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Reaction Motors XLR99 Rocket

The XLR99 powered the record-breaking X-15 on its fastest flights at nearly seven times the speed of sound. It was the first large, throttleable, restartable liquid propellant rocket engine to be used in a piloted vehicle. The engine was used only in the X-15 program, which rocketed humans to the edge of space. The X-15A-2 in this gallery has an XLR99 engine.

Developed and built by the Reaction Motors Division of Thiokol Chemical Corp., the XLR99 could deliver more than 57,000 pounds of thrust. The engine used liquid oxygen and anhydrous ammonia propellants fed into the engine by turbopumps at a flow rate of more than 10,000 pounds per minute.

The XLR99 engine had an operating life of one hour, after which it could be overhauled and used again, though operating times twice that long were demonstrated in tests. The XLR99 was theoretically capable of between twenty and forty flights before overhaul. The basic X-15 carried propellants for about 83 seconds of full-power flight, and the X-15A-2 carried enough in its twin orange external tanks for more than 150 seconds of full-power flight.

Like other large liquid fueled rocket engines, the walls of the XLR99's thrust chamber included hollow tubing so that fuel could be routed through the tubes to cool the chamber walls before being burned in the engine.

50,000 lbs. at sea level; 57,000 lbs at 45,000 ft; 57,850 lbs. at 100,000 feet

Propellants: Liquid anhydrous ammonia fuel and liquid oxygen oxidizer
Weight: 915 lbs. including turbopump

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North American X-15A-2