Category Rocketing into the Future

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

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

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 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.

“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.

“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.

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

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.

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

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

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.

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!

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

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.

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

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.

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.

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

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

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

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,

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!

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

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.

Like the Americans the British and the French, the Russians also understood that the Germans had made great advances in the development of jet and rocket technology during the Second World War. And in spite of the fascist origin of that knowledge the Soviets were not too proud to use it. At the end of the war they had captured the unfinished prototype and wind tunnel models of the German DFS 346, the advanced experimental research plane with swept wings, a pressurized cockpit, and the HWK 109-509C rocket engine. The cigar-shaped fuselage with sleekly embedded rivets was optimized for high speeds and the T-tail had all-moving horizontal stabilizers placed high on the vertical fin to prevent shock stall and disturbances by the wings. To minimize the plane’s frontal cross section the pilot was prone on his stomach and viewed through a Plexiglas nose. The Germans had designed the DFS 346 to be air-launched from a bomber so that the maximum of 2 minutes at full-thrust would suffice to break the sound barrier at high altitude. The plane was to land on a retractable skid, saving considerable weight in comparison to a conventional undercarriage using wheels. For measuring the speed of the aircraft a long spike with a pitot tube projected ahead from the nose. Now standard equipment on any aircraft, this tube measured the relative air pressure, which is a function of the velocity of an aircraft through the air. Poking this pitot tube out in front of the plane ensured that its measurements were not affected by airflow disturbances closer to the fuselage. At least as important as capturing hardware was the recruitment of many of the German engineers who had developed this revolutionary plane, by offering them privileges such as additional food rations as well as the opportunity to continue their research (apparently Stalin had finally understood that positive motivation resulted in more progress than brute force when it came to developing complex technology).

The Soviets planned to use the DFS 346 in order to gain a head start in the Cold War competition for speed and altitude, and therefore converted the German Siebel Flugzeugwerke company, which during the war had been tasked with developing the DFS 346, into the OKB-2 design bureau under the direction of the German engineer Hans Rossing. Soon the factory and its staff were moved from the original location in Germany to Russia, where the team continued their work on the DFS 346. Aleksandr Bereznyak, one of the original designers of Russia’s wartime BI rocket interceptor, was assigned to assist (and no doubt keep an eye on) Rossing. In order to disguise the German origin of the design, the project was renamed ‘Samolyot 346’ (Aircraft 346), and the Russian form of the German engine was designated ZhRD – 109-510.

Wind tunnel tests showed that at high angles of attack and low speeds the angle of the leading edge of the 346’s wing forced some air to flow sideways out towards the

wingtips instead of parallel to the fuselage. At the wing tips the airflow could end up flowing almost completely span-wise, sharply reducing the lift and resulting in a stall on the outer part of the wing and a loss of control of the aircraft. The solution was to add two so-called wing fences, low vertical ridges running from the leading edge to the back of the wing. This solution was later incorporated in most Soviet swept wing fighters of the 1950s and 1960s.

In 1947 the first prototype was completed. Since it had no engine installed it was designated 346P (for ‘Planer’, meaning glider). This version was meant to test flight stability, practice landings, and also try out the mating to and release from the carrier aircraft. It lacked a pressurized cockpit, propellant tanks and other propulsion-related equipment. In 1948 four test flights were carried out with the 346P being dropped from under the right wing of a confiscated American B-29 bomber that had suffered damage during a raid over Japan and then gone on to make an emergency landing in Soviet territory. Interestingly, during these tests the 346P was piloted by Wolfgang Ziese, who had previously been a test pilot for the Siebel company in Germany. In Russia he had prepared for the flights using a modified DFS ‘Kranich’ (Crane) glider that had been fitted with a prone-pilot cockpit and could be towed into the air behind a Petlyakov Pe-2 bomber.

Flying the 346 into unknown areas of aerodynamics, virtually encased in the tiny aircraft in an uncomfortable prone position and having to rely upon its complicated escape system, must have taken a lot of courage. Especially since at that time some aerodynamicists predicted that at Mach 1 an aircraft would slam into a virtual wall of air and inevitably be ripped apart by violent shock waves. The successful breaking of the sound barrier in the US by the Bell X-l leaked by Aviation Week in December 1947 did tell the Soviets that faster-than-sound flight was possible, but exactly what kind of phenomena they would encounter in the 346 was still unknown; naturally, the Americans kept the X-l flight data secret.

On one flight, Ziese forgot to check that the ailerons were in their neutral position before his aircraft was released by the B-29 carrier, so the 346P immediately flipped inverted. Only after losing almost 2,000 meters (6,600 feet) of altitude did he manage

to regain control of the plane. On the whole however, the 346P drops, gliding flights and landings went very well, and it was decided to proceed with the construction of a powered prototype. This 346-1 was completed in May 1949, and had a launch weight of 3,145 kg (6,935 pounds).

On 30 September 1948 the B-29 drops Ziese in the 346-1 equipped with a dummy engine from an altitude of 9.7 km (32,000 feet). He experiences some difficulties in controlling the aircraft and is obliged to land at an excessive speed (the fact that the aircraft does not have flaps for additional lift at low speeds exacerbates the problem). After the first hard touchdown the plane bounces several meters into the air, flies a further 700 or 800 meters (2,300 or 2,600 feet) across the ground, then touches down again. At that moment the ski is pushed back into the fuselage and the plane slides along the runway on its belly prior to coming to a standstill. It is slightly damaged, and the pilot is knocked unconscious but only lightly injured when his head hits the front of the cabin (apparently his seat and safety belt system were not up to the rough landing). Investigators conclude that Ziese had not fully released the skid during his approach, probably because he was fully occupied keeping the aircraft under control.

After repairs and improvements, the plane is redesignated 346-2 and glide flight testing resumes in October 1950 with Russian pilot P. Kazmin. The plane still proves tricky to fly, and on the first flight the skid once again fails to lock when lowered for landing. However, this time the landing takes place on a snow covered field and the belly-sliding does not cause any significant damage. On its second flight the 346-2 is towed by a Tu-2 bomber to an altitude of 2 km (6,600 feet) and released for a free gliding flight. This time Kazmin lands short of the runway. The aircraft is damaged and more repairs are needed. Meanwhile Ziese has recovered from his injuries and, starting on 10 May 1951, resumes flying the engineless 346-2, and starting on 6 June also the newly constructed but still unpowered 346-3 which has thinner wings better suited to transonic flight speeds. During the 346-3 flight tests the confiscated B-29 is replaced by a Soviet copy designated the Tupolev Tu-4 (reputedly copied so literally that rivets missing from the original were omitted).

Finally Ziese and the aircraft are judged to be ready for a powered flight, and on 15 August 1951 the 346-3 is driven through the air on rocket power for the first time. For around 90 seconds Ziese is the ruler of the sky. But the flight is no treat because the plane still has a tendency to roll. And due to a malfunctioning heating regulator the temperature in the cockpit rises to 40 degrees Celsius (104 degrees Fahrenheit), all but making the pilot faint. During this mission, as well as the following flight on 2 September, only the weaker cruise chamber of the engine is used in order to hold the speed below Mach 0.9 because tests in the T-106, the Soviet’s first supersonic wind tunnel have led the designers to fear that the aircraft’s control surfaces will freeze up at transonic speeds. And their fears are soon proven well-founded. On 14 September Ziese is dropped for the third low-thrust flight, ignites the smaller thrust chamber and accelerates into a climb. Shortly thereafter things go wrong at an altitude of just over 12 km (39,000 feet). Ziese reports to the ground that the aircraft is not responding to his control inputs, is rolling uncontrollably and rapidly losing altitude. Evidently the rocket thrust has pushed the plane into the transonic ‘no-go’ zone, resulting in locked control surfaces. On falling to a lower altitude Ziese manages to regain some control and ends up in a dive from which he pulls up at about 7 km (23,000 feet). When the airplane starts to roll wildly once again, Ziese realizes that he is running out of time and altitude. The controllers on the ground tell him to bail out. For the first time he triggers the explosive bolts to separate the cockpit section from the rest of the plane. The system works perfectly. The stabilizing parachute puts the cockpit into a smooth descent, enabling him to scramble out and land safely under his own parachute. The aircraft is obviously lost, along with all the flight measurements recorded and stored inside (there was no real time telemetry link with the ground, as is standard for test flights today). Nevertheless the limited data available enables investigators to figure out what probably happened. It is concluded that when it shot up into thinner air the aircraft entered the transonic flight regime and experienced shock stall at its tailplane and wings, freezing up its controls. Once the plane started to fall it accelerated out of the transonic area and exceeded Mach 1, at which moment the shock waves at the tail moved further to the rear, releasing the elevators. And when Ziese pulled out of the dive the aircraft slowed down and again entered the transonic regime, freezing up its controls once more.

It was clear that the 346 was not well suited to transonic speeds, and the aircraft shape’s aerodynamic speed limit had been achieved even without igniting the rocket engine’s more powerful main combustion chamber. The 346 project was abandoned. In any case, not much valuable data was expected to be gained from further flights because by the late 1940s Soviet jet aircraft were already flying faster than Mach 1. One ‘glass half full’ project report stated that within the speed limits imposed by the obsolete aerodynamic design all the 346-3 hardware had functioned well, including the rocket engine, the skid landing gear, and finally the escape capsule. The German engineers involved in the 346 project were repatriated to East Germany in 1953 (this was apparently standard procedure once Russian engineers felt that they had learned everything they could from their German colleagues.)

