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

THE ENGINES

Rocket engines carry their own fuel and oxidizer and have large thrust, and by launching at high altitude the airplane will encounter small drag. This will enable the aircraft to quickly reach hypersonic speeds and altitudes where it can obtain the desired data.

The design called for the XLR 99 engine, similar to the XLR11 engines that powered the X-1 airplane past Mach 1. The XLR99 had a thrust at sea level of 57,000 pounds, while the XLR11 had a thrust of 6,000 pounds in 1,500-pound increments. The scaling upward of
the engine was significant. This new engine was throttleable to about 30 percent of maximum thrust. Unfortunately, the engine shut down prematurely at partial thrust, so almost all flights were conducted at full thrust. It was later restricted to operate at a minimum of 43 percent max because of unwanted shutdown occurring followed by an inability to restart. The dry weight of the engine is 915 pounds.

The fuel for the XLR99 is anhydrous ammonia, with liquid oxygen as the oxidizer. The specific impulse of this fuel is 230 seconds at sea level and 276 seconds at 100,000 feet altitude. Specific impulse is defined as the thrust of the engine per

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Front view of the X-15A-2 with external fuel tanks. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

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image86Rear view of the loading process for mounting the X-15 under the wing of the B-52. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

weight of propellant used per second, and it is a measure of the efficiency of the fuel.

The engines were developed and supplied by Reaction Motors, Inc. (RMI), through NACA and the USAF as government-furnished equipment (GFE). The XLR99 was not ready in time for the X-15’s first flight, and a drop flight without an engine was performed to learn about the airplane’s flying and handling qualities. Since the XLR99 still wasn’t ready, the next series of flights were performed using two XLR11 engines. The XLR11 had been used singly at 6,000 pounds thrust in the X-1 and X-1A series of flights. The two XLR11s that were used in the early X-15 flights had only 12,000 pounds of thrust, much less than the 57,000 that would be available later in the XLR99. Even with the reduced acceleration, the two XLR11s enabled flights through the transonic speeds and to a supersonic speed of about a Mach number of 3. The two smaller engines were mounted in a cradle that was then mounted in the same attachments used for the XLR99. Both configurations used the same fuel tanks, even though the fuel used for the XLR11 was water alcohol instead of anhydrous ammonia. After the twenty-fifth flight, all X-15 flights used the XLR99 engine.

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X-15 rocket nozzle exit. NASM

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Rear view of the X-15 mounted under the wing of the B-52. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

The advantage of flying first with a proven engine was to ensure that both the airplane and the engine were not new and untested. It also prevented a delay in the program, which allowed continuity in flight testing.

RMI, which won a competition that included Bell Aircraft, and Aerojet encountered several problems in developing the new engine: leaks, pumps, fuel lines, vibration, liner failures, etc.

Costs increased, which delayed schedules. Scott Crossfield, the first X-15 test pilot, did not want to proceed with a temporary engine, preferring to wait for the XLR99. Fearing that the new engine would not be completed, both NAA Vice President Harrison Storms and Program Manager Charlie Feltz supported using the XLR11. Said Feltz, “I’ve been a little concerned about busting into space all at once with both a brand new

airplane and a brand new untried engine. . . . We’re trying to crack space, with a new pressure suit, reentry, landing, new metal, everything at once. I’ve got a real good buddy who’s going to be flying that airplane for the first time, and I’d just as soon have him around for a while.” [citation: Dennis Jenkins, X-15: Extending the Frontiers of Flight, NASA SP-2007-562, 1967, p. 203]

The engine was reliable, in part because it had thirty-seven dedicated people in the engine – maintenance shop at Edwards Air Force Base who obtained good results with the engine; 165 out of 169 successful engine operations indicated a
reliability of 97.6 percent. The total engine costs were initially estimated to be about $12.2 million, as originally bid. Because of many increases in scope during the design, the final costs were about $300 million.

Author Dennis Jenkins noted, “In retrospect the engine still casts a favorable impression.

The XLR99 pushed the state of the art further than any engine of its era, yet there were no catastrophic failures in flight or on the ground. There were, however, many minor design and manufacturing deficiencies. . . .”

