Category Rocketing into the Future

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

In preparation for the development of a reusable space glider or space shuttle, in the early 1960s NASA started to investigate so-called ‘lifting body’ aircraft. As the name implies, such planes have fuselages shaped to provide all or most of the necessary lift, dispensing with the need for large wings. For a glider or spaceplane returning from orbit such a configuration was thought to be ideal: wings need to be relatively thin, making it hard for them to handle the brutal aerodynamic forces of hypersonic flight and the extreme heat of re-entry, whereas a lifting body can be very robust and have a large volume relative to the surface that is heated up. Keeping a lifting body cool and in one piece is therefore theoretically simpler than preventing protruding wings from melting or being ripped off at hypersonic speeds. A lifting body basically combines the robustness and structural simplicity of a ballistic capsule with the maneuverability, flight range and landing accuracy of an aircraft. The questions to be answered were whether a lifting body was sufficiently controllable at all speeds and could be landed safely.

In 1962 the NASA Flight Research Center at Edwards (now called Dryden Flight Research Center) approved a program to build a very simple, unpowered lifting body prototype for low-speed flight tests on a shoe-string budget of $30,000; equivalent to about $220,000 in 2011. It had to be so lightweight that it could be towed into the air behind a car for the initial take-off and low-speed flights. In this mode it would also be used to train pilots, before progressing to towing to higher altitudes with Dryden’s C-47 transport aircraft and release for gliding trials. The resulting contraption, which looked like a horizontal cone cut in half, was designated the M2-F1 with the ‘M’ referring to ‘Manned’ and ‘F’ indicating it concerned a ‘Flight’ version. However, its unusual shape quickly earned it the nickname ‘Flying Bathtub’.

The M2-Fl’s structure consisted of a tubular steel frame made by Dryden, which was then covered over with a plywood shell by the Briegleg Glider Company, a local glider manufacturer. The fixed undercarriage was taken from a Cessna sports plane. The Uttle aircraft had a maximum take-off weight of about 570 kg (1,250 pounds).

For towing the M2-F1 over the hard, flat surface of Rogers Dry Lake a powerful and fast, but not very expensive car was required. Members of the flight test team bought a Pontiac Catalina convertible with the largest engine available, which was then fitted with a special gearbox and racing slicks by a renowned hot-rod shop near Long Beach. With these modifications the Pontiac could tow the M2-F1 into the air

The M2-F1 in tow behind a C-47 [NASA],

in 30 seconds at a speed of 180 km per hour (110 miles per hour). The first car-tow test run in April 1963 did not go all that well because the M2-F1 started to bounce uncontrollably on its two main wheels the moment that NASA research (and X-15) pilot Milt Thompson raised the nose off the ground. The problem was quickly found to be in the rudder control and was fixed. About 400 successful car-tow tests were made, all with Thompson at the controls. They produced enough flight data about the aircraft to proceed with flights behind the C-47 starting in August of that same year. For this the M2-F1 was equipped with a simple ejection seat as well as a small solid propellant rocket motor in the rear base. If required, this “instant L/D rocket” could provide a thrust of 300 Newton for 10 seconds; just enough to keep the plane in the air for a little bit longer if the pilot were to find himself descending too rapidly just prior to touchdown. The tow plane typically released the M2-F1 at an altitude of 3.7 km (12,000 feet) for a 2 minute glide down to Rogers Dry Lake at a speed of 180 to 190 km per hour (110 to 120 miles per hour). Apart from Thompson, who flew most of the glide tests, several other pilots took the controls of this strange aircraft, among them the famous test pilot Chuck Yeager.

A total of 77 aircraft-tow flights were performed, the success of which convinced NASA and the Air Force of the feasibility of the lifting body concept for horizontal landings of atmospheric entry vehicles. The solid rocket motor was only used once, on the last flight when USAF pilot Captain Jerauld Gentry accidentally rolled the relatively unstable M2-F1 onto its back just after take-off. Flying inverted behind the C-47 a mere 100 meters (300 feet) above the lakebed he released the tow line, finished the barrel roll into level flight, fired the rocket and made a perfect landing. The roll instability was caused by a lack of wingspan.

The success of the M2-F1 encouraged NASA to put some real money into lifting body research and gave rise to both the M2-F2 by NASA Ames Research Center and the HL-10 by NASA Langley Research Center (with ‘HL’ for ‘Horizontal Landing’ and TO’ referring to the design number). Unlike the lightweight M2-F1 glider these new aircraft were all-metal and were fitted with an XLR11 rocket engine (previously used in the X-l series, the D-558-2 and early X-15 missions) for testing their lifting body shapes at high speeds and high altitudes. As with the rocket propelled X-planes the XLR11 was used for a short powered flight phase, after which the plane would glide to a landing. In emergencies the engine could be reignited just before landing, eliminating the need for the solid propellant rocket of the M2-F1.

To be able to attain high speeds and long flight times the rocket propelled lifting bodies were carried to about 14 km (45,000 feet) under the wing of a B-52 and then released at a speed of about 720 km per hour (450 miles per hour). In fact they used the same carrier aircraft as the X-15, whose flight program was running concurrently. Special adapters were designed to enable the lifting bodies to use the wing pylon that was developed to carry the X-15. Both the M2-F2 and the HL-10 were equipped with pressurized cockpits and ejection seats.

The M2-F2 was built by Northrop and was similar in shape to the M2-F1. It made its first unpowered free flight on 12 July 1966 with Milt Thompson in the cockpit. Another 14 successful glide flights followed and revealed that, just like the M2-F1, the aircraft had a stability problem which often caused it to violently oscillate in roll during the ascent. The last planned unpowered flight in May 1967 ended in disaster when, just prior to landing, Bruce Peterson suffered from a pilot – induced oscillation problem. The situation was similar to that experienced by Scott Crossfield during his first X-15 flight, except that rather than a pitch oscillation the M2-F2’s problem was roll control. With the craft rolling from side to side and also having to avoid a rescue helicopter that was in his way, Peterson fired the XLR11 rocket engine to prolong the landing approach but nevertheless smashed onto the lake bed before the landing gear was fully down and locked. The aircraft skidded

across the ground in a cloud of dust, rolled over six times and came to rest upside down. Peterson was severely injured; he later recovered but had permanently lost the vision in his right eye. Some of the film footage of the crash was used by the 1970s’ TV series The Six Million Dollar Man. It took three years to rebuild and improve the M2-F2, which was redesignated the M2-F3. The most important modification was the addition of a third, central vertical fin to improve low-speed roll control. NASA pilot Bill Dana made the M2-F3’s first glide flight on 2 June 1970 and found it to possess much better lateral stability and control characteristics than the M2-F2. After only three glide flights the aircraft was taken on its first powered flight on 25

November. The M2-F3 subsequently made a total of 24 flights under rocket thrust, during which it attained a top speed of Mach 1.6 and, on its last flight in December 1972, a maximum altitude of 21.8 km (71,500 feet). This unique vehicle is now hanging in the Smithsonian Air and Space Museum in central Washington D. C.

