The Role of the Auxiliary Power Unit in Future Airplane Secondary Power Systems

Use of primary propulsion engines to supply secondary power for future airplane systems will incur increasing penalties due to technological and ecological considerations. This paper compares the relative merits of four secondary power system (SPS) concepts most likely to mature in the 1975-1985 time period. It also indicates that a promising payoff is available by the use of a dedicated auxiliary power unit (APU) SPS concept (in-flight APUs) for future near-sonic, long-range transports. Improvements to current technology APUs will be required before the APU can be further integrated in the SPS and can be depended upon for in-flight system power.