Tag Archives: Reaction Motors XLR99-RM-1

9 November 1962

McKAY, John B. (Jack) with X-15 56-6672, 13 March 19649 November 1962: Flight 74 of the X-15 Program was the Number Two aircraft’s 31st flight. X-15 56-6671 was carried aloft by Balls 8, the Boeing NB-52B Stratofortress, 53-008, for launch over Mud Lake, Nevada. NASA test pilot John Barron (“Jack”) McKay was to take the rocketplane to 125,000 feet at Mach 5.5 to investigate the stability and handling of the X-15 with the lower half of the ventral fin removed, and to investigate aerodynamic boundary layer phenomena.

North American Aviation X-15 56-6671 under the right wing of a B-52 Stratofortress at 45,000 feet. (NASA)
North American Aviation X-15 56-6671 under the right wing of a B-52 Stratofortress at 45,000 feet. (NASA)

The B-52 mothership dropped Jack McKay and the X-15 right on schedule at 10:23:07.0 a.m., local time, from an altitude of 45,000 feet (13,716 meters) and speed of approximately 450 knots (833 kilometers per hour). McKay advanced the throttle to ignite the Reaction Motors XLR99-RM-1 rocket engine. It fired immediately but when McKay advanced the throttle for the full 57,000 pounds of thrust, the engine remained at just 30%.

The X-15 could have flown back to Edwards Air Force Base, about 200 miles (320 kilometers) to the south, but with the engine not responding to the throttle, it was uncertain that it would continue running. The decision was made to make an emergency landing at Mud Lake.

Having reached a peak altitude of 53,950 feet (16,444 meters) and Mach 1.49 (1,109 miles per hour/1,785 kilometers per hour), Jack McKay continued to circle the lake burning off propellants as he lost altitude. The engine was shut down at 70.5 seconds. McKay positioned the aircraft for landing as he continued to dump unused propellant and liquid oxygen, but a considerable amount remained on board.

As he neared touchdown, he tried to lower the flaps but they did not deploy. The X-15 touched down on the dry lake bed at 296 miles per hour (476.4 kilometers per hour), 66 miles per hour (106 kilometers per hour) faster than normal.

Duration of the flight from air launch to touchdown was 6 minutes, 31.1 seconds.

The high speed and extra weight caused the X-15’s rear skids to hit harder than normal. When the nose wheels hit, a rebound effect placed even higher loads on the rear struts. At the same time, with the elevators in an extreme nose-up position, the higher aerodynamic loads pushed the skids deeper into the lake bed. This higher loading caused the left rear strut to collapse. The X-15 rolled to the left and the left elevator dug into the lake bed. This caused the aircraft to start sliding to the left. Jack McKay jettisoned the canopy and as the right wing tip dug into the surface, the X-15 flipped over and came to rest upside down.

A Piasecki H-21 rescue helicopter lands near the overturned X-15 at Mud Lake, 9 November 1961. (NASA)
A Piasecki H-21 rescue helicopter lands near the overturned X-15 at Mud Lake, 9 November 1961. (NASA)
The X-15 rolled over when the left landing skid collapsed because of the high-speed, overweight emergency landing at Mud Lake, Nevada. Jack McKay was trapped in the cockpit and suffered serious spinal injuries. (NASA)
The X-15 rolled over when the left landing skid collapsed because of the high-speed, overweight emergency landing at Mud Lake, Nevada. Jack McKay was trapped in the cockpit and suffered serious spinal injuries. (NASA)
The Number Two X-15, 56-6671, lies upside down and severely damaged at Mud Lake, Nevada, 9 November 1962. (NASA)
The Number Two X-15, 56-6671, lies upside down and severely damaged at Mud Lake, Nevada, 9 November 1962. (NASA)

McKay was seriously injured. He was trapped in the upside down X-15 and was in danger from the vapors of the ammonia propellants and liquid oxygen. An H-21 rescue helicopter hovered overhead to blow the vapor away.

Prior to the flight, an Air Force C-130 had brought a fire engine and crew to standby at Mud Lake, returned to Edwards and picked up a second fire engine and its crew, then remained airborne should an emergency landing be made at another intermediate dry lake.