Northrop also built the HL-10, which made its first glide flight in December 1966 with Bruce Peterson at the controls. Peterson found the plane to be very unstable and only managed to maintain control by keeping the speed up: once again actual flight testing had proven its worth as proof-of-concept of aerodynamics and control theory. The HL-10 was grounded while NASA engineers studied data recorded during the flight, as well as from additional wind tunnel tests. It was found that so – called flow separation at the outboard fins was the culprit: because the air was not flowing over them properly they were ineffective in providing lateral stability. The leading edges of the outboard fins were modified and the ensuing glide tests established that this fix worked. After 11 unpowered flights the first powered flight was made on 23 October 1968 by Jerauld Gentry, the pilot who made the last (and unusually exciting) flight of the M2-F1. During this first powered flight the XLR11 engine malfunctioned shortly after launch, forcing him to jettison the remaining propellant and make an emergency landing on a conveniently located dry lake bed. On 13 November NASA pilot John Manke made the first successful powered flight. The HL-10 was flown a total of 37 times, logging (on different flights) the highest speed and altitude in the entire lifting body program: a top speed of Mach 1.86 and a maximum altitude of 27,440 meters (90,030 feet). Although it had its share of teething problems, pilots eventually found the HL-10 to be more stable and easier to fly than the M2-F3 and in this respect also better than the later X-24A (see below). On its two last flights the XLR11 engine was replaced with three small hydrogen – peroxide engines that provided a continuous low thrust of 4,000 Newton in total during the final approach for landing, reducing the glide angle from 18 down to 6 degrees. However, it was found that this provided few benefits over a completely unpowered glide landing and led to the conclusion that adding low-thrust landing engines to aircraft with relatively low lift over drag ratios didn’t make much sense. It added complexity and weight, offered little assistance to the pilot, and the weak thrust did not enable him to abort a landing and fly around for another try. This HL-10 result later helped engineers to decide not to put any landing engines on the Space Shuttle Orbiter.

In his book Wingless Flight: The Lifting Body Story, HL-10 engineer Dale Reed describes a plan that he proposed for sending this vehicle into orbit. It would require to be fitted with reaction control thrusters for attitude control in space and an ablative heat shield for re-entry. It would be launched unmanned on a Saturn Y moonrocket along with a manned Apollo capsule; the HL-10 would basically take the place of the lunar module in the adapter of the upper stage. Once in orbit, one of the astronauts would make a spacewalk from the Apollo capsule and enter the cockpit of the lifting body. On the first of two such missions the pilot would make in­orbit checks of the vehicle and return to his spacecraft, whereupon the HL-10 would return to Earth automatically. On the second mission an astronaut would actually pilot the vehicle all the way back to Edwards. This very adventurous plan was never

implemented, although Wernher von Braun was apparently enthusiastic about using his Saturn Y for these missions.

The HL-10 can be found guarding the entrance of NASA Dryden Flight Research Center, mounted on a pedestal as if coming in for a landing. The Six Million Dollar Man also used footage of the HL-10.

The M2-F2 and HL-10 were followed by the joint USAF/NASA X-24A. It was built by the Martin Aircraft Company and looked somewhat like a fat version of the M2-F3 with a curved back, three vertical fins and an XLR11 engine. Jerauld Gentry piloted this “potato with three fins” on its first unpowered flight on 17 April 1969, as well as on its first powered flight on 19 March the next year (it thus flew before the wrecked M2-F2 reappeared as the M2-F3). During its flight test phase this aircraft was flown 28 times, achieving a top speed of Mach 1.6 and a maximum altitude of

21.8 km (71,400 feet).

The Martin Marietta Corporation (which the Martin Aircraft Company became) went on to strip the X-24A down and rebuild it as the X-24B, which looked rather different. Whereas the X-24A was round and fat the X-24B had a triangular ‘double delta’ shape with a flat bottom and pointed nose, and was affectionately called the

‘Flat Iron’. The double-delta planform meant it had delta wings which incorporated a bend (as on the Space Shuttle). This shape, derived from a study by the Air Force Flight Dynamics Laboratory of possible future re-entry vehicles, resulted in a more stable aircraft with a much better lift over drag ratio owing to its greater area of lift­generating surface. The reuse of much of the X-24A’s equipment and airframe saved the Air Force a lot of money in comparison to what it would have cost to build a new vehicle from scratch.

The first to take the X-24B up (or rather, down) for a glide flight was NASA pilot John Manke on 1 August 1973, and he was also at the controls on the first powered flight on 15 November. During its total of 36 flights the X-24B managed to reach a speed of Mach 1.76 and a maximum altitude of 22.6 km (74,100 feet). The X-24B also made two landings on the main concrete runway of Edwards to demonstrate that accurate runway landings were possible for a lifting body glider with a low lift over drag ratio (it had nose-wheel steering, unlike the X-15 and the other lifting bodies).

In 1975 Bill Dana made the final flight of the X-24B, drawing to a close not only the lifting body test program but also rocket aircraft flying at Edwards in general. For the occasion the team prepared a sign depicting the X-l and the X-24B and the text “End of an era; Sept. 23, 1975; Last rocket flight.” It truly was the end of one of the most interesting periods in aviation history, with rocket planes pushing technology, flight speeds and altitudes to levels that had been mere dreams when the X-l broke the speed of sound almost three decades earlier.

There were various proposals for an X-24C, including one by the Lockheed

Skunk Works for an aircraft that would use scramjets (able to function at higher speeds than a ramjet) to reach Mach 8, but in the post-Vietnam era the mihtary had Uttle money to spare and NASA was developing the Space Shuttle. The lifting body program thus ended with the last flight of the X-24B, after which the aircraft found a home in the National Museum of the US Air Force at Wright-Patterson Air Force Base. Although the original X-24A no longer exists, a very similar vehicle was put next to the X-24B in the museum to represent it. It is not a replica, but actually a conversion of a never-flown, jet-powered version of the X-24A lifting body called the SV-5J.

The lifting body program taught NASA invaluable lessons for the Space Shuttle, which it started to develop in the early 1970s. Although it was decided that the Space Shuttle Orbiter would have a relatively conventional fuselage with wings, rather than a lifting body shape, its low lift to drag ratio would produce a similarly steep gliding descent for landing. The lifting body flights showed that accurate and safe landings could be made with such a vehicle, without the need for a means of propulsion. The earher planned jet engines for the landing were discarded, simplifying the design and lowering the vehicle’s weight. The Orbiter went on to routinely land on the runway at Kennedy Space Center, and its pilots never found themselves wishing they had the jet engines available.

