XLR-99 Credit: USAF. 37,585 bytes. 440 x 256 pixels.

The XLR-99 was the first large, man-rated, throttleable, restartable liquid propellant rocket engine. The throttle setting could be varied from 50 percent to 100 percent of thrust, and the restart capability allowed it to be shut down in flight with the assurance that power would be available later if needed. The XLR-99 was used in the X-15 manned research rocketplane and was developed and built by Reaction Motors Division of Thiokol Chemical Company. The propellants were liquid oxygen (LOX) and anhydrous ammonia, fed into the engine by turbine pumps at a flow rate of more than 4,500 kg per minute.

The XLR-99 engine had a rated operating life of one hour, after which it could be overhauled and used again. Operating times twice that long were demonstrated in tests. Each XLR-99 was capable of between 20 and 40 flights before overhaul (The X-15A carried fuel for 83 seconds of full-power flight; the X-15A-2 carried fuel for more than 150 seconds of full-power flight).

In common with other large scale liquid fueled rocket engines, the walls of the XLR-99's thrust chamber were constructed of hollow tubing through which fuel was routed to cool the chamber walls before being burned in the engine.