Fuel for the engines, liquid hydrogen and liquid oxygen, is contained in the external tank, the largest element of the Shuttle. Fuel is supplied from the tank at a rate of about 178,000 liters per minute of hydrogen and 64,000 liters per minute of oxygen. The SSME utilizes a two stage combustion process. This arrangement allows lower ullage pressures to be used in the propellant tanks and higher pump speeds. A pogo supression system accumulator is attached to the oxidizer low-pressure duct.
Oscillations can cause unstable combustion by disturbing the injection process.
The exiting GOX is recondensed to LOX in the vehicle oxidizer feed system. Each engine has three primary levels of thrust or power - minimum, rated, and full power. Engine thrust, however, can be varied throughout the range from minimum to full power level depending on mission needs. During the later part of ascent, engine thrust will be reduced to insure that an acceleration force of no more than three times that of Earth's gravity is reached. The SSME is the first rocket engine to use a built-in electronic digital controller.