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The United States Space Shuttle Orbiter Auxiliary Power Unit (APU) provides power to the orbiter vehicle hydraulic system. The system operates critical flight functions during a mission including aerodynamic control surface actuation, Space Shuttle main engine thrust vector control, and landing gear deployment, steering, and braking. The APU was designed in the mid-1970s, then flight-certified in support of the approach and landing tests (ALTs) in 1977 and initial orbital flight tests (OFTs) in 1981. During APU design, development, and flight certification, it was necessary to develop a number of technology items representing the state of the art to meet the challenging requirements of space operation. Improvements in the APU are continuing to further enhance life and reliability. More than 1,500 hours of operating time have been accumulated during APU development, certification, and Space Shuttle flights.

The Space Shuttle Orbiter auxiliary power units (APUs) provide hydraulic power for the Orbiter vehicle control surfaces (rudder/speed brake, body flap, and elevon actuation systems), main engine gimbaling during ascent, landing gear deployment, and steering and braking during landing. Operation occurs during launch/ascent, in-space exercise, re-entry/descent, and landing/rollout. Operational effectiveness of the APU is predicated on reliable, failure-free operation during each flight, mission life (reusability) and serviceability between flights (turnaround). The flight data from the Orbiter Flight Tests (STS-1 through-4) is presented along with improvements to ensure that these long-run objectives are achieved.

The Space Shuttle Orbiter auxiliary power units (APUs) provide hydraulic power for the Orbiter vehicle control surfaces (rudder/speed brake, body flap, and elevon actuation systems), main engine gimbaling during ascent, landing gear deployment and steering and braking during landing. Operation occurs during launch/ascent, in-space exercise, reentry/descent, and landing/rollout. Operational effectiveness of the APU is predicated on reliable, failure-free operation during each flight, mission life (reusability) and serviceability between flights (turnaround). Along with the accumulating flight data base, the status and results of efforts to achieve these long-run objectives is presented.