In parallel with OKB-2 and its 346 project, OKB-256 under Pavel Vladimirovich Tsybin was working on a transonic rocket plane called the Tsybin LL (with the LL standing for ‘Letayushchaya Laboratoriya’, which means Flying Laboratory). Even though this aircraft was kept very simple in terms of construction and propulsion, it was meant to approach Mach 1 and if possible surpass it. After models were tested in the TsAGI wind tunnels, two prototypes were constructed. They were made almost entirely of wood, with ailerons and flaps operated by a pneumatic system powered by compressed air (the forces on the control surfaces were expected to be very high at transonic speed, and so require more than pilot muscle power to operate). The rocket engine in the tail was a straightforward solid propellant booster called the PRD – 1500, and it could provide an average of 15,000 Newton for a duration of 10 seconds. The first prototype, LL-1, had conventional straight wings and an ejectable dolly take-off undercarriage similar to that of the Me 163. From mid-1947 pilots M. Ivanov, Amet-Khan Sultan, S. Anokhine and N. Rybko together completed a total of 30 flights with this prototype. After being towed by a Tu-2 bomber to an altitude of 5 to 7 km (16 to 23,000 feet) the pilot pushed it into a steep dive of 45 to 60 degrees in order to gain as much speed as possible prior to leveling off and igniting the rocket

motor. Then a very short, horizontal high-speed powered flight was followed by a gliding return to land on a retractable skid.

During the winter of 1947-1948 the second prototype was equipped with forward – swept metal wings, the benefits of which the Russians had learned of from German wartime research, and which had also initially been planned to be incorporated in the previously described Lavochkin 162. Water tanks were installed in the fuselage so as to be able to adjust the center of gravity of the aircraft. This was designated the LL – 3. It made over 100 flights and achieved a maximum speed of 1,200 km per hour (750 miles per hour), corresponding to Mach 0.97, without any significant problems. After the LL-3 tests, the LL-1 was turned into the LL-2 by retrofitting it with swept wings, but it never flew because by then swept-winged jet fighter prototypes had already undergone extensive testing.

A more ambitious project was the Bisnovat 5 developed by aircraft manufacturer Matus Ruvimovich Bisnovat. This was intended to continue where the Samolyot 346 project had ended, providing data on transonic and low-supersonic flight speeds up

to 1,200 km per hour (750 miles per hour) at an altitude of 12 km (39,000 feet), which was Mach 1.1. Bisnovat had prior experience of rocket planes because he had been responsible for the production of the Kostikov 302 prototypes by OKB-55 during the war and later had been involved in a number of missile projects. Similar to the DFS 346-based Samolyot 346, the Bisnovat 5 was an all-metal monoplane that had wings swept back at 45 degrees and augmented by fences, and a pressurized cockpit. It was also to be dropped from a carrier aircraft, in this case from under the right wing of a Petlyakov Pe-8, and then land using a simple ski undercarriage. The main ski under the fuselage was set at an angle to enable the aircraft to land with its nose slightly up to ensure sufficient low-speed lift for a soft impact. UnUke the uncomfortable prone-pilot position and complicated escape capsule of the 346, the pilot had a conventional ejection seat and sat upright, although slightly reclined in order to reduce the plane’s cross section. A single Dushkin-Glushko RD-2M-3VF dual-chamber rocket engine was installed in the tail and fed nitric acid and kerosene propellants by a turbopump powered by hydrogen peroxide. This engine was similar to those of the Florov 4303, Kostikov 302P, the Polikarpov Malyutka and the MiG 1-207 but the combined thrust chambers provided a maximum thrust of 16,500 instead of 15,000 Newton at sea level.

Models were tested in the TsAGI T-104 wind tunnel at up to Mach 1.45 and then one-third-scale models that were powered by small liquid propellant rocket engines were launched from carrier aircraft. After these tests had validated the aerodynamics of the new design, the first flight prototype was constructed and prepared for gliding flights. The first flight of this ‘5-1’ aircraft on 14 July 1948 almost ended in disaster when it hit the Pe-8 carrier shortly after being released. But test pilot A. K. Pakhomov managed to keep the 5-1 under control and made an emergency landing in a rough field. This incident severely damaged the prototype but it was repaired,

and the pylon under the wing of the Pe-8 was revised to carry the Bisnovat 5 with its nose pointing slightly downward to reduce the risk of the aircraft flying up and hitting the carrier after the drop. The next glide flight showed that the aircraft had poor roll and yaw stability. This problem had not yet been resolved when the third flight was made on 5 September 1948 and caused the plane to land tilted to one side, hit the ground with a wingtip and topple over. The plane was almost broken in two and beyond repair, but Pakhomov was okay.

The ‘5-2’ prototype was modified based on the lessons learned during the gliding tests with the 5-1. The vertical tail was swept back further aft to improve directional stability, and the simple metal wingtip bows were replaced by shock-absorbing skids better suited to dampening the impact of touchdown. The test campaign was resumed on 26 January 1949 with pilot Georgi Shiyanov taking the 5-2 on its first glide flight. Again the mission ended in a hard landing with severe damage to the aircraft, this time because the pilot had difficulty in finding the proper approach to the rather short runway and therefore came down beyond it. The 5-2 was repaired and further improvements made. The main ski, which had previously been set at an angle in the vertical direction for improved lift prior to landing, was now put horizontal and thus parallel to the fuselage to improve the pilot’s view of the runway. This meant that the small ski on the tail could be removed and replaced with a ventral keel fin to further improve flight stability. No major problems occurred during the next flight but roll and yaw stability were still insufficient. This led the engineers to install downwards angled fins at the wingtips like those of the Florov 4302. The next six ghde flights showed that the stabihty had improved, and that the plane was controllable at least up to the highest speed of Mach 0.77 that was attained in a dive.

But before the powered flight test campaign could commence, the authorities had shifted their interest to further developing supersonic jet aircraft. On 26 December 1948 test pilot I. E. Fedorov had opened the throttle on his swept-wing Lavochkin La-176 (derived from the La-168 jet fighter), pushed the plane into a shallow dive and attained Mach 1.0, marking the Soviet Union’s entry into the world of supersonic flight (just over a year after Chuck Yeager made his historic flight in the X-l). Hence the authorities did not see much use for a Mach 1 rocket aircraft.

There was never a Russian equivalent to the American Douglas D-558-2 and Bell X-l series of experimental rocket planes, and no subsequent evolution into a vehicle Uke the X-15. The Samolyot 346 flew until September 1951 but never managed to exceed Mach 1 and (as noted above) this project was also terminated after the loss of the 346-3 aircraft.

Nevertheless Soviets engineers continued to develop many supersonic aircraft that were as good as anything in the West, and during the Cold War proved themselves to be masters of aerodynamic theory and design. It is however clear that Soviet spies in the US aviation industry and NACA provided data that was of great assistance to the Russian designers, and at least partly made up for their lack of a supersonic research aircraft program.

Basic Flying Rules: “Try to stay in the middle of the air. Do not go near the edges of it. The edges of the air can be recognized by the appearance of ground, buildings, sea, trees and interstellar space. It is much more difficult to fly there. ”

– Anonymous

At the end of the 1950s the idea of the pure rocket fighter was dead and the role envisaged for a mixed jet/rocket interceptor already very limited. However, as far as American designers of research aircraft were concerned, the evolution of supersonic extreme-altitude rocket aircraft had barely started. After the successful X-l and X-2 series and the D-558-2 Skyrocket, the next step was a rocket plane that could surpass all of its predecessors in terms of speed and altitude.

Whereas the early X-l aircraft had investigated the transonic and low supersonic flight regimes, the later X-ls and the D-558-2 had explored speeds around Mach 2, and the X-2 had marginally exceeded Mach 3, the new goal was to venture into the hypersonic area of aerodynamics: Mach 5 and above. The definition of ‘hypersonic’ is somewhat nebulous since there is no clear and sudden change with respect to the supersonic flight regime (as occurs between subsonic, transonic and supersonic). In general, with respect to supersonic aerodynamics, what happens at hypersonic speeds is much more complex and far more difficult to model and predict. Many of the simplifications about the behavior of the atmosphere, aerodynamic heating and shock waves that can safely be used for supersonic theory are no longer valid at speeds over Mach 5. Laboratory tests for hypersonics are hampered by the fact that it is virtually impossible to generate a continuous Mach 5 + airflow in a wind tunnel. Hypersonic wind tunnels depend on extremely brief, explosive bursts of gas that only facilitate measurements on very small models during a fraction of a second. Once again, the only way to get large amounts of reliable data on this flight regime is to fly research aircraft at hypersonic speeds.

As regards altitude, with its maximum attained altitude of 38.5 km (126,000 feet) the X-2 had already reached into the upper stratosphere. But how a spaceplane or a shuttle-like vehicle would behave in a virtual vacuum, and what it would experience on returning from orbit, had yet to be investigated. Many aviation experts at the time expected the airplane to evolve into an orbital spaceplane, initially launched on top of a conventional rocket but later on capable of taking off and landing like a normal aircraft. As early rocket pioneers such as Yalier had foreseen, a space plane was part of an inevitable evolution. The next step, the X-15, was therefore expected to act as a bridge between aircraft and spacecraft.

