The RS-25 was the rocket engine that powered the Space Shuttle Orbiter during ascent.
Each Space Shuttle Orbiter carried three RS-25 engines, also known as Space Shuttle Main Engines. They burned liquid hydrogen and liquid oxygen supplied from the large external tank. The engines ignited before liftoff and operated throughout the shuttle’s powered ascent, working together with the two solid rocket boosters.
The RS-25 was notable for its high efficiency, sophisticated turbopumps, and ability to be recovered and reused with the orbiter. It was developed by Rocketdyne and produced thrust that could be adjusted during flight. After the shuttle program ended, remaining engines and upgraded derivatives were assigned to NASA’s Space Launch System.
The RS-25 is not the same as the solid rocket boosters. It is also different from the F-1 engines used on Saturn V and the J-2 engines used on Saturn V’s upper stages.