These propositioned emergency assets were able to rescue McKay and to transport him to the hospital back at Edwards.

McKay eventually recovered sufficiently to return to flight status, but ultimately his injuries forced him to retire.

The Number Two X-15 was severely damaged. It was taken back to North American and was rebuilt into the X-15A-2, intended to reach speeds up to Mach 8. It would be more than a year and a half before it flew again.

North American Aviation X-15A-2 56-6671, after a 19-month repair, redesign and modification program. The fuselage was lengthened, additional propellant and reaction control tanks installed internally, the nose wheel and rear landing skid struts lengthened, and external tanks installed. (NASA)
North American Aviation X-15A-2 56-6671, after a 19-month repair, redesign and modification program. The fuselage was lengthened, additional propellant and reaction control tanks installed internally, the nose wheel and rear landing skid struts lengthened, and external tanks installed. (NASA)

© 2016, Bryan R. Swopes

9 November 1961

Major Robert M. White was the first pilot to exceed Mach 4, Mach 5, and on 9 November 1961, he flew to Mach 6.04. (NASA)
Major Robert M. White was the first pilot to exceed Mach 4, Mach 5, and on 9 November 1961, he flew to Mach 6.04. (U.S. Air Force)

9 November 1961: Major Robert M. White, U.S. Air Force, became the first pilot to fly faster than Mach 6 when he flew the number two North American Aviation X-15 hypersonic research rocketplane, 56-6671, to Mach 6.04.

This was the 45th flight of the X-15 program, and Bob White’s 11th flight. The purpose of this test flight was to accelerate 56-6671 to its maximum velocity, to gather data about aerodynamic heating at hypersonic speeds, and to evaluate the rocketplane’s stability and handling.

Boeing NB-52A Stratofortress 52-003 carries a North American Aviation X-15 piloted by Major Bob White. (NASA)
Boeing NB-52A Stratofortress 52-003 carries a North American Aviation X-15 piloted by Major Bob White. (NASA)

The X-15 was carried to approximately 45,000 feet (13,716 meters) while mounted to a pylon under the right wing of the “mothership,” a Boeing NB-52B Stratofortress, 52-008, nicknamed Balls 8. White was dropped over Mud Lake, Nevada, approximately 200 miles (322 kilometers) north of Edwards Air Force Base. Once clear of the B-52, he ignited the Reaction Motors XLR99-RM-1 rocket engine, and with it producing 57,000 pounds of thrust (253.549 kilonewtons) at full throttle, the X-15 accelerated for 86.9 seconds. The rocketplane reached a peak altitude of 101,600 feet (30,968 meters). Its speed was Mach 6.04 (4,094 miles per hour/6,589 kilometers per hour).

White stated in his post-flight report, “When I leveled off at about 101,000 feet, I made a little downward pressure [on the control stick], because I didn’t want to be climbing. I remember . . . going along watching that [Mach] meter reading roughly 6,000 feet per second, [and] saying to myself, ‘Go, go, go, go!’ We did just crack it, because we knew that bringing all the proper things together, we could or should get just about Mach 6.”

In order to achieve the goal, the flight plan called for pushing the LR-99 to the point of exhaustion instead of manually shutting down the engine at an arbitrary point. White said, “The shutdown seemed to be a little bit different this time, compared with a shutdown by closing the throttle. It seemed to occur over a longer time interval.” 

The X-15 Rocket Plane: Flying the First Wings into Space, by Michelle Evans, University of Nebraska Press, Lincoln and London, 2013, Chapter 3 at Page 87.

The number two North American Aviation X-15, 56-6671, is dropped from the Boeing NB-52A Stratofortress, 52-003. The XLR99 rocket engine is just igniting. Frost from the cryogenic fuels coats the fuselage. (NASA)
The number two North American Aviation X-15, 56-6671, is dropped from the Boeing NB-52A Stratofortress, 52-003. The XLR99 rocket engine is just igniting. Frost from the cryogenic fuels coats the fuselage. (NASA)

“The airplane really did get hot on those flights. Temperatures in excess of 1,300 °F. were recorded. Parts of the airplane glowed cherry red and softened up a bit during those flights. The airplane got so damned hot that it popped and banged like an old iron stove. It spewed smoke out of its bowels and it twitched like frog legs in a skillet. But it survived.”