If you have any interest in aviation or spaceflight, then you’ll have heard of the X-15. Entering ‘X-15’ into Google in early 2011 resulted in 56 million Internet hits. By comparison ‘Space Shuttle’ got a mere 23 million hits. The popularity of the X-15 is understandable. When this amazing machine was developed it represented the next step in rocket aircraft, flying at hypersonic speeds and reaching altitudes so high that it was considered to be outside the ‘sensible’ atmosphere. Eight of its twelve pilots earned the right to wear the coveted US Air Force ‘astronaut wings’ for achieving an altitude of 50 miles (80.5 km); strangely, such badges were not awarded to NASA pilots who flew the aircraft to such heights. Its extensive flight program took it to an altitude of 107.8 km (353,700 feet) and a speed of 7,274 km per hour (4,520 miles per hour), equivalent to Mach 6.7. The unofficial altitude record set by the X-15 on 22 August 1963 stood until the SpaceShipOne rocket plane broke it in 2004, and the unofficial aircraft speed record which it set on 3 October 1967 still stands. The X-15 was the ultimate rocket plane. It couldn’t achieve orbit but in terms of altitude, flight dynamics, instrumentation, heat shields and propulsion it was very much a suborbital spaceplane.

The X-15 originated from a suggestion by Bell Aircraft’s Walter Dornberger (who had been von Braun’s boss in Germany during the war) in the early 1950s to develop a rocket plane to explore the realm of hypersonic flight. As with the previous rocket X-planes, the X-15 was to be carried aloft by a bomber to maximize the usefulness of the available rocket propellant for reaching high altitudes and speeds. The reason for developing the X-15 was similar to the goals that had inspired the X-l, D-558-2 and X-2: to provide experimental data on high-speed flight for improving and validating aerodynamics theories and models (which translated into sets of equations to enable the aerodynamic behavior of aircraft to be predicted). The X-15 would extend this knowledge to speeds in excess of Mach 5.

NACA (soon to become NASA), the Air Force and the Navy all had an interest in the program, but the Navy eventually opted out in order to concentrate on advanced planes for it fleet of aircraft carriers. The Air Force was in charge of the development of the X-15 and NACA was to lead the flight research campaign after the acceptance flight tests were completed. The request for proposals for the airframe was issued on 30 December 1954 based on a preliminary NACA design. Invitations for the rocket engine went out on 4 February 1955. North American, Republic, Bell, and Douglas all responded with designs that closely resembled the reference concept. In late 1955 North American won the contract to develop and build the X-15 aircraft, and shortly thereafter Reaction Motors was hired to supply the engine.

North American’s design had a long, cylindrical fuselage with short stubby wings

and a tail section that combined thin, all-moving horizontal stabilizers and the thick, all-movable wedge-shaped dorsal and ventral fins that NACA suggested would work better at hypersonic speeds than the thin fins that were commonly used on supersonic planes. The two horizontal stabilizers could be moved differentially (i. e. pointing one up and the other down) for roll control, thereby eliminating both the need for ailerons and potential shock-wave interaction problems at the wings. In unpowered flight the pilot would use a standard center-stick, but while running on rocket power he would employ a small joy-stick at the right of his seat, where his elbow would be blocked in order to prevent him from pulling the nose up as the strong acceleration forced his arm backwards.

Most of the internal volume of the aircraft was taken up by the propellant tanks and the rocket engine, prompting designers and pilots to dub it “the missile with a cockpit” and “the flying fuel tank”. The retractable landing gear comprised a nose – wheel and two skids in the rear fuselage. Because the ventral fin protruded below the extended skids the pilot had to jettison part of it just before landing. The ejected fin landed by parachute to be recovered and reused. Later in the flight campaign it was discovered that the aircraft was actually more stable without the fin extension during re-entry into the atmosphere on a high-altitude mission, so from then on the ejectable part was no longer used.

The X-15 would be carried under the wing of the large and powerful B-52 jet bomber, with its dorsal fin rising through a notch in the trailing edge of the carrier

ANHYDROUS AMMONIA TANK (FUEL)

LIQUID OXYGEN TANK (OXIDIZER)

LIQUID NITROGEN

^AUXILIARY ; POWER UNITS

HYDROGEN-

PEROXIDE

aircraft’s wing and its cockpit section protruding forward from under the wing. This arrangement required the X-15 pilot to be on board his aircraft from the moment the B-52 started taxiing, but the big advantage was that in an emergency his ejection seat could be used immediately; previous rocket planes of the X series were housed in the bomb bays of their carriers and in an emergency an already seated pilot was unable to eject until after his plane had been dropped. To overcome the boil-off of the liquid oxygen supply in the X-15 during the long climb to the release altitude the B-52 was able to continuously top up the X-15’s tank. After its powered flight the X-15 would
glide back for a landing at Rogers Dry Lake near Edwards. The small wings were well suited for high-speed, low-drag flights, but not particularly good for gliding. To generate enough lift the aircraft had to glide rather rapidly, which meant landing at 320 km per hour (200 miles per hour). But the dry lake had so much roll-out space that the nose landing wheel did not have to be equipped for steering, simplifying the design and further lowering the X-15’s weight.

The engineers developing the X-15 had to overcome many challenges. A big one was aerodynamic heating. Since shock-wave compression would heat up the air (in much the same way as air is heated in a bicycle pump) the skin of the aircraft would get very hot, not only when flying horizontally at hypersonic speeds for a prolonged time but also when re-entering the atmosphere after being boosted into the vacuum of space (albeit to a lesser extent due to the relatively brief duration of this phase of the mission). It was calculated that the temperatures of the upper fuselage would reach 238 degrees Celsius (460 degrees Fahrenheit) and the nose and the leading edges of the wings almost 700 degrees Celsius (1,300 degrees Fahrenheit). In addition to heat – resistant titanium, the project required the new high-temperature nickel alloy called Inconel X. This alloy would retain sufficient strength at such temperatures but it was a difficult material to work with (as was titanium, in fact). The wings and fuselage of the X-15 consisted of titanium frames with an Inconel X skin. The aircraft remained relatively cool by using the ‘heat sink’ principle: rather than applying active cooling, its structure would simply absorb the aerodynamic heat for the duration of the high – temperature phase of the flight and later radiate it away. The skin was painted black in order to maximize this heat loss. The internal structure would get fairly hot, which is why it was made of titanium rather than standard aircraft aluminum. The fact that different areas of the aircraft would have different temperatures and would therefore expand irregularly, necessitated innovative design solutions such as flexibly mounted wings which could deform span-wise and chord-wise, wing leading edges that were segmented so they could expand without buckling, and the incorporation of a variety of metals having different thermal expansion rates. The intense heating and high air pressures in hypersonic flight also meant that conventional boom-type sensors could not be used, because they would soon bend, break and melt off. An innovative ‘Ball Nose’ (officially a ‘high-temperature flow-direction sensor’) was installed instead. It was protected by a thick Inconel X skin and cooled by liquid nitrogen to prevent it from melting at high speeds. It was automatically aligned with the airflow to give the pilot data on angle of attack, sideslip (the direction of the airflow around the aircraft) and the impact pressure of the air (a measure of the velocity).

The pilot was housed in a pressurized aluminum cabin that was isolated from the aircraft’s skin and insulated by heat radiation shielding and insulation blankets. He wore a full pressure suit that would instantly inflate in the event of a loss of cabin pressure or ejection from the aircraft. The cabin and the suit were both pressurized and cooled by nitrogen, an inert gas that would help to prevent fire in the cockpit but meant the pilot had to breathe from a separate oxygen supply system; when opening the visor to scratch his nose he had to be careful to hold his breath. Nevertheless, the cockpit could become rather hot and pilots usually landed drenched in sweat despite having opened the nitrogen cooling supply all the way prior to launch.

Instead of a heavy, complicated escape capsule such as on the X-2 the designers of the X-15 chose to incorporate an ejection seat. As a concept this escape system was fairly conventional but the extreme situations in which it would be required to operate were definitely not. Design specifications stated that the seat must enable a pilot to leave the aircraft whilst flying at speeds up to Mach 4, in any attitude, and at altitudes up to 37 km (120,000 feet). These were much more extreme conditions than faced by any other aircraft escape system, with the result that it became possibly the most elaborate ejection seat ever developed. When a pilot felt the urgent need to get out he drew his feet into the foot rests, his ankles striking a set of bars that activated ankle restraints and extended a set of airflow deflectors in front of his toes. He then raised the ejection handles, activating a set of thigh restraints as well as rotating elbow restraints that drew in his arms. This would protect him from the imminent onslaught of the high-speed air outside the cockpit which, depending upon the flight speed and altitude, might be several times the force of a major hurricane. At the same time the seat’s oxygen supply would be activated to enable the pilot to breathe independently of the aircraft. When the ejection handles reached 15 degrees of rotation the cockpit canopy was automatically ejected and solid rocket motors boosted the seat out of the aircraft. Immediately, a pair of fins folded out and two telescopic booms extended backwards to stabilize the assembly. The seat automatically released the pilot either at an altitude of 4.6 km (15,000 feet) or 3 seconds after ejection if already below that altitude. The system jettisoned the headrest, and released the seat belt, power and oxygen lines and other restraints so that he would be free to land under his own parachute. If the automatic release system failed the pilot could release himself, and to enable him to judge his altitude his visor was kept clear of ice by a battery powered heater. The X-15 ejection seat was tested on a rocket sled track at Edwards, but neither tested or used for real in the extreme conditions for which it was designed.

