne also became the first privately funded aircraft to exceed Mach 2. All of the flights took place from the Mojave Airport Civilian Flight Test Center, the runway close to Scaled Composites’ premises. The four pilots that flew SpaceShipOne came from a variety of aerospace backgrounds: Mike Melvill was a test pilot, Brian Binnie a former Navy pilot, and both Doug Shane and Peter Siebold were company engineers. They all trained to fly SpaceShipOne using a flight simulator (like the X-15 pilots) as well as by flying the White Knight and other aircraft produced by Scaled Composites.

After Melvill’s space flight, everything was deemed ready to try for the X Prize by making two such flights within a fortnight. On 29 September 2004 Melvill shot up to an altitude of 103 km (338,000 feet), which was slightly less than planned due to a serious roll instability during the rocket-boost phase, but was still above the 100 km requirement. It was quickly followed on 4 October (specifically chosen to mark the 47th anniversary of the launch of Sputnik) by Brian Binnie’s fully successful flight to the record altitude of 112.014 km (367,500 feet) that won the X Prize for Scaled Composites and also made SpaceShipOne the first privately funded aircraft to exceed Mach 3: when the motor cut off at over 61 km altitude (200,000 feet) the maximum speed was Mach 3.09, an equivalent velocity of 3,490 km per hour (2,170 miles per hour). Melvill and Binnie, the two pilots who flew above the 100 km (330,000 feet) mark were issued the first commercial ‘astronaut wings’ by the US Federal Aviation Administration.

No further flights were made, as the prize had been won and the concept and the technology proven. For commercial space tourism flights, Rutan wanted to develop a larger rocket plane that could seat more passengers and incorporate more

SpaceShipOne in the National Air and Space Museum [Photo by Eric Long, National Air and Space Museum, NASM WEB 10516-2005, Smithsonian Institution].

redundant systems and aerodynamic stability for increased safety. In addition, he did not wish to risk damaging the unique and now historic SpaceShipOne. Since 2005 the small rocket plane has hung on display in the main atrium of the National Air and Space Museum in Washington D. C., between the Wright Flyer, the Spirit of St. Louis and the Bell X-l, and near the first X-15. As a tribute to SpaceShipOne’s achievement, in 2006 a small piece of its carbon fiber material was cut off and launched on the New Horizons probe heading for Pluto. An attached inscription reads: “To commemorate its historic role in the advancement of spaceflight, this piece of SpaceShipOne is being flown on another historic spacecraft: New Horizons. New Horizons is Earth’s first mission to Pluto, the farthest known planet in our solar system. SpaceShipOne was Earth’s first privately funded manned spacecraft. SpaceShipOne flew from the United States of America in 2004.”

A fiberglass replica of SpaceShipOne created using the same molds used to make the original can be found in the AirVenture Museum in Oshkosh. Another full-scale replica is on display in the William Thomas Terminal at Meadows Field Airport in Bakersfield, while a third is in the Mojave Spaceport’s Legacy Park, and a fourth is hanging above the stairs in the main entrance of Building 43 of Google’s Googleplex campus (Google cofounder Larry Page was a trustee on the X Prize board) and a card taped to the nozzle implores, “Attention Googlers: Please do NOT launch. Thanks.” SpaceShipOne also became a popular model rocket, with Estes Industries currently offering several SpaceShipOne models that you can launch from your own back yard repeatedly by replacing the little solid propellant rocket motor.

SpaceShipTwo and White Knight Two [Scaled Composites, LLC],

Rutan’s company has now teamed up with the Virgin Group, famous for its airline and its entertainment and communications companies, as well as its charismatic and adventurous head, Sir Richard Branson. Under the name ‘The Spaceship Company’, the Virgin Group and Scaled Composites have set up a joint venture to develop the SpaceShipTwo and White Knight Two aircraft which will be operated by a company called Virgin Galactic. At the time of writing, the ‘spaceline’ plans to operate a fleet of five SpaceShipTwo vehicles starting no earlier than 2012. They have been taking bookings at $200,000 per passenger for the early flights, and by late 2011 had over 450 paid customers. It is expected that ticket prices will drop significantly as flight operations mature, increasing the size of the space tourist market.

