First Man’s first Rocket Engine, from the X-15

“XLR-99 restartable liquid propellant rocket engine nozzle for use in the North American X-15 experimental aircraft, flown by such men as Neil Armstrong, Scott Crossfield and Joe Engle. This one has definitely been fired.

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.

This engine measures about five feet in overall length with end exhaust diameter of 30″, and weighs approximately 200 pounds.” — Regency Superior

The X-15 engines needed to be serviced frequently, creating flown spares:
“After one hour of operation, the XLR99 required an overhaul. Operating times nearly twice that were recorded in tests, but declared largely unsafe.

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.” — Wikipedia