At the Edge of Space: The X-15 Flight Program, by Milton O. Thompson, Smithsonian Institution Press, Washington, 1992, at Page 98.

North American Aviation X-15 56-6671 accelerates after the XLR99 engine is ignited. (NASA)
North American Aviation X-15 56-6671 accelerates after the XLR99 engine is ignited. (NASA)

As the X-15 decelerated through Mach 2.4, the right side windshield shattered, leaving it completely opaque. On Bob White’s previous flight, the left windshield had also broken. Fortunately, in both cases, only the outer layer of the dual pane glass broke. The reduced visibility made the approach difficult to judge, but White made a successful landing, touching down on Rogers Dry Lake after a flight of 9 minutes, 31.2 seconds duration.

The number three North American Aviation X-15 rocketplane, 56-6672, just before touchdown on Rogers Dry Lake. A Lockheed F-104 Starfighter chase plane escorts it. The green smoke helps the pilots judge wind direction and speed. Frost on the X-15's belly shows residual propellants in the tanks. (NASA)
The number three North American Aviation X-15 rocketplane, 56-6672, just before touchdown on Rogers Dry Lake. A Lockheed F-104 Starfighter chase plane escorts it. The green smoke helps the pilots judge wind direction and speed. Frost on the X-15’s belly shows residual propellants in the tanks. (NASA)
NASA ET62-0270
The shattered windshield of X-15 56-6671, 9 November 1961. (NASA)

A P-51 Mustang fighter pilot in World War II, Robert M. White was shot down on his 52nd combat mission in February 1945 and captured. He was held as a prisoner of war until the war in Europe came to an end in April 1945. White was recalled to active duty during the Korean War. He was a graduate of the Air Force Experimental Test Pilot School and flew tests of many aircraft at Edwards before entering the X-15 program.

Major White had been the first pilot to fly faster than Mach 4, Mach 5 and Mach 6. He was the first to fly over 200,000 feet, then over 300,000 feet. He made at total of sixteen X-15 flights.

Major Robert M. White, U.S. Air Force, with one of the three North American Aviation X-15s on Rogers Dry Lake, 1961. (NASA)
Major Robert M. White, U.S. Air Force, with one of the three North American Aviation X-15s on Rogers Dry Lake, 1961. (NASA)

After leaving the X-15 program, Bob White returned to operational duties. Later, he flew 70 combat missions over North Vietnam in the Republic F-105 Thunderchief supersonic fighter bomber, including leading the attack against the Paul Doumer Bridge at Hanoi, 11 August 1967, for which he was awarded the Air Force Cross. He next went to Wright-Patterson AFB where he was director of the F-15 Eagle systems program. He returned to Edwards AFB as commander of the Air Force Flight Test Center. White was promoted to Major General in 1975.

General White retired from the U.S. Air Force in 1981. He died 10 March 2010.

A North American Aviation support crew deactivates X-15 56-6671 on Rogers Dry Lake after a flight, while the mothership, NB-52A Stratofortress 52-003 flies overhead. (NASA)
A North American Aviation support crew deactivates X-15 56-6671 on Rogers Dry Lake after a flight, while the mothership, NB-52A Stratofortress 52-003 flies overhead. (NASA)

© 2016, Bryan R. Swopes

3 November 1965

North American Aviation X-15A-2 56-6671 on Rogers Dry Lake. In addition to the lengthened fuselage and external tanks, the nose wheel strut is longer and the windshields have been changed to an oval shape. A wheeled dolly supports the aft end of the rocketplane. (NASA)
North American Aviation X-15A-2 56-6671 on Rogers Dry Lake. In addition to the lengthened fuselage and external tanks, the nose wheel strut is longer and the windshields have been changed to an oval shape. A wheeled dolly supports the aft end of the rocketplane. (NASA)

3 November 1965: Major Robert A. Rushworth made the first flight of the modified X-15A-2 rocketplane, Air Force serial number 56-6671. After a landing accident which caused significant damage to the Number 2 X-15, it was rebuilt by North American Aviation. A 28-inch (0.71 meter) “plug” was installed in the fuselage forward of the wings to create space for a liquid hydrogen fuel tank which would be used for an experimental “scramjet” engine that would be mounted on the the ventral fin. The modified aircraft was also able to carry two external fuel tanks. It was hoped that additional propellant would allow the X-15A-2 to reach much higher speeds.