SpaceShipTwo, based on the same principle, concept and shape as SpaceShipOne is roughly twice the size in order to house two pilots and six passengers. It will be propelled by a larger hybrid rocket motor named ‘RocketMotorTwo’ delivering over

230,0 Newton of thrust. Development of the new rocket plane was delayed when in 2007 an explosion occurred during an oxidizer flow test that was being conducted at the Mojave Air & Space Port. Three staff were killed and another three severely injured; rocket engines are still potentially dangerous devices that have to be handled with great care. White Knight Two is an innovative twin-hull aircraft that carries the SpaceShipTwo rocket plane between its fuselages. It is also designed to operate as a zero-G parabolic-flight aircraft for SpaceShipTwo passenger training or micro­gravity science flights, and as a high-altitude research plane. It could potentially launch other rockets than SpaceShipTwo, such as small sounding rockets with instruments for scientific research.

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SpaceShipTwo technical diagram [Virgin Galactic].

Unlike previous rocket plan projects, environmental impact is now an important issue in aviation. With respect to carbon dioxide (C02) emissions the hybrid engine is not exactly ‘green’ but according to Virgin Galactic, “C02 emissions per passenger on a spaceflight will be equivalent to approximately 60% of a per-passenger return commercial London/New York flight.” This is about 500 kg of carbon dioxide per passenger per flight. So even if SpaceShipTwo flights eventually number 1,000 per year the resulting carbon dioxide emissions would be in the order of one-thousandth of what a major airline typically expels into the atmosphere during a year. Virgin Galactic nevertheless accepts that the environmental impact of their operations could have serious implications for the image and success of their business, and the larger Virgin empire is committed to being as environmentally friendly as is practical. The company therefore plans to run its spaceport(s) with as much renewable energy as possible, which may even make them a net energy producer and potentially “carbon negative” by preventing more emissions of carbon dioxide than its vehicles produce. White Knight Two’s jet engines will initially burn kerosene but are also capable of running on butanol, a biofuel that can be made from algae.

The first SpaceShipTwo, christened VSS (Virgin Space Ship) ‘Enterprise’ (after the legendary Star Trek starship) made its first glide flight on 10 October 2010, being launched by the first White Knight Two aircraft VMS (Virgin Mother Ship) ‘Eve’, and it performed its first ‘feathered’ flight on 4 May 2011. To date, a total of 16 glide flights have been made, and round 100 test flights are expected before the first
passengers will be carried. The first commercial flight is expected no earlier than 2012. The company will initially operate from Spaceport America, a brand new $210 million airport for suborbital vehicles located in New Mexico. There are also plans for a sister spaceport in northern Sweden. Singapore and the United Arab Emirates have both also shown interest in establishing suborbital flight facilities.

A SpaceShipTwo flight will be an incredible adventure offering the possibility, albeit brief, to experience what astronauts (and X-15 pilots) feel and see, without the heavy workload. You will be dropped from the carrier aircraft at an altitude of 15 km (50,000 feet) and then go supersonic within 8 seconds. After 70 seconds of powered flight, during which you attain a maximum speed of just over Mach 3 (equivalent to about 3,500 km per hour, or 2,100 miles per hour) the rocket plane will coast to a peak altitude of 110 km (360,000 feet). The virtually drag-free parabolic trajectory will last for 3.5 minutes, during which you will be able to float about in the relatively spacious cabin and admire the view of Earth below and the curvature of the horizon through the large windows.

Other companies are also working on suborbital rocket planes for space tourism, with microgravity science and high-altitude experiments (as on the X-15) forming a secondary market. XCOR Aerospace, which is based on the same Mojave airfield as Scaled Composites, is developing its ‘Lynx’ rocket plane (superseding its earlier and similar ‘Xerus’ design). Unlike SpaceShipTwo this double-delta-winged vehicle will take off from a runway on its own power and hence will not require a carrier aircraft. This simplifies the development and operations (one rather than two planes) but it means the rocket aircraft has to carry all the propellant for the entire flight itself. The Lynx Mark-I prototype aircraft is considerably smaller than SpaceShipTwo and will only be able to reach an altitude of 60 km (200,000 feet) carrying a pilot and a single paying passenger. A more advanced Mark-II production version is to be able to reach the milestone of 100 km (330,000 feet). The passenger will have to remain strapped in his seat, as the cockpit is too small for weightless acrobatics. On the other hand, the initial ticket price announced by the company is about half that of a SpaceShipTwo flight. XCOR appears to be well advanced in the general development of liquid propellant rocket engines, and has reported that its 13,000 Newton XR-5K18 liquid oxygen and kerosene rocket engine (four of which will be needed to power the Lynx) is almost ready for flight. But propulsion is only one part of a rocket plane, and although the company has done extensive wind tunnel testing using a scale model of the Lynx, its announcement that it expects to start the test flight campaign of its Mark-I prototype in 2012 appears rather optimistic.

