Category X-15 THE WORLD’S FASTEST ROCKET PLANE AND. THE PILOTS WHO USHERED IN THE SPACE AGE

The X-15 flights would not have been possible without the B-52A, which carried the airplane under its right wing. Edwards Air Force Base is huge, and it includes the whole of Muroc Dry Lake. Not only did the flights originate at Edwards, both the X-15 and its mother ship, the B-52, landed
there also, although on different plots of ground at the site. The B-52 started on the runway at zero velocity, accelerated to takeoff, and carried the X-15 to its launch position with a speed of approximately M=0.85 and an altitude of about 45,000 feet.

While the X-15 achieved a record speed of M=6.7, the first 0.85 was accomplished by the B-52 in the first phase of the flight. The B-52 also sometimes positioned the drop location as far away from Edwards as 300 miles, whereas the flight profile dictated for the X-15 to land at Edwards. The X-15 expended no fuel for such a running start, which was required to obtain the data sought by the test.

It took about an hour and a half from takeoff to get to the launch position; the rest of the X-15’s flight to its landing was an additional 10 minutes.

Both the X-1 and the X – 2 rocket-powered research aircraft were also carried aloft from

Listed below are the number of landings that took place at alternate fields, to be compared with the 188 normal landings at Rogers Dry Lake.

2 Cuddeback

1 Delamar

4 Mud

1 Rosamond

1 Silver

1 Smith Ranch

Since these were emergency fields, they had to have equipment there and personnel on site to act in case they were needed. Prior to the flights, equipment such as a fire truck with 500 gallons of water, a helicopter, firemen, an Air Force pilot to act as the lake controller, an AF crew chief, an AF doctor, an AF pressure-suit technician, and a NASA X-15 specialist were deployed. A test flight was a big operation, and a cancelation was a waste of time for many.

Edwards Air Force Base by carrier or “mother” aircraft, the B-29 for the X-1 and the B-50 for the X-2. The mechanical alterations required to the carrier aircraft were principally in the bomb bay area in order to securely hold the research aircraft and to provide a reliable launch mechanism.

The research aircraft pilots rode to the launch altitude and speed in the carrier aircraft, did the checkout before launch within the carrier aircraft, and replaced the liquid oxygen that had boiled off during the climb, all before entering the research airplane. For the X-15, the mother ship was supposed to have been the B-36, and the X-15 would have been carried to its launch position in the bomb bay opening. Some of the reasons the B-52 made the cut instead were related to differences in the availability and cost of each aircraft and the parts required for its maintenance during the flight-test program.

The B-36, then in the process of being phased out as an active bomber in the Air Force inventory, was a maintenance nightmare, whereas the then – modern B-52 was (and still is today) the main bomber for the Strategic Air Command. Moreover, the weight of the X-15 increased during the design phase, and the extra capability of the B-52 could

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more easily achieve the speeds and altitudes required by the data regions. Changing from the B-36 to the B-52 meant that the X-15 pilot could not ride inside the carrier aircraft. Using the B-52 meant that the X-15 had to be mounted on a pylon under the B-52’s right wing.

There was no way for the pilot to transfer from the B-52 to the X-15 after takeoff, which meant that he had to remain inside the X-15 during takeoff and for the roughly hour-and-a-half climb to position. This increased the pilot’s risk significantly. In an emergency during the launch-to – climb phase, the B-52 would have to drop the X-15 and its pilot rather than risk the lives of the entire operation’s crew. If the X-15 could be dropped, its pilot could possibly glide to a dry lakebed, or eject if the altitude was high enough. There were a number of captive flights—i. e., while the X-15 was still attached to its mother ship—where problems arose of such a nature that the launch was aborted, such as the auxiliary power unit (APU) not functioning in checkout or electrical signals not transmitting properly. In these circumstances, the B-52 landed safely with the X-15 still tucked under its wing. On such occasions, it must have seemed like a long, fruitless mission for the captive X-15 pilot. Luckily, neither the B-52 nor the X-15 pilots ever had to face such an unplanned drop.

The B-52 required numerous modifications to allow both airplanes to replenish the liquid oxygen, to accommodate the mating of the two aircraft, to assure that the B-52 had adequate control for the mission, and to assure that structural sufficiency was proper for both aircraft. (The X-15’s fuel was anhydrous ammonia, which does not boil off and does not require topping off, meaning that only the liquid oxygen required replenishment.) Twenty-seven B-52 pilots supported the X-15 flights. Two of the first were Capt. Charles Bock and Capt. John Allavie.