Another major design issue was how the aircraft should maneuver itself when the aerodynamic controls lost their effectiveness in the near-vacuum at extreme altitudes. Attitude control required incorporating a reaction control system consisting of small thrusters that the pilot could control using a small stick placed on the left side of his console. This steering method was also in development for the Mercury spacecraft but the X-15 would be the first aircraft to depend on such reaction control (the X-1B had tested a similar system as an experiment). The assembly consisted of four 500 Newton thrusters for pitch, four 500 Newton thrusters for yaw, and four 190 Newton thrusters for roll (the roll thrusters needed less thrust because the aircraft was easier to roll than it was to pitch or yaw). Each wing had one upward and one downward pointing roll thruster near its tip (the farther a thruster was from the plane’s center of gravity the more effective it was because of the cantilever torque effect), while the aircraft’s nose housed the two sets of yaw thrusters and two sets of pitch thrusters. For every impulse in each direction two thrusters would fire in parallel (e. g. for pitch there was one pair to push the nose up and one pair to push it down), with the system continuing to function if one thruster of each set malfunctioned. The thrusters ran on the gas (super-heated steam and oxygen) provided by the decomposition of hydrogen peroxide.

Reaction control in a space-like environment is very different from maneuvering an aircraft using normal aerodynamic control surfaces. If you make a turn in an airplane in the atmosphere, all you have to do to return to straight flight is to push the controls back to neutral. It is just like on a boat. This is called ‘static stability’ because the airflow around the aircraft ensures that it automatically assumes a stable attitude when the pilot lets go of the stick and foot pedals. However, in space there is no such thing. In a (near) vacuum, if you use a bit of rocket thrust in the plane’s nose to push it to the left, the aircraft will not stop turning after you cease thrusting. The thrust has accelerated the nose and thus given the aircraft a leftward rate of rotation that will remain constant if nothing interferes with it. To point the nose in a certain direction you have to start it rotating in that direction and then, when the moment is right, fire the thrusters on the other side of the nose to cancel the rate of turn. Thus if you ignite the thrusters on the right side of the nose for 2 seconds to start a turn, you will then require to fire the thrusters on the left side for the same duration (presuming that they have the same thrust) to stop the rotation. Of course, while the thrusters are firing either to start or stop a rotation the rate will be either increasing or decreasing, making pointing the aircraft in a certain direction using reaction control thrusters a very delicate and difficult task. The dynamics are completely different from a normal aircraft with air flowing over its wings and tail, and they do not come naturally even to an experienced pilot.

The first two X-15s to be delivered had conventional hydraulically actuated flight controls, aided by a simple З-axis stability augmentation system that would weakly counteract any unintended motions. But the X-15-3, which was specifically meant to fly at extreme altitudes, had ‘fly-by-wire’ adaptive flight control. This would monitor the pilot’s movements of the stick and rudder pedal and adjust them prior to passing the actions to the aerodynamic control surfaces and the reaction control system, thereby making the plane handle in a similar manner in all flight regimes. At higher flight speeds it reduced the sensitivity of the controls and seamlessly integrated the reaction control thrusters with the aerodynamic controls: the lower the ambient air density the more the thrusters would be called upon. It was believed that at extreme altitudes the X-15 would not be controllable without this adaptive control system, until pilot Pete Knight experienced a total electrical failure in X-15-3 during a high – altitude mission and still managed to land safely.

Powering the X-15 would be a Reaction Motors XLR99 rocket engine, generating an awesome 227,000 Newton of thrust (the equivalent of half a million horsepower) at sea level, and 262,000 Newton in a near-vacuum. As the weight of a fully fueled X- 15 was 15,400 kg (34,000 pounds) this meant the engine’s thrust was about twice the plane’s weight at the moment it was dropped from its B-52. The X-15 could thus fly straight up and still accelerate. When it ran out of propellant the aircraft’s weight was a mere 6,600 kg (14,600 pounds), which meant that just before shutting down its engine it had a tremendous thrust to weight ratio of 4 at high altitudes; accelerating at 4 G! At that time the mighty XLR99 was the most powerful, most complex yet safest man-rated rocket engine in the world. It could be throttled from 50 to 100% of thrust, shut down and restarted in flight. The restart capabihty was useful if the engine failed to ignite upon the aircraft’s release from the carrier aircraft but other than during the early demonstrations intentional stops and restarts were deemed unnecessary and too risky.

The XLR99 used ammonia and liquid oxygen as propellants, and the turbopumps were driven by hot steam produced by the decomposition of hydrogen peroxide using a silver catalyst bed. A kind of spark plug ignited propellant in a small combustion chamber, which then acted as a blow torch for an instant start of the

rocket engine itself. Ammonia is toxic and expensive but gives better performance than the alcohol used in for instance the XLR11, whilst not burning as hot as for instance kerosene. It proved a good compromise combining a high performance with a relatively simple and therefore reliable engine cooling system. The standard X-15 carried sufficient propellant to run the XLR99 at full power for 85 seconds but the modified X-15A-2 with two external drop tanks could fly at maximum thrust for just over 150 seconds. The XLR99 was a big unit weighing 415 kg (915 pounds) and required an overhaul after every accumulated hour of operation, so a standard X-15 could make about 40 missions before the engine needed to be replaced. For electrical and hydraulic power the aircraft relied on a pair of redundant auxiliary power units driven by steam from decomposed hydrogen peroxide, just like the engine turbopumps.

Because of delays in the development of the XLR99, early X-15 flights used two XLR11 engines (running on ethyl alcohol and liquid oxygen) similar to that which had powered the X-l, and they provided a combined thrust of only 71,000 Newton.

X-15-1 was rolled out from North American Aviation’s plant outside Los Angeles on 15 October 1958 applauded by some 700 spectators, amongst them Vice President Nixon. Here was (part of) America’s answer to the Soviet Sputnik satellite, which had beaten the US into orbit a year earlier and caused them to suddenly realize that they were in a technological race for space supremacy with the USSR. The aircraft’s first glide flight was made on 8 June 1959 piloted by Scott Crossfield, who had left NACA to become chief test pilot at North American. His job was to demonstrate the rocket plane’s airworthiness at speeds up to Mach 3, which needed to be verified

before the aircraft could be handed over to the government. As an aeronautical engineer as well as a test pilot, Crossfield had also played a major role in the design and development of the aircraft.

The first free flight of the X-15-1 came close to disaster shortly prior to landing. Crossfield pulled the nose up to slow his descent, then found he had to push the stick

forward again because the nose had come up too far. This was the start of a severe divergent pitching oscillation. The more he tried to correct the motion the worse it got. Only his superb piloting skills enabled him to smack the X-15 onto the desert airstrip at the bottom of a cycle without damage to the plane or injury to himself. Afterwards it was found that the aircraft’s pitch controls had been set at too sensitive a level, resulting in ‘pilot-induced oscillations’, a situation in which inputs from the pilot tend to overcorrect and cause a pendulum motion of increasing magnitude.

Crossfield was also at the controls of the second aircraft, the X-15-2, when it made the program’s first powered flight on 17 September. Once clear of the B-52 carrier he ignited first one XLR11 and then, when satisfied, added the second engine. The flight plan had called for a ‘safe’ maximum speed of Mach 2 but even with the air brakes fully extended he could not keep the X-15 from creeping up to Mach 2.1. He ended this promising, brief powered phase of the flight with a lazy barrel role for the benefit of the two jet planes that were flying ‘chase’. Later missions soon had the X-15 going much faster, especially when flights with the XLR99 engine were started in November 1960.

North American built three X-15 aircraft, with the second being specifically set up for high-speed missions and the third for high-altitude missions. The X-15 program made a total of 199 powered flights over a period of nearly 10 years (a planned 200th flight in November 1968 was canceled due to technical problems and bad weather). Thirteen flights were to altitudes exceeding 50 miles (80 km), earning eight pilots the right to wear US Air Force ‘astronaut wings’, but only two of these qualified as true space flights by the rules of the International Aeronautical Federation and they were by Air Force pilot Joe Walker of the X-l series and Skyrocket fame. The FAI defines spaceflight as occurring above 100 km (62 miles) altitude; Walker flew up to 105.9 km (347,000 feet) on 19 July 1963 and to 107.8 km (354,000 feet) on 22 August. At such altitudes 99.9% of the atmosphere was below the aircraft (the decision on where the atmosphere ends and space begins is pretty arbitrary because there is still atomic oxygen even above the altitude at which the Space Shuttle orbits). In terms of speed records, the X-15 enabled Air Force pilot Robert White to become the first person to fly at Mach 4, 5 and 6. It had taken 44 years for aircraft to reach Mach 1 but White increased his maximum achieved velocity from Mach 4 to Mach 6 in the span of only 8 months.

