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A sign which, at one time, accompanied the SSME. It reads

Space Shuttle Main Engines

The engines, fueled with liquid hydrogen and liquid oxygen, fire for about 8.5 minutes during lift off and ascent - long enough to burn through more than a half million gallons of super-cold cryogenic liquid propellants stored in the huge external tank attached to the underside of the orbiter. The liquid oxygen is kept at -298°F (-183.33°C) and the liquid hydrogen is kept at at -423°F (-252.77°C). The engines shut down just before the orbiter reaches low Earth orbit.