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. 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)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 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)
Captain Milburn Grant Apt, United States Air Force, with a Bell X-2. (U. S. Air Force)
27 September 1956: Captain Milburn G. (“Mel”) Apt, United States Air Force, was an experimental test pilot assigned to the Air Force Flight Test Center at Edwards Air Force Base, California. After Frank Everest and Iven Kincheloe had made twelve powered flights in the Bell X-2 supersonic research aircraft, Mel Apt was the next test pilot to fly it.
The X-2 was a joint project of the U.S. Air Force and NACA (the National Advisory Committee on Aeronautics, the predecessor of NASA). The rocketplane was designed and built by Bell Aircraft Corporation of Buffalo, New York, to explore supersonic flight at speeds beyond the capabilities of the earlier Bell X-1 and Douglas D-558-II Skyrocket.
In addition to the aerodynamic effects of speeds in the Mach 2.0–Mach 3.0 range, engineers knew that the high temperatures created by aerodynamic friction would be a problem, so the aircraft was built from stainless steel and K-Monel, a copper-nickel alloy.
The Bell Aircraft Corporation X-2 was 37 feet, 10 inches (11.532 meters) long with a wingspan of 32 feet, 3 inches (9.830 meters) and height of 11 feet, 10 inches (3.607 meters). Its empty weight was 12,375 pounds (5,613 kilograms) and loaded weight was 24,910 pounds (11,299 kilograms).
The second of two Bell X-2 supersonic research rocketplanes, 46-675, on its transportation dolly at Edwards Air Force Base, California, 1952. On 12 May 1953 this X-2 exploded during a captive test flight, killing Bell’s test pilot Jean L. “Skip” Ziegler. (NASA)
The X-2 was powered by a throttleable two-chamber Curtiss-Wright XLR25-CW-1 rocket engine that produced 2,500–15,000 pounds of thrust (11.12–66.72 kilonewtons)
Rather than use its limited fuel capacity to take off and climb to altitude, the X-2 was dropped from a modified heavy bomber as had been the earlier rocketplanes. A four-engine Boeing B-50D-95-BO Superfortress bomber, serial number 48-096, was modified as the drop ship and redesignated EB-50D.
The launch altitude was 30,000 feet (9,144 meters). After the fuel was exhausted, the X-2 glided to a touchdown on Rogers Dry Lake at Edwards Air Force Base.
Bell X-2 46-674 after drop from Boeing EB-50D Superfortress 48-096. (U.S. Air Force)
With Mel Apt in the cockpit on his first rocketplane flight, the B-50 carried the X-2 to 31,800 feet (9,693 meters). After it was dropped from the bomber, Apt ignited the rocket engine and began to accelerate. He passed Mach 1 at 44,000 feet (13,411 meters) and continued to climb. Apt flew an “extraordinarily precise profile” to reach 72,200 feet (22,007 meters) where he put the X-2 into a dive. The rocket engine burned 12.5 seconds longer than planned, and at 65,589 feet (19,992 meters) the X-2 reached Mach 3.196 (2,094 miles per hour, 3,377 kilometers per hour).
Milburn Apt was the first pilot to exceed Mach 3. He was The Fastest Man Alive.
Bell X-2 46-674 in flight over Southern California, 1955–56. Note the supersonic diamond-shaped shock waves in the rocket engine’s exhaust. (Bell Aircraft Corporation)
It was known that the X-2 could be unstable in high speed maneuvers. The flight plan called for Apt to slow to Mach 2.4 before beginning a gradual turn back toward Rogers Dry Lake where he was to land, but he began the turn while still at Mach 3. Twenty seconds after engine burn out, the X-2 began to oscillate in all axes and departed controlled flight. His last radio transmission was, “There she goes.” ¹
Mel Apt was subjected to acceleration forces of ± 6 Gs. It is believed that he was momentarily unconscious. Out of control, the X-2 fell through 40,000 feet (12,192 meters) in an inverted spin. Apt initiated the escape capsule separation, in which the entire nose of the X-2 was released from the airframe. It pitched down violently and Mel Apt was knocked unconscious again. He regained consciousness a second time and tried to parachute from the escape capsule, but was still inside when it hit the desert floor at several hundred miles per hour. Mel Apt was killed instantly.
Since 1950, Milburn G. Apt was the thirteenth test pilot killed at Edwards Air Force Base.