The first flight with the new configuration was an “envelope expansion” flight, intended to test the handling characteristics of the X-15A-2, and to jettison the tanks (which were empty on this flight) to evaluate the separation and trajectory as they fell away from the rocketplane in supersonic flight.

Boeing NB-52A Stratofortress 52-003, The High and Mighty One, with North American Aviation X-15A-2 56-6671 mounted to the pylon under its right wing. The external propellant tanks have been brightly painted to aid tracking after they are jettisoned. (U.S. Air Force)
Boeing NB-52A Stratofortress 52-003, The High and Mighty One, with North American Aviation X-15A-2 56-6671 mounted to the pylon under its right wing. The external propellant tanks have been brightly painted to aid tracking after they are jettisoned. (U.S. Air Force)

The X-15A-2 was dropped from the Boeing NB-52A Stratofortress 52-003, over Cuddeback Lake, 37 miles (60 kilometers) northeast of Edwards Air Force Base in the Mojave Desert of southern California. This was the only time during the 199-flight X-15 Program that this lake was used as a launch point.

The X-15 was released at 09:09:10.7 a.m., PST. Bob Rushworth ignited the Reaction Motors XLR99-RM-1 rocket engine and it ran for 84.1 seconds before its fuel supply was exhausted. This engine was rated at 57,000 pounds of thrust (253.549 kilonewtons).

The X-15 climbed to 70,600 feet (21,519 meters) and reached Mach 2.31 (1,514 miles per hour/2,437 kilometers per hour.)

The test flight went well. The external tanks jettisoned cleanly and fell away. The recovery parachute for the liquid oxygen tank did not deploy, however, and the tank was damaged beyond repair.

Rushworth and the X-15A-2 touched down on Rogers Dry Lake after a flight of 5 minutes, 1.6 seconds.

© 2016, Bryan R. Swopes

24 October 1968

William Harvey Dana, NASA Research Pilot. (National Aeronautics and Space Administration E-5327)

24 October 1968: William Harvey Dana takes the first North America Aviation X-15 hypersonic research rocketplane, 56-6670, for the 199th and final flight of the X-15 program.

Carried aloft by NASA’s Boeing NB-52A Stratofortress mothership, 52-003, the first X-15A was launched over Smith Ranch Dry Lake, about half-way between the city of Reno and the NASA High Range Tracking Station at Ely, Nevada, at 10:02:47.3 a.m., Pacific Daylight Time (17:02:47.3 UTC). Bill Dana started the Reaction Motors XLR99-RM-1 rocket engine for a planned 84 second burn. The flight plan called for the X-15 to reach Mach 5.45 and 250,000 feet (76,200 meters).

The rocketplane’s performance was very close to plan, a tribute to Dana’s piloting skill. The engine burned out after 83.8 seconds. The maximum speed was slightly lower than planned at Mach 5.38, while the peak altitude was a little higher, at 255,000 feet (77,724 meters).

Dana glided back to Edwards Air Force Base. The total duration of the flight was 11 minutes, 28.3 seconds.

Dana made his first flight in the North American Aviation X-15 hypersonic research rocketplane on 4 November 1965. He reached a maximum speed of Mach 4.22, and a peak altitude of 80,200 feet (24,445 meters). He made a total of sixteen flights in the X-15s. Dana’s highest speed was Mach 5.38, and his highest altitude, 306,900 feet, (93,543 meters), on 1 November 1966.

This had been 56-6670’s 81st flight, and the 141st time it had been carried aloft aboard a B-52.