Above: X-15 in the process of being mated to the B-52. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

X-15 being dropped from the B-52. USAF, Air Force Flight 75

Test Center History Office, Edwards Air Force Base

The activities of the B-52 airplanes and their USAF pilots over nine years were integral to the success of the X-15 program. It was not a minor expense.

In all, 199 flights were conducted over a nine-year period from June 1959 to October 1968. Three airplanes were built, repaired, and rebuilt during that period. The third airplane was a significant modification. This longer version included external fuel tanks to extend the flight time, the range of altitude, and the Mach number to be investigated. Most of the initial objectives for the airplane were reached in the early years. But because the X-15 could fly in the hypersonic regime, NASA wanted to conduct many experiments, some examining various materials using the airplane as a test bed.

One of the thermal protection techniques used to protect hypersonic vehicles from the intense aerodynamic heating environment is the covering of the vehicle surface with an ablative material. This material would directly absorb the heat and burn away (ablate), thus protecting the surface underneath. Some of the later X-15 test flights tested a specific ablative material, namely MA-25S developed by Martin Marietta. This silicon-based material was sprayed on the surface of the X-15. After several hours of curing, it was sprayed with a coating of Dow Corning DC90-090, a silicon – based sealer, which gave the X-15 a white color.

Some of these caused problems in flight. For example, for some flights an ablative material was put on the airplane for testing purposes and for additional heat protection. As the material vaporized, it coalesced on the windshield, making it opaque, seriously affecting the visibility of the pilot. For further tests of the ablating material, the engineers had to install an external shield on half the windshield that could be moved away after ablation had obscured the other side in order to allow the pilot to have clear vision for the remainder of the flight.

Chase aircraft are high-speed aircraft whose pilots observe the physical status of the X-15 during its mission, principally during its climb with the B-52 and then toward the end of the X-15’s test flight. They are positioned near alternate landing fields, at approach to landing through touchdown, and during the landing run-out.

During the climb, while the X-15 is attached to the B-52 mother ship, the chase pilot observes the X-15’s external features, makes control-
surface checks, and observes any irregularities during the climb. In making control-surface checks, the chase pilot observes the physical deflection of the control surfaces, which for the X-15 are the rudder and the horizontal tail, as deflected by the pilot in the cockpit and observed by the pilot in the chase plane. The pilot in the cockpit cannot see these control surfaces, and so it falls to the pilot of the chase plane to observe them. This check is done before the X-15 is dropped from the B-52. It is an essential safety check; if the control surfaces are not working, the flight is scrubbed.

At drop, the chase pilot watches the engine start up, observes the power levels, notes the clearance from the B-52 as the X-15 separates, and

is there to assist in descent to landing at Edwards AFB if, for example, the engine doesn’t start and the X-15 heads for an emergency landing at the dry lake designated for that particular launch.

He can note all the external features of the X-15, its sink rate, its progressive proximity to the ground, and anything unusual that would help the pilot during the landing, such as anomalies in configuration if the flaps did not deploy. The chase pilot can quickly land during an accident in order to physically assist or help rescue the pilot. In emergencies, he would perform the same functions when stationed near alternate landing sites.

With flights varying from launch close to Edwards Air Force Base to launch 300 miles distant, different numbers of chase planes were needed. Usually there were four, one for the climb of the X-15 and the B-52 mother ship, another
at drop, one at an intermediate station above an alternate landing field, and one to cover the descent and landing at Edwards. During the most distant launch, an additional chase plane was needed to cover additional emergency field locations. As a result, there were either four or five chase planes used per X-15 flight. These chase pilots were usually other X-15 pilots, NASA research pilots, or Air Force pilots from the Air Force Flight Test Center.

The chase airplanes that were chosen best matched the X-15’s flight characteristics required by the X-15 testing program. For the early flights launched at Edwards Air Force Base, an F-100 answered the call. Later, the team chose a Northrop T-38A because it better matched the B-52’s speed during its right turns. Both the F-100 and the T-38A could fly in the low supersonic

▲ Another view of the X-15 landing with the F-104 chase plane alongside. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

▼ Another view of the F-104 chase plane. USAF, Air Force Flight Test
Center History Office, Edwards Air Force Base

range, around Mach 1.5. If there was a problem in climb and cruise to launch, the chase pilot was thus in position to help in the landing.

For the launch at a distance from Edwards Air Force Base, an F-104 chase aircraft stayed with the X-15 until it accelerated out of sight. The F-104 was the first fighter airplane capable of sustained flight at Mach 2. The pilot of the F-104 observed the
separation from the B-52 at drop and watched the engine for proper light-up. If the engine did not fire properly, the F-104 would descend with the X-15 to landing and be on hand to help on the ground.

