Reaction Motors XLR99
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The XLR99 engine was the first large, throttleable, restartable liquid propellant rocket engine. It was developed beginning in the 1950s 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 (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 XLR99 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 XLR99 was capable, in theory, of between 20 and 40 flights before an overhaul.
Like many other liquid-fuel rocket engines, the XLR99s used regenerative cooling, in that the thrust chamber and nozzle had tubing surrounding it, in which the propellant and oxidizer passed before being combusted. This both kept the engine cool, and preheated the fuel. The basic engine itself weighs 910 lb, or 413 kg.
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