XCOR modified an existing canard configuration (i. e. tailless) ‘Long EZ’ sports aircraft to demonstrate its rocket engine capabilities by installing two 1,800 Newton restartable, pressure-fed, regeneratively cooled rocket engines which burn isopropyl alcohol and liquid oxygen. This ‘EZ-Rocket’, which is a modest-performance rocket plane in its own right, has made a total of 26 flights including a number of air show demonstrations. In December 2005 the EZ-Rocket set the world record for ‘Distance without Landing’ for a ground-launched rocket powered aircraft with a flight from Mojave to California City, a distance of 16 km (9.94 miles). “That was the shortest long-distance record flight ever!” pilot Dick Rutan exclaimed. XCOR also built and

Artistic impression of the Lynx rocket plane [XCOR Aerospace].

flew the ‘X-Racer’, a sleek rocket aircraft based on the airframe of the ‘Velocity SE’ canard sports plane. This was a prototype for aircraft to compete in rocket plane races organized by the Rocket Racing League, an organization that seeks to promote rocket aircraft development by flying competitions. The X-racer is equipped with an XR-4K14 restartable, pump-fed rocket engine that burns liquid oxygen and kerosene with a thrust of 6,600 Newton. It made its first flight on 25 October 2007. The test program has now been completed after a total of 40 flights and demonstrations. The X-Racer holds claim to several (unofficial) records including the most flights made in a single day by a manned rocket powered aircraft, and the fastest turn-around for a manned rocket powered vehicle.

Armadillo Aerospace, the small aerospace company of computer game developer John Carmack, who made his fortune by developing popular games such as Doom and Quake, has also made a rocket engine for the Rocket Racing League. It equipped the Rocket Racing League’s current Mark-II and Mark-Ill Rocket Racers, which are also based on the Velocity airframe (in this case the Velocity XL FG version) with a home-grown rocket engine that is fed with liquid oxygen and ethanol and develops a maximum thrust of 11,000 Newton. Seven successful test flights were made by the Mark-II aircraft during August 2008 and both machines are currently used for flight demonstrations. The Rocket Racing League hopes to generate sufficient interest for a number of teams to build or purchase similar rocket aircraft in order to participate in rocket propelled air races. In the meantime, you can download a video game that puts you in the cockpit of a Rocket Racer.

EZ-Rocket PCCOR Aerospace].

In March 2002 the Space Adventures company that organizes ‘flight participant’ missions to the International Space Station, unveiled a mockup of the ‘Cosmopolis ХХГ (C-21) lifting body-type suborbital rocket plane at Zhukovskiy Air Base near Moscow. This was to be developed by the Russian Myasishchev Design Bureau, be launched from the design bureau’s existing M-55X ‘Geofizika’ high altitude aircraft, and be able to carry a pilot and two passengers into space at $98,000 per ticket with the first flight in 2004. The carrier aircraft with the C-21 attached would first slowly climb to an altitude of 17 km (56,000 feet) and then gather speed in order to make a vertical climb to 20 km (66,000 feet) to release the C-21. The C-21 would then ignite its expendable sohd propellant rocket motor. When this motor burned out it would separate and fall away, leaving the C-21 to follow a ballistic arc to a peak altitude of 100 km (330,000 feet). The rocket plane would glide back to the airport and make a parachute-assisted touchdown. But Space Adventures has abandoned its plans to use the C-21 and instead contracted Armadillo Aerospace to develop a vertical launched, vertically landing suborbital rocket capsule to implement its planned suborbital flight services.