In the 1990s the shape of the X-24A returned in the form of the X-38 technology demonstrator that was expected to lead to a Crew Return Vehicle to enable astronauts aboard the International Space Station to return to Earth in an emergency. The X-38 made several unmanned ghde test flights after release from a B – 52 but the program was canceled in 2002. Currently the SpaceDev company is developing the somewhat similar Dream Chaser mini-shuttle partly funded by NASA under the Commercial Orbital Transportation Services program intended to encourage private companies to develop space transportation vehicles for servicing the International Space Station. If introduced, the Dream Chaser lifting-body vehicle will be launched atop an Atlas Y rocket to take crew and cargo into low orbit.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Even before the X-15 took to the air the Air Force, NACA and North American were making plans for an orbital version. This X-15B would be launched using a multi-stage rocket derived from the launch booster of the SM-64 Navaho missile; a project that had just been canceled and had left North American with a warehouse of rocket boosters. But the X-15B was canceled when Project Mercury was approved, a much simpler capsule concept that promised early results in the developing space race with the Soviets. Then the national goal of being first to land a man on the Moon gave rise to the Gemini and Apollo capsule-style spacecraft.

However, the Air Force, regarding a capsule as merely a step on the way to more routine access to space, saw the benefit of a reusable shuttle-type vehicle for manned missions. During the 1950s Werner von Braun had proposed a reusable canard space glider to be launched atop a rocket using two expendable stages. This was explained in Collier’s magazine in 1952, in one of a series of articles on future spaceflight that von Braun wrote with Willy Ley between 1952 and 1954. Collier’s printed 4 milhon copies per issue, so these articles did much to spur enthusiasm for spaceflight in the US. The articles were enlivened by beautifully detailed illustrations by leading space artists, including Chesley Bonestell, that effectively dramatized von Braun’s manned spaceflight development blueprint for the general public. Further publicity came soon afterwards with von Braun presenting a hugely popular three-part Disney television show on the future of space travel, which also featured his designs. At one point von Braun presents his winged spacecraft design: “Now here is my design for a four-stage orbital rocketship. First we would design and build the fourth stage and then tow it into the air to test it as glider. This is the section that must ultimately return the men to the Earth safely.” His ‘Ferry Rocket’ is basically an upper rocket stage with multiple engines and long, slightly swept-back wings, each with an elongated vertical stabilizer mid-way. Large horizontal stabilizers were fitted to the nose, resulting in a canard design. The launcher’s first stage had huge fins as well, not for flying but for counteracting the imbalance caused by the rocket glider’s wings on top of the rocket, as otherwise a slight wind during take-off or buffeting while ascending through the atmosphere would easily blow the whole assembly off course.

By 1959 the Air Force was promoting a new program as the means of performing military manned space missions. Like von Braun’s concept, this ‘Dyna-Soar’ would not be a real rocket plane but a reusable space glider (its name was a contraction of Dynamic Soaring) bolted to the nose of a conventional launch rocket. It would only use its wings during the unpowered descent back through the atmosphere and make a controlled landing near its launch site. There it would be quickly readied for its next launch on a new expendable rocket. In its operational form Dyna-Soar would be able to perform all kinds of missions and even put things into orbit because it would have a cargo bay; it was basically to be a small, early version of the Space Shuttle.

In contrast, Mercury and the later Gemini and Apollo spacecraft were completely single-use; very Uttle of what was launched would come back, and with their ablative heat shields that burned away during re-entry the capsules were not reusable. They also had very little means of maneuvering once they started their ballistic fall back to Earth and at best could be expected to land within a radius of several kilometers of a specific point. Because of this uncertainty in the landing spot, as well as to ensure a soft landing, they had to come down in the ocean, which meant a fleet of search and recovery ships was required. The X-20 pilot would fly his craft back to its airbase, making the return much more economical. Of course a controllable, winged machine was also much more appealing to the Air Force than a ‘spam-in-a-can’ capsule using parachutes. It facilitated a dignified landing on a runway, rather than an inglorious splash into the ocean.

The Air Force forecast numerous versions and missions for the X-20, involving payloads for gathering aerodynamic flight data, satellite inspection, electronic and photographic intelligence, and even dropping nuclear bombs with greater precision than was possible using a balhstic missile! Dyna-Soar was to be a research aircraft, spy plane, orbital bomber and transportation shuttle all in one.

In June 1959 Boeing was awarded the development contract. Boeing’s design was a 5,200 kg (11,400 pounds) delta-winged vehicle with large vertical winglets instead of a more conventional tail for lateral (yaw) control. Most of the internal structure as well as the upper surface was to be made from Rene 41, a ‘super alloy’ that was able to withstand extreme temperatures. However this material would not suffice for the areas that would see the highest temperatures during re-entry, when slamming into the atmosphere at some 28,000 km per hour (17,000 miles per hour), or Mach 28. Capsule spacecraft solved this issue by using expendable ablative heat shields, but as the X-20 was to be a completely reusable space glider its underside would require to be made by placing molybdenum sheets over insulated Rene 41, while its nose-cone was to be made from pure graphite incorporating zirconia rods.

The X-20 would be controlled by a single pilot. Behind him was an equipment bay that could contain either data-collection equipment, reconnaissance equipment, weapons, or seats for up to another four astronauts. The X-20 would be connected to a small rocket stage to enable the craft to shoot itself away from the Titan III booster during launch in case of an abort or, once in space, to change its orbit. At the end of a mission this ‘transition stage’ would fire its main rocket engines against the velocity vector so that the X-20 would fall back to Earth. It would then be jettisoned and the aircraft would descend through the atmosphere and use aerodynamic drag to further slow down (‘aerobraking’). During the initial, high-temperature re-entry, the pilot’s windows would be protected by an opaque heat shield that could be jettisoned once the aerothermodynamic onslaught was over. Because rubber tires would burn during re-entry, the X-20 would land using wire-brush skids made of Rene 41. The Titan III

launcher would be based on the Titan intercontinental ballistic missile and be fitted with exceptionally large stabilizing fins to compensate for the disturbances caused by the X-20’s wings on top of the assembly during ascent through the atmosphere.

In April 1960 seven astronauts were secretly chosen to fly the X-20, among them (future) X-15 pilots Neil Armstrong, Bill Dana, Pete Knight and Milt Thompson.

But the X-20 Dyna-Soar never made it out of the factory, let alone into orbit. In the early 1960s it was already becoming evident that many of its foreseen missions could be performed sooner and more cheaply by unmanned spacecraft or manned capsules. The high cost and questionable military utility led to the cancellation of the program in December 1963. This made sense at the time in terms of immediate technological and military priorities, but in hindsight it is definitely a pity. Although the X-20 was a vertically-launched reusable glider with some orbital maneuvering capabihty rather than a real rocket plane, it was a logical next step in the evolution of rocket aircraft. The X-15 was a true rocket plane, but suborbital. It could reach orbital altitudes but lacked the propellant to accelerate to orbital speed. With the X – 20 the Air Force was developing the technology needed to fly a plane from orbit back to Earth, with all the aerodynamic and thermodynamic complications, and some new materials capable of withstanding extreme temperatures. Had the X-20 continued, it would have delivered valuable experience that could have made the later Space Shuttle a more economical launch vehicle and perhaps opened the way to fully reusable spaceplanes.

