XLR-99 Rocket Engine at the Smithsonian

Developed by the Reaction Motors Division of Thiokol for North American, it was the first large man-rated rocket engine that could be throttled and with restart ability, and could deliver 57,000 pounds of thrust. Using this engine, the X-15 set many speed and altitude records in the 1960s. The propellants for the XLR99 were liquid oxygen and anhydrous ammonia, fed into the engine by turbine pumps at a flow rate of more than 10,000 pounds per minute.

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.