X-15 Flight Summary

X-15 Pilots

Number of

Maximum Mach

Maximum Altitude

Flights

Number Achieved

Achieved (feet)

Scott Crossfield

14

2.97

86,116

Joseph A. Walker

25

5.92

354,200

Robert M. White

16

6.04

314,750

Forest S. Peterson

5

5.3

101,800

John B. McKay

29

5.65

295,600

Robert A. Rushworth

34

6.06

285,000

Neil A. Armstrong

7

5.74

207,500

Joe H. Engle

16

5.71

280,600

Milton O. Thompson

14

5.48

214,100

William J. Knight

16

6.7

280,500

William H. Dana

16

5.53

306,950

Michael J. Adams

7

5.59

266,000

Total Flights

199

Total Flight Time: 30 hours, 13 minutes, 49.4 seconds Total Distance Flown: 41, 763.8 statute miles

Times above Mach:

1

2

3

4

5

6

Hrs:Mins:Secs:

(cumulative)

18:16:28

12:10:44

8:51:33

5:58:52

1:25:33

0:1:18

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image91Front view of the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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

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04

 

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For the X-15 program to be a success,

the airplane and the pilots had to have a home—a physical facility for servicing the aircraft and a takeoff and landing area. Each flight required teams of support people on the ground as well as other pilots and airplanes in the air. All of these constituted the test arena.

EDWARDS AIR FORCE BASE

The X-15 flight tests occurred at Edwards Air Force Base, located about 100 miles northeast of Los Angeles. It is located on Rogers Dry Lake, a 44-mile-long pluvial lake in the Mojave Desert, which is the world’s largest pluvial lake (sometimes called paleolakes because they are caused by heavy rain during periods of glaciation). This dry lake maintains a smooth surface because winds consistently sweep the winter rains back and forth across the lakebed. Most of the year, the lakebed is dry and flat with a variation of height of only about 18 inches from one end to the other.

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

 

► X-15 run-up area at Edwards Air Force Base, 1958. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

There are a number of dry lakes in this high desert region, some of which made suitable alternate sites for the emergency landings that might occur, and occasionally did occur, during the flight-testing program. The lakebed had to be smooth enough and hard enough to support an airplane that landed on skids, without digging in and causing an accident, but also long enough for a normal landing. The maximum travel distance from launch to landing was set by the high – altitude flight, where the glide from altitude to landing required a 300-mile distance from launch to Edwards Air Force Base. The alternate fields selected were located within glide range at launch

along the path from the launch site to Rogers Dry Lake at Edwards.

The U. S. Army Air Force had used Rogers Dry Lake, then known as Muroc, since the 1930s. During World War II, the Army used the site for flight testing. The advantages of the site include the long, effective runway offered by the lakebed and the 15,000-foot concrete runway that had been built during the war. Other advantages that Rogers afforded were the good weather that enabled many flying days and the security of being essentially in the middle of nowhere, both of which ensured control over the flights. It also provided security for classified aircraft.

While Air Force personnel maintained tight security during the X-1 and X-2 flights, they were more relaxed with the X-15, primarily because it was a research airplane, not intended for combat. Edwards Air Force Base was where all the new military airplanes were tested, including airplanes of super-secret nature, earmarked for eventual combat. Thus, security was at a maximum. By the time of the X-15, however, research airplanes were viewed as just that, research tools. They were thus lower in the hierarchy of security. Most details of the X-15 airplane, the flight tests, and the data were not kept secret. Security for the X-15 was more in the nature of “watchman” and “housekeeping.” Those responsible made certain that no unauthorized people had access to the airplane, that tools were not left in the cockpit by accident, etc.

The first U. S. jet airplane, the Bell P-59, was tested on October 2, 1942, at Muroc by Bell’s chief
test pilot, Bob Stanley. When the X-1 outgrew the initial test site at Pinecastle, Florida, the Air Force selected Rogers Dry Lake for its subsequent flights. There, on October 14, 1947, Chuck Yeager flew the X-1 to the first supersonic flight, reaching a Mach number of 1.06 at 43,000 feet altitude. The NACA High Speed Flight Section under Walter Williams, who was responsible for the X-1 testing, continued in the testing of the Douglas D-558-2 and the Bell X-2 rocket-propelled aircraft, as well as other aircraft flown for test purposes before the creation of the X-15. The site also boasted the presence of the USAF Test Pilot School, whose pilots and aircraft supported the X-15 test flights in many ways, including flying chase aircraft deployed along the X-15 flight path.

