The Reaction Motors LR99 engine was the first large, throttleable, restartable liquid-propellant rocket engine. Development began in the 1950s by the Reaction Motors Division of Thiokol Chemical Company to power the North American X-15 hypersonic research aircraft. It could deliver up to of thrust with a specific impulse of or at sea level. Thrust was variable from 50 to 100 percent, and the restart capability allowed it to be shut down and restarted during flight when necessary. Design and development The engine is propelled by liquid oxygen and anhydrous ammonia, pumped into the engine by turbopumps at a mass flow rate of over per minute. After one hour of operation, the XLR99 required an overhaul. Operating times nearly twice that were recorded in tests, but declared largely unsafe. The basic X-15 aircraft carried fuel for about 83 seconds of 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, through which the propellant and oxidizer passed before being burned. This kept the engine cool, and preheated the fuel. The basic engine has a mass of . Operational history The LR-99 was used exclusively to power the X-15 research aircraft after initial trials that used a pair of Reaction Motors XLR11s. Applications North American X-15 Variants XLR99-RM-1 prototype engines for initial testing and flight trials. YLR99-RM-1 Service test engines fitted to X-15s for later flights. Specifications (YLR-99-RM-1) See also Reaction Motors XLR11 References External links Reaction Motors XLR99 Rocket – National Museum of the United States Air Force XLR-99 Aircraft rocket engines Rocket engines of the United States Rocket engines using the gas-generator cycle