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 APU has successfully supported the more than 40 Space Shuttle orbital missions flown to date, and it continues to be the only flight-certified APU capable of man-rated space operation.Sundstrand is contracted to provide the APU to the Space Division of Rockwell International for the Orbiter Vehicle. The Space Shuttle Orbiter prime contract is administered by the NASA Johnson Space Center.