The area was known as the high desert because Edwards Air Force Base was at 2,500 feet altitude and the alternate fields ranged up to 5,700 feet. Landing at an altitude higher than sea level requires

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DC-3 and C-130 support aircraft at Mud Lake. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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Flyover by the B-52. On the ground are the X-15, Piasecki X-21 helicopter, and ground support personnel and equipment. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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B-52 with the X-15 attached, taxiing before takeoff for its flight on November 3, 1965, with pilot Bob Rushworth in the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

a longer ground distance, since the air is less dense; thus, speed at landing has to be higher. Decelerating to stopping from a higher speed at landing by necessity requires a longer landing distance.

On November 9, 1962, X-15 pilot John McKay embarked on a routine flight to reach a Mach number of 5.5 and an altitude of 125,000 feet. Though McKay’s flight plan called for full power, the engine was putting out only 35 percent power, and ground control directed McKay to shut off the engine and land at Mud Lake, one of the emergency landing sites. McKay jettisoned some of the remaining fuel as required by protocol, but the routine emergency landing was complicated when the flaps didn’t deflect downward to increase lift, resulting in a dangerously high-speed landing at 257 knots. This caused a failure to the main landing skid, which in turn caused the left wing and stabilizer to dig into the lakebed, flipping the X-15 upside down.

McKay jettisoned his canopy during this flip – over, but his helmet was the first thing to hit the ground. The rescue crew and the fire truck sped to

the airplane. Fumes from the crash prevented them from approaching, but the H-21 helicopter pilot used his rotor blades to blow the fumes coming from the anhydrous ammonia fuel that leaked from the aircraft, so that rescue could proceed. The rescue crew was able to dig the ground out from under McKay and extract him.

A C-130 arrived with paramedics and more rescue personnel, and they flew McKay to Edwards Air Force Base before tending to the damaged X-15. The emergency preparation and actions saved McKay’s life and showed the crucial importance of alternate fields and the support teams who staffed them.

The X-15 pilots did not want to land at these alternate fields. They were for emergencies only. Landings there were the same as those as at Edwards—dead-stick landings with no power to make adjustments for height or location during landing, nor to abort the landing approach and go around to try again. In his book At the Edge of Space: The X-15 Flight Program, Milt Thompson summed up the pilots’ preferences:

Подпись: X-15 after engine failure forced pilot Jack McKay to crash-land upside down at Mud Lake, November 9, 1962. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
Rogers (dry lake) was where God intended man to land rocket airplanes. It was big. It had many different runways. It was hard.

It had no obstructions in any of the many approach paths. It had all of the essential emergency equipment. It was territory that we were intimately familiar with, and it had a lot of friendly people waiting there. In other words, it was home.

HIGHLIGHTS OF THE FLIGHTS

What is it like for a research test pilot to fly an X-15 airplane into unknown areas of speed and altitude? He arrives early in the morning, a good time for flight since the winds and temperature are lower at that time in this desert area. He goes to the physiological van at Edwards Air Force Base and there puts on his David Clark full-pressure

suit. He walks across the ramp to the airplanes, the B-52 and X-15. He climbs a large ladder to a platform next to the X-15, and then he enters the small X-15 cockpit. He prepares the airplane and himself for takeoff while the X-15 is attached to the B-52 mother plane.

The B-52 crew goes through a preflight list that includes the location, altitude, and velocity at which the X-15 was to be launched. They then start the engines and check that everything is okay with the pilot, who is captive in the X-15 under the wing. (All this was a much less severe routine than that required by the X-15 pilot in preparation for the flight, but their job to make sure the X-15 was safely launched was just as important.)