Conceptually, a follow-on to the X-20 could have been to replace the expendable launching rocket with a reusable rocket powered carrier aircraft to create a two-stage rocket plane (similar to the X-15B/Valkyrie combination but with the carrier aircraft able to fly higher and faster than the Valkyrie, and with the secondary vehicle able to

boost itself into orbit). That would have been considerably simpler than developing a single-stage rocket spaceplane, whilst retaining the benefits of an all-reusable system with (hopefully) aircraft-like operations. Prior to the cancellation of the X-20 there were many outline designs for orbital rocket plane and shuttle-type vehicles, most of them involving multiple stages. There were all kinds of combinations: orbital gliders with or without integrated rocket stages; with expendable or reusable, winged or non-winged stages using either liquid or solid propellants; and with horizontal or vertical take-offs.

Martin offered the very ambitious ‘Astroplane’, a horizontal-take-off, horizontal­landing, single stage spaceplane which, powered by “nuclear magnetohydrodynamic engines”, sounds like something straight out of Star Trek. This intriguing propulsion system would extract nitrogen from the atmosphere, rapidly cool it, then accelerate the resulting liquid using powerful electromagnetic forces generated by an onboard nuclear reactor. The vehicle had a long, slim shape that would have been perfect for low drag at high speed but unsuitable for horizontal runway landings at reasonable speeds. The designers therefore envisaged deployable wings that would be extended at low speeds to provide additional lift. The bat-like airfoils would be composed of rigid ribs with a flexible membrane stretched between them. What would happen if a wing failed to deploy properly and this flying radiation hazard fell out of the sky was obviously something that would have to be worked on a bit.

In general, however, it was understood that without resorting to exotic, far­fetched propulsion systems, a reusable launcher would have to be a multi-stage vehicle. For conventional multi-stage expendable launchers the useful payload that can be placed into orbit typically represents only 3% of the total weight of the rocket that leaves the pad, and around 18% in terms of hardware (i. e. without propellant). A single-stage vehicle cannot shed the weight of empty tanks and no-longer-needed rocket engines on the way up, so its payload is necessarily considerably less than for a multi-stage launcher. The wings, heat shields and control systems that are required to enable a single-stage vehicle to return to Earth can all too easily reduce its payload to zero, or even to negative values implying the vehicle will not even be able to reach orbit. To compensate, Single Stage To Orbit (SSTO) vehicles must carry more propellant per unit of hardware, which leads to larger vehicles. All this makes the development of a single-stage spaceplane with a reasonable payload extremely challenging, expensive and risky, and certainly beyond the technology of the 1960s and 1970s (and indeed beyond today’s technical capabilities; more on this later in this book).

The Martin ‘Astrorocket’ concept studied in the early 1960s therefore consisted of two vertically launched, winged rocket stages mated belly to belly. The task of the larger vehicle was to get its smaller sibling to high altitude and high speed, release it and then glide back to Earth while the second stage continued into orbit. The orbital vehicle would later make a gliding return (very much like the Space Shuttle) and be reunited with the first stage and prepared for another mission. This VTHL (Vertical Take-off and Horizontal Landing) concept would be proposed many times in various forms and by several different companies.

Douglas offered the ‘Astro’ design. This was based on the same idea but with the orbital vehicle mounted on the nose of the carrier booster rather than attached to its belly, and with both vehicles having lifting body configurations.

Lockheed’s ‘Reusable Orbital Carrier’, which was studied at about the same time, also involved two winged rockets but this combination would take off horizontally using an accelerated sled. The orbital vehicle of this HTHL (Horizontal Take-off and Horizontal Landing) launcher would ride on the back of its large carrier stage, which would use jet engines to return to its launch base. Interestingly NASA had stated in the specifications for this carrier stage that it “should offer a potential commercial application in the late 1970s, such as operating the vehicle over global distances for surface-to-surface transport of cargo and personnel”. In other words, it must be able to be converted into a hypersonic, rocket propelled airliner; a kind of super – Concorde similar to that proposed by Max Valier in the 1930s.

Lockheed and North American both had similar concepts for three instead of two winged stages, which meant more complicated operations but less stringent vehicle mass-minimization requirements. A proposed further development of Lockheed’s ‘System ІІГ would involve a giant first stage booster aircraft with ramjet propulsion and was called ‘System IV’ (System I was the Apollo Saturn IB combination as later used on the Apollo 7 mission, while System II would consist of a Saturn IB rocket with a reusable 10-man spaceplane that would also form the basis for the System III and System IV orbital stages).

In the mid-1960s NASA, along with the Department of Defense, adopted a similar phased approach involving a ‘Class ІІГ vehicle based on the Lockheed’s System IV design but with a hypersonic booster aircraft powered by combined turbofan-ramjet-scramjet engines. Such engines would run as regular turbojets at subsonic and low supersonic speeds, and as ramjets at supersonic velocities where the airstream could still be slowed down to subsonic speeds; at hypersonic flying velocities the airflow through the engine would be supersonic and so the engines would go into scramjet mode. A scramjet (meaning ‘supersonic combustion ramjet’) is a ramjet in which the combustion takes place in a supersonic airflow so that the air coming into the engine does not need to be slowed down to subsonic speeds (as is required for ramjets). The big benefit is that the incoming air does not lose useful energy due to deceleration, energy that would otherwise be converted into heat that would require the engine to be actively cooled. The combined engine system NASA envisaged represented an enormous technological challenge: at the time the only existing hybrid jet engine was the J58 of the SR-71 Blackbird, whose turbojet and ramjet modes enabled the aircraft to reach about Mach 3.2. The hypersonic stage of the Class III vehicle would require to fly much faster and its engine would be considerably more complicated. The joint Aeronautics and Astronautics Coordinat­ing Board Subpanel on Reusable Launch Vehicle Technology, formed by NASA and the Department of Defense, concluded that the required technology could not be expected to be operational before 1982. (In fact, it still does not exist today.) Preliminary calculations for the Class III carrier vehicle indicated it would weigh

306,0 kg (675,000 pounds) at liftoff, if required to carry a 132,000 kg (290,000 pound) orbiter plane with a 16,000 kg (35,000 pound) low-Earth-orbit payload. This meant the total liftoff weight would have been about a quarter of that of the Space Shuttle for a payload only about one-third less. Of course the Space Shuttle actually flew whereas this Class III vehicle remained a paper study so we must be wary of such comparisons. Nevertheless, the numbers do indicate the potential mass benefits to be gained by using airbreathing engines.