Generally an X-15 mission would fall into one of two categories: high-speed or high-altitude, but for most phases of the flight they adopted similar procedures. The pilot followed a strict pre-determined flight plan defining exactly what combinations of thrust, speed, altitude and heading were required as functions of time. This depended on the type of vehicle tests or experimental measurements to be made and therefore was different for each flight, but a typical X-15 mission would proceed as follows:

After mating of the X-15 to the B-52, propellant loading and pre-flight checks, the ground crew disconnects the servicing carts and the big bomber with its heavy load taxies for several miles along the dry lake bed at Edwards to the start of the runway. The X-15 pilot is already fully enclosed in the cockpit of his rocket plane, doing his own checks in preparation for the mission. Initiating the take-off run, accompanying ground support vehicles are soon left behind as the B-52 rapidly accelerates. On a hot day in the Mojave Desert, when the air density is relatively low, more than 3.7 km (12,000 feet) of runway is required to get the combination into the air. Slowly the jet bomber climbs to an altitude of about 14 km (45,000 feet), where it continues to fly at around 800 km per hour (500 miles per hour). The underside of the X-15 builds up a coating of frost at the location of the liquid oxygen tank due to the intense cold of that propellant. The cruise to the release point takes up to an hour, during which

the cryogenic liquid warms up and evaporates; if it were not constantly replenished from a tank on the B-52 the X-15 would boil off 80% of its oxygen. The sleek rocket plane is carried a suitable distance so that it can be launched straight into the direction of its intended landing site (Rogers Dry Lake), eliminating the need for the rocket plane to make any turns during powered flight. Mission planners have made sure that there are enough dry lake beds along the X-15’s flight route for emergency landings in the event that the rocket engine does not ignite or extinguishes in flight. Twelve minutes prior to launch the X-15 pilot starts the auxiliary power units, which then produce an exhaust trail behind the aircraft. He also checks all onboard systems, tries the flight controls, tests the reaction control system, sets all the switch positions, activates the main propulsion system, and powers up the data recorders and cameras. The X-15 is accompanied by several jet fighter planes during the various phases of the mission to help and advise its pilot, who is unable to see any part of his own aircraft through the tiny windows (although otherwise the view was not too bad because his helmet was very close to the windows). Several ground stations along the flight path are ready to relay radar measurements of the rocket plane’s location, speed and direction of flight, as well as telemetry data from the X – 15, to the ground control center, which is in turn in contact with the pilot. This enables the control center to help the pilot to verify that the instrumentation is giving accurate information, and also help him to keep up with the often complex flight plan and offer advice if something goes awry.

Release from the B-52 is sudden, since the X-15 is aerodynamically a very poor glider at low speeds and, moreover, very heavy with a full propellant load. It drops like a streamlined brick, falling clear of its carrier in seconds. The rocket engine must be ignited promptly; if it does not start after two attempts then the pilot has barely enough time to dump the propellant and prepare for an emergency landing. But when the mighty XLR99 ignites, the X-15 rapidly accelerates and leaves the B-52 and the chase planes for that part of the mission far behind. Climbing at nearly 1,200 meters (4,000 feet) per second at an angle of 42 degrees it shoots up into the thin atmosphere at full power.

As the X-15 lightens due to its voracious propellant consumption, the acceleration gradually increases from 2 G to 4 G. This subjects the pilot to a peculiar sensation in which, although he is holding a steady pitch and climb attitude, he feels he is pulling G in a sharp pitch maneuver that is increasing the climb angle and even rotating the aircraft over onto its back, as if looping. The instruments in the cockpit tell him it is an illusion but even an experienced test pilot like Robert White once could not help himself from momentarily pushing the nose down to check whether the horizon was still in the right place; because of this little maneuver he actually failed to reach his planned maximum altitude on that flight.

The relatively long high-G acceleration was pretty uncomfortable; Milt Thompson once said that the X-15 was the only airplane he ever flew where he was glad when the engine quit. A G-suit integrated into his flight suit would help a pilot to cope with the acceleration by inflating bladders to press on his abdomen and legs and prevent a black-out from blood draining away from the brain into the lower parts of the body.

If the flight is a high-speed mission, the pilot levels off at an altitude below 30 km (100,000 feet) so that the X-15 can employ its standard aerodynamic controls and fly as a conventional airplane. The remaining propellant is used to accelerate to the top speed required, with the pilot varying the thrust and flight angle to control speed and altitude. But if a high altitude is the objective the X-15 continues to climb until the engine exhausts its propellant supply, some 85 seconds after launch and at an altitude of about 50 km (160,000 feet). The aircraft then continues to climb unpowered for up to 2 minutes until gravity reduces the vertical speed to zero, at which point the plane has achieved its maximum altitude of up to 108 km (354,000 feet). With an optimum flight profile the X-15 was capable of flying even higher, but the re-entry would have been too fast and too steep for the structural limits.

During the unpowered ascent the pilot is in a zero-drag, zero-thrust ‘free fall’ (in effect falling upwards). Although essentially weightless, he remains strapped firmly into his seat. The view is spectacular, as described by Robert White: “My flights to

217,0 feet and 314,750 feet were very dramatic in revealing the Earth’s curvature. At my highest altitude I could turn my head through a 180 degree arc and wow! The Earth is really round. At my peak altitude I was roughly over the Arizona-California border in the area of Las Vegas, and this was how I described it: looking to my left I felt I could spit into the Gulf of California. Looking to my right I felt I could toss a dime into San Francisco Bay.” But an X-15 pilot had little time for sightseeing, not so much because the flight was brief but because keeping the plane under control required the utmost attention.

After reaching the top of its ballistic arc the aircraft falls back to Earth in zero- gravity conditions until the deceleration by the increasing aerodynamic drag of the atmosphere becomes noticeable at lower altitudes. During the time the air density is too rarefied for the aerodynamic controls to work, the pilot orients the X-15 using the reaction control system. Initial penetration of the atmosphere has to be done holding the plane’s nose high up, presenting the broad underside to the air to create a strong shock wave that slows the vehicle down and deflects the resulting heat away from its skin (the Space Shuttle Orbiter would adopt a similar re-entry attitude when coming back from space). This requires very precise steering, as too high an angle of attack will put the plane into a flat spin that is extremely difficult to escape from, whilst too low an angle will plunge the X-15 into the denser atmosphere too fast and result in pressures and temperatures that will destroy it. The weightless ballistic part of the flight lasts at most 5 minutes. Together with its extreme altitude this makes the X-15 very similar to a spaceplane, albeit one that cannot achieve orbit.

As the thickening air slows the falling X-15, the pilot experiences a maximum of 5 G of deceleration for about 15 seconds. Electronic stability augmentation helps to keep the aircraft in a proper attitude during re-entry, preventing inertia coupling such as killed Mel Apt in the X-2. The aerodynamic control surfaces are banging against their stops and sending loud noises reverberating through the empty propellant tanks. Once the speed stabilizes, the pilot pulls out into level flight and initiates a shallow supersonic gliding descent to the landing site. He adjusts the glide path by extending or retracting the air brakes: the further these are deployed, the greater is the drag, the lower is the speed, the lower is the lift, and thus the steeper is the rate of descent.

The round trip of up to 640 km (400 miles) has brought the X-15 back to where it started. At 11 km (35,000 feet) altitude the pilot guides the aircraft into an approach pattern for a landing on Rogers Dry Lake, banking to visually check the landing site. He dumps any remaining propellants to make sure he is not too heavy for landing, jettisons the ventral rudder if it is present (as otherwise it would dig into the ground), lowers the landing flaps and undercarriage and closes the air brakes to avoid running out of necessary flight speed so near the ground. Lowering the aircraft gingerly at a sink rate of about 0.6 meters per second (2 feet per second) he touches down with a forward speed of 320 km per hour (200 miles per hour). When pilot Joe Walker was asked whether he thought it would be possible to land the X-15 very accurately while coming out of a very steep gliding approach he responded, “There’s no question of where you’re going to land, it’s how hard.” Generally X-15 pilots managed to touch down gently and very close to the intended landing spot.

Because the main skids are located far back on the fuselage, once they touch the ground the rest of the aircraft slams down fairly hard onto the single nose wheel. The X-15 then skids on the dry lake bed surface for about 2 km (1 mile) before stopping, with the high friction of the skids eliminating the need for active braking. While the

jettisoned ventral rudder (which landed under a small parachute) is retrieved, ground support personnel drive up to the X-15, assist the pilot in getting out of the cockpit, and prepare the aircraft for transport back to the hangar. The pilot, now relaxed after the tensions of the flight, enters the transportation van to have his flight suit removed and post-flight physiological checks. The B-52 roars overhead at low level and then makes a 180 degree turn while climbing (a so-called chandelle maneuver) in order to celebrate ‘mission accomplished’. So ends another X-15 mission that has added more data points to the collection of aerodynamic data on flight at hypersonic speeds and extreme altitudes.

Of course not all missions went according to plan. On the fourth powered test flight of the program, Scott Crossfield had to make an emergency landing in the second X- 15 due to a small fire in the engine compartment. Because he did not have enough time to dump all of the propellant he had to land with a much higher angle of attack and hence a nose-high attitude to generate sufficient lift. Once the skids hit the ground, the nose wheel smacked down hard, since it was impossible to keep the nose up with the skids being all the way at the rear of the plane (normal aircraft have their main wheels under the wings, near the center of gravity in order that the nose can be lowered slowly after main gear touchdown). Because of this, as well as the weight of the propellant, the airframe buckled just aft of the cockpit. Crossfield was unharmed but the plane needed extensive repairs. He also survived a fuel tank explosion during a test of the third X-15’s XRL99 in 1960. He was sitting in the cockpit wearing his normal clothes when suddenly he was blasted forward. The aircraft was engulfed by a fire but because of the hermetically sealed cockpit Crossfield survived unscathed. The remainder of the test team had been safe inside a control bunker, so nobody was harmed.

Many missions failed in less dramatic ways, often due to the XLR99 not igniting or quitting early and other malfunctions of onboard equipment. Even on what were deemed successful flights not everything always worked perfectly. On several high­speed flights hypersonic air penetrated the X-15 via small gaps between access doors and panels, burning tubes and wires and allowing smoke into the cockpit. Typical of the less safety-conscious manner in which experimental programs were run in those days, quick fixes were implemented with minimal disruption to the schedule.