For the chase aircraft covering the intermediate emergency fields, F-104s assisted in the descent and landing of the X-15 and provided any assistance needed after touchdown. These aircraft

delayed their takeoff for about 30 minutes after the B-52 took off so they would have enough fuel to loiter at their positions.

Other flight vehicles participated as well. A helicopter, the Piasecki H-21, ferried personnel to and from emergency fields as required. It also blew fumes away from damaged aircraft, as when Jack

McKay flipped over during his emergency landing. This allowed emergency personnel to extricate him from his airplane and perform other functions during his rescue.

Air Force C-130s transported equipment and personnel to emergency fields, including fire engines. Safety was taken seriously.

X-15 on the lakebed after the flight on October 17, 1961, with pilot Joe Walker still in the cockpit and the Pasecki H-21 helicopter in the background. USAF, Air Force Flight Test Center History Office,

Edwards Air Force Base

Rear view of the B-52 on the ground with the X-15 attached to its right wing. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Support trucks and personnel at an X-15 landing site. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

As expected on the basis of experience with the earlier supersonic X-airplanes, the lateral – directional stability of the X-15 decreased as the Mach number rose to supersonic and hypersonic
speeds. Honeywell’s adaptive control system automatically compensated for the aircraft’s unstable lateral-directional behavior in various flight regimes, and it utilized the combined operation of the aerodynamic control surfaces and the rocket reaction controls in their respective regions of flight.

Originally, the vertical tail sections above and below the airplane were large. That section, located below the airplane, is called the ventral tail. Wind tunnel data showed a need for a large ventral tail, so large that it would hit the ground first before the landing skids. This necessitated designing the bottom part of the ventral to be ejected prior to landing. The flight data showed a lesser need for the large area of the ventral tail, and in subsequent flights the bottom half was left off.

A relationship between the wind tunnel data and the flight data was thus established. The Honeywell MH-96 adaptive control system allowed the airplane, unstable in certain regions of flight, to be operated in a conventional manner throughout. Moreover, it provided an automatic transition from the conventional aerodynamic control system (rudder, elevator, etc.) used within the sensible atmosphere to the reaction control system for high- altitude flight, where the aerodynamic forces were too weak. This relieved the pilot from manually making this change, both on ascent to high altitudes and back again for descent.

he X-15 program was a success, thanks in no small part to the men who flew the airplane. Each of the X-15 test flights was an example of intense man-machine interaction, and each of the twelve pilots who flew the X-15 were as finely tuned and technologically sophisticated as the machine itself. They set speed and altitude records for a manned airplane that still stand today, and they pioneered new piloting techniques for hypersonic aircraft that were not only adapted for the Space Shuttle but will continue to be used for future manned hypersonic aircraft. The X-15 pilots were brave and professional, venturing into a totally unknown regime of flight, and they helped to write the book on manned hypersonic flight for the next generation.

All of the X-15 pilots at one time or another were members of the elite NASA Flight Research Center at Edwards Air Force Base. The flight research team was under the direction of Walter C. Williams, who managed a group that planned all the flights, determined what data to acquire, gave the pilots what they needed to obtain the data in an effective and safe manner, and determined how to react in emergencies. Williams and his team were in charge of the flight testing of all the X-airplanes through transonic and supersonic regimes leading up to the X-15, namely the X-1, X1A, D558-2, and the X-2. This center had started out as a small group of about 27 people in 1946 dealing with the X-1 and grew to about 500 at the time of the X-15. These people collectively:

1) Maintained the aircraft, housed, repaired, modified, and prepared the airplane for each flight.

2) Provided for each flight. This included ground crew efforts to ready the airplane, provide the instrumentation, assure the safety for the airplane, provide the chase aircraft and their pilots, and provide emergency gear like the fire trucks and helicopters, as well as the communication links.

3) Provided plans and procedures for each flight, including a detailed pilot checklist for the X-15 and the B-52 mother ship.

4) Provided a flight plan for the X-15 to obtain the requisite data. This sequence included the drop from the B-52, rocket firing and powered flight, climb and transition to level flight, unpowered flight to the speed and altitude required for the data, and finally return to base and landing.

5) Provided a simulation plan to train the pilot for obtaining the data in flight, alternate flight paths to the desired data points if the airplane was over or under the speeds and altitudes planned, and emergency response to various potential problems during the flight. The Flight

Research Center had a special flight simulator designed for the hypersonic regime.

6) Conducted the flights with all the equipment, chase pilots and planes, and communication lines to assist the X-15 pilot to assure safety and performance.

7) Reduced and evaluated the flight data, and utilized the results in future activities.

