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Future fighters will require more compact, lighter weight, small gas turbine auxiliary power units (APUs) capable of faster starting, and operation, up to altitudes of 50,000 ft. The US Air Force is currently supporting an Advanced Components Auxiliary Power Unit (ACAPU) research program to demonstrate the technologies that will be required to accomplish projected secondary power requirements for these advanced fighters. The requirements of the ACAPU Program represent a challenging task requiring significant technical advancements over the current state-of-the-art, prominent among which are: Small high heat release high altitude airbreathing combustors. High temperature monolithic ceramic and metallic small turbines. Capability to operate, and transition from non-airbreathing to airbreathing modes. This paper discusses these challenging requirements and establishes technology paths to match and exceed the required goals.

A frequently used method of starting aircraft prime propulsion gas turbine engines is by means of a small auxiliary power unit (APU) providing compressed (bleed) air to an accessory drive gearbox-mounted air turbine starter. This APU may be carried on-board or mounted externally on ground support equipment. The majority of these APU's deliver a bleed pressure of approximately 4.0 at standard day conditions, as constrained by oil auto-ignition temperature limits no higher than 450-500°F for mostly commercial aircraft applications. The requirements for in-flight cross bleed starting of military engines at higher Mach numbers have necessitated increasing bleed internal air duct temperatures up to 1200°F and have afforded the feasibility of either higher APU bleed pressure ratios or power augmentation with the “bleed and burn” option.