Reaction Motors XLR-99

File:XLR-99 Rocket Engine USAF.jpg

The XLR-99 engine.

The XLR-99 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 I_sp 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 original X-15 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 many other liquid-fuel rocket engines, the XLR-99s used regenerative cooling, where 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 weighed 910 lb (or 413 kg).

External links

• NMUSAF REACTION MOTORS XLR99 ROCKET fact sheet
• XLR-99