The second X-15 was badly damaged in a crash landing by pilot John McKay on 9 November 1962 due to failing wing flaps and the weight of unjettisoned propellant braking a landing skid on touchdown. McKay suffered several cracked vertebrae and the aircraft was virtually destroyed. However, both lived to fly another day. McKay’s injuries healed and he returned to flight status. The plane was rebuilt and modified by North American for flying even faster than previously. A big improvement was the installation of attachment points for two large drop tanks (one for liquid oxygen and the other for ammonia). The propellants in these tanks were to be used for the initial phase of a high-speed mission, then the empty tanks would be discarded to lower the aircraft’s aerodynamic drag. The X-15-2’s fuselage was also slightly lengthened to accommodate an additional liquid hydrogen tank intended to power a small prototype ramjet engine that was to be placed on the ventral fin. A dummy engine was carried to determine how it affected the aerodynamics, but the X-15 program finished before a real ramjet could be installed.

For protection from the extreme temperatures of the high-speed flights a special ablative heat shield material was applied to the upgraded X-15’s surface. This would slowly bum off, removing heat so that it would not reach the aircraft’s structure.

One issue was that the melted material formed an opaque coating on the windows. The simple solution was to cover one window with protective doors during the powered phase of the flight. It would be uncovered for landing, so that the pilot at least had one clean window to look through. A smaller issue was that the ablative material was pink. No self-respecting test pilot was willing to fly in a pink aircraft, but luckily a protective white coating was also necessary to protect the ablative material from liquid oxygen.

The improved aircraft was renamed X-15A-2 and first flew with the ablative coating on 21 August 1967, when it achieved a speed of 5,419 km per hour (3,368 miles per hour). A new layer of coating was then applied in preparation for the next, much faster flight. On 3 October of that same year Pete Knight flew the aircraft to a maximum speed of 7,274 km per hour (4,520 miles per hour), Mach 6.72. It was the highest speed of the X-15 program and still represents the highest speed achieved by any aircraft except the Space Shuttle. However, after landing, the plane was found to be in a sorry state. Some of the skin of the ventral fin was burned and excessive heat had also damaged the nose and the leading edges of the wings and equipment inside the ventral fin, particularly the dummy ramjet. In fact, Knight didn’t need to eject the ramjet prior to landing, it fell off by itself due to the heavy damage to the pylon onto which it was mounted (it was later discovered that the ramjet created a shock wave that impinged on the pylon, locally causing extremely high temperatures). Clearly the limit of the thermal protection system, and as such the aircraft’s speed limit had been reached. The X-15A-2 was repaired but never flew again.

A total of a dozen test pilots flew the X-15, including Neil Armstrong, who would become the first man to walk on the Moon, and Joe Engle, who would command a Space Shuttle mission. One pilot, USAF test pilot Major Michael J. Adams, lost his life flying this challenging machine on 15 November 1967. He flew the X-15-3, the one specifically built for high-altitude missions and the aircraft in which seven pilots had already earned their ‘astronaut wings’. During the climb aiming for an altitude of 81 km (266,000 feet), an electrical disturbance from an onboard experiment caused the reaction control system to function only intermittently. The glitch also caused the inertial system and boost-guidance computers to display incorrect data on the cockpit instruments. As the X-15 began to deviate from its proper direction of flight, Adams, possibly disoriented and confused by the false instrument data, made control inputs which actually increased the heading error. Soon the aircraft was flying sideways, a situation that was not serious while in near-vacuum but would spell disaster once the plane fell back into the atmosphere. Adams reported to the ground control team that the aircraft seemed “squirrely”, then said “I’m in a spin.” It sent a chill up the spine of the control engineers. Since no pilot had ever experienced a hypersonic spin, there was little advice they could offer.

Adams managed to recover from the spin, but then found himself in an inverted (upside-down) dive. But this attitude was stable and there was sufficient altitude for

Adams to regain control of the aircraft. Next the fly-by-wire control system began to try and correct the erroneous attitude, resulting in a violent out-of-control oscillation. With the flip of a single switch Adams could have shut off this runaway system, but no one thought to suggest it as the plane rapidly plummeted into the ever denser air. At an altitude of 19 km (62,000 feet) and falling at almost 6,400 km per hour (4,000 miles per hour), the X-15-3 was ripped apart by the rapidly increasing aerodynamic pressures and forces which exceeded 8 G. Adams did not eject, probably because he lost consciousness or was otherwise incapacitated, and was killed when the aircraft’s forward section struck the desert floor near Johannesburg, California. Wreckage was found scattered over an area of 130 square kilometers (50 square miles). Adams was posthumously awarded ‘astronaut wings’ for this flight.

There were several concepts for an even more advanced version of the X-15 with delta wings, uprated engines, increased propellant volumes, and structures that could withstand higher temperatures. A plan to launch such an X-15A-3 from the top of a Mach 3 high altitude XB-70 Valkyrie bomber in order to achieve even higher speeds and altitudes came to nothing, primarily due to a lack of funding. In any case, by then the priority was switching to achieving orbital flight by launching capsules or winged vehicles on top of expendable ballistic missiles.

The many accomplishments of the X-15 program include the first application of hypersonic aerodynamics theory and wind tunnel data to an actual flight vehicle, the first use of a reaction control system in space, the first apphcation of a reusable high-

temperature alloy structure, and the development of the first practical full pressure suit for flying in space (the direct ancestor of the suit the Mercury astronauts would wear). The X-15 pilots showed that it was possible to safely land an unpowered plane that had a very poor lift to drag ratio, time after time, and this greatly influenced the Space Shuttle Orbiter concept. Many technologies developed for the X-15 were later incorporated into airplanes, missiles, and spacecraft. Experience gathered during the development of the reusable XLR99, for instance, was extremely useful developing the Space Shuttle Main Engine. The Shuttle also incorporated some key parts made of Inconel X, the ‘super’ alloy that formed the skin of the X-15. The idea of a ground control center actively assisting a pilot during his flight was picked up by the orbital space program, laying the foundation for the famous NASA Mission Control Center that played such a vital role in the Mercury, Gemini, Apollo and Shuttle projects.

The X-15 was also the first aircraft to make extensive use of a ‘man-in-the-loop’ simulator, the so-called ‘Iron Bird’ that allowed pilots and flight planning engineers to test and evaluate flight procedures and explore the aircraft’s behavior whilst safely on ground. The simulator was initially set up with calculated, theoretical figures for the X-15’s flight characteristics but once real flights began it was constantly updated with actual measurements. Nowadays such simulators are used in any new aircraft project and enable designers and pilots to ‘fly’ it long before any hardware leaves the factory. An X-15 pilot typically trained for weeks in the simulator prior to his flight. This was necessary because each mission had its own unique set of requirements (in terms of speed, altitude, attitude, durations of different flight phases, etc) to ensure that between them the missions covered the full flight envelope that the program was meant to explore. Once every movement that was planned for the nominal flight had become second nature, the simulator would run a pilot through strings of unexpected emergencies. A pilot would typically fly 200 simulated missions before taking to the air. Mike Adams actually got so bored with his training sessions that he started to fly them upside down! As the free-flight time was only 8 to 10 minutes, the preparations lasted much longer than the actual mission (as is typical for any type of crewed space flight). Apart from rehearsing in the simulator, new pilots would also make several unpowered X-15 flights to familiarize themselves with the demanding procedures for making a landing.

Arguably, no other aircraft in aviation history has expanded our knowledge about high-speed flight as much as the X-15. During its 199 powered flights it accumulated a total flight time of 30 hours and 14 minutes, of which 9 hours were spent flying faster than Mach 3 (powered as well as gliding) and 82 minutes at speeds over Mach 5. Data was gathered by an array of sensors, telemetered to the ground during flight and recorded for detailed analyses. In addition, the pilots were closely monitored by various sensors, providing the US with the first biomedical data on the effects of weightlessness on the human body. Whether the X-15 was an aircraft able to reach space or a spacecraft with wings remains a matter of opinion, but arguably it was the world’s first reusable spacecraft.

The X-15 program proved key elements of hypersonic theory as it was understood at the time, but also showed several inconsistencies. This led to improved theories for the prediction of lift, drag, stability, control, and temperatures that were

fundamental to developing the Space Shuttle. The data that the three X-15 aircraft gathered is still being used today in the development of new spaceplanes and hypersonic missiles. In fact, because the program provided such a wealth of information, and aerodynamics change little between Mach 6 and orbital velocities, there has been no need for a new X-plane capable of flying faster than the X-15 in the atmosphere. The X-15 data goes so far beyond what is required for the development of a normal aircraft that the data such a successor could yield has not been thought worth the cost up until today.

In addition, the experience gained in the development and flying of the X-15 was of tremendous value for the fledgling US manned space program; in the words of NACA scientist and X-15 advocate John Becker, the project led to “the acquisition of new manned aerospace flight ‘know how’ by many teams in government and industry. They had to learn to work together, face up to unprecedented problems, develop solutions, and make this first manned aerospace project work. These teams were an important national asset in the ensuing space programs.” The experience that North American acquired in developing and building the X-15 helped it to win the contract for the role of prime contractor for the Space Shuttle two decades later.