The giant European space company EADS Astrium announced in 2007 that it was to develop a suborbital rocket plane for space tourism. This single-stage, straight-winged plane would take a pilot and four passengers to the edge of space and offer a great view through many large windows and a roomy cabin for weightless antics. It would take off from a normal airport and climb to an altitude of 12 km (39,000 feet) with jet engines, then ignite a Romeo liquid oxygen-methane rocket

The Mark-Ill Rocket Racer in flight [Rocket Racing League],

engine to reach 60 km (200,000 feet) in just 80 seconds with enough velocity to continue unpowered to its 100 km (330,000 feet) apogee. As the plane fell back the pilot would use small thrusters to control its attitude for re-entry into the atmosphere prior to restarting the jet engines to return to the airport. Jet engines use 10 to 20 times less propellant than rocket motors of the same thrust over the same time and are much more efficient for the first and final phases of a flight (SpaceShipTwo’s carrier aircraft uses jet engines for the same reason) but when they are not providing thrust at high altitudes they are dead weight. SpaceShipTwo effectively leaves them behind once it separates from its carrier. Jet engines are also handy in case of a failure of the rocket engine, as well as for ferry flights between airfields. Astrium expected to require around 1 billion euro to develop their system (much more than SpaceShipTwo is estimated to cost), flights to begin in 2012, and tickets to cost up to

200,0 euro. “The development of a new vehicle able to operate in altitudes between aircraft (20 km) and below satellites (200 km) could well be a precursor for rapid transport point-to-point vehicles, or quick access to space,” the company said.

Artistic impression of the take-off of the EADS Astrium suborbital rocket plane [EADS Astrium & Marc Newson Ltd],

Famous designer Marc Newson was to take care of the aesthetics of the design, and the images in the brochure published by Astrium sure are beautiful. As Astrium builds the Ariane 5 launcher and its mother company EADS develops and produces the famous Airbus airliners as well as the Eurofighter military jet, the company seems ideally suited to pursuing a suborbital rocket plane project: it has all the necessary knowledge, experts and facilities in-house, and could incorporate a lot of existing EADS aircraft and spacecraft equipment such as cockpit instrumentation, undercarriage and control thrusters.

After their 2007 announcement, however, Astrium remained awfully quiet about their rocket plane, making it appear to have been merely a publicity stunt rather than a real project. But early in 2011 the company announced that it was indeed working on the concept and that after having placed work on hold for several years due to the global economic downturn it was planning to spend a further 10 million euro on it in 2011; a considerable sum but not much in comparison with the 1 billion euro that it had predicted for full development. “We continue to mature the concept, maintaining the minimum team in order that when we find the relevant partnership we are ready and have progressed sufficiently,” Astrium CEO Franfois Auque told reporters in January 2011. Once it has secured the required financial and industrial partners, the company expects to be able to put the rocket plane into service within five years.

In 2004 another big European aeronautics company, Dassault Aviation in France, announced its own suborbital rocket plane design called VSH. This was based on an earlier design for an automated air-launched reusable hypersonic vehicle known as YEHRA (‘Vehicule Hypersonique Reutilisable Aeroporte’) but was intended to be manned and therefore VSH stood for ‘YEHRA Suborbital Habite’. The delta-winged rocket plane would be carried into the air by a commercial aircraft, be released at an altitude of 7.6 km (25,000 feet) and a speed of Mach 0.7, and ignite a liquid oxygen-kerosene rocket engine to climb to the milestone altitude of 100 km (330,000 feet). Design work is progressing in the context of the K-1000 project that Dassault is self-financing with several industrial partners in Switzerland.

Bristol Spaceplanes, mentioned earlier for its Spacecab and Spacebus projects, is working on a rocket plane called ‘Ascender’. This is a delta-winged aircraft with two jet engines and a single rocket motor similar in concept to that of EADS Astrium but only able to seat a pilot and a single passenger. Ascender’s rocket engine, a prototype of which is to fly on a sounding rocket, will use hydrogen peroxide and kerosene as propellants. As such it resembles the Spectre rocket engines developed in the 1950s to power the SR.53 and SR.177. Ascender is also to pave the way for the company’s orbital spaceplane concepts (discussed above). However progress is slow because the company is waiting for a serious investor so that it can afford to appoint a full-time team of engineers.

Virgin Galactic would seem to be the most advanced company in terms of making and flying suborbital rocket planes, but if the space tourism market really takes off there ought to be room for several aircraft manufacturers and operators to compete. This would hopefully lower ticket prices further, resulting in ever more people being able to afford a flight to the edge of space.