A. Scott Crossfield, wearing a David Clark Co. XMC-2 full-pressure suit, which he helped to design and test, with the first of three North American X-15s, 56-6670. (North American Aviation, Inc.)

56-6670 is the first of three X-15s built by North American Aviation’s Los Angeles Division for NASA, the United States Air Force, and the United States Navy, to investigate the effects of hypersonic flight (Mach 5+). On 8 June 1959, North American Aviation Chief Engineering Test Pilot, and former NACA research test pilot, Albert Scott Crossfield, had made the first glide flight of 56-6670, also launched by 52-003.

NASA Research Pilot William H. Dana with North American X-15A 56-6672 on Rogers Dry Lake. (NASA)

From 8 June 1959 to 24 October 1968, the three X-15s were flown by twelve test pilots, three of whom would qualify as astronauts in the X-15. Two would go on to the Apollo Program, and one, Neil Alden Armstrong, would be the first human to set foot on the surface of the Moon, 20 July 1969. Joe Engle would fly the space shuttle. Four of the test pilots, Petersen, White, Rushworth, and Knight, flew in combat during the Vietnam War, with Bob White being awarded the Air Force Cross. Petersen, Rushworth and White reached flag rank.

Overhead view of X-15A-1 56-6670 shows the rocketplane’s overall fineness ratio and short wings. (North American Aviation, Inc.)

Flown by a single pilot/astronaut, the X-15 is a mid-wing monoplane with dorsal and ventral fin/rudders and stabilators. The wing had no dihedral, while the stabilators had a pronounced 15° anhedral. The short wings have an area of 200 square feet (18.58 square meters) and a maximum thickness of just 5%. The leading edges are swept to 25.64°. There are two small flaps but no ailerons. The entire vertical fin pivots for yaw control.

Above 100,000 feet (30,840 meters) altitude, conventional aircraft flight control surfaces are ineffective. The X-15 is equipped with a system of reaction control jets for pitch, roll and yaw control. Hydrogen peroxide was passed through a catalyst to produce steam, which supplied the control thrusters.

The forward landing gear consists of a retractable oleo strut with steerable dual wheels and there are two strut/skids at the rear of the fuselage. The gear is retracted after the X-15 is mounted on the NB-52 and is extended for landing by its own weight.

The rocketplane’s cockpit featured both a conventional control stick as well as side-controllers. It was pressurized with nitrogen gas to prevent fires. The pilot wore an MC-2 full-pressure suit manufactured by the David Clark Company of Worcester, Massachusetts, with an MA-3 helmet. The suit was pressurized below the neck seal with nitrogen, while the helmet was supplied with 100% oxygen. This pressure suit was later changed to the Air Force-standardized A/P22S.

North American Aviation, Inc. X-15A 56-6670 on Rogers Dry Lake, Edwards Air Force Base, California. (NASA)

The X-15 is 50.75 feet (15.469 meters) long with a wing span of 22.36 feet (6.815 meters). The height—the distance between the tips of the dorsal and ventral fins—is 13.5 feet (4.115 meters). The stabilator span is 18.08 feet (5.511 meters). The fuselage is 4.67 feet (1.423 meters) deep and has a maximum width of 7.33 feet (2.234 meters).

The X-15s were built primarily of a nickel/chromium/iron alloy named Inconel X, along with corrosion-resistant steel, titanium and aluminum. Inconel X is both very hard and also able to maintain its strength at the very high temperatures the X-15s were subjected to by aerodynamic heating. It was extremely difficult to machine and special fabrication techniques had to be developed.

X-15 56-6670 with NB-52A 52-003, 13 April 1960. (NASA)

Since the X-15 was built of steel rather than light-weight aluminum, as are most aircraft, it is a heavy machine, weighing approximately 14,600 pounds (6,623 kilograms) empty and 34,000 pounds (15,422 kilograms) when loaded with a pilot and propellants. The X-15s carried as much as 1,300 pounds (590 kilograms) of research instrumentation, and the equipment varied from flight to flight. The minimum flight weight (for high-speed missions): was 31,292 pounds (14,194 kilograms) The maximum weight was 52,117 pounds (23,640 kilograms) at drop (modified X-15A-2 with external propellant tanks).