Wreckage of the Bell X-2, 46-674, in the Kramer Hills, east of Edwards Air Force Base. (U.S. Air Force)Wreckage of the Bell X-2, 46-674. (NASM 9A08208)
Milburn Grant Apt was born at Buffalo, Kansas, 8 April 1924. He was the third child of Oley Glen Apt, a farmer, and Ada Willoughby Apt.
“Mel” Apt enlisted as a private in the Air Corps Enlisted Reserve, United States Army, 9 November 1942. On 23 June 1943, Private Apt was appointed an Aviation Cadet. After completing flight training, Cadet Apt was commissioned a Second Lieutenant, Army of the United States (A.U.S.). He was promoted to First Lieutenant, A.U.S., 4 September 1945. Apt was released from active duty on 11 August 1946. On 10 October 1947, he was reclassified as a Second Lieutenant, Air Corps, United States Army, with date of rank 8 April 1945.
In February 1950, Lieutenant Apt, then stationed at Williams Air Force Base, Arizona, married Miss Faye Lorrie Baker of Phoenix. They would have two children.
Mel Apt earned a Bachelor of Science degree from the University of Kansas, Lawrence, Kansas, in 1951, and a second bachelor’s degree in aeronautical engineering from the Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio. He then attended the U.S. Air Force Experimental Test Pilot School at Edwards Air Force Base, California, graduating in September 1954. Apt was assigned to the Fighter Operations Branch, Air Force Flight Test Center, as a test pilot.
On 22 December 1954, Captain Apt was flying a chase plane during a test at Edwards. The test aircraft crash-landed on the dry lake and caught fire with its pilot trapped inside. Mel Apt, with his bare hands, rescued the other test pilot, saving his life. For this courageous act, he was awarded the Soldier’s Medal.
Captain Apt was posthumously awarded the Distinguished Flying Cross for his flight in the X-2. The medal was presented to his widow in a ceremony at Edwards in March 1957.
Captain Milburn Grant Apt, United States Air Force, was 32 years old at the time of his death. His remains were buried at the Buffalo Cemetery, Buffalo, Kansas.
Captain Iven Carl Kincheloe and Captain Milburn Grant Apt (seated in cockpit) with the Bell X-2 at Edwards Air Force Base, 1956. (Jet Pilot Overseas)
¹ Recommended: Coupling Dynamics in Aircraft: A Historical Perspective, by Richard E. Day, Dryden Flight Research Center, Edwards AFB, California NASA Special Publications 532, 1997.
Captain Iven Carl Kincheloe, Jr., United States Air Force
7 September 1956: At Edwards Air Force Base, California, test pilot Captain Iven Carl Kincheloe, Jr., U.S. Air Force, flew the Bell X-2 rocketplane, serial number 46-674, to a speed of Mach 1.7 and an altitude of 126,200 feet (38,465 meters). He was the first pilot to fly above 100,000 feet (30,480 meters) and was called “The First of the Spacemen.”
The X-2 was a joint project of the U.S. Air Force and NACA (the National Advisory Committee for Aeronautics, the predecessor of NASA). The rocketplane was designed and built by Bell Aircraft Corporation of Buffalo, New York, to explore supersonic flight at speeds beyond the capabilities of the earlier Bell X-1 and Douglas D-558-2 Skyrocket. In addition to the aerodynamic effects of speeds in the Mach 2.0–Mach 3.0 range, engineers knew that the high temperatures created by aerodynamic friction would be a problem, so the aircraft was built from Stainless Steel and K-Monel, a copper-nickel alloy.
Bell X-2 46-674 on approach to land. (NASA ED06-0174)
The Bell Aircraft Corporation X-2 was 37 feet, 10 inches (11.532 meters) long with a wingspan of 32 feet, 3 inches (9.830 meters) and height of 11 feet, 10 inches (3.607 meters). Its empty weight was 12,375 pounds (5,613 kilograms) and loaded weight was 24,910 pounds (11,299 kilograms).
The X-2 was powered by a throttleable Curtiss-Wright XLR25-CW-1 rocket engine that produced 2,500–15,000 pounds of thrust (11.12–66.72 kilonewtons) burning alcohol and liquid oxygen. The engine used two rocket chambers and had pneumatic, electrical and mechanical controls. The smaller chamber could produce a maximum 5,000 pounds of thrust, and the larger, 10,000 pounds (22.24 and 44.48 kilonewtons, respectively).