The next step foreseen by Burt Rutan is an orbital rocket plane for space tourism, but that poses a tremendous challenge because although the 100 km (330,000 feet) altitude reached by SpaceShipTwo will be sufficiently above the atmosphere to circle the Earth a couple of times, the speed of the vehicle falls far short of that required to

Artistic impression of the Ascender rocket plane [Bristol Spaceplanes].

enter orbit. To achieve orbit at that height, a vehicle must have a horizontal speed of

7.8 km per second (4.8 miles per second); i. e. 28,000 km per hour (17,500 miles per hour). SpaceShipTwo reaches Mach 3 at engine burn-out in a steep climb but at the top of its parabolic arc its speed is virtually zero (as all its energy has been converted into altitude). Compared to SpaceShipTwo’s maximum speed of 0.9 km per second (0.6 miles per second) an orbital rocket plane needs to go over 8 times faster; and as kinetic energy increases with the square of the speed that means a propulsion system capable of delivering almost 70 times as much energy! This is why satelhte launchers and orbital spaceplane concepts are so much larger than suborbital rocket planes such as SpaceShipTwo and Lynx; even though they all reach space, in terms of energy and thus propellant volume the difference is huge. Weight constraints are also much more demanding for an orbital spaceplane. Whereas a suborbital rocket plane’s dry weight can be approximately 40% of the vehicle’s overall weight including propellant, the energy needed to go into orbit demands that a plane’s empty weight be no more than 10% of its take-off weight (for both types of vehicle these percentages diminish if multiple stages and/or airbreathing propulsion are employed but the large difference remains). This also has consequences for safety: where normal aircraft structures are usually designed to be able to withstand 1.5 times the highest load expected to occur during the plane’s lifetime (and even 2 times for the undercarriage) this margin will be extremely difficult to meet for reusable orbital spaceplanes. Even for expendable launchers, which are less constrained regarding empty weight, this factor is typically only 1.2, except for crewed launchers where it is 1.4 according to NASA standards.

In short, the difficulty in achieving orbit is not so much to get up to high altitude, it is rather to attain the necessary high velocity with a structure weight that provides a reasonable amount of rehability and safety. Factoring in the much more extreme re-entry temperatures that will require heat shields, and that ‘feathering’ cannot be used for hypersonic re-entry, clearly indicates that an orbital SpaceShipThree will not be merely an upgrade of SpaceShipTwo but a completely new, much larger, and more complicated spaceplane that will be vastly more expensive to develop and operate.

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.

Man versus robot

“Man is the best computer we can put aboard a spacecraft… and the only one

that can be mass produced with unskilled labor.” – Wernher von Braun

Will a future hypersonic plane have pilots on board or be fully automatic? Nowadays aerodynamics and rocket propulsion are fairly well understood and can be accurately modeled and simulated. As a result, the areas where direct pilot intervention may be needed due to unexpected behavior of an aircraft/spaceplane are rapidly decreasing. This is especially true for vehicles that have a fixed, pre­determined trajectory such as missiles and launch vehicles. So-called Unmanned Aerial Vehicles have become important operational military assets, and these aircraft are steered from the ground or fly their missions completely autonomously as aerial robots. It is therefore likely that future space planes will be flown by a computer under human supervision from the ground rather than directly by a human pilot, particularly as hypersonic vehicles tend to be aerodynamically unstable and therefore require sophisticated avionics for efficient and safe control. For instance, Skylon is to fly automatically; any astronauts to be transported into orbit will be housed inside its payload bay.

Especially on a satellite launch vehicle with relatively little margin for errors and malfunctions, operating without pilots results in a simpler and thus cheaper design; a crew requires a comfortable cabin with regulated pressure and temperature, requires to have an escape capability if the spaceplane is less reliable than a regular aircraft (which rocket vehicles invariably are) and requires higher safety margins to be built into the design. Not having any people on board potentially makes the vehicle less expensive and saves weight and space that can be used for more payload. Moreover, catastrophic failures have less grave consequences; compare the dramatic aftermaths of the losses of the Space Shuttles Challenger and Columbia with those of the many but almost forgotten failures of unmanned expendable launchers.

