6 November 1958: NASA Research Test Pilot John B. (Jack) McKay made the final flight of the X-1 rocketplane program, which had begun twelve years earlier.
Bell X-1E 46-063 made its 26th and final flight after being dropped from a Boeing B-29 Superfortress over Edwards Air Force Base on a flight to test a new rocket fuel.
When the aircraft was inspected after the flight, a crack was found in a structural bulkhead. A decision was made to retire the X-1E and the flight test program was ended.
The X-1E had been modified from the third XS-1, 46-063. It used a thinner wing and had an improved fuel system. The most obvious visible difference is the cockpit, which was changed to provide for an ejection seat. Hundreds of sensors were built into the aircraft’s surfaces to measure air pressure and temperature.
The Bell X-1E was 31 feet (9.449 meters) long, with a wingspan of 22 feet, 10 inches (6.960 meters). The rocketplane’s empty weight was 6,850 pounds (3,107 kilograms) and fully loaded, it weighed 14,750 pounds (6,690 kilograms). The rocketplane was powered by a Reaction Motors XLR11-RM-5 rocket engine which produced 6,000 pounds of thrust (26.689 kilonewtons). The engine burned ethyl alcohol and liquid oxygen. The X-1E carried enough propellants for 4 minutes, 45 seconds burn.
The early aircraft, the XS-1 (later redesignated X-1), which U.S. Air Force test pilot Charles E. (“Chuck”) Yeager flew faster than sound on 14 October 1947, were intended to explore flight in the high subsonic and low supersonic range. There were three X-1 rocketplanes. Yeager’s Glamorous Glennis was 46-062. The X-1D (which was destroyed in an accidental explosion after a single glide flight) and the X-1E were built to investigate the effects of frictional aerodynamic heating in the higher supersonic ranges from Mach 1 to Mach 2.
The X-1E reached its fastest speed with NASA test pilot Joseph Albert Walker, at Mach 2.24 (1,450 miles per hour/2,334 kilometers per hour), 8 October 1957. Walker also flew it to its peak altitude, 70,046 feet (21,350 meters) on 14 May 1958.
There were a total of 236 flights made by the X-1, X-1A, X-1B, X-1D and X-1E. The X-1 program was sponsored by the National Advisory Committee on Aeronautics, NACA, which became the National Aeronautics and Space Administration, NASA, on 29 June 1958.
17 September 1959: After previously making one glide flight, North American Aviation Chief Engineering Test Pilot Albert Scott Crossfield made the first powered flight of an X-15 hypersonic research rocket plane.
Carried aloft under the right wing of an eight-engine Boeing NB-52A Stratofortress bomber, USAF serial number 52-003, the first of three North American Aviation X-15s, 56-6670, was airdropped from 35,000 feet (10,668 meters) over Rosamond Dry Lake, 40 miles (64 kilometers) north of Edwards Air Force Base. Launch time was 08:08:48.0 a.m., Pacific Daylight Savings Time (15:08.48.0 UTC).
The X-15 was designed to use the Reaction Motors XLR-99 rocket engine, but early in the test program that engine was not yet available so two smaller XLR-11 engines were used. This was engine the same type used in the earlier Bell X-1 rocket plane that first broke the sound barrier in 1948. Though producing just one-fourth the thrust of the XLR-99, it allowed the functional testing of the X-15 to proceed.
Scott Crossfield wrote:
Two minutes after launch I reached 50,000 feet and pushed over in level flight. Then I dropped the nose slightly for a speed run, meanwhile maneuvering the ship through a series of turns and rolls, conscious of a deep rumbling noise of the rocket and a great rush of wind on the fuselage. It was obvious the black bird was in her element at supersonic speeds. She responded beautifully. I stared in fascination at the Mach meter which climbed from 1.5 Mach to 1.8 Mach and then effortlessly to my top speed for this flight of 2.3 Mach or about 1,500 miles and hour. Then, because I was under orders not to take the X-15 wide open, I shut off three of the rocket barrels. As I slowed down, I recalled the agony at Edwards many years before when we had worked for months pushing, calculating, polishing and who knows what else to achieve Mach 2 in the Skyrocket. Now with the X-15 we had reached that speed in three minutes on our first powered flight and I had to throttle back.
—Always Another Dawn, The Story Of A Rocket Test Pilot, by A. Scott Crossfield with Clay Blair, Jr., The World Publishing Company, Cleveland and New York, 1960. Chapter 39 at Pages 362.
