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 <a href="Glossary">valves</a> in the SSMEs as well as the OrbiterÕs landing gear and brakes.
<p>
The hydraulic systems are each pressurized by a main pump to 20 685 <a href="Glossary">kilopascal</a>s (3000 pounds per square inch). These pumps are powered by the OrbiterÕs <a href="text">auxiliary power units (APUs)</a>. 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.