The B-52 takes off and climbs to altitude, about 45,000 feet. There the flight crew inside the B-52 prepares for the drop launch of the X-15, going through their checklist and topping off the liquid oxygen in the X-15, some of which has boiled off during the climb to launch altitude.

When all is ready, the B-52 drops the X-15, located underneath its right wing. The X-15 smoothly separates from the mother ship, usually with a roll to the right to compensate for the local airflow located under the right wing of the B-52. The X-15 pilot levels his airplane and lights up his engine. He accelerates away from the B-52 and, once clear, the pilot rotates his airplane to increase the angle of attack for climb to altitude.

Although the primary purpose of the X-15 was the acquisition of research data on the aerodynamics, thermodynamics, and flight dynamics of hypersonic flight, the quest for speed and altitude has been the driving force in the historical advancement of the airplane over the past 120 years. Therefore, obtaining maximum speed and maximum altitude was also important. However, the flight conditions required to obtain maximum speed are different than those to obtain maximum altitude.

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MAXIMUM SPEED

Here, the pilot continues his climb to altitude, then pushes over at zero lift until the airplane is in level flight at the desired altitude. He continues to fly at that altitude at full thrust until the maximum speed is obtained, which occurs when the fuel is used up. Zero lift means that the pilot adjusts the orientation of the airplane relative to the airflow ahead of the airplane (the angle of attack) so that the aerodynamic lift becomes zero, and he holds this until the X-15 is now moving in horizontal flight (level flight).

The airplane then starts to fall back to earth under the force of gravity, and it decelerates as the aerodynamic drag builds up at lower altitudes. During this return to earth, the airplane is in a steep glide, with a plan to reach an altitude of about 35,000 feet with a velocity of 290 to 350 miles per hour (called high key, which was the highest approach to the runway at Edwards Air Force Base). From there, he descends to an altitude of 18,000 feet, flying in the opposite direction of the landing runway (called low key on the flight trajectory). At this point, the airplane is about 4 miles from touchdown. The pilot continues in a 180-degree turn and then lands, probably at a speed of 200 miles per hour.

MAXIMUM ALTITUDE

After launch from the B-52, the X-15 continues to climb until the fuel is used up and then continues in an upward ballistic trajectory, reaching a maximum altitude determined by its kinetic energy at the point of engine burnout and the force of gravity. The airplane then begins to descend. The pilot then heads for home, reaches high key above Edward, descends, and lands as above. Because of the high altitude, the glide return is over a larger distance than the lower-altitude flights. For these flights, the airplane would be dropped at a greater distance from Edwards Air Force Base, sometimes as far as away as 300 miles, so that his glide ends at Edwards.

For most of the X-15 flights, the data gathering was done in the regions bounded by the maximum speed and the maximum altitude flights. The variation of Mach number and altitude during these flights is shown in the two Mach number/ altitude versus time-of-flight figures shown, one for a maximum speed flight and one for a maximum altitude flight.

The data obtained in the hypersonic region of these flights provided vital flight data points that were calibrated against analytical predictions and against wind tunnel data. The designing of aircraft

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ROLLOUT FROM THE NORTH AMERICAN FACTORY

Arrival of the first X-15 to Edwards Air Force Base. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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Unloading the X-15 upon arrival at Edwards Air Force Base. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

The welcoming crowd upon arrival of the X-15 to Edwards Air Force Base. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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PREPARATION FOR FLIGHT-UNDER THE WING OF THE B-52

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

TAKEOFF

 

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Detail of the mating of the X-15 with the B-52 for its first flight with external fuel tanks (empty), November 3, 1965. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Takeoff of the B-52 with the X-15 with external tanks, November 3, 1965. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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MATED WITH B-52 IN FLIGHT

 

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X-15 mated with the B-52 for one of its early contractor flights. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Takeoff of the B-52 with the X-15 mounted under the wing. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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LAUNCH

X-15 mounted under the wing of the B-52 mother ship at altitude. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

IN FLIGHT

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

LANDING ON LAKEBED AND RESTING ON

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

LAKEBED

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

X-15 after landing. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

Подпись: X-15A-2 with external fuel tanks on the ramp of the NASA Flight Research Center at Edwards. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

to fly in these regions, as well as vehicles to return from space, could proceed with confidence by knowing what corrections to make to the analyses and wind tunnel data. This data gathering and its correlation to analysis and wind tunnel results was the purpose of the X-15 research airplane program.

