RS-25 Engine

The RS-25, also known as the Space Shuttle Main Engine, is the world’s most reliable rocket booster engine. For three decades, these liquid hydrogen/liquid oxygen engines powered humans and payloads on all 135 Space Shuttle flights, enabling advancements such as the construction of the International Space Station, deployment and servicing of the Hubble Space Telescope, and furthered our scientific knowledge of the Universe.

As the nation shifts its vision toward deep space destinations, repurposed and modified RS-25 engines will support initial missions of NASA’s 70-metric ton Space Launch System (SLS). On the SLS, expendable versions of the engine will be used in a cluster of four to provide thrust for the launch vehicle’s core stage. While engines on the shuttle ran at 491,000 pounds vacuum thrust (104.5-percent of rated power level), the power level was increased for SLS to 512,000 pounds vacuum thrust (109 percent of rated power level) to augment the vehicle's heavy lift capability.

A total of 16 RS-25 flight engines are stored at NASA's Stennis Space Center. Teams began hot-fire testing in 2015.



(109% Power Level)

Specific Impulse
(109% Power Level)


Fuel: Hydrogen
Liquid Hydrogen Oxidizer: Liquid Oxygen Mixture Ratio (O/F): 6.0


Vacuum: 512,300 lb. Sea Level: 418,000 lb. Chamber Pressure: 2,994 psia

Vacuum: 452 sec.
Sea Level: 366 sec.

Length: 168 in.
Diameter: 96 in.
Weight: 7,775 lb.
Area Ratio: 69:1



Quick Facts & Key Milestones

  • May 1975 – First test firing
  • Nov. 1980 – 500th test firing
  • April 1981 – First Space Shuttle mission
  • July 1995 – First Block I engine upgrade flight
  • July 2001 – First Block II engine upgrade flight
  • Jan. 2004 – 1 millionth accumulated seconds test firing
  • June 2007 – First advanced health management upgrade flight
  • July 2011 – Last Space Shuttle mission
  • Jan. 2015 – First hot-fire test for SLS
  • 2019 – Planned first SLS Exploration Mission (EM-1) test flight