In September 1959, Walter Williams left the Flight Research Center for the first of many executive positions in the space program, beginning with director of operations for Project Mercury. He was replaced at the Flight Research Center by Paul F. Bikle, who continued Williams’s rigorous professional standards. All the important accomplishments of the X-15 program were performed under Bikle.

The first flight of the X-15 took place on June 8, 1959. Carried aloft under the wing of a B-52, the experimental vehicle was released with its pilot at an altitude of 37,550 feet. Unlike all subsequent X-15 flights, however, there was no roar of the rocket engine. Indeed, there were no propellants aboard; this was intended to be a gliding flight, pure and simple. Its purpose was as a familiarization flight, the first checkout of the flight characteristics of the airplane in its glide down to landing, the response to the control system, the stability of the airplane, the handling of the control forces by the pilot, the response rate of the airplane to the controls, and its motion at touchdown and landing.

Nevertheless, the X-15 reached a speed of Mach 0.79 on its maiden descent to the desert floor. Moreover, as with all the other 198 X-15 test flights, a problem occurred. The airplane began to pitch up and down, a longitudinal oscillation that rapidly increased in amplitude.

The pitch damper designed to avoid this oscillation was discovered to be inoperable. Fortunately, the X-15 touched down safely at the bottom of an

oscillation, suffering damage only to the landing gear. A. Scott Crossfield, the pilot who had the most influence of all the X-15 pilots on the design and flight performance of the airplane, performed the difficult maneuver. In all other aspects, the plane performed as anticipated by the designers.

In the early days of flight, the aerodynamic controls (ailerons, elevators, rudder) were directly connected to the cockpit via cables, and the pilot had to use physical force to operate these controls. As the speeds of airplanes increased, the aerodynamic forces became larger and required more physical force from the pilot to operate the controls. With the advent of high-speed jet flight, these forces
became too large for the pilot to overcome, and hydraulically boosted controls were introduced (much like power steering in your automobile). For the X-15, the power assist controls that gave force amplification to the pilot were effective; they were used by the pilots when the aerodynamic forces were high at the lower altitudes.

The power assist controls were used throughout by some of the pilots who did not use the conventional center stick and who only used the force amplification controls. The MH-96 also blended this control with the rocket controls, which were used when the air density was so low that the aerodynamic controls were ineffective because of the high altitude and resulting low dynamic pressure. It made the transition from aero control to rocket automatic. For use in future hypersonic aircraft, and in the Space Shuttle that actually followed, it simplified the piloting when flying in these varied regions of aerodynamic force. The X-15 demonstrated that airplanes in these regions, even while rapidly traversing from one region to another with high accelerations and decelerations, could be flown safely by trained pilots.

Scott Crossfield was more than just the first man to fly the X-15; he was the only one of the twelve test pilots who contributed directly to the
airplane’s design and to the design of its flight-test program. Crossfield successfully combined his master’s degree in aeronautical engineering with his exceptional piloting ability and experience to enhance the design and operation of an experimental vehicle that would go far beyond the known atmospheric flight spectrum, to speeds of almost Mach 7 and to altitudes higher than 350,000 feet.

Scott Crossfield was born on October 2, 1921, in Berkeley, California, and attended college at the University of Washington in Seattle, beginning in 1940. The outbreak of World War II interrupted

his studies in 1942, when he joined the Navy. After he received his pilot’s wings and ensign’s commission in 1943, the Navy assigned him to be a flight instructor and maintenance officer.

He served in the South Pacific for six months but did not see combat duty. His piloting skills put him at the helm of a Navy aerobatic team, and he flew Corsair fighters for a short period following the war. Crossfield was, however, an aeronautical engineer at heart, and he returned to the University of Washington in 1946 to finish his bachelor’s degree in aeronautical engineering, as well as his M. S., in 1949. During that time, he obtained valuable experience working in the Kirsten Wind Tunnel at Washington.

It was not a good time to graduate with an aeronautical engineering degree; the industry

was suffering from large government cutbacks in defense after World War II. However, the advent of the Korean War in 1950 reversed this situation, and suddenly the aircraft industry was back on its feet. Crossfield found a position as an aeronautical research pilot with the NACA High Speed Flight Station (now the NASA Dryden Flight Research Center) at Edwards Air Force Base in June 1950. The time and opportunity were ripe for Crossfield; over the next five years, he was to fly virtually all the experimental airplanes at Edwards, including the Bell X-1, the delta-wing XF-92, the X-4, the X-5, and the Douglas D-558- 1 Skystreak. On November 20, 1953, he became the first person to fly at Mach 2 while piloting the rocket-powered Douglas D-558-2 Skyrocket to a speed of 1,291 miles per hour in a shallow dive.

