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The North American X-15 rocket-powered aircraft was part of the X-series of experimental aircraft, initiated with the Bell X-1, that were made for the USAF, NASAmarker, and the USN. The X-15 set speed and altitude records in the early 1960s, reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. It currently holds the world record for the fastest speed ever reached by a manned aircraft.

During the X-15 program, 13 of the flights (by eight pilots) met the USAF spaceflight criteria by exceeding the altitude of 50 miles (80.47 km, 264,000 ft), thus qualifying the pilots for astronaut status. The USAF pilots qualified for USAF astronaut wings, while the civilian pilots also qualified for NASA astronaut wings.

Of all the X-15 missions, two flights (by the same pilot) qualified as space flights per the international (Fédération Aéronautique Internationale) definition of a spaceflight by exceeding a 100 kilometer (62.137 mi, 328,084 ft) altitude.

Engines & fuel

The X-15 had a long fuselage with short stubby wings and an unusual tail configuration. A Reaction Motors Inc XLR99 rocket engine generating of thrust powered the aircraft. This engine used ammonia and liquid oxygen for propellant and hydrogen peroxide to drive the high-speed turbopump that pumped fuel into the engine. This rocket could be throttled like an airplane engine and was the first such throttleable engine that was "man-rated" or declared safe to operate with a human aboard. Early flights used two Reaction Motors XLR11 engines.

Design and development

X-15 just after release.

The X-15 was based on a concept study from Walter Dornberger for the NACA for a hypersonic research aircraft.The requests for proposal were published on 30 December 1954 for the airframe and on 4 February 1955 for the rocket engine. The X-15 was built by two manufacturers: North American Aviation was contracted for the airframe in November 1955, and Reaction Motors was contracted for building the engines in 1956.

The first X-15 flight was an unpowered test flight by Scott Crossfield, on 8 June 1959; he also piloted the first powered flight, on 17 September 1959, with his first XLR-99 flight on 15 November 1960.

Like most X-series aircraft, the X-15 was designed to be carried aloft, under the wing of a B-52 bomber plane. The X-15 fuselage was long and cylindrical, with rear fairings that flattened its appearance, and thick, dorsal and ventral wedge-fin stabilizers. Parts of the fuselage were heat-resistant nickel alloy (Inconel-X 750). The retractable landing gear comprised a nose-wheel carriage and two rear skis. The skis did not extend beyond the ventral fin, which required the pilot to jettison the lower fin (fitted with a parachute) just before landing. The two XLR-11 rocket engines for the initial X-15A model delivered of total thrust; the main engine (installed later) was a single XLR-99 rocket engine delivering at sea level, and at peak altitude.

Before 1958, USAF and NACA, (later NASA), officials discussed an orbital X-15 spacecraft — the X-15B — for launching to outer space atop an SM-64 Navajo missile, that was cancelled when the NACA became the NASA, and Project Mercury was approved. By 1959, the X-20 Dyna-Soar space-glider program became the USAF's preferred means for launching military manned-spacecraft into orbit; the program was cancelled in the early 1960s.

Operational history

Three X-15s were built, flying 199 test flights, the last on 24 October 1968. Twelve test pilots flew the X-15; among them were Neil Armstrong (first man on the moon) and Joe Engle (a space shuttle commander). In July and August 1963, pilot Joe Walker crossed the 100 km altitude mark, joining the NASA astronauts and Soviet Cosmonauts as the only men to have crossed the barrier into outer space (Alan Shepard was the first American in space, reaching 187 km during suborbital flight, while Soviet Yuri Gagarin was the first human being in space, reaching 327 km in apogee of his orbital flight) and becoming the first to exceed this threshold twice.

U.S. Air Force Test pilot Major Michael J. Adams was killed on 15 November 1967 in X-15 Flight 191 when his craft (X-15-3) entered a hypersonic spin while descending, then oscillated violently as aerodynamic forces increased after re-entry. As his craft's flight control system operated the control surfaces to their limits, the craft's acceleration built to 15 g vertical and 8 g lateral. The airframe broke apart at 60,000 ft altitude, scattering the craft's wreckage for 50 square miles. On 8 June 2004, a monument was erected at the cockpit's locale, near Randsburg, Californiamarker. Major Adams was posthumously awarded Air Force astronaut wings for his final flight in craft X-15-3, which had reached 266,000 ft (81.1 km, 50.4 mi.) of altitude. In 1991, his name was added to the Astronaut Memorialmarker.

Bomber NB-52A (s/n 52-003), permanent test variant, carrying an X-15, with mission markings; horizontal X-15 craft silhouettes denote glide flights, diagonal silhouettes denote powered flights.

