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The XLR99 was the first large, "man rated," throttleable, restartable liquid propellant rocket engine. The throttle setting could be varied from about 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 again be available later, if needed. The XLR99 was one of the rocket engines used in the X-15 manned research aircraft which was capable of propelling man to the fringes of space.

Developed and built by Reaction Motors Division of Thiokol Chemical Company, the XLR99 could deliver up to 57,000 lbs. of thrust, or the equivalent of about 500,000 horsepower. The propellants for the XLR99 were liquid oxygen (LOX) and anhydrous ammonia, fed into the engine by turbine pumps at a flow rate of more than 10,000 lbs. per minute.

The XLR99 engine had a rated operating life of one hour, after which it could be overhauled and used again, though operating times twice that long were demonstrated in tests. Since the basic X-15 carried fuel for about 83 seconds of full-power flight, and the X-15A-2 carried fuel for more than 150 seconds of full-power flight, each XLR99 was theoretically capable of between 20 and 40 flights before overhaul.

In common with other large scale liquid fueled rocket engines, the walls of the XLR99's thrust chamber were constructed of hollow tubing so that fuel could be routed through the tubes to cool the chamber walls before being burned in the engine. The basic weight of the engine is 910 lbs.

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