RS-25

The RS-25, also known as the Space Shuttle Main Engine (SSME), is a <a href="/facts/Liquid-fuel_rocket/TAonrVDn">liquid-fuel</a> <a href="/facts/Cryogenic_rocket_engine/MCUrNQ51">cryogenic rocket engine</a> that was used on <a href="/facts/NASA/TIMCV0yV">NASA</a>'s <a href="/facts/Space_Shuttle/7BoyetEv">Space Shuttle</a> and is used on the <a href="/facts/Space_Launch_System/DbGCgH1W">Space Launch System</a>.
Designed and manufactured in the United States by <a href="/facts/Rocketdyne/3sGkwgd8">Rocketdyne</a> (later <a href="/facts/Pratt_%2526_Whitney_Rocketdyne/HdQuCSzF">Pratt & Whitney Rocketdyne</a> and <a href="/facts/Aerojet_Rocketdyne/hN0hGfHI">Aerojet Rocketdyne</a>), the RS-25 burns <a href="/facts/Cryogenic_fuel/4RHxUvG8">cryogenic</a> (very low temperature) <a href="/facts/Liquid_hydrogen/Vpk3Gb8P">liquid hydrogen</a> and <a href="/facts/Liquid_oxygen/qnI0KDTb">liquid oxygen</a> propellants, with each engine producing 1,859 kN (418,000 lbf) <a href="/facts/Thrust/EM04CqV7">thrust</a> at liftoff. Although RS-25 heritage traces back to the 1960s, its concerted development began in the 1970s with the first flight, <a href="/facts/STS-1/DLnJNV3G">STS-1</a>, on April 12, 1981. The RS-25 has undergone upgrades over its operational history to improve the engine's thrust, reliability, safety, and maintenance load.
The engine produces a <a href="/facts/Specific_impulse/JKGgw2I4">specific impulse</a> (Isp) of 452 seconds (4.43 kN-sec/kg) in vacuum, or 366 seconds (3.59 kN-sec/kg) at sea level, has a mass of approximately 3.5 tonnes (7,700 pounds), and is capable of throttling between 67% and 109% of its rated power level in one-percent increments. Components of the RS-25 operate at temperatures ranging from −253 to 3,300 °C (−400 to 6,000 °F).
The Space Shuttle used a cluster of three RS-25 engines mounted at the stern of the <a href="/facts/Space_Shuttle_orbiter/pD50Txk9">orbiter</a>, with fuel drawn from the <a href="/facts/Space_Shuttle_External_Tank/RApJkcAq">external tank</a>. The engines were used for propulsion throughout the spacecraft ascent, with total thrust increased by two <a href="/facts/Space_Shuttle_Solid_Rocket_Booster/ETayF0Xh">solid rocket boosters</a> and the orbiter's two <a href="/facts/AJ10/3qW5la7q">AJ10</a> <a href="/facts/Space_Shuttle_Orbital_Maneuvering_System/r2fyByAg">orbital maneuvering system</a> engines. Following each flight, the RS-25 engines were removed from the orbiter, inspected, refurbished, and then reused on another mission.
Four RS-25 engines are installed on each Space Launch System, housed in the engine section at the base of the core stage, and expended after use. The first four Space Launch System flights use modernized and refurbished engines built for the Space Shuttle program. Subsequent flights will make use of a simplified RS-25E engine called the Production Restart, which is under testing and development.

RS-25 open-in-new

RS-25