A concept called ‘Mustard’ (Multi-Unit Space Transport And Recovery Device) was studied by the British Aircraft Corporation (ВАС) in the mid-1960s. It involved three similar winged vehicles that would be launched vertically as a single stack. A number of configurations were examined: a belly-to-belly-to-back sandwich, triangular belly-to-belly, two belly-to-belly and the third inline on top. Two of the vehicles would serve as boosters to put the third one into orbit, with the boosters pumping their remaining propellant into the orbital vehicle to top up its tanks prior to separation. In this way all of the engines could fire simultaneously for the first phase of the flight, yet the orbital shuttle would still initiate its solo flight with a full propellant load. The concept would potentially have enabled the orbiter to reach the Moon, which is a unique capability for a winged reusable design. All three stages

would have flown back to base after their mission, landing horizontally to be readied for the next launch. Around this same time the Warton Division of ВАС also studied several launch vehicle concepts in which a very large hypersonic aircraft carried on its back a small shuttle mounted on top of an expendable rocket. The ‘European Space Transporter’, which originated as the Nord Aviation Mistral and was studied by French and West German aerospace companies, was rather similar; it looked like a giant airbreathing fighter with enormous ramjet intakes and a rocket propelled lifting-body shuttle strapped onto its belly. Dassault’s ‘Aerospace Transporter’ was also based on essentially the same idea, involving an orbital ‘space taxi’ shuttle carried under a turboramjet Mach 4 aircraft that resembled the Concorde. Dropping a rocket vehicle rather than (as envisaged for the X-15A-3) launching it from the back of a carrier aircraft reduces the risk of the shuttle striking its motherplane, but the downside is a longer and more complex undercarriage on the carrier in order to give the shuttle enough clearance above the ground during take-off and aborted landings of the aircraft combination.

In 1967-1968, the US Air Force awarded several study contracts for an ‘Integral Launch and Re-entry Vehicle’ (ILRV) which would consist of a reusable single-stage VTHL that would jettison relatively inexpensive propellant tanks. This represented a compromise between a fully reusable single-stage launch vehicle (which would be large, heavy and extremely challenging and expensive to create) and a fully reusable two-stage vehicle (that would require the development and operation of two separate vehicles). The McDonnell-Douglas ILRV design featured a VTHL lifting body that resembled a steam iron, mated to four jettisonable tanks: two large ones for liquid hydrogen and two smaller ones for liquid oxygen. It would fold out small wings to increase its lift over drag ratio in the final descent and landing. Lockheed’s proposed ‘Starclipper’ was a VTHL lifting body design in which a single integrated A-shaped propellant tank would ‘wrap around’ the reusable lifting body. General Dynamics proposed a ‘Triamese’ solution with three reusable rocket stages which had fold-out wings and jet engines to land using a standard airport runway. The concept did partly satisfy the ILRV concept of a single vehicle design because the three winged stages would have been virtually identical.

The primary mission of all these reusable spaceplane concepts dreamed up in the 1960s was to transport crews to a large Earth orbiting space station. However, other missions such as the launching, inspection, repair and retrieval of satellites were also envisaged. The reusability was expected to translate into very low operating costs: for instance Lockheed claimed that their Starclipper would have a turn-around time (the time necessary to prepare the vehicle for its next mission) of only 24 hours and would reduce the costs of launching cargo into space to less then $100 per kg in 2011 terms. In fact, launching satellites into low orbit currently still costs $10,000 per kg for expendable launchers (and double this for the Space Shuttle when it was retired that year).

By mid-1969 NASA was planning an extremely ambitious and financially rather unrealistic, manned space exploration program to follow up on Apollo. The wish list included space stations, interplanetary ‘space tugs’ and eventually bases on the Moon and Mars. The first task was to build a 12-person space station in orbit by 1975 and to expand this into a 50-person ‘space base’ by 1980. Smaller ‘way-stations’ would then be deployed in geostationary Earth orbit and around the Moon and Mars. Naturally all this infrastructure would require an efficient, dependable and inexpensive means of transportation in the form of a space shuttle.

In a meeting in April 1969 Maxime Faget, the renowned designer of the Mercury capsule and by then director of engineering and development at NASA’s Manned Spacecraft Center, presented a balsa-framed, paper-skinned airplane model that had short, straight wings and a shark-style nose. It looked remarkably similar to Sanger’s Silbervogel concept of the Second World War. When introducing the design, Faget explained, “We’re going to build America’s next spacecraft. It’s going to launch like a spacecraft; it’s going to land like a plane.”

Back in January NASA had awarded four Phase-А study contracts to McDonnell-Douglas, North American Rockwell, Lockheed and General Dynamics to begin the development of the shuttle launch system. Martin Marietta, whose bid had been rejected, decided to participate using its own funds in order not to be left out of what promised to be a very lucrative project. The NASA requirements called for a vehicle able to launch 12 people as well as some 11,300 kg (25,000 pounds) of supplies to a space station in low orbit. Soon these became even more demanding and complex when it was decided that the vehicle must also be capable of launching satellites and interplanetary probes. Then the Air Force demanded that the stated payload capacity be doubled.

NASA had by then decided that the shuttle was going to be a fully reusable YTHL Two Stage To Orbit (TSTO) vehicle, since that was expected to produce the lowest cost per flight and highest payload capability in combination with good operability and mission flexibility. This meant the next generation launch vehicle was not going to be a real single-stage spaceplane but rather a combination of vertically launched winged rocket stages, each capable of gliding back to Earth and making a horizontal, unpowered landing. In an article optimistically titled ‘The Spaceplane That Can Put YOU in Orbit’ in the July 1970 issue of Popular Science, Wernher von Braun (at the time NASA Deputy Associate Administrator) commented on the decision: “It would be ideal, of course, if we could build a single-stage-to-orbit shuttle, which, without shedding any boost rockets or tanks, could fly directly up to orbit and return in one piece to the take-off site for another flight. Although we may well know how to build such a vehicle some day, most studies unfortunately show that with the present state of propulsion and structural technology this objective would be just a shade too ambitious.”

Out of the 120 different concepts studied, five emerged as the most promising. ‘Concept A’ of North American as well as ‘Concept C’ of Lockheed involved a huge winged, vertically launched booster stage carrying a smaller orbital shuttle stage on its back. ‘Concept B’ of McDonnell-Douglas and ‘Concept D’ of General Dynamics both had a winged booster stage and a shuttle fitted belly-to-belly. Martin Marietta’s ‘Concept E’ involved a shuttle vehicle mated to a twin-body winged booster stage. For the orbital elements, North American, General Dynamics and Martin Marietta favored conventional, fuselage-with-wings shapes, whereas McDonnell-Douglas and Lockheed proposed lifting bodies. However, McDonnell-Douglas soon changed their shuttle design to a more conventional shape which would re-enter with folded wings. General Dynamics’ shuttle was based on a similar idea but with retractable switch-blade wings. Not having the wings extended during re-entry would save on thermal protection weight, but of course the extension system would be complicated, heavy, and involve a fair amount of risk (for example if they did not open).

For the ensuing Phase-В step of the project Lockheed teamed up with Boeing to further develop their concept, but did not win a contract. North American Rockwell and General Dynamics did, and joined forces to work on the concept of a North American shuttle stage to be launched on the back of a large winged booster designed by General Dynamics. McDonnell-Douglas and Martin Marietta, the other winning team, jointly studied a concept that was very similar but employed re-entry thermal protection based on metal alloys rather than the sihca tiles favored by North American Rockwell and General Dynamics. Both teams considered two basic orbiter designs (NASA having not expressed a preference either way): one involved Maxime Faget’s design with straight wings and lower re-entry heating but a relatively small cargo capability and a limited cross-range, and the other a delta-winged design with a greater cross-range and a larger cargo bay but higher heat loads. All Phase-B concepts were for a fully reusable launch system.