On October 3, 1967, Pete Knight achieved the maximum Mach number for the X-15, and he did it flying the modified version of the X-15, the X-15-A2, with additional fuel in the extended fuel tanks and with extra external fuel tanks. The extra fuel allowed more full thrust time, totaling 141 seconds—50 seconds more than the basic X-15 Nos. 1 and 2. After being in the X-15 for more than an hour under the wing of the B-52 while on the ground, Knight performed the preflight checklist and was lifted when the B-52 took off at 1:20 p. m. They headed for Mud Lake, over which the B-52 dropped him an hour later.

It took two launch attempts before the drop actually worked. Knight stated later that he “reached up and hit the launch switch and immediately took my hand off to [go] back to the throttle and found that I had not gone anywhere. It did not launch.” [citation: Jenkins, X-15: Extending the Frontiers of Flight, NASA SP-2007-562, 1967, p. 459] A second attempt 2 minutes later resulted in a smooth release. Pete then accelerated and climbed at an angle of attack of 12 degrees (angle between the wing chord and the free-stream airflow direction) at high lift until he reached a climb angle (angle between the horizontal and the flight path) of 32 degrees. He leveled off at 102,100 feet and reached a speed of 6,600 feet per second (Mach 6.7). This speed remains the fastest for a manned-powered airplane forty-seven years later, with no competitor airplane in sight.

Then, some unpleasant excitement occurred after burnout. Pete performed some rudder pulses to get data with the yaw damper off. As he decelerated through M=5.5, the “Hot Peroxide” warning light came on. On this particular flight, the X-15 was carrying a dummy supersonic combustion ramjet engine (scramjet) below its fuselage as part of a NASA hypersonic propulsion project. This was not an operating engine; it was a dummy engine being carried under the X-15 to examine the aerodynamic characteristics of the engine shape in full-scale hypersonic flight. The warning was caused by the aerodynamic heating generated by the shock wave from the dummy scramjet impinging on the bottom surface of the X-15. It severely damaged the airplane. Pete jettisoned the remaining peroxide to prevent it from exploding. The dummy scramjet was externally mounted in anticipation of future experiments. Shock waves also impinged on the vertical tail, with some melting and skin rollback.

The hot-peroxide event distracted Knight from energy management of the X-15, and he arrived at high key at supersonic speed rather than the desired, slower, subsonic speed. With this airspeed, the X-15 had too much kinetic energy. Pete then tried to jettison the ramjet, but nothing seemed to happen. He dissipated the excess kinetic energy by flying past the landing site, allowing aerodynamic drag to slow the airplane, and then landed at the proper speed. The dummy ramjet didn’t release at once when jettisoned, and it was later located on the lakebed after some clever reasoning and analysis by Johnny Armstrong of the Flight Planning Group.

Joe Walker flew the maximum altitude flight on August 22, 1963. In his prior flight on July 19, 1963, the maximum altitude planned by NASA for that flight had been 315,000 feet, but he unintentionally overshot that mark and achieved an altitude of 347,800 feet, close to the maximum altitude of 360,000 feet that NASA was ultimately seeking for the X-15. The airplane could go over

400.0 feet, but there was concern about the reentry from that altitude. It was deemed difficult but possible for the pilot to make a successful reentry from there, but NASA set a limit at

400.0 feet. Because of the risks of reentry from higher altitudes, they set the flight at 360,000 feet to allow for the inaccuracies of the engine and the ability of the pilot to hold to the tight limits of controlling the angle of attack.

The flight path was selected, with climb angles and fuel cut-off that were calculated to achieve their goal. The engine thrust could vary from 57,000 pounds to 60,000 pounds, and a difference of 1,500 pounds would result in a 7,500-feet altitude change. One second in fuel cut-off time would result in a 4,000-foot altitude change, and if the climb angle were off by one degree, a 7,500-foot change in altitude would result. The planned maximum altitude of the flight was set at 360,000 feet because it allowed a factor of safety. If some of the slight variations in engine thrust, fuel cut-off time, and climb angle took place, the inadvertent increase in altitude would not take the X-15 to over 400,000, where reentry was more dangerous.

