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KENNEDY SPACE CENTER, FLA. - In this view from the floor of the Orbiter Processing Facility, the first of three Space Shuttle Main Engines (SSME) is seen after installation. Discovery is the vehicle designated for the Return to Flight mission STS-114. Overall, an SSME weighs approximately 7,000 pounds. An SSME operates at greater temperature extremes than any mechanical system in common use today. The liquid hydrogen fuel is -423 degrees Fahrenheit, the second coldest liquid on Earth. When the hydrogen is burned with liquid oxygen, the temperature in the engine's combustion chamber reaches +6000 degrees Fahrenheit -- that's higher than the boiling point of Iron. The maximum equivalent horsepower developed by the three SSMEs is just over 37 million horsepower. The energy released by the three SSMEs is equivalent to the output of 23 Hoover Dams.