The RS-25, otherwise known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is planned to be used on its successor, the Space Launch System. Built in the United States of America by Pratt & Whitney Rocketdyne, the RS-25 burns cryogenic liquid hydrogen & liquid oxygen propellants, with each engine producing 1,859 kN (418,000 lbf) of thrust at liftoff.
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After each flight the engines would be removed from the orbiter and transferred to the Space Shuttle Main Engine Processing Facility (SSMEPF), where they would be inspected and refurbished in preparation for reuse on a subsequent flight. A total of 46 reusable RS-25 engines, each costing around US$40 million, were flown during the Space Shuttle program, with each new or overhauled engine entering the flight inventory requiring flight qualification on one of the test stands at Stennis Space Center prior to flight.