Encyclopedia Astronautica

Credit: USAF
Thiokol Lox/Ammonia rocket engine. 262.4 kN. Out of production. Isp=276s. The first large, man-rated, throttleable, restartable liquid propellant rocket engine, boosted the X-15A. First flight 1959.

The XLR-99 was the first large, man-rated, throttleable, restartable liquid propellant rocket engine. The throttle setting could be varied from 50 percent to 100 percent of thrust, and the restart capability allowed it to be shut down in flight with the assurance that power would be available later if needed. The XLR-99 was used in the X-15 manned research rocketplane and was developed and built by Reaction Motors Division of Thiokol Chemical Company. The propellants were liquid oxygen (LOX) and anhydrous ammonia, fed into the engine by turbine pumps at a flow rate of more than 4,500 kg per minute.

The XLR-99 engine had a rated operating life of one hour, after which it could be overhauled and used again. Operating times twice that long were demonstrated in tests. Each XLR-99 was capable of between 20 and 40 flights before overhaul (The X-15A carried fuel for 83 seconds of full-power flight; the X-15A-2 carried fuel for more than 150 seconds of full-power flight).

In common with other large scale liquid fueled rocket engines, the walls of the XLR-99's thrust chamber were constructed of hollow tubing through which fuel was routed to cool the chamber walls before being burned in the engine.

The XLR99-RM-1 Rocket Engine Development Program originated with a NACA study of an advanced research airplane utilizing a liquid rocket engine. When this program was approved as the X-15, a committee, composed of USAF, Bureau of Aeronautics and NASA representatives, made a survey of all rocket engines available and in development. As a result of this study, three engines were selected which were best suited as a starting point for meeting the X-15 engine performance requirements and the X-15 airplane schedule. These were the Bell XLR81-BA-1, Aerojet XLR73-AJ-1, and the Reaction Motors XLR30-RM-2. Final selection of the X-15 power plant was made by the airframe manufacturer. North American Aviation won the X-15 airframe competition and included the XLR30-RM-2 rocket engine as part of the winning proposal.

The development of the X-15 engine on an associated contractor basis and supply of necessary engines as GFE was the procurement policy established for this program. The engine contractor was Reaction Motors Division of Thiokol Chemical Corporation (RMD). The development program began February 1956. Early component tests proved the XLR30 rocket engine to be inadequate as a starting point; therefore, a completely new design was initiated and designated the XLR99-RM-1 rocket engine.

Thrust (sl): 227.300 kN (51,099 lbf). Thrust (sl): 23,174 kgf. Engine: 415 kg (915 lb). Chamber Pressure: 40.82 bar. Area Ratio: 9.8. Thrust to Weight Ratio: 64.4920327868853. Coefficient of Thrust vacuum: 1.87489910824557. Coefficient of Thrust sea level: 1.63479910824557.

Status: Out of production.
Diameter: 1.42 m (4.65 ft).
Thrust: 262.40 kN (58,990 lbf).
Specific impulse: 276 s.
Specific impulse sea level: 239 s.
Burn time: 90 s.
Number: 339 .

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Associated Countries
Associated Spacecraft
  • X-15B American manned spacecraft. Study 1958. North American's proposal for the Air Force initial manned space project was to extend the X-15 program. The X-15B was a 'stripped' X-15A with an empty mass of 4500 kg. More...
  • X-15A American manned spaceplane. 174 launches, 1959.06.08 (X-15 Flight 1) to 1968.10.24 (X-15 Flight 199). The X-15 was the first USAF and NASA project for manned spaceflight, initiated years before Mercury. More...
  • X-15A-3 American manned spaceplane. Cancelled 1962. It had been proposed that X-15 number 3 would be reworked to install a delta wing and designed to reach Mach 8. More...
  • X-15A-2 American manned spaceplane. Study 1965. The crash-damaged X-15 number 2 was rebuilt to attain even higher speeds. The body frame was stretched, and two drop tanks were added, increasing propellant load by 75%. More...
  • Mini-shuttle American manned rocketplane. Study 1972. In August 1972 it was proposed to test a subscale version of the shuttle to test the aerodynamics. The 13,750 kg vehicle would be 11 m long and have a wingspan of 7 m. More...

See also
Associated Launch Vehicles
  • X-15A American air-launched rocketplane. Manned hypersonic research rocket aircraft. More...
  • X-15A-2 American air-launched rocketplane. Manned hypersonic research rocket aircraft. Stretched rebuild of crashed X-15A, with drop tanks. Reached Mach 6.7 and 108 km altitude. More...

Associated Manufacturers and Agencies
  • Thiokol American manufacturer of rocket engines and rockets. Thiokol Corporation, Ogden, UT, USA. More...

Associated Propellants
  • Lox/Ammonia Liquid oxygen was the earliest, cheapest, safest, and eventually the preferred oxidiser for large space launchers. Its main drawback is that it is moderately cryogenic, and therefore not suitable for military uses where storage of the fuelled missile and quick launch are required. Ammonia (NH3) is a colourless gas and liquid with a strong irritating characteristic odour. More...

Associated Stages
  • X-15A Lox/Ammonia propellant rocket stage. Loaded/empty mass 14,184/5,158 kg. Thrust 262.45 kN. Vacuum specific impulse 276 seconds. More...
  • X-15A-2 Lox/Ammonia propellant rocket stage. Loaded/empty mass 16,069/7,766 kg. Thrust 262.45 kN. Vacuum specific impulse 276 seconds. More...

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