The X-15 dropped 2,000 feet (610 meters) while Scott Crossfield ignited the two XLR-11 engines and then started “going uphill.” During the 224.3 seconds burn duration, the X-15 reached Mach 2.11 (1,393 miles per hour/2,242 kilometers per hour) and climbed to 52,300 feet (15,941 meters), both slightly higher than planned.
Problems developed when the rocket engine’s turbo pump case failed, and fire broke out in the hydrogen peroxide compartment, engine compartment and in the ventral fin. Crossfield safely landed on Rogers Dry Lake at Edwards Air Force Base. The duration of the flight was 9 minutes, 11.1 seconds. Damage to the rocket plane was extensive but was quickly repaired. 56-6670 flew again 17 October 1959.
Over the next nine years the three X-15s would make 199 flights, setting speed and altitude records nearly every time they flew, and expanding NASA’s understanding of flight in the hypersonic range. The first two X-15s, 56-6670 and 56-6671, survived the program. 670 is at the Smithsonian Institution National Air and Space museum and 671 is at the National Museum of the United States Air Force.
22 August 1953: After one successful glide flight with Bell Aircraft Corporation test pilot Skip Ziegler, the X-1D rocketplane, serial number 48-1386, was scheduled for its first powered flight with the Air Force project officer, Lieutenant Colonel Frank Kendall (“Pete”) Everest.
The Bell X-1D was one of four second-generation X-1 rocketplanes, each designed and built to investigate a different area of supersonic flight. The X-1D was instrumented for aerodynamic heating research.
After being carried to altitude by the Boeing EB-50A Superfortress mothership, Pete Everest saw that the rocketplane’s nitrogen pressure was dropping. (Pressurized nitrogen was used to push the ethyl alcohol/liquid oxygen propellant to the Reaction Motors XLR11-RM-5 engine.) With insufficient pressure, the X-1D’s flight had to be cancelled. Everest tried to jettison the fuel so that a landing could be made safely. There was an internal explosion.
Fearing that a larger explosion or fire would jeopardize the bomber and its crew, Everest abandoned the X-1D, climbing up into the bomber. The X-1 was then dropped. It crashed onto the desert floor and exploded.
At first it was assumed that vapors from a fuel leak had exploded from contact with an electrical source inside the rocketplane. There had been three similar explosions which resulted in the destruction of the X-1A, X-1-3 and the number two Bell X-2. That explosion, which occurred while the X-2 was on a captive test flight near the Bell Aircraft Corporation Factory, Buffalo, New York, 12 May 1953, killed test pilot Skip Ziegler and flight test engineer Frank Wolko aboard the B-29 mothership.
Investigators discovered that leather gaskets which were used in the rocketplanes’ fuel systems had been treated with tricresyl phospate (TCP). When this was exposed to liquid oxygen, an explosion could result. The leather gaskets were removed from the other rocketplanes and the explosions stopped.
Colonel Everest’s close call was dramatized in the 1956 Toluca Productions motion picture, “Toward The Unknown,” which starred Academy Award-winning actor William Holden as “Major Lincoln Bond,” a fighter pilot, test pilot and former prisoner of war, all of which could describe Pete Everest.
Frank Kendall (“Pete”) Everest, Jr., was born 10 August 1920, at Fairmont, Marion County, West Virginia. He was the first of two children of Frank Kendall Everest, an electrician, and Phyllis Gail Walker Everest. He attended Fairmont Senior High School, Fairmont, West Virginia, graduating in 1938, and then Fairmont State Teachers College where he was a member of the Tau Beta Iota (ΤΒΙ) fraternity. Everest also studied engineering at the University of West Virginia in Morgantown.
Pete Everest enlisted as an aviation cadet in the United States Army Air Corps at Fort Hayes, Columbus, Ohio, 7 November 1941, shortly before the United States entered World War II. His enlistment records indicate that he was 5 feet, 7 inches (1.70 meters) tall and weighed 132 pounds (60 kilograms). Everest graduated from pilot training and was commissioned as a second lieutenant, Air Reserve, 3 July 1942.
Lieutenant Everest married Miss Avis June Mason in Marion County, West Virginia, 8 July 1942. They would have three children, Frank, Vicky and Cindy.
