RS-25

RS-25

RS-25 test firing. The bright area at the bottom of the picture is a Shock diamond
Country of origin	United States
First flight	April 12, 1981 (1981-04-12) (STS-1)
Designer	Rocketdyne
Manufacturer	Rocketdyne (1981–2005) Pratt & Whitney Rocketdyne (2005–2013) Aerojet Rocketdyne (2013–present)
Associated LV	Space Shuttle Space Launch System
Predecessor	HG-3
Status	In use
Liquid-fuel engine
Propellant	LOX / LH2
Mixture ratio	6.03:1
Cycle	Fuel-rich dual-shaft staged combustion
Configuration
Nozzle ratio	78:1[1]
Performance
Thrust, vacuum	2,279 kN (512,300 lbf)[1]
Thrust, sea-level	1,860 kN (418,000 lbf)[1]
Throttle range	67–109%
Thrust-to-weight ratio	73.1[2]
Chamber pressure	2,994 psi (20.64 MPa)[1]
Specific impulse, vacuum	452.3 s (4.436 km/s)[1]
Specific impulse, sea-level	366 s (3.59 km/s)[1]
Mass flow	514.49 kg/s (1,134.26 lb/s)
Dimensions
Length	4.3 m (168 in)
Diameter	2.4 m (96 in)
Dry mass	3,177 kg (7,004 lb)[2]
References
References	[3][2]
Notes	Data is for RS-25D at 109% of rated power level.

The RS-25, also known as the Space Shuttle Main Engine (SSME),^[1] is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is used on the Space Launch System (SLS).

Designed and manufactured in the United States by Rocketdyne (later Pratt & Whitney Rocketdyne and Aerojet Rocketdyne), the RS-25 burns cryogenic (very low temperature) liquid hydrogen and liquid oxygen propellants, with each engine producing 1,859 kN (418,000 lb_f) thrust at liftoff. Although RS-25 heritage traces back to the 1960s, its concerted development began in the 1970s with the first flight, STS-1, 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 specific impulse (I_sp) 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).^[1]

The Space Shuttle used a cluster of three RS-25 engines mounted at the stern of the orbiter, with fuel drawn from the external tank. The engines were used for propulsion throughout the spacecraft ascent, with total thrust increased by two solid rocket boosters and the orbiter's two AJ10 orbital maneuvering system 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.