In addition to the primary Phase-В study contracts, in June 1970 NASA also let three contracts to Grumman/Boeing, Chrysler, and Lockheed to study an ‘Alternate Space Shuttle Concept’ (ASSC). This was intended as a backup plan in case the fully reusable concepts turned out to be too expensive to develop (as would soon prove to be so). The Grumman/Boeing team received the most important contract (which was later upgraded into a full Phase-В contract) for studies of a shuttle with expendable propellant tanks and of a reusable orbiter boosted by existing liquid propellant rocket engines and expendable solid rocket motors. For its ASSC contract, Lockheed further refined its Starclipper concept, which already envisaged drop tanks. The ASSC study results showed that the use of expendable tanks greatly reduced the size and weight of the orbital vehicle as well as the booster vehicle, whilst also significantly lowering the total system’s development costs.

While Phase-В was underway, NASA’s original grand plan involving large space stations, lunar bases and manned missions to Mars evaporated. This left the Space Shuttle “a project searching for a mission”, as critics in the US Congress derided it. NASA and the Air Force then began to focus on the shuttle as a stand-alone project, as a low-cost ‘space truck’ for launching, repairing and retrieving satellites, and for flying all kinds of onboard experiments. Based on what we now know to have been wildly optimistic assumptions, NASA’s plan was for a total of 445 flights during the 10 year period 1979 through 1988 (the Space Shuttle as we know it actually flew only 135 missions during its 30 year lifetime).

In May 1971 it also became clear that the government was not going to allocate sufficient funding to enable NASA to develop a fully reusable Space Shuttle. NASA and its contactors then spent the next 6 months frantically revising their concepts by incorporating expendable propellant tanks, solid propellant rocket boosters, and even modified forms of the first stage of the Saturn Y moonrocket. At the end of 1971 the manned, winged flyback booster options were discarded on the basis of the expected development cost and system complexity (both for development as well as in-service operations). By mid-1972 NASA had finally decided on the general concept for the Space Shuttle as a reusable, winged orbiter with a single expendable propellant tank and two huge solid propellant rocket boosters which would be retrieved by parachute and refurbished for reuse. Based on the expected sizes of future military spy satellites and the orbits they would require, the Orbiter had to have a huge payload bay as well as the ability to put large satellites into polar orbit from Vandenberg Air Force Base in California (from which no Shuttle ever launched, even although a nearly complete launch facility was created there). The Orbiter also required a fairly large cross-range during its descent to Earth, since its landing site in California would be rotating away from under it when returning from a reconnaissance mission involving flying a single polar orbit. Consequently the Orbiter had to be equipped with large delta-wings, and Faget’s straight-wing concept was discarded. In mid-1972 the Phase-C/D contract for the design and construction of the Orbiter was awarded to North American Rockwell. Its Rocketdyne subsidiary would supply the main engines that would burn hydrogen and oxygen, Martin Marietta would supply the external propellant tank and Thiokol would supply the sohd rocket boosters. The concept illustrations of the time show a vehicle very similar to the Space Shuttle we are familiar with. Early in 1974 NASA also decided that the Orbiter would not require jet engines, because the X-24 and other lifting bodies had proven that ‘dead-stick’ glide landings would be sufficiently safe. Discarding the (deployable) jet engines further simplified the design and resulted in considerable weight and volume savings. It did mean that Shuttle pilots had only one chance to land, but the decision was correct because not a single Orbiter ever crashed during landing.

The selected configuration had a major impact on the test flight philosophy. Up to then all crewed launch systems had been unmanned on their maiden flights. For the early Shuttle concept, when it involved a reusable winged booster stage, von Braun said in his aforementioned article that the first stage and the orbiter would each be tested separately. Although the system as a whole would be launched vertically when operational, both winged stages would initially make a series of horizontal take-offs and subsonic flights using their own jet engines. In the next step, each vehicle would make individual vertical take-offs and supersonic flights using their rocket engines. Only after these test flights had been satisfactorily concluded would the orbiter be strapped to the booster and the combination launched into space. This logic could not be applied to the Space Shuttle. The Orbiter was able to make unpowered, subsonic glide tests from the back of a Boeing 747 but the first powered flight simply had to be an all-out launch with two pilots on board because it was not designed to fly in an unmanned role. The Orbiter was not able to take off horizontally since it lacked jet engines. There was no way to flight test the large External Tank and the two Sohd Rocket Boosters separately because the Orbiter’s computers provided the commands. When astronauts John Young and Bob Crippen first flew the Space Shuttle in April 1981 NASA took an enormous risk with a new system that was so radically different from the previous manned capsules (for their later Buran shuttle the Soviets actually flew their orbiter with jet engines installed for flight tests, launched the large Energia carrier rocket separately to verify its performance, and only then launched and landed the entire Buran system unmanned).

The chosen Space Shuttle design was an awkward compromise between mission requirements, technology, costs, schedule, the need for the project to provide jobs all across the US, and the political wish for something ambitious to reassert the nation’s technological leadership. Nevertheless, NASA still believed its partial reusability and great payload capabilities would revolutionize spaceflight. Access to space would be relatively cheap, opening Earth orbit and beyond for all manner of exciting scientific, industrial and even commercial activities. In a speech to the National Space Club on 17 February 1972 NASA Deputy Administrator George Low said that now the great challenge was to develop a “productive Space Shuttle, one that performs as required, can be developed at a reasonable cost, and is economical to operate. If we meet the first two of these objectives, but not the third, we will have developed a white elephant.”

During its 30 year life the Space Shuttle performed an amazing array of missions but it never fulfilled its promise of regular, safe, economic flights into space. Instead of weekly launches NASA managed to fly an average of fewer than five missions per annum. Two of the Shuttle flights have ended in disaster; an accident rate of 1 in 68, which is rather dismal compared to the fewer than two crashes per million flights for commercial airliners in the US. Instead of costing around $40 million per flight as predicted in 1975 (equivalent to about $160 million in 2011), actual costs were closer to $500 million per launch. When also taking into account the development costs and the production costs for the five Orbiters, the actual average cost per flight was well over $1 billion!

During the 1970s NASA claimed the Orbiter’s spacious cargo bay would relax the constraints on the size and weight for its payloads, resulting in lower costs. However in reality the stringent safety constraints imposed on spacecraft and equipment meant to be transported on crewed space vehicles generally resulted in higher development and production costs than when launching on an unmanned expendable rocket.

