Reaction Motors XLR-99
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The XLR-99 engine was the first large, throttleable, restartable liquid propellant rocket engine. It was developed in the late 1940s by the Reaction Motors Division of Thiokol Chemical Company to steer the X-15 research aircraft. It could deliver up to 57,000 pounds force or 254 kN of thrust with an Isp of 279 seconds (239 seconds sl). The thrust could be varied from 50 to 100 percent, and the restart capability allowed it to be shut down and restarted during flight when necessary.
The engine was propelled by liquid oxygen and anhydrous ammonia, pumped into the engine by turbines at a flow rate of over 10,000 lb (4,500 kg) per minute.
After operating for one hour, the XLR-99 would have to be overhauled before another usage. Operating times nearly twice that were recorded in tests, but declared largely unsafe. The basic X-15 plane carried about 83 seconds of fuel for full-powered flight, while the X-15A-2 carried fuel for just over 150 seconds. Therefore each XLR-99 was capable, in theory, of between 20 and 40 flights before an overhaul.
Like other massive liquid-fuel rocket engines, the XLR-99's thrust chamber walls were made of hollow tubing so fuel could be routed through the tubes to cool the chamber walls before being burned in the engine. The basic engine itself weighs 910 lb, or 413 kg.
