The word ÒhydraulicÓ means something that is moved or operated by a fluid under pressure. The hydraulic systems on the Orbiter are used to operate the vehicleÕs exterior steering and flying structures (the rudder, speed brake , elevons and body flaps ). They also operate the control valves in the SSMEs as well as the OrbiterÕs landing gear and brakes.

The hydraulic systems are each pressurized by a main pump to 20 685 kilopascals (3000 pounds per square inch). These pumps are powered by the OrbiterÕs auxiliary power units (APUs). The fluid used in the OrbiterÕs three hydraulic systems is a special compound made of hydrogen and carbon and has been designed to reduce fire hazards.