The second X-15A was rebuilt after a landing accident. It was lengthened , a pair of auxiliary fuel tanks attached under the fuselage, and a heat-resistant surface treatment applied. Re-named the X-15A-2, it first flew on 28 June 1964, reaching 7,274 km/h (4,520 mph, 2,021 m/s).

The altitudes attained by the X-15 aircraft do not match that of Alan Shephard's 1961 NASA space capsule flight nor subsequent NASA space capsules and space shuttle flights. However, the X-15 flights did reign supreme among rocket-powered aircraft until the third spaceflight of Space Ship One in 2004.

Five aircraft were the X-15 program: three X-15s, two B-52 bombers:

  • 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 in October 1969)
  • NB-52B - 52-008marker (retired in November 2004)

A 200th flight over Nevada was slated for 21 November 1968, piloted by William J. Knight. Technical problems and bad weather delayed the flight six times, and on 20 December 1968, the 200th flight was finally cancelled. The X-15 was unfastened from the wing of bomber NB-52A, and prepared for indefinite storage.


Current Static Displays

X-15 at the National Air and Space Museum


Stratofortress Motherships:
  • NB-52A (s/n 52-003) is at the Pima Air Museum, Tucson, Arizona - launched the X-15 #1 thirty times, the X-15#2 eleven times, and the X-15#3 thirty-one times (as well as the M2-F2 four times, the HL-10 eleven times and the X-24A twice).
  • NB-52B (s/n 52-008) is at the Dryden Flight Research Center, Edwards AFB, CA - Launched the majority of X-15 flights.

Specifications (X-15)

Record flights

Highest flights

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 (80.47 km) or more. However the FAI set the limit of space at 100 km. Thirteen X-15 flights went higher than 50 miles (80.47 km) and two of these reached over 62.137 miles (100 km).

X-15 flights higher than
Flight Date Top speed Altitude Pilot
Flight 62 17 July 1962 Robert M. White
Flight 77 17 January 1963 Joe Walker
Flight 87 27 June 1963 Robert Rushworth
Flight 90 19 July 1963 Joe Walker
Flight 91 22 August 1963 Joe Walker
Flight 138 29 June 1965 Joseph H. Engle
Flight 143 10 August 1965 Joseph H. Engle
Flight 150 28 September 1965 John B. McKay
Flight 153 14 October 1965 Joseph H. Engle
Flight 174 1 November 1966 Bill Dana
Flight 190 17 October 1967 Pete Knight
Flight 191 15 November 1967 Michael J. Adams
Flight 197 21 August 1968 Bill Dana

Fastest flights

X-15 10 fastest flights
Flight Date Top Speed Altitude Pilot
Flight 45 9 November 1961 Robert M. White
Flight 59 27 June 1962 Joe Walker
Flight 64 26 July 1962 Neil Armstrong
Flight 86 25 June 1963 Joe Walker
Flight 89 18 July 1963 Robert Rushworth
Flight 97 5 December 1963 Robert Rushworth
Flight 105 29 April 1964 Robert Rushworth
Flight 137 22 June 1965 John B. McKay
Flight 175 18 November 1966 Pete Knight
Flight 188 3 October 1967 Pete Knight

X-15 pilots

X-15 pilots and their achievements during the program
Pilot Organization Total










Michael J. Adams U.S. Air Force 7 1 0 5.59 3,822 50.3
Neil Armstrong NASA 7 0 0 5.74 3,989 39.2
Scott Crossfield North American Aviation 14 0 0 2.97 1,959 15.3
Bill Dana NASA 16 2 0 5.53 3,897 58.1
Joseph H. Engle U.S. Air Force 16 3 0 5.71 3,887 53.1
Pete Knight U.S. Air Force 16 1 0 6.70 4,519 53.1
John B. McKay NASA 29 1 0 5.65 3,863 55.9
Forrest S. Petersen U.S. Navy 5 0 0 5.3 3,600 19.2
Robert A. Rushworth U.S. Air Force 34 1 0 6.06 4,017 53.9
Milt Thompson NASA 14 0 0 5.48 3,723 40.5
Joe Walker U.S. Air Force 25 3 2 5.92 4,104 67.0
Robert M. White* U.S. Air Force 16 1 0 6.04 4,092 59.6
Killed * White was backup for Captain Iven Kincheloe

See also



  1. " | Aircraft Museum X-15"., 24 November 2008.
  2. Jenkins, Dennis R. Space Shuttle: The History of the National Space Transportation System: The First 100 Missions, 3rd edition. Stillwater, Minnesota: Voyageur Press, 2001. ISBN 0-9633974-5-1.
  3. "NASA astronaut wings award ceremony". NASA Press Release, 23 August 2005.
  4. Käsmann 1999, p. 105.
  5. X-15A Crash site
  6. United States Air Force Museum 1975, p. 73.


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