What are the APUs and why are they needed?
The orbiter has three independent hydraulic systems. Each consists of a main hydraulic pump, hydraulic reservoir, hydraulic bootstrap accumulator, hydraulic filters, control valves, hydraulic/Freon heat exchanger, electrical circulation pump, and electrical heaters. Each system provides hydraulic pressure to position hydraulic actuators for: • Thrust vector control of the main engines by gimbaling the three SSMEs • Actuation of various control valves on the SSMEs • Movement of the orbiter aerosurfaces (elevons, body flap, rudder/speed brake) • Retraction of the external tank/orbiter 17-inch liquid oxygen and liquid hydrogen disconnect umbilicals within the orbiter at external tank jettison • Main/nose landing gear deployment (system 1)/(system 1 or 2) • Main landing gear brakes and anti-skid • Nose wheel steering (system 1 with backup from system 2). Each hydraulic system is capable of operation when exposed to forces or conditions caused by acceleration, deceleration, normal gravity,
