Space Shuttle Orbiter Auxiliary Power Unit Flight Performance

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.