Lieutenant Everest was appointed first lieutenant, Army of the United States (A.U.S.), 11 November 1942. He was assigned as a Curtiss-Wright P-40 Warhawk fighter pilot. Everest flew 94 combat missions with the 314th Fighter Squadron, 324th Fighter Group, in North Africa, Sicily and Italy. He was credited with shooting down two Luftwaffe Junkers Ju-52 transports, 18 April 1943, and damaging a third. Everest was promoted to the rank of captain, A.U.S., 17 August 1943.
In 1944, Captain Everest was returned to the United States to serve as a flight instructor. He requested a return to combat and was then sent to the China-Burma-India theater of operations, commanding the 17th Provisional Fighter Squadron at Chenkiang (Zhenjiang), China, where he flew 67 missions in the North American P-51 Mustang, and shot down four Japanese airplanes. He was himself shot down by ground fire in May 1945. Everest was captured by the Japanese and suffered torture and inhumane conditions before being freed at the end of the war. He was promoted to the rank of major, A.U.S., 1 July 1945. He was returned to the control of the United States military 3 October 1945.
After the war, Major Everest was assigned as a test pilot at Wright-Patterson Air Force Base, Ohio, before going west to the Air Force Flight Test Center at Edwards Air Force Base, California.
Everest’s permanent rank was advanced from second lieutenant, Air Reserve, to first lieutenant, Air Corps, 19 June 1947, with date of rank retroactive to 3 July 1945.
At Edwards, Pete Everest was involved in nearly every flight test program, flying the F-88, F-92, F-100 (he flew the YF-100A prototype to an FAI world speed record, 29 October 1953 ¹), F-101, F-102, F-104 and F-105 fighters, the XB-51, YB-52, B-57 and B-66 bombers. He also flew the pure research aircraft, the “X planes:” the X-1, X-1B, X-2, X-3, X-4 and X-5. Pete Everest flew the Bell X-1B to Mach 2.3, and he set an unofficial world speed record with the Bell X-2 at Mach 2.87 (1,957 miles per hour, 3,150 kilometers per hour), which earned him the title, “The Fastest Man Alive.” He was the pilot on thirteen of the twenty X-2 flights.
Frank Everest returned to operational assignments in March 1957, commanding the 461st Fighter Squadron, 36th Fighter Wing, equipped with the F-100 Super Sabre, at Hahn Air Base, Germany. Later, Colonel Everest commanded the 4453rd and 4520th Combat Crew Training Wings, and was assigned staff positions at the Pentagon. On 20 November 1963, Colonel Everest, commanding the 4453rd Combat Crew Training Squadron, flew one of the first two operational McDonnell F-4C Phantom II fighters from the factory in St. Louis to MacDill Air Force Base, Florida.
On 1 November 1965, Pete Everest was promoted to the rank of brigadier general. Between 1966 and 1972, General Everest flew 32 combat missions over Southeast Asia.
He served as commander of the Aerospace Rescue and Recovery Service from 1970 to 1973. He retired from the Air Force 1 March 1973 after 33 years of service. Pete Everest later worked as a test pilot for Sikorsky Aircraft.
During his military career, General Everest was awarded the Air Force Distinguished Service Medal; Legion of Merit with two oak leaf clusters (three awards); Distinguished Flying Cross with two oak leaf clusters (three awards); Purple Heart; Air Medal with one silver and two bronze oak leaf clusters (seven awards); Air Force Commendation Medal with one oak leaf cluster (two awards); Presidential Unit Citation with two bronze oak leaf clusters (three awards); Air Force Gallant Unit Citation; Prisoner of War Medal; American Campaign Medal; European-African-Middle Eastern Campaign medal with four bronze stars; Asiatic-Pacific campaign Medal with two bronze stars; World War II Victory Medal; national Defense Service Medal; Armed Forces Expeditionary Medal; Vietnam Service Medal; Air Force Longevity Service Award with one silver and two bronze oak leaf clusters (eight awards); Air Force Small Arms Expert Marksmanship Ribbon; and the Republic of Vietnam Campaign Medal with 1960– device. General Everest was rated as a Command Pilot, and a Basic Parachutist.
Brigadier General Frank Kendall Everest, Jr., United States Air Force, died at Tucson, Arizona, 1 October 2004, at the age of 84 years.
¹ FAI Record File Number 8868: World Record for Speed Over a Straight 15/25 Kilometer Course, 1,215.298 kilometers per hour (755.151 miles per hour)
4 June 1954: at Edwards Air Force Base, California, Major Arthur W. “Kit” Murray flew the experimental Bell X-1A research rocketplane to an altitude of 89,810 feet (27,374 meters). He flew high enough that the sky darkened and he was able to see the curvature of the Earth. Newspapers called him “America’s first space pilot.”
