RS-25
The Aerojet Rocketdyne RS-25, also 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 currently used on the Space Launch System (SLS).
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 42 years ago (STS-1) |
| Manufacturer | Rocketdyne, Pratt & Whitney Rocketdyne, Aerojet Rocketdyne |
| Associated LV | Space Shuttle Space Launch System |
| Predecessor | HG-3 |
| Status | Used on SLS after refurbishment |
| Liquid-fuel engine | |
| Propellant | Liquid oxygen / liquid hydrogen |
| Mixture ratio | 6.03:1 |
| Cycle | Fuel-rich dual-shaft staged combustion |
| Configuration | |
| Nozzle ratio | 78:1 |
| Performance | |
| Thrust, vacuum | 512,300 lbf (2.279 MN) |
| Thrust, sea-level | 418,000 lbf (1.86 MN) |
| Throttle range | 67–109% |
| Thrust-to-weight ratio | 73.1 |
| Chamber pressure | 2,994 psi (20.64 MPa) |
| Specific impulse, vacuum | 452.3 seconds (4.436 km/s) |
| Specific impulse, sea-level | 366 seconds (3.59 km/s) |
| Mass flow | 1,134.26 lb/s (514.49 kg/s) |
| Dimensions | |
| Length | 168 inches (4.3 m) |
| Diameter | 96 inches (2.4 m) |
| Dry weight | 7,004 pounds (3,177 kg) |
| References | |
| References | |
| Notes | Data is for RS-25D at 109% of rated power level. |
Designed and manufactured in the United States by Rocketdyne (later Pratt & Whitney Rocketdyne and Aerojet Rocketdyne), the RS-25 burns cryogenic liquid hydrogen and liquid oxygen propellants, with each engine producing 1,859 kN (418,000 lbf) 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 reliability, safety, and maintenance load.
The engine produces a specific impulse (Isp) of 452 seconds (4.43 kN-sec/kg) in a 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 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 are 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 the simplified RS-25E engine, also known as the Production Restart, currently under testing and development.