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The North American X-15 rocket plane was perhaps the most important of the USAF/USN X-series of experimental aircraft, after only possibly the Bell X-1. The X-15 set numerous speed and altitude records in the early 1960s, reaching the edge of space and bringing back valuable data that was used in the design of later aircraft and spacecraft.
During the X-15 program, 13 flights (by eight pilots) met the US criterion for a spaceflight by passing an altitude of 50 miles (80 km) and the pilots were accordingly awarded astronaut status by the USAF. Out of these, two flights (by the same pilot) also qualified for the international FAI definition of a spaceflight by passing the 62.1 mile (100 km) mark.
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History
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The original Request for Proposals was issued for the airframe December 30, 1954, and for the rocket engine on February 4, 1955. North American received the airframe contract in November 1955, and Reaction Motors contracted in 1956 to build the engines.
As with many of the X-aircraft, the X-15 was designed to be carried aloft under the wing of a B-52. The fuselage was long and cylindrical, with fairings towards the rear giving it a flattened look, and it had thick wedge-shaped dorsal and ventral fins. The retractable landing gear consisted of a nose wheel and two skids — to provide sufficient clearance part of the ventral fin had to be jettisoned before landing. The two XLR-11 rocket engines of the initial model X-15A delivered 36 kN (8,000 lbf) of thrust; the "real" engine that came later was a single XLR-99 that delivered 254 kN (57,000 lbf) at sea level, and 311 kN (70,000 lbf) at peak altitude.
The first flight was an unpowered test made by Scott Crossfield on June 8, 1959, who followed up with the first powered flight on September 17. The first flight with the XLR-99 was on 15 November 1960.
Three X-15s were built in all, and they made a total of 199 test flights, the last one on October 24, 1968. Plans were made for a 200th X-15 flight to be launched over Smith Ranch, Nevada. It was scheduled for November 21, 1968 with William J. Knight as the pilot. Various technical and weather delays caused the planned launch to slip at least six times until late December, 1968. Finally after a cancellation on December 20, 1968 due to weather, it was decided there would not be a 200th flight. The X-15 ground crew de-mated the aircraft from the NB-52A, and prepared it for indefinite storage. X-15
Twelve test pilots flew the plane, including Neil Armstrong, later the first man on the Moon and Joe Engle who went on to command Space Shuttle missions.
In July and August, 1963, pilot Joe Walker crossed the 100 km altitude mark twice, becoming the first person to enter space twice.
Test pilot Michael J. Adams was killed on November 15, 1967 when his X-15-3 began to spin on descent and then disintegrated when the acceleration reached 15 g (147 m/s²), scattering wreckage over 50 square miles. On June 8, 2004 a memorial monument was erected at the location of cockpit crash sitenear Randsburg, California. Michael Adams was posthumously awarded astronaut wings for his last flight in the X-15-3, which had attained an altitude of 266,000 feet (81.1 km). In 1991 Adams' name was added to the Astronaut Memorial at the Kennedy Space Center in Florida.
The second X-15A was rebuilt after a landing accident. It was lengthened by about 0.74 m (2.4 ft), received a pair of auxiliary fuel tanks slung under the fuselage, and was given a heat-resistant surface treatment, the result being called the X-15A-2. It first flew June 28, 1964, and eventually reached a speed of 7,274 km/h (4,520 mi/h or 2,021 m/s).
The altitudes attained by the X-15 remained unsurpassed by any piloted aircraft except the Space Shuttle until the 3rd spaceflight of SpaceShipOne in 2004. The speeds and altitudes have, also, frequently been exceeded by unpiloted air-launched rockets, such as the Pegasus rocket which has carried several satellites all the way into orbit. The widely reported record achieved by the diminutive X-43A scramjet testbed on November 16, 2004 of nearly Mach 10 (6,600 mph or 10,620 km/h or 2.95 km/s) at 95,000 ft (29 km) is only a record for an air-breathing jet engine.
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Specifications (X-15)
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General characteristics
Length: 50.7 ft (15.45 m)
Wingspan: 22.3 ft (6.8 m)
Wing area: 200 ft² (18.6 m²)
Empty: 14,600 lb (6,620 kg)
Loaded: 34,000 lb (15,420 kg)
Maximum takeoff: 34,000 lb (15,420 kg)
Powerplant: 1x Thiokol XLR99-RM-2 liquid-fuel rocket engine, 70,400 lbf (313 kN) thrust (at 30 km)
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Performance
Maximum speed: 4,520 mph (7,274 km/h) Mach 6.85
Range: 280 miles (450 km)
Service ceiling: 67 miles (108 km), 354,330 ft
Rate of climb: 60,000 ft/min (18,000 m/min)
Wing loading: kg/m² ( lb/ft²)
Serial Numbers: (Five main aircraft were involved in the X-15 program. The three X-15's and two B-52 carrier aircraft.)
X-15A-1 - 56-6670, 82 powered flights
X-15A-2 - 56-6671, 53 powered flights
X-15A-3 - 56-6672, 64 powered flights
NB-52A - 52-003 (retired October 1969)
NB-52B - 52-008 (retired November 2004)
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Highest flights
In the United States there are two definitions of how high a person must go to be referred to as an astronaut. The USAF decided to award astronaut wings to anyone who achieved an altitude of 50 miles (80 km) or more. However the FAI set the limit of space at 100 km. Thirteen X-15 flights went higher than 50 miles (80 km) and two of these reached over 100 km.
† fatal
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Fastest flights
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X-15 Pilots
† Capt Iven Kincheloe had been selected in September 1957, as the Air Force's project test pilot for the X-15 Program (with Capt Bob White as his alternate) but was killed the following year in an F-104 crash before the X-15 ever flew.
†† Killed on X-15 Flight 191.
These two pilots crossed the 100 km mark after leaving the X-15 program, Armstrong in Gemini and Apollo and Engle in the Shuttle program.
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Related content
Related development:
Comparable aircraft:
SpaceShipOne
See also:
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