Professor Robert H. Goddard, “The Father of Modern Rocketry,” authorized Curtiss-Wright to use his patents, and his rocketry team went to work for the Curtiss-Wright Rocket Department. Royalties for use of the patents were paid to the Guggenheim Foundation and Clark University. Professor Goddard died before he could also make the move to Curtiss-Wright.
Rather than use its limited fuel capacity to take off and climb to altitude, the X-2 was dropped from a modified heavy bomber as had been the earlier rocketplanes. A four-engine Boeing B-50A Superfortress bomber, serial number 46-011, was modified as the ”mothership.” A second Superfortress, B-50D-95-BO 48-096, was also modified to carry the X-2, and was redesignated EB-50D
The launch altitude was 30,000 feet (9,144 meters). After the fuel was exhausted, the X-2 glided to a touchdown on Rogers Dry Lake at Edwards Air Force Base.
A Bell X-2 carried by Boeing EB-50D Superfortress 48-096. (U.S. Air Force)
Iven Kincheloe was awarded the Mackay Trophy for this flight. His altitude record remained unbeaten until the X-15 Project.
Iven Kincheloe stands in front of the Bell X-2 and the entire support team at Edwards Air Force Base. The “mothership” is a highly-modified Boeing EB-50D Superfortress. Chase aircraft are a North American F-86 Sabre, Lockheed T-33 Shooting Star, North American F-100 Super Sabre. The rescue helicopter is a Sikorsky H-19. (NASA)
Major William J. Knight, U.S. Air Force, with the modified X-15A-2, 56-6671, at Edwards Air Force Base, California. (U.S. Air Force)
21 August 1967: On the 186th flight of the X-15 program, the modified North American Aviation X-15A-2, 56-6671, made the first of two flights with a heat-protective ablative coating, designed to protect the steel structure of the rocketplane from the extreme heat of flight at high Mach numbers.
After a landing accident which caused significant damage to the Number 2 X-15, it was rebuilt by North American. 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 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 external tanks were not carried on the 21 August 1967 flight.
With Major William J. (“Pete”) Knight, U.S. Air Force, in the cockpit, the X-15A-2 was airdropped from the Boeing NB-52B Stratofortress, 52-008, known as Balls 8, over Hidden Hills Dry Lake, just on the California side of the border with Nevada. This was Knight’s 11th X-15 flight, and the 52nd flight for 56-6671. The launch time was 10:59:16.0 a.m., PDT. Knight fired the 57,000-pound-thrust Reaction Motors XLR99-RM-1 rocket engine and accelerated for 82.2 seconds. The purpose of this flight was to attain a high speed rather than altitude. The X-15A-2 reached Mach 4.94 (3,368 miles per hour, 5,420 kilometers per hour) at 85,000 feet (25,908 meters) and reached a peak altitude of 91,000 feet (27,737 meters). Pete Knight touched down on Rogers Dry Lake at Edwards Air Force Base, just 7 minutes, 40.0 seconds after launch.
4 August 1960: NASA research test pilot Joseph Albert Walker set an unofficial world speed record when he flew the number one North American Aviation X-15, 56-6670, to 2,195 miles per hour (3,532.5 kilometers per hour). This was the 18th flight of the X-15 Program. It was 56-6670’s eighth flight and Walker’s fourth X-15 flight. The purpose of this test was to gradually increase the rocket plane’s speed toward its design limit.
Airdropped from the Boeing NB-52A Stratofortress mothership, 52-003, over Silver Lake, near the California-Nevada border, at 08:59:13.0 a.m., PDT, Walker fired the X-15’s two Reaction Motors XLR11-RM-13 rocket engines for 264.2 seconds. The X-15 accelerated to Mach 3.31 and climbed to a peak altitude of 78,112 feet (23,810 meters). [The two XLR11s were used as an interim powerplant until the Reaction Motors XLR99 was ready. The combined thrust of both LR11s was only slightly more than the idle thrust of the XLR99.]
Walker touched down on Rogers Dry Lake at Edwards Air Force Base, California, after a flight of 10 minutes, 22.6 seconds.
Joe Walker with X-15 56-6670 on Rogers Dry Lake. (NASA)
