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The YLR99 was the standard engine used in the X-15 series research aircraft. Developed and produced by the Reaction Motors Division of Thiokol Chemical Corp., the YLR99 delivered over 50,000 pounds of thrust, the equivalent of about 500,000 hp. Propellants for the YLR99 were liquid oxygen and anhydrous ammonia, fed by turbine pumps at a flow rate of more than 10,000 pounds per minute (20 times faster than an F-100 jet fighter). The throttle settings could be varied from about 50 percent to 100 percent of thrust, and the engine had an in-flight shut-down and restart capability.
The YLR99 had a rated operating life of one hour, but operating times twice that were demonstrated in tests. After one hour of operation the engine could be overhauled for reuse. Since the basic X-15 carried fuel for about 83 seconds of full-powered flight, and the X-15A-2 carried fuel for over 150 seconds of full-power flight, each YLR99 was theoretically capable of 20 to 40 flights before overhaul, depending upon the aircraft in which it was installed.
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