However, on many occasions having an exceptionally skilled pilot on board saved the X-15 and earlier rocket aircraft. So any fully automatic flight control system on a versatile hypersonic aircraft intended for various types of missions must be smart and capable of reacting very rapidly to unexpected situations and emergencies. That may be difficult, as programming a computer for unforeseen events is near to impossible whilst the human brain excels at improvisation (although developments in so-called neural networks may result in self-learning computers that can quickly react to new situations). And what if a spaceplane carries astronauts onboard? Even if they are not flying the vehicle themselves, the aircraft will still need to incorporate the additional equipment and various reliability enhancing redundancies that an unmanned vehicle can do without. Would it be acceptable for them to ride into space in a fully robotic hypersonic launcher? Or would a pilot with a manual override capability be required, as for astronauts launched on current expendable rockets like the Soyuz and even the Space Shuttle, if only for psychological reasons? The impact on the design would be limited if one of the transported crew could fly the vehicle in an emergency, in order that no additional seat need be assigned to a pilot.

Talking of people on board spaceplanes and rocket planes in general, what about vehicle safety? The early rocket propelled aircraft like the Me 163 were extremely hazardous. Four pilots died and two were severely injured during the X-l, X-2 and X-15 programs and there were also many less serious accidents. Of the 16 individual airframes involved, 10 were completely or largely destroyed in accidents: not a very good safety record given that the X-planes only made a total of some 415 flights, a total that can be readily accumulated by a single airliner in 6 months of operations. Does this mean that rocket planes are inherently dangerous and hence ought never to be used for suborbital space tourism and/or mass transportation into orbit? Surely we have learned much about high-speed, high-altitude flight since those days, and rocket propulsion has also greatly matured. Suborbital flight in particular, benefits not only from the experience gained from the experimental rocket planes but also from high-performance jet aircraft in general.

Furthermore, whilst the high losses among pilots flying rocket planes may appear high today, they were not particularly exceptional compared to the accident rate in experimental aviation and the general testing of prototype aircraft. In the late 1940s and the 1950s test pilot loss rates in the US were in the order of one per week. And crashes of military jets in operational service occurred frequently. Nowadays crashes and aircraft explosions are very rare, even for new types, so there is no real reason to expect suborbital rocket aircraft like SpaceShipTwo to suffer from anything like the loss rates of early jets. However, a suborbital launch is certainly more hazardous than a regular airline flight, and orbital spaceflight even more so. In part this is due to the extreme speeds, altitudes and temperatures involved, in combination with the need to keep the vehicle as light as possible, and in part due to the still experimental nature of human spaceflight. At the time of writing, the number of crewed space missions is less than 290, well below the number of planes in the air on a typical day. There have been even fewer suborbital rocket plane flights into FAI-certified space. Indeed, only two X – 15 flights and the recent three missions of SpaceShipOne ascended above the milestone altitude of 100 km (62 miles), and another eleven X-15 flights exceeded 80 km (50 miles). In today’s world of health and safety regulations, the relatively low trustworthiness of rocket vehicles is certainly a business risk. People have come to expect that even radically new aircraft will not kill anyone, and that suborbital space tourists riding rocket planes should not feel that they are putting their lives on the line. On the other hand, perhaps it is the risk that provides the sense of adventure.

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.

Conclusions

“Nothing ever built arose to touch the skies unless some man dreamed that it should, some man believed that it could, and some man willed that it must.” – Charles Kettering

The ‘golden age’ of the rocket plane, whether it is defined in terms of the number of aircraft, speed of progress or number of flights, kicked off with the He-176 in 1939, essentially at the same time as the jet age, and arguably ended with the final flight of the X-15 in 1968. Successful rocket aircraft projects of that period were based upon three vital ingredients: a good aircraft design, a good rocket engine, and a great pilot. If any part of this fundamental triangle was lacking, the outcome was often disaster: aircraft pitched over due to Mach tuck, engines blew up, and pilots overshot landing fields and crashed their expensive aircraft. The extreme speeds that rocket aircraft achieved and the new aerodynamic territories they ventured into meant things could go wrong very fast and very unexpectedly. Pilots who let their powerful, sleek planes get ahead of them often did not make it back. And extensive flight experience did not mean that pilots were safe from making mistakes. For each new, experimental rocket aircraft every pilot was essentially inexperienced. The same applied to the designers, but at least they rarely lost their fives due to a fault in their aircraft or engine.

