X-15, HIGHER AND FASTER

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

Models of the competing designs for the X-15 arranged around the earlier Bell X-1A: clockwise North American, Republic, Bell and Douglas [US Air Force].

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.

Diagram of the X-15 ejection seat [North American Aviation].

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

Reaction Motors XLR99 rocket engine being installed in the engine test stand [NASA],

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

The dual XLR11 engine setup used for the early X-15 flights [NASA].

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

Scott Crossfield gets ready for the X-15’s first captive-carry flight during which it was not released from its carrier plane [NASA],

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.

An X-15 is dropped from its carrier plane [NASA].

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

X-15 just before touchdown [NASA].

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.

X-15-2 after its crash in 1962 [NASA].

Launch of the X-15A-2 with its white painted thermal protection and dummy ramjet [NASA],

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

Pilot Neil Armstrong and X-15-1 [NASA].

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-

A delta-winged X-15 launched from an XB-70 Valkyrie [North American Aviation].

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.