Powered by a rocket engine, and developed by Douglas for the U. S. Navy, the Douglas D-558-2 explored transonic and supersonic flight and the flight characteristics of swept-wing supersonic aircraft. Flight tested at the Muroc Flight Test Facility alongside other research aircraft such as the X-1, X-1A, and X-2, the D-558-2 was the Navy’s venture into the mysteries of supersonic flight. Controversy persists as to who deserves credit for the first Mach 2 flight. Crossfield reached Mach 2 in the D-558-2, but in a shallow dive. Just twenty-two days later, Chuck Yeager flew the Bell X-1A to Mach 2.44 in level flight.

This beautiful, swept-wing airplane now hangs in the Milestones of Flight Gallery at the National Air and Space Museum.

On June 24, 1952, the NACA Committee on Aerodynamics called for an airplane that could probe the unknown problems of flight at Mach numbers between 4 and 10 and at altitudes between 12 and 50 miles. On October 5, 1954, this same committee, in executive session, made the final decision to proceed with this manned hypersonic research airplane, which would eventually become the X-15; Crossfield was a
member of the committee. On May 9, 1955, four aircraft companies submitted proposals to the Air Force (which was paying for the airplane):

Bell, Douglas, North American, and Republic. After North American won the contract, Scott Crossfield left the NACA and joined North American as chief engineering test pilot and design consultant on the X-15.

After piloting the first test flight of the X-15 on June 8, 1959, Crossfield flew the airplane thirteen more times, his last X-15 flight taking place on December 6, 1960—the thirtieth test flight of the X-15 program. At this point, North American finished its contractor check flights and turned the aircraft over to the Air Force. Although Crossfield had expected to fly the X-15 during its entire program, because he was a NAA employee, not a NACA employee, his flight participation in the X-15 came to an end.

Crossfield continued with North American, first as the director responsible for systems tests, reliability engineering, and quality assurance for several aircraft and space vehicles, and then as its technical director, Research Engineering and Test. In 1967, he left the company to serve as a division vice president for Research and Development for Eastern Airlines until 1973, and he then served as senior vice president for Hawker Siddeley Aviation in 1974 and 1975. In 1977, nine years after the X-15 program ended, he became a technical consultant to the House Committee on Science and Technology. He served in this capacity for sixteen years, during which he was a steadfast proponent of manned hypersonic flight. He especially supported the massive U. S. X-30 supersonic combustion ramjet engine-(scramjet) powered single-stage to orbit aerospace plane project during the 1980s and early ’90s. He retired in 1993.

Scott Crossfield earned a number of prestigious awards during his life, including being a joint recipient of the 1961 Collier Trophy, the

International Clifford B. Harmon Trophy for 1960, the Lawrence Sperry Award for 1954, the Octave Chanute Award for 1954, and the Iven C. Kincheloe Award for 1960. He was inducted into the National Aviation Hall of Fame in 1983 and the International Space Hall of Fame in 1988. As a reflection on his aeronautical engineering accomplishments, the American Institute of Aeronautics and Astronautics elected him to the rank of Honorary Fellow in 1999, the highest recognition in that society.

In 2000, the National Air and Space Museum awarded him its most prestigious award, the Lifetime Achievement Award. An elementary school in Herndon, Virginia, and the terminal of the Chehalis-Centralia Airport in Washington State both bear his name.

On April 19, 2006, Crossfield got into his Cessna 210A to return home from Maxwell Air Force Base in Montgomery, Alabama, where he had just finished giving a speech to a class of young Air Force officers. Amid severe thunderstorms, his airplane broke up in midair; recovery teams found wreckage in three different locations within a quarter-mile region. Later, the National Transportation Board ruled the probable cause of his crash to be a combination of two failures: Crossfield had not obtained updated weather information en route, and the air traffic controller failed to provide adverse-weather avoidance assistance. Crossfield was survived by his wife of sixty-three years, Alice Crossfield, as well as six children and nine grandchildren. He is buried in Arlington National Cemetery.

Crossfield was unique among the X-15 pilots. He always considered himself an aeronautical engineer, although he was also an exceptional test pilot. Being an honorary fellow of the AIAA is indicative of his status within the aeronautical engineering profession. Although he flew the X-15 only fourteen times, never exceeded Mach 2.97 (Flight 26, November 15, 1960), and never flew any higher than 88,116 feet (Flight 6, February 11, 1960), he was arguably the most influential of all the pilots in the X-15 program.