However, even while they were in the limelight, airplanes with rocket propulsion were rapidly rendered obsolete by improved turbojet engines. The Me 163B was the only pure rocket fighter that ever entered military service, while the only operational mixed-power fighters were the Mirage IIIC, – E, and – S, and for most of the time even these flew without their optional rocket packs. Altogether the rocket propelled fighter plane does not have a very impressive track record when taking into account all the development effort on experimental aircraft and prototypes.

Rocket planes were soon realized only to be really useful as research aircraft to fly at extremely high speeds and altitudes. The X-15 set incredible records for aircraft speed and altitude but the data it collected at the extremes of its flight envelope was so far beyond what was required for operationally useful manned aircraft that there was no need to make a successor to push the boundaries even further. Orbital rocket planes, the logical next step, proved to be too complex, too costly, and ultimately not really needed. Rocket aircraft development therefore stopped at the end of its

infancy and at the peak of its success, and so never matured into really operationally useful series-produced planes. Instead, new military planes relied on advanced jet engines and spacecraft kept using vertical take-off launchers that were usually expendable or at best included a reusable shuttle that was able to glide back from orbit.

By the mid-1980s it seemed that a second golden age was about to begin, with a number of ambitious spaceplanes and hypersonic airliners such as the Sanger-II and HOTOL following up on the experimental rocket planes of the 1950s and 1960s. But these new vehicles would not be pure rocket planes, as they were to rely on airbreathing propulsion for the first part of their flight. Indeed, NASP was initially expected to do without rocket motors. In that respect, they were more similar to the various mixed-power interceptors of the 1960s.

But the revival proved to be a false start. While routine hypersonic flight into orbit appeared to many people to be imminent, the unforgiving numbers in the engineers’ weight budgets and the managers’ cost estimates said otherwise. In part the optimism appears to have been inspired by the ease of imagining a spaceplane flying into orbit as a natural extension of high-speed and high-altitude aviation: an X-15, just flying a bit faster. Looked at Uke this, the intrinsic difficulties seemed smaller than they really are. Spaceplanes are inherently large because of the enormous volumes of propellant required. It is possible to make a small, relatively low cost aircraft, but not a small, cheap orbital spaceplane (indeed, people build simple aircraft in their garage but it is very unlikely that one day your neighbor will roll a hypersonic satellite launcher out of his shed).

Dr. Richard HalUon, a former Chief Historian of the US Air Force recently said of the apparent lack of progress in hypersonic flight (and thus the spaceplanes discussed in this book): “The hope of hypersonics thus became inextricably caught up in what might be termed a hypersonic hype. This led, over time, to a cycle of fits and starts that has largely worked to discredit the potential of the field and taint it with an image of waste and futility. Typically, a program has begun with great fanfare and promise, increased in complexity, and when realistic performance, schedule, and cost estimates are derived, its appeal quickly fades.”

In addition to the canceled X-20 Air Force project, NASA has a long history of abandoned hypersonic projects, including the X-30, X-33, X-34 and X-38. The space agency seems essentially to have given up on spaceplanes, shuttle-type space gliders, and indeed reusable launchers in general for the near future. The Russians had their single Buran flight but never progressed beyond paper studies for real spaceplane concepts. At present, neither NASA nor the Russian Space Agency, nor indeed any other space agency, is willing to risk burning its hands on another shuttle, let alone a spaceplane project.

A modest resumption of interest in the rocket plane was kicked off by the success of SpaceShipOne. Hopefully other suborbital rocket propelled aircraft will soon fly. However, it seems that brief suborbital flights represent the last niche in aeronautics for the pure rocket powered plane to play a useful role: any future hypersonic aircraft or orbital spaceplane will primarily rely on advanced forms of jet propulsion, perhaps in combination with rocket power if really necessary. In fact, even the early

X-planes like the X-l, X-2 and X-15, as well as the D-558-2 Skyrocket, were launched from large turbojet aircraft that can be regarded as airbreathing first stages.