The X-1A reached Mach 1.97. Encountering the same inertial coupling instability as had Chuck Yeager, 20 November 1953, though at a lower speed, the X-1A tumbled out of control. The rocket plane lost over 20,000 feet (6,100 meters) altitude before Murray could regain control. For this accomplishment, Major Murray was awarded the Distinguished Flying Cross.
One week earlier, 28 May 1954, Murray had flown the X-1A to an unofficial world record altitude of 90,440 feet (27,566 meters).
Arthur Warren Murray was born at Cresson, Cambria County, Pennsylvania, 26 December 1918. He was the first of two children of Charles Chester Murray, a clerk, and Elsie Espy Murray.
Arthur Murray attended Huntingdon High School, Huntingdon, Pennsylvania, graduating 4 June 1936, and then studied Juniata College, also in Huntingdon, 1937–1938.
Kit Murray enlisted in the Field Artillery, Pennsylvania National Guard, 17 November 1939. (Some sources state that he served in the U.S. Cavalry.) Murray had brown hair and blue eyes, was 5 feet, 10 inches (1.78 meters) tall and weighed 150 pounds (68 kilograms). Following the United States’ entry into World War II, Sergeant Murray requested to be trained as a pilot. He was appointed a flight officer (a warrant officer rank), Army of the United States, on 5 December 1942. On 15 October 1943 Flight Officer Murray received a battlefield promotion to the commissioned rank of second lieutenant, A.U.S.
Between 6 January and 22 October 1943, Murray flew over 50 combat missions in the Curtiss-Wright P-40 Warhawk across North Africa. After about ten months in the Mediterranean Theater, he returned to the United States, assigned as an instructor flying the Republic P-47 Thunderbolt fighter bomber, stationed at Bradley Field, Hartford, Connecticut.
Lieutenant Murray married Miss Elizabeth Anne Strelic, who had immigrated from Czechoslovakia with her family as an infant, at Atlantic City, New Jersey, 29 December 1943. They would have six children, and foster a seventh. They later divorced. (Mrs. Murray died in 1980.)
Murray was promoted to 1st lieutenant, A.U.S., 8 August 1944. His next assignment was as a maintenance officer. He was sent to Maintenance Engineering School at Chanute Field, Rantoul, Illinois, and from there to the Flight Test School at Wright Field, Dayton, Ohio.
Murray was the first test pilot to be permanently assigned to Muroc Army Air Field (later, Edwards Air Force Base). Other test pilots, such as Captain Chuck Yeager, were assigned to Wright Field and traveled to Muroc as necessary.
Murray’s A.U.S. commission was converted to first lieutenant, Air Corps, United States Army, on 19 June 1947, with date of rank retroactive to 15 October 1946. The U.S. Air Force became a separate military service in 1947, and Lieutenant Murray became an officer in the new service.
Murray was involved in testing new Air Force fighters such as the Bell P-59 Airacomet, Lockheed P-80 Shooting Star, Republic P-84 Thunderjet, McDonnell XF-88 Voodoo; and the Douglas XB-43 Jetmaster and North American Aviation B-45 Tornado jet bombers. He also flew the experimental aircraft such as the X-1A, X-1B, X-4 and X-5. Murray spent six years at Edwards before going on to other assignments.
Later, 1958–1960, Major Murray was the U.S. Air Force project officer for the North American Aviation X-15 hypersonic research rocketplane at Wright Field.
Colonel Murray retired from the U.S. Air Force in 1961. He next worked for Boeing in Seattle, Washington, from 1961 to 1969, and then Bell Helicopter in Texas.
On 4 April 1975, Kit Murray married his second wife, Ms. Ann Tackitt Humphreys, an interior decorator, in Tarrant County, Texas.
Colonel Arthur Warren Murray, United States Air Force (Retired), died at West, Texas, 25 July 2011, at the age of 92 years.
The Bell X-1A was a follow-on project to the earlier X-1. It was designed and built by the Bell Aircraft Corporation at Buffalo, New York, to investigate speeds above Mach 2 and altitudes above 90,000 feet (27,432 meters). It was carried to altitude by a modified Boeing B-29 Superfortress, then dropped for the research flight.