The use of rocket controls in flight was demonstrated earlier on the Bell X-1B airplane. Therefore, it was natural that rocket controls would be used for the X-15 as the only effective controls in space, where the aerodynamic forces are inadequate or nonexistent. These low-thrust rocket engines, using a monopropellant (hydrogen peroxide), provided useful control in space and have been used by the Space Shuttle in outer space.

ACCOMPLISHMENTS

All the design goals of the X-15 were met during its flight-test program, and some were surpassed.

The design maximum altitude and Mach number were both reached. The hypersonic research data obtained provided a rich database that confirmed the viability of hypersonic wind tunnel data as well as the usefulness of the limited theoretical analyses available at that time. The airplane proved to be a successful hypersonic vehicle, and the X-15 pilots performed admirably over an almost ten-year period. The program ended when the funding ran out and research experiments no longer justified the associated costs of the flights.

The flight region explored and extended the known range to M=6.7 and an altitude of 354,200 feet. The X-15 pilots explored this hypersonic range and provided data for future manned flights and for manned space vehicles flying from space through the atmosphere to landing, such as the Space Shuttle.

The new large RMI rocket motor performed well, providing the acceleration needed and with an operating efficiency of about 97 percent in support of obtaining mission data. There were no blowups in flight, and although the partial thrust use and subsequent restart capability were not reliable, the engine was able to position the airplane in the flight regions to be studied.

The MH-96 adaptive control system proved adequate and useful for stability on all three axes of flight. Some form of adaptive controls (controls that adapt automatically to the changing flight environment that was encountered during the flight of the airplane) have been used by high- performance aircraft in the fifty-plus years since the X-15.

All three control systems worked. The pilots preferred the power assisted controls over pure manual controls for use in the atmosphere, and the reaction rocket controls performed well in space and where the aerodynamic forces were insufficient. They have since been incorporated into the design of the Space Shuttle. The transition
in use of the control system from space to the atmosphere where aerodynamic controls took over was easily effected.

The high-temperature material, Inconel X, maintained its strength as predicted at the high temperatures obtained in flight, and it supported the flight loads. This design approach, which allowed for thermal expansion of the hot structure while the cold understructure remained unstressed, was ultimately successful after the engineering team made a few corrections following initial hot flights.

The aero-thermodynamic analytical predictions were considerably higher than the actual measurements; analytics can now reliably use empirical data obtained from these flights. The research team also learned that the predicted high stagnation temperatures occurred where air could enter small gaps in wing construction, which then burned internal wires and structural features.

A ball nose instrument was attached at the extreme nose of the airplane and utilized Inconel X to withstand the high temperatures of hypersonic flight. This instrument, which provided angle of attack and angle of yaw data to the pilot, was necessary for flying and controlling the airplane at the high-speed and high-temperature conditions.

Replacement of ailerons was accomplished by using the horizontal stabilizer differentially deflected (i. e., right stabilizer angle increased while the left stabilizer angle decreased, and vice versa), providing satisfactory roll control and simplifying the knowledge of airflow conditions at the tail.

As a research airplane, the X-15 was also a useful platform for doing experiments at hypersonic speeds. Most important, the repeated and successful utility of this airplane over highly accelerated and decelerated flight from space to landing demonstrated that piloted aircraft are suitable for manned controlled return from space and for missions in the hypersonic regime.

1921-1966

Joe Walker flew the X-15 for his first time on March 25, 1960, during which he achieved Mach

2.0 and an altitude of 48,630 feet. He was the first NASA pilot in the test program.

Joe Walker was born on February 20, 1921, in Washington, Pennsylvania. He graduated with a bachelor of arts degree in physics from Washington & Jefferson College in 1942. He was caught up in the storm of World War II, joined the Army Air Force, and flew P-38 fighters in North Africa, for which he earned the Distinguished Flying Cross and the Air Medal with seven Oak Leaf clusters. In March 1945, he joined the NACA and became involved in the icing research program at the Aircraft Engine Research Laboratory
(now the NASA Glenn Research Laboratory) in Cleveland, Ohio. There, in the words of Milton Thompson, himself an X-15 pilot, Joe Walker “spent many hours droning around in the crappiest winter weather that they could find in the Great Lakes region.” [citation: Milton O. Thompson,

At the Edge of Space: The X-15 flight Program (Smithsonian Institution Press, 1992, p. 4)]

