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SPACE SHUTTLE MAIN ENGINES Fact Sheet
Written and Edited by

 

Length: 14 feet

Diameter: 7 feet, 6 inches



Space Shuttle Main Engines (SSME) are the most advanced liquid-fueled rocket engines ever built. They are manufactured by the Rocketdyne Division of Rockwell, located at Canoga Park, California.

Each Space Shuttle Orbiter has three SSME's mounted on the aft fuselage in a triangular pattern. Each SSME is designed for 7.5 hours of operation over an average lifespan of 55 starts.

SSME's burn a combination of liquid oxygen and liquid hydrogen fed from the Space Shuttle External Tank (ET). The SSME's employ a staged combustion cycle, in which the fuels are first partially burned at high pressure and low temperature, then burned completely at high pressure and high temperature.

The staged burning of fuel allows the SSME's to produce thrust more efficiently than other rocket engines. A rapid burning of fuel in a staged burn gives SSME's a combustion efficiency of about 99%.

SSME thrust is variable, which is extremely important in Space Shuttle mission applications. SSME thrust can be varied from a range of 65% minimum to 109% maximum of their 100% rated power levels at precise increments of 1% as needed.

A thrust value of 100%, called rated power, corresponds to a thrust of 375,000 pounds per each SSME at sea level, or 470,000 pounds of thrust per each SSME in a vacuum.

A thrust value of 104%, called full power, which is typically employed as the Space Shuttle ascends, corresponds to a thrust of 393,800 pounds per each SSME at sea level, or 488,800 pounds of thrust per each SSME in a vacuum.

In an emergency, each SSME may be throttled up to 109% power. This corresponds to a thrust of 417,300 pounds of thrust per each SSME at sea level, or 513,250 pounds of thrust per each SSME in a vacuum.

All three SSME's receive identical throttle commands at the same time. Throttle commands usually come from general purpose computers aboard the Orbiter. In an emergency, however, throttle commands may be controlled manually from the flight deck.

Firing of the three SSME's begins at Launch Minus 6.6 seconds, at which time general purpose computers aboard the Orbiter command a staggered start of each SSME. The first to fire is Main Engine Number Three (right), followed by Main Engine Number Two (left) and Main Engine Number One (center) at intervals of 120 milliseconds.

If all three SSME's do not reach a mandatory thrust of 90% over the course of the next three seconds, a Main Engine Cutoff command is initiated automatically, followed by the cutoff of all three SSME's and a number of safety functions.

If all three SSME's are performing normally, the Space Shuttle can be launched. The SSME's achieve full power at launch, but are throttled back at about Launch Plus 26 seconds in order to protect the Space Shuttle from aerodynamic stress and excessive heating.

The SSME's are throttled back up to full power at about Launch Plus 60 seconds, and typically continue to produce full power until shortly before the Space Shuttle reaches orbit. During ascent, each SSME may be gimbaled plus or minus 10.5 degrees pitch and yaw to help steer the Space Shuttle.

The SSME's typically burn for about 8.5 minutes after launch. At about Launch Plus 7 minutes, 40 seconds the SSME's are throttled down so that the Space Shuttle will not experience gravitational force in excess of three g's. Gravitational forces in excess of three g's might adversely affect the Space Shuttle and its crew.

At about ten seconds before Main Engine Cutoff (MECO), a MECO sequence begins. About three seconds later, the SSME's are commanded to begin throttling back at intervals of 10% thrust per second until they reach a thrust of 65% of rated power, called minimum power. Minimum power is maintained for just under seven seconds, then the SSME's shut down.

The SSME's are controlled during flight by digital computer systems mounted on each engine. These operate in conjunction with engine sensors, valve actuators and spark igniters to provide a redundant, self-contained system for monitoring engine control, checkout and status.

The SSME digital computer systems, called Main Engine Controllers, are mounted on the top of each SSME, on the outside of the combustion chamber. Each SSME has one Main Engine Controller, which consists of two digital computers and their related electronics.

In association with general purpose computers aboard the Orbiter, the SSME Main Engine Controllers are able to provide flight readiness verification, engine start and shutdown sequencing, closed-loop thrust and propellant mixture ratio control and sensor operation.

The Main Engine Controllers also produce valve actuator and spark igniter control signals, perform engine performance monitoring and limiting functions, respond to Orbiter commands plus transmit and store engine status, performance and maintenance data.

Following MECO, the SSME's have completed their function and are no longer needed during a Space Shuttle mission. SSME's are thoroughly inspected, checked and tested after each Space Shuttle flight. Each SSME may be replaced or changed out as necessary.

On rare occasions, SSME's are static tested on the launch pad in what is called a Flight Readiness Firing (FRF). An FRF is typically performed when engines that have never been flown are to be used for the first time during a Space Shuttle mission. In a typical FRF, all three SSME's are simultaneously fired for about ten seconds.


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