The rocketplane was 35 feet, 7 inches (10.846 meters) long with a wingspan of 28 feet (8.534 meters) and overall height of 10 feet, 8 inches (3.251 meters). It had an empty weight of 6,880 pounds (3,120.7 kilograms) and gross weight of 16,487 pounds (7,478.3 kilograms).
The X-1A was powered by a Reaction Motors XLR-11-RM-5 four-chamber rocket engine which produced 6,000 pounds of thrust. It had a maximum speed of Mach 2.44 (Yeager) and reached an altitude of 90,440 feet (27,566.1 meters) (Murray).
The X-1A was destroyed by an internal explosion, 20 July 1955.
10 March 1959: With North American Aviation’s Chief Engineering Test Pilot Albert Scott Crossfield in its cockpit, the X-15 hypersonic research rocket plane was airborne for the first time. X-15A 56-6670 was carried aloft under the wing of the Boeing NB-52A Stratofortress drop ship, 52-003, for a series of captive flights. The purpose was to verify that all the systems on both the X-15 and the B-52 were properly functioning up to the point that the drop would occur.
Fully settled in my tiny flight office, I could speak by radio to the B-52 pilot, Charlie Bock, who was about thirty feet away in the nose of the mother plane, out of sight. . . .
As we sat, waiting at the end of the long runway while chase planes took off and circled, the clock on the instrument panel of the X-15 showed 0955. . . On signal, B-52 pilot Charlie Bock cobbed the eight engines, standing hard on the brake pedal. As the engines wound up to full military power, the X-15 trembled and the noise was tremendous. Through my radio earphones I heard Bock call a countdown for the benefit of the official movie cameramen who would record every inch of the takeoff:
“Five . . . four . . . three . . . two . . . one. BRAKE RELEASE.”
One hundred thirty tons of aluminum, fuel, Inconel X, five men and the hope of a nation began rolling down the long runway. . .
As we rolled, the huge runway distance markers flashed by, clocking our path: 14,000 . . . 13,000 . . . 12,000 . . . 8,000. When the X-15 air-speed indicator reached 170 knots, I noted only a minor vibration. We would continue the takeoff. 6,000 . . . 5,000 . . . 4,000, and we broke ground. It was smooth and gentle, like the take-off of an airliner. The air-speed indicator crept up to 260 knots. The parched brown desert fell away. . . .
—Always Another Dawn: The Story of a Rocket Test Pilot, by A. Scott Crossfield and Clay Blair, Jr., The World Publishing Company, Cleveland and New York, 1960, Chapters 34 and 35 at Pages 316–321.
The gross weight of the combined aircraft was 258,000 pounds (117,000 kilograms). After a takeoff roll of 6,200 feet (1,890 meters) the B-52/X-15 lifted of at 168 knots (193 miles per hour/311 kilometers per hour). During the 1 hour, 8 minute flight the the B-52 climbed to 45,000 feet (13,716 meters) and reached a speed of 0.83 Mach (548 miles per hour/881 kilometers per hour).
The X-15A rocketplane was designed and built for the U.S. Air Force and the National Advisory Committee for Aeronautics (NACA, the predecessor of NASA) by North American Aviation, Inc., to investigate the effects of hypersonic flight (Mach 5+). Design work started in 1955 and a mock-up had been completed after just 12 months. The three X-15s were built at North American’s Los Angeles Division, at the southeast corner of Los Angeles International Airport (LAX), on the shoreline of southern California.
The first flight took place 8 June 1959, again, with Scott Crossfield in the cockpit of the Number 1 ship, 56-6670.
While earlier rocketplanes, the Bell X-1 series, the the Douglas D-558-II, and the Bell X-2, were airplanes powered by rocket engines, the X-15 was a quantum leap in technology. It was a spacecraft.
Like the other rocketplanes, the X-15 was designed to be carried aloft by a “mothership,” rather than to takeoff and climb to the test altitude under its own power. The carrier aircraft was originally planned to be a Convair B-36 intercontinental bomber but this was soon changed to a Boeing B-52 Stratofortress. Two B-52s were modified to carry the X-15: NB-52A 52-003, The High and Mighty One, and NB-52B 52-008, Balls 8.
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.
One pilot, John B. (“Jack”) McKay, was seriously injured during an emergency landing at Mud Lake, Nevada, 9 November 1962. Michael James Adams, was killed when the Number 3 ship, 56-6672, went into a hypersonic spin and broke up on the program’s 191st flight, 15 November 1967.
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/rudder 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.
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.
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.
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.
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.