There are several reasons why the Space Shuttle cost far more to operate and took much longer to ‘turn around’ than expected. An important reason is the maintenance of the Orbiter. For a start it had 35,000 brittle thermal protection tiles, and these had to be individually inspected after every flight and often replaced. Structural elements, instrumentation, and electrical wiring all had to be thoroughly inspected after every flight; a task that is only done periodically for normal aircraft. The Orbiter required a team of some 90 people working more than 1,030 hours in total on maintenance and refurbishment after each mission, costing about $8 million per flight in 2011 prices. Not included in this was the heavy maintenance work on the three very complicated Space Shuttle Main Engines that was done separately from the vehicle. Even though the engines only operated for 8 minutes per flight they were not nearly as reliable as airliner jet engines; they suffered more rapid wear and required a lot of checking and maintenance after each mission. In contrast, jet engines run for hundreds of hours before preventive maintenance is needed. In addition the two Solid Rocket Boosters, which landed in the ocean by parachute and were retrieved by specialized ships, had to be completely disassembled, cleaned and repaired before they could be filled with solid propellant and reassembled for reuse. Most of the things that on a conventional rocket are thrown away, with the Shuttle had to be retrieved, refurbished and than put together again. Only the large External Tank was discarded once the Shuttle had almost reached orbit and left to burn up upon falling back into the atmosphere. But that meant a new one was needed for each flight.

The idea that the Space Shuttle would launch any and all US payloads, whether from NASA, the military or commercial users, never materialized. It was simply too expensive, and it was deemed too risky to have the nation’s access to space depend on a single launch system. To stimulate the diversity of launch systems following the loss of Challenger, President Reagan even decided that commercial satellites would no longer be launched using the Shuttle. This meant a dramatic decline of the number of Shuttle launches relative to the rate advertised in the 1970s (although on the other hand that rate was unattainable). Another important reason for the reduced number of launches required was the increased lifetime of satellites, yielding lower replacement rates than were assumed during the development of the Shuttle. Another mission that was claimed for the Shuttle, retrieving satellites, was undermined by the fact that the Shuttle could not fly high enough to retrieve geostationary communications satellites (potentially the biggest market); it could only reach satellites in low orbit. In any case the retrieval of a malfunctioning satellite was usually more expensive than building a replacement. The relatively low flight rate in itself also increased the cost per flight, because the so-called fixed costs for facilities and personnel had to be shared across fewer missions. This was foreseen by several people even before the Shuttle started flying, as by, for instance, Gregg Easterbrook in the very insightful article ‘Beam Me Out Of This Death Trap, Scotty’ published by the Washington Monthly in 1979.

Roger Launius, former Chief Historian for NASA and now Senior Curator at the Smithsonian Air and Space Museum, has perfectly summarized the Shuttle program: “In 135 missions, with two catastrophic failures, the US Space Shuttle proved itself a vehicle filled with contradictions and inconsistencies. It demonstrated on many occasions remarkable capabilities, but always the cost and complexity of flying the world’s first reusable space transportation system ensured controversy and difference of opinion.”

The Soviets also investigated the development of reusable spaceplanes in the early 1960s. In 1962 the Mikoyan Design Bureau presented its ‘50-50’ concept, so-named since it would have comprised a reusable, air-breathing, hypersonic aircraft carrying a two-stage expendable rocket with a small orbital space glider. A first version of the carrier aircraft, using kerosene as fuel, would have accelerated to Mach 4 prior to releasing the rocket/spaceplane combination at an altitude of about 23 km (14 miles). A variant for the longer term would have reached Mach 6 at 30 km (19 miles) using hydrogen fuel. The orbital spaceplane was to be a lifting body with extendable wings for use during the subsonic part of the return flight. It was officially named the MiG

105 ‘Spiral’ but unofficially was the ‘Lapot’ (meaning ‘flat shoe’) for its somewhat awkward appearance. The MiG 105 would only have been able to accommodate a single cosmonaut-pilot, housed in an emergency escape capsule that could return to Earth independently even after being ejected in orbit.

The first flight of the complete 50-50 system was planned for 1977, and a group of cosmonaut-pilots started training in 1965 to fly test models and eventually the real spaceplane. But it proved difficult to maintain financial and political support because it was expensive, the technology was difficult to master and the lengthy development did not promise the quick results the Kremlin preferred. In 1973 the

cosmonaut team was disbanded and in 1976 the seriously underfunded project was finally halted. The large 50-50 carrier aircraft was never built, but a turbojet-powered prototype of the MiG 105 for subsonic tests flew eight times between 1976 and 1979, both taking off under its own power and being dropped from a Tupolev Tu-95 bomber. Today this MiG 105-11 test vehicle languishes in a muddy field at the Monino Soviet Air Force Museum outside Moscow.

Despite the 50-50 project’s demise the flight test program of the MiG 105 was continued to collect data for the new but less ambitious ‘Buran’ project. The Buran (Russian for ‘Snowstorm’) was the Soviet equivalent of the US Space Shuttle. It was developed purely to keep up with the US in technology and capability, and the Buran orbiter ended up looking remarkably similar to its American competitor. But unlike the Space Shuttle it did not use its own rocket engines for launch, it was mounted on the side of a giant Energia rocket. The Buran made only one, completely automatic, orbital flight without crew in 1988 and then the financial problems of the crumbling Soviet Union killed both the Buran and Energia projects.

The only Buran vehicle flown in space was destroyed in 2002 when the roof of its hangar collapsed due to poor maintenance, but a decrepit ground-test prototype is an attraction in a Moscow park. The atmospheric flight and landing test prototype with jet engines for a runway take-off found a more dignified home in the Technical Museum Speyer in Germany.

Being similar to the Space Shuttle, it is very unlikely that the Buran could have lowered launch costs relative to those of reliable expendable Russian launchers like Soyuz and Proton. In fact, the Russians regarded the optimistic Space Shuttle flight cost figures that NASA published during the 1970s as misinformation to disguise the Shuttle’s real purpose as an orbital bomber and “space pirate ship” for destroying or abducting Soviet satellites.

In the 1979 James Bond movie Moonraker, highjackers steal a Space Shuttle Orbiter during a transfer flight on top of its Boeing 747 carrier. They ignite its main engines and fly off, with the rocket exhaust blowing up the 747 (in reality the Orbiter was of course only transported with empty tanks for its maneuvering thrusters, and could not carry propellant for its main engines). From 1988 until 1991 the Russians briefly worked on a shuttle that would do something similar, without the destruction of its carrier. A concept called MAKS (Russian abbreviation for ‘Multipurpose Aerospace System’) involved launching a fairly small shuttle mated to a large expendable propellant tank from atop a giant, subsonic Antonov An-225 airplane (which had already been built for transporting the Buran orbiter). The standard version of the MAKS shuttle would have had a crew of two and a payload of 6,600

kg (14,500 pound) to low orbit. It was supposed to reduce the cost of transporting materials into orbit by a factor of ten. But the collapse of the Soviet Union and the ensuing poor Russian economy killed this project as well. In June 2010 Russia nevertheless announced that it was considering reviving the MAKS program.