Initial flights were flown with a 5 foot, 11 inch (1.803 meters)-long air data boom at the nose, but this would later be replaced by the “ball nose” air sensor system. The data boom contained a standard pitot-static system along with angle-of-attack and sideslip vanes. The boom and ball nose were interchangeable.

Delays in the production of the planned Reaction Motors XLR99 rocket engine forced engineers to adapt two vertically-stacked Reaction Motors XLR11-RM-5 four-chamber rocket engines to the X-15 for early flights. This was a well-known engine which was used on the previous rocketplanes. The XLR11 burned a mixture of ethyl alcohol and water with liquid oxygen. Each of the engines’ four chambers could be ignited individually. Each engine was rated at 11,800 pounds of thrust (58.49 kilonewtons) at Sea Level.

Thiokol Reaction Motors Division XLR99-RM-1 rocket engine. (U.S. Air Force)

The Reaction Motors XLR99-RM-1 rocket engine was throttleable by the pilot from 28,500 to 60,000 pounds of thrust. The engine was rated at 50,000 pounds of thrust (222.41 kilonewtons) at Sea Level; 57,000 pounds (253.55 kilonewtons) at 45,000 feet (13,716 meters), the typical drop altitude; and 57,850 pounds (257.33 kilonewtons) of thrust at 100,000 feet (30,480 meters). Individual engines varied slightly. A few produced as much as 61,000 pounds of thrust (271.34 kilonewtons).

The XLR99 burned anhydrous ammonia and liquid oxygen. The flame temperature was approximately 5,000 °F. (2,760 °C.) The engine was cooled with circulating liquid oxygen. To protect the exhaust nozzle, it was flame-sprayed with ceramic coating of zirconium dioxide. The engine is 6 feet, 10 inches (2.083 meters) long and 3 feet, 3.3 inches (0.998 meters) in diameter. It weighs 910 pounds (413 kilograms). The Time Between Overhauls (TBO) is 1 hour of operation, or 100 starts.

The XLR99 proved to be very reliable. 169 X-15 flights were made using the XLR99. 165 of these had successful engine operation. It started on the first attempt 159 times.

The highest speed achieved during the program was with the modified number two ship, X-15A-2 56-6671, flown by Pete Knight to Mach 6.70 (6,620 feet per second/4,520 miles per hour/ kilometers per hour) at 102,700 feet (31,303 meters). On this flight, the rocketplane exceeded its maximum design speed of 6,600 feet per second (2,012 meters per second).

The maximum altitude was reached by Joe Walker, 22 August 1963, when he flew 56-6672 to 354,200 feet (107,960 meters).

The longest flight was flown by Neil Armstrong, 20 April 1962, with a duration of 12 minutes, 28.7 seconds.

The North American Aviation, Inc., X-15A-1, 56-6670, being brought into the Arts and Industries building, June 1969. (Smithsonian Institution Archives SI-A-4145-23-A)

North American Aviation X-15A-1 56-6670 is on display at the Smithsonian Institution National Air and Space Museum. X-15A-2 56-6671 is at the National Museum of the United States Air Force.

North American Aviation Inc./U.S. Air Force/NASA X-15A 56-6670, hypersonic research rocketplane on display at the National Air and Space Museum. (NASM)

© 2023, Bryan R. Swopes

3 October 1967

Major William J. Knight, United States Air Force, with the North American Aviation X-15A-2, 56-6671. (U.S. Air Force)

3 October 1967: The 188th flight of the X-15 Program was the 53rd for the Number 2 aircraft, 56-6671. It had been extensively modified by North American Aviation to an X-15A-2 configuration following a landing accident which had occurred 9 November 1962. The fuselage was lengthened 28 inches (0.711 meters) to accommodate a liquid hydrogen fuel tank for a scramjet engine that would be added to the ventral fin, a new tank for additional hydrogen peroxide to generate steam for the rocket engine turbo pump, and external propellant tanks to allow the rocketplane to reach higher speeds and altitudes. The entire surface of the X-15 was covered with an ablative coating to protect the metal structure from the extreme heat it would encounter on this flight.

