Reaction Motors/Thiokol XLR99

The XLR99 was used to power the X-15 research aircraft and was the first large liquid propellant rocket capable of being throttled and restarted. The engine used up to 10,000 lbs of fuel per minute.
The XLR99 was also known as the Pioneer.


  • XLR99-RM-2: 57,000 lb (250 kN) thrust (at high altitude)


  • Liquid-fuel rocket
  • Fuel: anhydrous ammonia
  • Oxidizer: liquid oxygen (LOX)

used by

  • North American X-15