An acronym used by NASA to mean Space Shuttle Main Engine. The Shuttle has three of these per orbiter; they burn hydrogen and oxygen (thus producing water). They are only used while the External Tank is attached to the orbiter, as they use far too much fuel to be operated from internal stores. In order to feed them, turbopumps on the orbiter push approximately 7,000 gallons of fuel and oxidizer per second from the External Tank into their combustion chambers. That's enough to empty an Olympic swimming pool in around seven seconds. They have a specific impulse of around 500 seconds.

Fuel for the SSMEs, which is at cryogenic temperatures, is pumped through the walls of the engine bells before being burned. This way the fuel is prewarmed and the engines themselves are cooled.

The Space Shuttle can abort a launch in the seconds between SSME ignition (heard in the countdown as 'main engine start') and SRB ignition, since they are liquid fueled. Once the SRBs ignite, you're going somewhere, that's for sure.

Finally, the SSMEs can be throttled, or their thrust varied, due to the ability to control fuel flow. This is necessary to maintain a steady acceleration, since the mass of the shuttle decreases rapidly during ascent as it burns fuel; two minutes into the flight, the shuttle weighs less than half of what it did at launch. It is also used to manage dynamic pressure on the orbiter during the launch in order to not exceed structural load limits; fortunately, maximum dynamic pressure (called Max Q in shuttlespeak) is reached fairly early. Then atmospheric pressure begins to drop off rapidly, allowing the engines to be spooled up; this is what the phrase "Go for throttle-up" indicates during the ascent. During peak output, the SSMEs put out 109%of their rated thrust; a 5% safety margin means that maximum power used is 104%. This is possible because their 100% power output level was a fixed specification during the design phase of the Orbiter, but Rocketdyne produced engines which output 109% of that power level; NASA retained the power rating as designed and introduced the 5% safety margin.

SSME Block II Stats: (thank you, Boeing/Rocketdyne)

  • Specific Impulse: 452 seconds at full throttle
  • Max Thrust: 418,000 lbs. (STP) / 512,300 lbs. (in vacuum) ( > 12,000,000 horsepower)
  • Chamber pressure: 2,912 psi
  • Mixture Ratio - fuel:oxidizer: 6.03:1
  • Propellant/Oxidizer: Hydrogen/Oxygen
  • Size: 168 in. long / 90 in. wide
  • Weight: 7,774 lbs.