This flight was delayed for about two weeks because of weather and airplane APU problems. The actual launch went well, and Walker stayed close to the flight plan. The propellants were depleted at 176,000 feet at a speed of 5,600 feet per second. The airplane continued to soar upward on a ballistic trajectory to 354,200 feet—two minutes after fuel burnout. At that point, Walker and the X-15 were 67 miles high.

After reaching peak altitude, the airplane headed home, some 306 miles away, and was moving at 5,500 feet per second when it passed through 176,000 feet. This was a mirror image of

Подпись:

Подпись: ASTRONAUT WINGS

its ballistic climb after fuel burnout. The pullout force at 5 g occurred at 95,000 feet, and the pilot maintained the high g pullout in order to level flight at 70,000 feet. The rest of the flight back to landing at Edwards Air Force Base was uneventful. The total time of flight was 11 minutes and 8 seconds. While 67 miles is well above the 50 miles required for the pilot to achieve official astronaut rating, it was not awarded to Joe Walker until forty-two years later, after he had died.

There was only one fatal accident during the whole X-15 flight-test program. On November 15, 1967, Michael Adams lost his life when a possible electrical disturbance affected his flight control

The Air Force pilots who flew the X-15 to altitudes above 50 miles all received Astronaut Wings, but NASA had decided not to give the same award to the civilian pilots who had made the same achievement. This caused controversy within the aerospace community. Finally, NASA reversed this policy, and in a ceremony on August 23,

2005, the three NASA pilots who flew the X-15 above 50 miles—William Dana, Jack McKay, and Joe Walker—were awarded Astronaut Wings.

image180Only Bill Dana was alive at that time to receive the certificate. However, the families of McKay and Walker were present to receive the honor.

Подпись: ABLATIVE COATING

Подпись: Mike Adams in the cockpit of the X-15 (mated to the B-52), in preparation for his first X-15 flight, October 6, 1966. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base Подпись: The X-15A-2 with its ablation coating. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
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system. This, combined with his possible vertigo, caused his X-15 to go out of control and break up at an altitude of approximately 62,000 feet during descent and crash to the desert floor. This flight underscored the risk involved in such flight testing. The details of this flight are given in Chapter 5.

THE B-52 CARRIER AIRCRAFT

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

Подпись:Подпись: X-15 landing with an F-104 chase plane alongside. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
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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Подпись: Top: X-15 mating area. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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.

NUMBER OF FLIGHTS CONDUCTED

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.

Подпись: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

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

B-52 in flight with the X-15 attached and the F-100 chase plane alongside. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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B-52 in flight with the X-15 attached and the T-38 chase plane alongside. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

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

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▲ 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

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

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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

Подпись: B-52 in flight with the X-15 attached and the T-38 chase plane nearby. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base
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,

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Edwards Air Force Base

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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

LATERAL DIRECTIONAL STABILITY AND THE MH-96 FLIGHT CONTROL SYSTEM

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.

PILOTS

T

Подпись: Bob White standing beside the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force Basehe 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.

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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

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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.

POWER ASSIST CONTROLS

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.

1921-2006

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

Подпись: Scott Crossfield in his pressure suit for a preflight Crossfield in the X-15 cockpit. USAF, Air Force briefing. USAF, Air Force Flight Test Center History Flight Test Center History Office, Edwards Office, Edwards Air Force Base Air Force Base
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.

Подпись: DOUGLAS D-558-2 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.

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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

Подпись: X-15 at rollout. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

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.

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Joe Walker in his flight suit going to the X-15. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base

 

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.

ROCKET CONTROLS IN SPACE

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.

Подпись: The stable platform used to mate the X-15 to the B-52 malfunctioned at the start of the first X-15 government flight on March 25, 1960. Nevertheless, the flight took place. It was also test pilot Joe Walker’s first X-15 flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base Подпись: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.