Minor issues delayed the takeoff but finally, after they were corrected, and with Pete Knight in the X-15’s cockpit, it was carried aloft under the right wing of Balls 8, a Boeing NB-52B Stratofortress, 52-008.

At 45,000 feet (13,716 meters) over Mud Lake, Nevada, the X-15 was droppeded at 14:31:50.9 local time. Knight fired the Reaction Motors XLR99-RM-1 rocket engine and began to climb and accelerate. After 60 seconds, the ammonia and liquid oxygen propellants in the external tanks was exhausted, so the the tanks were jettisoned to eliminate their weight and aerodynamic drag.

The X-15A-2 climbed to 102,100 feet (31,120 meters) and Pete Knight leveled off, still accelerating. After 140.7 seconds of engine burn, Knight shut the XLR99 down. He noticed that thrust seemed to decrease gradually and the X-15 continued to accelerate to 6,630 feet per second (2,021 meters per second), or Mach 6.72.

North American Aviation X-15A-2 56-6671 is carried to launch altitude under the right wing of the Boeing NB-52B Stratofortress 52-008. (U.S. Air Force)
North American Aviation X-15A-2 56-6671 is carried to launch altitude under the right wing of the Boeing NB-52B Stratofortress 52-008. The scramjet is attached to the ventral fin. (U.S. Air Force)
North American Aviation X-15A-2 56-6671 immediately after being released from the mothership, Boeing NB-52B Stratofortress 52-008, Balls 8, over Mud Lake, Nevada, 3 October 1967. The steam trail is hydrogen peroxide used to power the rocket engine turbopump. (U.S. Air Force)
North American Aviation X-15A-2 56-6671 immediately after being released from the mothership, Boeing NB-52B Stratofortress 52-008, Balls 8, over Mud Lake, Nevada, 3 October 1967. The steam trail is hydrogen peroxide used to power the rocket engine turbopump. (U.S. Air Force) 
The North American Aviation X-15A-2 56-6671 ignites the XLR99 engine after being released from the mothership, Balls 8, 3 October 1967. (U.S. Air Force)
The X-15A-2’s XLR99-RM-1 rocket engine ignites after release from the mothership, Balls 8, 3 October 1967. (U.S. Air Force) 

Shock waves from the dummy scramjet mounted on the ventral fin impinged on the fin’s leading edge and the lower fuselage, raising surface temperatures to 2,700 °F. (1,482 °C.) The Inconel X structure started to melt and burn through.

Pete Knight entered the high key over Rogers Dry Lake at 55,000 feet (16,764 meters) and Mach 2.2, higher and faster than normal. As he circled to line up for Runway One Eight, drag from the scramjet caused the X-15 to descend faster and this set him up for a perfect approach and landing. Because of heat damage, the scramjet broke loose and fell away from the X-15.

Knight touched down after an 8 minute, 17.0 second flight. His 4,520 mile per hour (7,274 kilometers per hour) maximum speed is a record that still stands.

Firefighters cool down the ventral fin of the North American Aviation X-15A-2 56-6671 after its last landing on Rogers Dry Lake, 3 October 1967.(U.S. Air Force)
Firefighters cool down the ventral fin of the North American Aviation X-15A-2 56-6671 after its final landing on Rogers Dry Lake, 3 October 1967.(U.S. Air Force)

The X-15A-2 suffered considerable damage from this hypersonic flight. It was returned to North American for repairs, but before they were completed, the X-15 Program came to an end. This was 56-6671’s last flight. It was sent to the National Museum of the United States Air Force where it is part of the permanent collection.

In a ceremony at the White House, President Lyndon B. Johnson presented the Harmon International Trophy to Major William J. Knight.

The Harmon International Trophy at the Smithsonian Institution National Air and Space Museum. (NASM)
The Harmon International Trophy at the Smithsonian Institution National Air and Space Museum. (NASM)

© 2016, Bryan R. Swopes