Walker transferred to the NACA High Speed Flight Station (later the Dryden Flight Research Center) in 1951, and his flying skills earned him the position of chief pilot in 1955. He flew as project pilot on some of the early, important
high-speed experimental airplanes, including the Douglas D-558-1 and 2 and the Bell X-1A and X-1E, X-3, X-4, and X-5. During his first year at the NACA High Speed Flight Station, Walker received an NACA medal for heroism. He was in the cockpit of the X-1A mounted in the bomb bay of a B-29 in flight. In preparation for his research flight, he pressurized the X-1A’s propellant tank. An explosion immediately occurred, and Walker passed out. Regaining consciousness as the B-29 crew opened the X-1A canopy and pulled him out, Walker realized that the X-1A had to be deactivated before a bigger explosion occurred. Risking his life, Walker crawled back into the cockpit and depressurized the remaining tanks. The smell of hot peroxide started to fill the B-29. The X-1A now resembled a bomb about ready to go off. Scrambling back into the B-29, Walker decided to jettison the X-1A. The experimental airplane spun down to the desert floor and was destroyed, but the B-29 and its crew returned safely.

In 1959, the NACA became part of the newly formed National Aeronautics and Space Administration. Hence, on March 25, 1960, Walker became the first NASA pilot to fly the X-15. Remarkably, on his first flight, Walker took the X-15 to Mach 2 and an altitude of 48,630 feet. During the course of his remaining twenty – four flights in the X-15, Walker achieved the highest altitude of all the X-15 flights, 354,200 feet on Flight 91, August 22, 1963. This is still the unofficial world record for winged vehicles.

During his twenty-five flights in the X-15, Walker collected data on stability and control, aerodynamic heating, flight performance, aerodynamics, thermostructural response, maximum speed, and maximum altitude characteristics. On Flight 91, in addition to setting the unofficial world altitude record, he obtained data on reentry flight with the ventral fin off, checked out an altitude predictor, and took physical atmospheric measurements with a Barnes spectrometer and a photometer. Collecting this scientific and engineering data was the core of the X-15’s research mission.

After his last X-15 flight on August 22, 1963, Walker continued in his position as chief pilot at the NASA High Speed Flight Station. Prior to his involvement with the X-15, he had logged a number of flights in the Lockheed F-104, the first airplane designed for sustained supersonic flight at Mach 2. It was in this airplane that he first carried out pioneering tests using reaction controls, taking the F-104 to altitudes of 90,000 feet. So it was natural that on June 8, 1966, he chose to pilot an F-104 on a routine photo shoot with the North American XB-70. General Electric had requested some promotional photographs of a family of airplanes powered by GE engines. Flying too close to the XB-70, his F-104 was caught in the trailing vortex of the large airplane and flipped onto the top of the bomber. Walker perished in the ensuing fireball. The XB-70 pilot, Al White, ejected, sustaining serious injury but surviving. Carl Cross, the copilot, was killed.

he 199th flight by Bill Dana in 1969 was the last for the X-15. The two of these revolutionary airplanes that still remained were readied for installation in national aviation museums after the completion of the X-15 research program.

As early as 1962, the Smithsonian Institution had requested an X-15 airplane for eventual display in Washington, D. C. The first X-15 was installed by the Smithsonian on May 13, 1969, in what was then known as Silver Hill and is now called the Garber Facility. It was moved to the Smithsonian’s Arts and Industries Building in June 1969 and placed near the 1903 Wright Flyer. The Arts and Industries Building served as the National Air and Space Museum at that time. After being loaned out to the FAA and then to the NASA Flight Research Center for display, it returned to the Smithsonian to be installed in the new National Air and Space Museum in Washington, on the Mall, for its opening on July 1, 1976. It hangs there now in the Milestones of Flight Gallery.

The X-15A-2 airplane went to the National Museum of the USAF at Wright-Patterson Air Force Base in Dayton, Ohio. A set of external tanks and a dummy supersonic combustion ramjet (scramjet) engine are part of that display.

The X-15-3 that crashed with Mike Adams was buried at an unknown location at Edwards Air Force Base.

The two B-52 carrier airplanes used by the X-15 program were reassigned by the Air Force after performing in the subsequent lifting body program at the NASA Flight Research Center. The X-15 pilots continued with their careers: Neil Armstrong became famous as one of the first three men to land on the moon. Selected to be in the second astronaut class, he left the X-15

program, commanded Gemini 8, and on July 20, 1969, as commander of Apollo 11, became the first human to walk on the moon. His next position in NASA was deputy associate administrator for aeronautics at NASA headquarters. He left NASA to become professor of aeronautics at the University of Cincinnati, after which he served on the boards of several corporations. Neil Armstrong passed away on August 25, 2012.

Bill Dana became chief pilot at the Flight Research Center, then had progressively higher positions in Flight Operations, in F-18 research, and finally as chief engineer at the Flight Research Center, a position he held until his retirement in 1998.