The development of hypersonic launch vehicles will be expensive but by using a one-step-at-a-time approach, also known as ‘crawl-walk-run’, it may be technically feasible as well as affordable with or without government funding. The logic is clear: start with a suborbital aircraft such as SpaceShipTwo, advance to a suborbital hopper that can launch payloads into low orbit at the apogee of its ballistic flight into space, and finally make an orbital spaceplane. Each of these steps could be a commercially viable project in its own right, earning the money needed to fund the next step on the road to a fully reusable launch vehicle. In this regard, NASA’s Commercial Orbital Transportation Services program (which encourages private companies to introduce crew and cargo transportation vehicles to service the International Space Station) is of interest since one of the participants, SpaceDev, is developing the aforementioned Dream Chaser mini-shuttle.

At the same time, the US military’s desire for a long-range hypersonic missile (or even an attack aircraft) able to reach any place on Earth in no-time and fly too fast to be shot down is generating a lot of spaceplane technology. Perhaps in the foreseeable future the quest for the first operational hypersonic aircraft able to routinely fly into orbit and back will finally be concluded. Meanwhile the old quip in the US military remains valid that “hypersonics is the future of airpower and always will be”.

A proper airplane should have pilots on board, but the more that time passes the lower the chance that future spaceplanes will be directly piloted by anyone present. For launch vehicles flying only cargo it seems certain no crew will be required, and pilots may not be needed even for transporting astronauts. The Sanger-II and NASP spaceplanes would have had cockpits and flight crews but HOTOL was specifically intended to fly without, as indeed is its Skylon successor. The technology of orbital spaceplanes will be just as exciting as that of the X-15, but without pilots the concept loses a lot of its glamour and sense of adventure.

So what are the chances of there being a fully reusable, crewed rocket plane (with or without airbreathing engines) like the Euro 5 discussed in the Preface of this book blasting through the air anytime soon? Unfortunately, I think it looks like it will take quite while. The only operational rocket aircraft in the near future will be suborbital. When orbital spaceplanes eventually come around (hopefully) it is likely they will be fully automatic vehicles rather than resembling a hypersonic fighter aircraft piloted by gallant astronauts.

Still, the required technology and the possibilities that are on offer are extremely exciting: if spaceplanes really can dramatically reduce the cost of putting things into orbit then they will at long last open up space for large-scale commerce, production, moonbases, space solar energy satellites, space hotels and other marvelous ideas that are currently wishful artistic impressions and science fiction.

I just cannot accept that the expensive, wasteful expendable rocket as we know it today is the best that we can do and therefore represents the final answer in access to space. Spaceplanes truly represent the last great aeronautical frontier.

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!

Aircraft maximum velocity and altitude evolution

The illustrations show the maximum velocity and the maximum altitude that aircraft have achieved over the years, and as such they encapsulate much of the story told in this book.

Since 1939 the (unofficial) maximum velocity records have all been set by rocket propelled aircraft, with the trend being steeply exponential then concluding with the X-15 in 1967. Around the same time that the X-15 program ended, the maximum velocities attained by turbojet and ramjet aircraft also reached their limits. It will be possible to fly faster using airbreathing propulsion but it will require scramjets (work on experimental versions of which continues to this day). It is also interesting to note that velocities that were initially achieved by mixed-propulsion interceptors using jet

Aircraft Maximum Velocity Evolution

Aircraft Maximum Altitude Evolution

as well as rocket engines were soon surpassed by jet-power-only aircraft (rendering mixed propulsion obsolete by about the end of the 1950s).

The (unofficial) maximum altitude records have been exclusively the province of rocket aircraft since 1948, with the exponential trend once again culminating with the X-15. Turbojet/ramjet aircraft cannot fly at altitudes above 30 km (100,000 feet) for extended times and are only able to surpass this during short zoom climbs. Sustained airbreathing flight at higher altitudes will require scramjets.

Given the exponential growth of the maximum velocity and altitude achieved by aircraft over time, it is understandable that many people expected these trend Unes to continue into the 1970s and beyond with aircraft reaching orbital altitudes as well as orbital velocities within a decade or two. Of course the Space Shuttle actually did so in 1981 but it was a vertical take-off, rocket-launched space ghder rather than a true rocket plane. Real spaceplanes possessing rocket engines, sophisticated airbreathing engines or combinations of the two, have yet to progress beyond the drawing board.

SpaceShipOne managed to exceed the highest altitude achieved by the X-15 but got nowhere near that aircraft’s record velocity; it travels about as fast as the fastest airbreathing aircraft. But SpaceShipOne was the first aircraft in four decades to reach the edge of space.