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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 Department of Transportation has initiated an effort to design, build, and test an organic Rankine cycle (ORC) bus engine. The objective of this effort is to demonstrate the feasibility of this engine in operating a 25-passenger bus. The engine design considerations and resulting configuration are described in this paper.