Joe Engle was selected to become an astronaut in 1966 and performed as support crew on Apollo 10, then as backup lunar module pilot on Apollo 14. He commanded the Space Shuttle Columbia
and manually flew the reentry from Mach 25 through reentry and landing (the only time it was manually flown for an entire flight). His last flight in space was as pilot of Discovery in August 1985.

Pete Knight went to Southeast Asia and flew 253 combat missions in the F-100. He was test director of the F-15 System Program Office and piloted the airplane. He returned to Edwards Air Force Base as vice commander of the Flight Test Center and as an active F-16 pilot. He retired from the Air Force in 1982 and entered politics, rising to California state senator. He died on May 8, 2004.

Jack McKay retired from NASA in October 1971 and died on April 27, 1975, largely from complications from his X-15 crash.

Pete Peterson left NASA in 1962 and returned to the U. S. Navy, rising in rank after combat in Vietnam to be commander of the Naval Air Systems Command. He retired from active duty as vice admiral in May 1980. He died on December 8, 1990.

Bob Rushworth returned to the USAF after flying the X-15, and in the Vietnam conflict he flew 189 combat missions. He rose through the command ranks to become a general, and he retired as a major general from the position of vice commander of the Aeronautical Systems Division at Wright-Patterson Air Force Base. He died of a heart attack on March 17, 1993.

Milt Thompson remained with NASA after piloting the X-15, becoming chief of research projects. He then became chief engineer, a position he retained until his death on August 6, 1993. He wrote a wonderful book about his experiences flying the X-15, At the Edge of Space.

Joe Walker was helping obtain publicity shots of the XB70A while flying an F-104. Getting too close to the B-70 and caught in air currents between the two aircraft, he was killed in a midair collision on June 8, 1966.

Bob White continued in the United States Air Force. He became brigadier general and

commander of the Air Force Flight Test Center. He later became a major general and then chief of staff of the 4th Allied Tactical Air Force. He retired from the USAF in February 1981 and died on March 17, 2010.

Scott Crossfield, who left the NACA Flight
Research Center to join North American Aviation

to be a part of their X-15 design and flight-test team, ended his association with the X-15 program when the Air Force took it over. He then continued at NAA in many high-level and technical executive positions. He followed his NAA career with executive positions at Eastern Airlines and Hawker Siddeley Aviation. He then became a consultant to the House of Representatives Committee on Science and Technology. He lectured on aviation to many groups until his demise. It was after such a lecture at Maxwell AFB that he was killed in his Cessna 210 aircraft in a storm over Georgia while flying home on April 19, 2006.

At this writing, Joe Engle and Bill Dana are the only surviving X-15 pilots.

Collectively, the pilots who flew the X-15 airplane continued in their careers, flying for NASA in a research mode or for the military, where they progressed into positions of military leadership. Building upon their technical backgrounds and research piloting, they applied their work discipline to perform important responsibilities on behalf of the United States.

They were talented men, driven and successful in their endeavors.

The X-15 remains the fastest and highest – flying manned airplane in history. The fact that no

Test pilot Scott Crossfield in his pressure suit standing with colleagues in front of the B-52 mother ship. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Joe Walker ready to enter the cockpit for his first flight on the X-15, March 25, 1960. This was the first government flight in the X-15 program. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
other manned hypersonic airplane has followed in its wake is a testimonial to the difficulty and severity posed by the hypersonic flight regime. The authors remain convinced that the future will see manned hypersonic flight for sustained periods in the atmosphere, a development that will rely on the data produced during the X-15 program on hypersonic aerodynamics, flight dynamics, structures, flight control, and pilot behavior. These hypersonic airplanes will be powered by air­breathing jet engines, not rocket engines. Such air­breathing engines will be supersonic combustion ramjet engines (scramjets), which have been under development since the 1970s and which are still a subject of intense research.

Indeed, on May 1, 2013, the experimental X-51, an unmanned hypersonic vehicle, achieved the longest duration sustained flight powered by a scramjet of over 300 seconds at speeds above Mach 5. The future of practical, environmentally safe, and economically feasible hypersonic manned flight still lies before us, and when that happens, the X-15 will indeed be the “Wright Flyer” of its kind.

▲ X-15 in flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

▼ The X-51 hypersonic research vehicle, powered by a supersonic combustion ramjet engine (scramj et). The X-51 is unmanned and is a waverider configuration for high lift-to-drag ratio. Its first flight was on May 26, 2010. Its fourth and final flight was on May 1, 2013, when it flew at Mach 5.1 for 240 seconds under scramjet

propulsion, the longest air-breathing hypersonic flight to that time. USAF