Because it could fly so incredibly high and fast, the X-15 was also a very useful platform for carrying research experiments not specifically related to aerodynamics. These could be mounted in the cockpit, in a wing-tip pod, in a tail-cone box, or in a special skylight compartment with protective doors just behind the cockpit (giving a free view into space at high altitudes). Many types of experiments were flown, such as micrometeoroid collection pods, astronomical instruments, radiation detectors, star tracker sensors and ablative heat shield samples for the Apollo program, an electric side-stick controller, a landing computer, and high-temperature windows. Especially in the last six years of its operation the X-15 was more used as a platform to support a variety of technology programs than it was for the aerodynamic research for which it was conceived.

The two X-15s that survived the flight program can both be seen in museums: the X-15-1 (56-6670) is in the National Air and Space Museum in central Washington, D. C., hanging from the atrium ceiling close to the X-l. The X-15A-2 (56-6671) is in the Air Force Museum in Dayton, Ohio. There are also mockups at the Dryden Flight Research Center at Edwards, at the Pima Air Museum in Tucson, Arizona, and at the Evergreen Aviation Museum in McMinnville, Oregon.

With the advent of manned spaceflight in the early 1960s, the US Air Force foresaw an important role for its pilots in space, not only as part of the X-15 program but also as astronauts on Gemini and Apollo missions, manning an orbital USAF outpost, and flying military shuttle-like vehicles starting with the X-20 Dyna-Soar (which will be described later). Thus in 1962 the USAF Experimental Flight Test Pilot School at Edwards Air Force Base became the Aerospace Research Pilots School (ARPS) and its training for armed forces test pilots was expanded beyond the traditional aviation curriculum to include an 8-month aerospace course involving spacecraft operation. In line with the school’s change in scope, it soon required a high-performance but low-cost training aircraft that would be able to fly far up into the stratosphere. With such a plane the flight profiles of the X-15 and X-20 could be rehearsed in order to enable test pilot students to familiarize themselves with flight in very rarefied air, where the standard aerodynamic control surfaces become ineffective and control thrusters are needed. Re-entry is a particularly dangerous phase that leaves little margin for pilot error. For the X-15 and indeed any spaceplane, the correct orientation for re-entry into the atmosphere is of paramount importance because only the properly shielded part of the aircraft is able to protect it from the extreme temperatures that it will experience. In the early 1960s there were no computers that could take care of such delicate maneuvering and were also small enough to be accommodated inside an aircraft. The X-15 was flown manually all the way, and it would be the same for the X-20 once this had been inserted in orbit by its launch rocket. The ARPS training planes would also familiarize students with zero – gravity, since during the unpowered ascent and descent in a near-vacuum they would effectively be weightless.

North American proposed a modified version of their X-15 design, adding another cockpit for an instructor and a conventional undercarriage with wheels instead of the usual skids to enable the trainer to take off under its own power. But the X-15 was a complex, expensive machine. A more cost-effective solution was found in the shape of an F-104 Starfighter jet modified for mixed propulsion. An additional benefit of a modified F-104 was that it would enable students to rehearse reigniting a jet engine after a high-altitude parabohc flight during which it would either be deliberately shut down or left to flameout by being starved of oxygen.

The ARPS had already been using standard production Starfighters to simulate the very steep, low-lift and high-drag glide approaches of the X-15 and the planned X-20 Dyna-Soar. This involved climbing to an altitude of 3.7 km (12,000 feet), throttling the jet engine back to 80%, ‘dirtying up’ by extending the flaps, speed brakes and undercarriage to maximize the aerodynamic drag, and then establishing a 30 degree dive. This resulted in a very steep, low lift-over-drag descent similar to that of a spaceplane. (The lift-over-drag, or L/D, ratio is a measure of a plane’s aerodynamic efficiency: a ‘dirtied-up’ Starfighter had a ratio of 2.2, meaning the lift force its wings provided was only 2.2 times the amount of drag the aircraft caused. In contrast, a ‘clean’ Starfighter had an L/D of 9.2. The L/D of a typical airliner is 17, and that of the X-15 was about 4 during its glide phase.) The pilot would pull the nose up for the landing flare a mere 460 meters (1,500 feet) above the ground, which left very little room for error. It was a risky profile but it did prepare pilots for landing future spaceplanes.

The school’s new F-104 space trainers would need to be equipped with a reaction control system similar to that tested by the X-1B and used operationally by the X-15, to control the aircraft at high altitudes and allow students to rehearse attitude control for spacecraft. NASA already had experience in this, having modified an F – 104A in 1959 to use a hydrogen peroxide reaction control system. Following zoom climbs to altitudes up to 25 km (83,000 feet) this gave the NASA Starfighter controllability in the rarefied upper atmosphere. To achieve higher altitudes the ARPS trainer would have an auxiliary rocket engine in the tail.

In 1962 the ARPS awarded Lockheed (the Starfighter manufacturer) a contract to modify three F-104A single-seat fighters for the dedicated role of aerospace trainer. Three existing aircraft were subsequently taken out of long-term storage, relieved of all unnecessary equipment (such as the cannon) to reduce their weight, then equipped with improved instrumentation for high-altitude flight. The reaction control system was based on that of the X-15 and consisted of four 500 Newton thrusters for pitch, four 500 Newton thrusters for yaw and four 190 Newton thrusters for roll. As on the X-15, two thrusters would fire in parallel for every impulse in each direction. The thrusters ran on hydrogen peroxide from a dedicated tank and the pilot controlled them using a handle mounted on the instrument panel. The wings were extended at their tips both to make room for the roll thrusters and to increase lift in order to compensate for the modified aircraft’s greater weight. The standard vertical fin and rudder were substituted by the larger versions of the two – seat F-104 to increase their effectiveness in the thin air of the high stratosphere, and the fiberglass nose radome and its radar were replaced by an aluminum cone that housed the pitch and yaw thrusters. A battery was added to run the onboard systems when the jet engine cut out. A long nose probe was installed to measure the angle of attack and sideslip of the plane with respect to the airflow without disturbance from the flow around the body.

A normal jet engine only operates with subsonic air flowing into it. On the basic Starfighter, shock cones were installed in front of the air intakes to guarantee that at supersonic speeds a shock wave formed. This slowed the air passing through it into the intakes to subsonic speed. The inlet shock cones on a standard Starfighter created a proper shock shape up to Mach 2 but extensions were fitted to these cones because the new trainer would fly faster than that. Because the jet engine would not operate in very thin air and thus could not provide ‘bleed’ air to pressurize the cockpit at high altitude, an additional pressurization system was needed. The pilot would be wearing a pressure suit that would inflate in an emergency but an inflated suit would seriously hamper the precise control required from the pilot during normal operation, so it was decided to fully pressurize the entire cockpit using nitrogen from an added gas tank (oxygen would have offered the benefit that the pilot could breathe it, but would have been a serious fire hazard).

The F-104As were equipped with a standard J79 jet engine that gave the plane a normal maximum thrust of 43,000 Newton at sea level, which could be increased to

67,0 Newton on afterburner. For the required extra boost a compact Rocketdyne AR2-3 (LR121-NA-1) rocket engine was installed at the base of the vertical tail, just above the jet’s exhaust. It was canted slightly so that its thrust was aimed through the plane’s center of gravity and thus would not continuously push the nose down. This engine ran on a mixture of standard JP-4 kerosene jet fuel drawn from the aircraft’s standard fuel tank and 90% concentrated hydrogen peroxide oxidizer. It provided a thrust of 27,000 Newton and could be restarted and throttled in the 50% to 100% range using a specific throttle lever on the left side of the cockpit. The aircraft carried sufficient oxidizer for about 90 seconds of full-thrust rocket operation. With the need

to replace some of the AR2-3’s parts after an hour of operation some 40 flights could be made before a minor overhaul was required (a major overhaul was required after 2 hours and the total life of an engine was 4 hours). The heavily modified Starfighter design was designated the NF-104A (with the ‘N’ standing for ‘Nonstandard’) AeroSpace Trainer (AST).

Every aircraft has a so-called service ceiling, which is the maximum altitude at which it can operate under normal conditions and whilst in steady, horizontal flight. To reach altitudes that are well beyond their service ceiling for brief periods of time jet fighters use so-called zoom climbs in which the plane accelerates horizontally to great speed and then pitches up into a steep climb. Zoom climbs enable aircraft to exploit the good performance of their jet engine at relatively low altitudes to build up speed which can then be traded for height on the way up (the thrust rapidly diminishes due to a lack of air as the altitude increases). The NF-104A pilots used a similar approach but they had the benefit that their additional rocket engine would not lose thrust in the thin air of the stratosphere. An experienced NF-104A test pilot would typically use his jet engine with afterburner to accelerate to Mach 1.9 at an altitude of 11 km (35,000 feet), then ignite the rocket engine to full thrust. Shortly afterwards he would reach Mach 2.2, whereupon he would pitch the aircraft sharply up into a 70 degree climb at 3.5 G. As the amount of cooling air flowing through the engine dropped with the decreasing atmospheric density the jet engine’s temperature would gradually increase. To prevent this from reaching levels that would impair the engine’s structure the pilot would start to throttle down the afterburner at an altitude of about 21 km (70,000 feet) and completely shut down the engine at around 26 km (85,000 feet). After the rocket engine had depleted its hydrogen peroxide the aircraft would continue to climb ballistically farther into the stratosphere until its vertical

rate reached zero. Then it would fall back into the denser air, where the jet engine would be restarted.