In the early 1980s the US Air Force studied a very similar concept. It was initially called the ‘Space Sortie Vehicle’, then the ‘Air Launched Sortie Vehicle’, and finally the ‘Air Force Sortie Space System’ (AFSSS). The carrier was to be a souped-up version of the modified Boeing 747 used to transport Space Shuttle Orbiters. Several companies came up with designs based on the initial specification which involved a lifting-body mini-shuttle about 15 meters (50 feet) in length, either unmanned or with a single pilot, and powered by RL-10 rocket engines whose liquid oxygen and liquid hydrogen propellants would be in large external tanks that would be discarded when empty. Some consideration was given to wrapping these expendable tanks around the shuttle in order to create an aerodynamic assembly that would be capable of a lifting ascent, meaning generating lift to help it attain altitude after release from its carrier (in which case it would have been a real rocket plane). Rather than having its shuttle take off gently from a horizontal flying carrier (as was envisaged for MAKS) the Air Force Rocket Propulsion Laboratory had something more spectacular and effective in mind: the 747, normally not an aerobatic aircraft, would have the thrust of its jet engines augmented by the installation of afterburners: liquid hydrogen from tanks on the Boeing (also used for topping up the propellant tanks of the mini­shuttle shortly prior to launch) would be injected into the hot exhaust of the four otherwise standard turbojet engines to produce a massive increase in power of up to 400%. This would enable the giant airliner to zoom up at a 60 degree angle to reach a launch altitude of 15 to 17 km (50,000 to 55,000 feet). An alternative idea was to install a single Space Shuttle Main Engine or a cluster of RL-10 rocket engines in the tail of the 747. When standing on ‘alert status’, this rapid response system was to facilitate a flyover of any point on Earth within 75 minutes of the moment the 747 started to taxi. The shuttle was also to deliver small payloads into orbit, rendezvous with satellites or space stations, and fly “low altitude penetration of target area” missions to drop bombs before reigniting the engines to regain altitude and return to its base. As the cryogenic propellants would boil off whilst standing on alert, constant replenishment from tanks on the ground would have been required to keep the system ready for take off at a moment’s notice. This project never progressed beyond the preliminary design stage, in part because it was judged too expensive but primarily because there was no urgent need for it.

The French space agency (CNES) and the European Space Agency worked on a small, manned space shuttle named Hermes. The concept was similar to that of the X-20 Dyna-Soar, with a small space glider launched on top of a European Ariane 5 rocket. The project was approved in November 1987 but canceled in 1992 when it became apparent that neither the cost nor performance goals could be achieved. The Ariane 5 development continued as a conventional, expendable launch vehicle and is now very successful in the geostationary satellite market.

In the US, work on a successor for the Space Shuttle has not been very successful. During the 1990s NASA and prime contractor Orbital Sciences worked on the X-34, an unpiloted, experimental rocket plane powered by an inexpensive, non-reusable ‘Fastrac’ engine running on RP-1 kerosene and liquid oxygen. The X-34 was to test reusable launch vehicle technology. Like the X-15 it would have been dropped from a carrier airplane, but would have been able to reach Mach 8. But NASA canceled it in 2001 after the company refused to incorporate significant design changes without additional funding.

The most ambitious reusable launcher test project, NASA’s X-33 developed by Lockheed Martin, involved a suborbital, single-stage, unpiloted VTHL vehicle with a wedge-shaped lifting-body. It was to lift off vertically without making use of the lift generated by its shape, fly at Mach 15 and then land horizontally Uke an airplane. It was to be powered by a ‘linear aerospike’ engine that consisted of a series of small rocket motors along the outside edge of a wedge-shaped protrusion. The aerospike is essentially an inside-out bell-shaped rocket nozzle in which the ‘unwrapped’ bell (or ramp) serves as the inner wall of a virtual nozzle along which the expanding hot gas flow produces thrust. The other side of the nozzle is effectively being formed by the outside air. The advantage is that the expansion of the rocket exhaust automatically adjusts itself to the ambient pressure of the atmosphere, preventing thrust losses due to underexpansion or overexpansion. While a conventional rocket nozzle can only be optimized for a single altitude, and hence only one point in a rocket’s trajectory, an aerospike engine runs efficiently at all altitudes as well as in the vacuum of space.

Sadly the X-33 project was scrubbed in 2001 owing to major problems with the development of this aerospike engine and with the lightweight composite-material hydrogen tanks (which required complex shapes to fit the lifting-body curves). The cost of the project exceeded the $1.2 billion budget limit even before any test flights could be made. The X-33 was to have led to the development by Lockheed Martin of the operational single-stage reusable launch vehicle named ‘Venture Star’. However, it would probably have been very difficult to scale up the X-33 test vehicle without putting on too much weight, which (as explained earlier) is a frequent problem in the design of reusable single-stage launchers.

NASA’s Orbital Space Plane program to develop a crew transportation vehicle for the International Space Station was initiated in 2002. Four competing concepts from different industries emerged, three of which involved mini-shuttle designs and one a capsule spacecraft, all of which were to be launched on top of an expendable rocket. But by 2004 they had been superseded by the Orion Multi-Purpose Crew Vehicle, an Apollo-style capsule that is still under development.

In April 2010 the Air Force finally launched a small robotic demonstrator shuttle on top of an Atlas V rocket. The payload and operations of this X-37 ‘Orbital Test Vehicle’ were classified, but after 7 months in low orbit it made a gliding return. A second X-37 was launched in March 2011. The X-37 was intended to be carried into orbit inside the Space Shuttle cargo bay, but once it was realized that a Shuttle flight

would be uneconomic and moreover that the Shuttle would be retired in 2011, it was redesigned for an expendable launcher and in order to prevent its wings steering the rocket off course the mini-shuttle was covered by an aerodynamic shroud. The X-37, which was developed by Boeing, would appear to be a test vehicle for an operational military, reusable, robotic spacecraft rather than a sub-scale precursor for a crewed shuttle system.

The Space Shuttle taught us that to really lower the price of transporting people and cargo into space, we require a fully reusable launcher that is easy to maintain. Ideally, it would be a single integrated vehicle without expendable tanks or boosters that require retrieval and refurbishment. It should return to its launch site to preclude complicated and expensive transportation; if the Orbiter landed anywhere other than at Kennedy Space Center it had to be mated with a specially equipped Boeing 747, flown to Florida, then de-mated from its carrier. Instead of thousands of fragile heat resistant tiles, a limited number of readily replaceable metallic shingles ought to be employed. The propellants should be non-toxic, safe and relatively easy to handle to avoid complicated tanking and propulsion system maintenance procedures (although the various rocket planes have yielded a lot of experience in handling dangerous liquids such as hydrogen peroxide). Future reusable rocket planes may carry computers and sensors that constantly check the health of all subsystems and components in flight, both to warn the crew/flight operators of any problems during a mission and to make it easier for the turn-around team to determine when and what kind of maintenance is required. The reusable rocket engines should last longer than the Space Shuttle Main Engines, require less maintenance, and be easier to repair. The RS-25 engines of the Space Shuttle can only be operated for about 10 minutes before major maintenance, which involves time-consuming (and hence expensive) tasks and the replacement of a lot of equipment. In contrast, jet engines as used in modern airliners can operate for months of accumulated flight time with only very limited checking and maintenance. Ideally, a future space plane would have some kind of combined rocket/jet engine that can use oxygen from the atmosphere while flying at relatively low altitudes. This would mean less onboard propellant, smaller tanks, and therefore a smaller, Ughter vehicle. However, such engines tend to be complex, which potentially makes them hard to maintain.