Space Shuttle Main Engine

The Space Shuttle main engines (SSMEs) are the first rocket engines to be reused from one mission to the next. They are throttled during ascent, and the engines move - "gimbal" - to help steer the Shuttle.

The SSME uses a two-stage combustion process. Liquid hydrogen and liquid oxygen are pumped from the external tank and burned in two preburners. The hot gas from the preburners drives two high-pressure turbopumps - one for liquid hydrogen (fuel) and one for liquid oxygen (oxidizer).

The high-pressure pumps inject the fuel and oxidizer into the main combustion chamber, where the second stage of combustion occurs. The expansion of the hot gases through the chamber and the nozzle produces thrust. The resulting blue-white exhaust is mainly water vapor (super-hot steam).

In approximately 8 minutes, 40 seconds, the three SSMEs burn over 1.6 million pounds of propellant (approximately - 528,000 gallons).

Temperatures inside the main combustion chamber reach 6,000 degrees Fahrenheit, hot enough to melt steel. Meanwhile, liquid hydrogen circulates through miles of tubing at -423 degrees to cool the engines.

Turbomachinery in the pumps rotate at up to 37,000 revolutions per minute. These speed and temperature extremes are unmatched by any other machine in the world!

Each SSME is controlled by its own computer, which checks the health of the engines 50 times per second during countdown and ascent. The controller can shut an engine down if it detects a problem.

The SSMEs were designed in the late 1970s by Pratt & Whitney Rocketdyne - UTC. Pratt & Whitney Rocketdyne - UTC continues to build and assemble the majority of components in Canoga Park, California.

The SSME has undergone major upgrades to improve performance, safety and reliability. Improvements include the introduction of redesigned high-pressure turbopumps into the SSME fleet. The new pumps, designed and built by Pratt & Whitney at West Palm Beach Florida, have over 900 fewer welds than the previous design. The reduction in welds improves engine reliability and safety, and reduces the time spent on inspecting the engines.

The main engines are tested at Stennis Space Center in Mississippi. The new Pratt & Whitney Rocketdyne - UTC turbopumps are also tested at Stennis. Engines and engine components are delivered to Kennedy Space Center to be prepared for flight.

At approximately T-7 seconds, the engines are started, one at a time. When their controllers indicate they are running normally, the SRBs are ignited and the shuttle lifts off. The SSMEs throttle down to reduce stress during the period of maximum dynamic pressure, and then throttle up to 104.5 percent propelling the orbiter to 17,500 miles per hour. They then shut down and ride with the orbiter throughout the rest of the mission.

Engines that have completed a mission also undergo a series of inspections and testing prior to the next flight. After the orbiter lands at Kennedy Space Center, it’s towed to the orbiter processing facility. There, Pratt & Whitney Rocketdyne - UTC technicians remove the engines and conduct preliminary inspections. A transporter then takes the engines from the orbiter processing facility to the SSME shop. New or overhauled components, such as turbopumps, are integrated into the flight engines. Engines are then returned to the orbiter processing facility for installation into one of the orbiters.

The orbiter is then towed to the vehicle assembly building where it is mated to a new external tank and solid rocket boosters. Final checkouts are performed and the Space Shuttle main engines are now ready for another flight.

An SSME is fired during an acceptance test.	During launch, the three SSMEs reach full power an instant before booster ignition.

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