During the ballistic climb and descent the aircraft and pilot would effectively be weightless. The pilot remained strapped in but would feel an absence of body weight against his seat, and loose objects would fly through the cabin. However, he had little time to enjoy this experience or the view because during this phase he would need to use the reaction control system to nose the aircraft over through 140 degrees in order to cancel the climb attitude and push the nose 70 degrees down for re-entry into the atmosphere whilst maintaining zero roll and yaw. This ensured a stable position in the thicker atmosphere and also that sufficient air would flow into the jet’s intakes to windmill its compressor and thereby enable it to be restarted. A poor entry attitude would result in a pilot soon finding himself in an out-of-control aircraft and unable to restart the engine (as occurred to Chuck Yeager during a particularly hairy NF – 104A flight, as we will see later).

The Bell X-2 had reached altitudes very similar to those planned for the NF-104A but unlike the long, thin Starfighter with its tiny wings the X-2 was an excellent and stable glider designed to land without engine power. An additional complication that did not affect the all-rocket X-2 was the gyroscopic effect of the still rapidly turning turbine and compressor of the extinguished jet engine: this effect resisted the reaction control thrusters, and required careful compensation by the pilot whilst executing the pitch over maneuver.

The F-104 itself was, even by today’s standards, an impressive high-performance aircraft and with the additional rocket engine it truly became a near-spaceplane. But it was definitely something for experienced pilots (as all ARPS students were). The normal Starfighter was so difficult to handle that it had gained the nickname ‘Widow Maker’. The West German Air Force bought 917 of them in the early 1960s and by 1976 had lost 178 in accidents. The added rocket engine and the need to operate the reaction control system in the high stratosphere made it even more dangerous to fly. During the steep climb the plane’s attitude and the pilot’s rigid helmet meant that the horizon could not be seen, so all critical stability control and maneuvering had to be performed on cockpit instrumentation only. Furthermore the NF-104A was a single­seat aircraft so there was no instructor in the back ready to take over if things went wrong. To ready a student for this challenging aircraft he would first make a flight in a standard F-104 with his pressure suit fully inflated to familiarize himself with the movement restrictions this would impose in an accidental cockpit depressurization. Next he would make a zoom flight in a conventional Starfighter trainer supervised by an instructor in the rear seat. Following 4 hours of rehearsal in a flight simulator he would execute three solo zoom flights in a standard F-104 while being coached by an instructor in the back of an accompanying two-seat trainer. During these flights he would gradually build up the climb angle: 30 degrees on the first flight, 40 degrees on the second, and finally 45 degrees. Only then would he be deemed ready to zoom the rocket-equipped NF-104A, and even then the maximum allowed climb angle was 50 degrees (the optimum for reaching extreme altitudes was 70 degrees).

The first NF-104A (56-0756) was tested by Lockheed’s test pilot Jack Woodman and Major Robert W. Smith of the Test Division of the Air Force Flight Test Center at Lockheed’s factory near Palmdale Airport, prior to its formal handover to the Air Force. During this phase Smith broke the standing altitude record by zooming up to an astonishing altitude of 36,230 meters (118,860 feet) on 22 October 1963. During this flight he managed to keep the aircraft under control even though all three axes of the reaction control system had accidentally been wired incorrectly! After acceptance by the Air Force, and during the next phase of testing at Edwards, Smith surpassed his record by achieving an altitude of 36,800 meters (120,800 feet) on 6 December, and the unpowered parabolic arc provided no less than 73 seconds of weightlessness. Nearly half a century later, Smith’s achievement still stands as the highest altitude ever achieved by a US aircraft taking off from a runway. Although the plane left the ground under its own power (unlike the X-planes) and the altitudes achieved were accurately recorded by ground stations equipped with radar and powerful telescopes, both records remained unofficial because the Air Force had not requested the flights to be monitored by the International Aeronautical Federation.

The second NF-104A (56-0760) was delivered 25 days after the first, and the third and final aircraft (56-0762) on 1 November 1963. Unfortunately the third plane was lost barely a month later, on 10 December, when it crashed during a flight piloted by Chuck Yeager, who was Commander of the Aerospace Research Pilots School at the time. According to him, he was unable to push the nose back down once he reached the zenith of the zoom climb, possibly due to a malfunction of the reaction control system, causing the Starfighter to go into a disastrous flat spin at an altitude of 33 km (109,000 feet). During a normal descent the aircraft would be orientated (using the reaction control system) so that air would flow into the jet engine’s intakes and make its compressor windmill, thus providing hydraulic pressure for activating the control surfaces and enabling the engine to be restarted. However, in the flat spin no air was flowing into the engine, so there was no power to control the ailerons, elevators and rudder and no means to restart the engine. As the aircraft plummeted from the sky, Yeager had no option but to abandon the aircraft. He ejected just 2.6 km (8,500 feet) short of hitting the ground, was struck by his own discarded ejection seat on the way down and was badly burned by its glowing solid rocket motor, but managed to land by parachute. This is depicted in the movie The Right Stuff, although it shows him flying a standard F-104G without a rocket engine.

The ensuing Air Force investigation cleared Yeager of responsibility for the crash, blaming the accident on an aircraft malfunction. However, the NF-104’s primary Air Force test pilot Major Robert Smith, who had trained Yeager to fly the profile, insists that Yeager simply did not perform a proper zoom climb, pulling up to the full 70 degree climb angle too slow and too late. This meant he ran out of speed before reaching the intended maximum altitude, was too late trying to nose the aircraft down, and hence began to fall with the nose still 70 degrees up. Yeager may have believed he was still climbing because the rocket was still operating, but by then the thrust of the rocket alone was insufficient to prevent the plane from falling back essentially tail first. Still in relatively dense air, no amount of reaction control thrust could have maneuvered him out of the attitude in which it was impossible to restart the jet engine. According to Smith, Yeager’s fame and influence meant that the investigation ruled in his favor and unjustly labeled the NF-104A a dangerous aircraft. The differing accounts of this incident, Yeager in his famous autobiography and Smith on his highly informative NF-104A website, don’t even agree on the purpose of the disastrous flight: according to Yeager it was part of his investigation of a known pitch-up problem of the aircraft (caused by the T-tail being masked by the wings at high angles of attack, preventing the airflow from reaching the horizontal stabilizers; the same problem that troubled the British SR. 53) while Smith insists Yeager was merely trying to break the altitude record and had been assigned by the Air Force to fly the NF-104A solely for this purpose.

After the accident a restricted flight regime was enforced to ensure safe flights for the ARPS students using the two remaining NF-104As, part of which was imposing a limit of 50 degrees on the chmb angle (again, unjustly according to Smith, which in his opinion left the students with little opportunity to experience the peculiarities of reaction control at really extreme altitudes).

The remaining aircraft were used to train students, but not very often and only for zooming flights to relatively low altitudes. The dangerous hydrogen peroxide caused some trouble, as was to be expected from experiences with earlier hydrogen peroxide powered rocket aircraft. Once a tail tank ruptured on the ground and another time a small explosion occurred in a wing while in flight, both caused by hydrogen peroxide reacting with metal aircraft parts. Modifications where made to prevent recurrences but the 56-0756 suffered an inflight rocket motor explosion in June 1971 owing to a hydrogen peroxide leak. The rocket engine and most of the rudder where blown off but the student was able to land safely. The seriously damaged aircraft was scrapped.

By then, however, the Air Force’s human spaceflight ambitions had withered: the X-15 program was over, the X-20 had long since been canceled, and the task of the planned Manned Orbiting Laboratory (MOL), namely military reconnaissance, could be done better and at far lower cost by unmanned spy satellites; MOL was canceled in 1969. The Space Shuttle would not fly for another decade and it would be run by NASA. The remaining NF-104A was therefore retired, mounted on a pole and placed outside the Air Force Test Pilot School where it can still be seen today.

Various parts of this aircraft, including the extended wing tips and the metal nose cone, were loaned to Daryl Greenamyer for his civilian aviation record attempts with a highly modified Starfighter that was based on equipment from various F-104s. In 1977, after a practice zoom flight working up to his altitude record attempt, one main wheel of the plane’s undercarriage did not completely deploy and Greenamyer had to eject. The NF-104A parts were lost along with the rest of his aircraft.

Several test pilots who flew the NF-104A before the planes were handed over to the ARPS experienced severe difficulties in controlling the aircraft. To achieve high altitudes and to allow sufficient time in near-vacuum for the very large (140 degree) change in pitch angle required the pilot to fly a very precise zoom maneuver. The initial speed, the fast pull-up maneuver to the 70 degree climb angle, and maintaining this angle, were all extremely important. Near the peak of its parabohc trajectory the NF-104A moved from the aerodynamic control region into the space control region and back in less than a minute, giving the pilot little time to transition from the well – known aerodynamic controls to the less familiar reaction controls and back again.

Nevertheless, according to the Air Force’s primary test pilot, Robert Smith, it was

not a particularly dangerous aircraft to fly as long as the pilot flew the proper zoom trajectory, had sufficient understanding of the peculiarities of reaction control at such altitudes, and did not attempt to push the aircraft beyond its established boundaries. Indeed, X-15 test pilot Bob Rushworth flew the NF-104A to the impressive altitude of 34 km (112,000 feet) without trouble on his first and only flight in it. In total some 50 pilots flew the NF-104A during 302 flights and accumulated a total of 8.6 hours of rocket engine operation.