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Current gas turbine auxiliary power units (APUs) for aircraft main engine starting and secondary power operation are configured with pneumatic and electric links, and in some applications with a direct shaft power link. Respective examples are the pneumatic/electric link used in the S-70B helicopter and the shaft power link in the FSX fighter aircraft. Future high performance aircraft will require more compact and lighter weight APUs. These APUs will require higher thermal efficiency and durable self-sufficient secondary power equipment, capable of reliable faster starting over a wider aircraft operating range. Gas turbine auxiliary power design configurations and technology requirements to meet these objectives are examined here for both commercial and military aircraft applications.

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.

Advanced military fixed and rotary wing aircraft will require more compact, lighter weight small gas turbine auxiliary power units (APU's) and jet fuel starters (JFS), capable of faster starting and delivering high specific power outputs over wider operating envelopes. APU start system weights can be a significant fraction of the total installed APU weight, especially if fast starts are demanded under sub-Arctic environments. It is shown that both start system weight, and rotor containment armor weight are proportional to the product of rotational speed squared and rotating assembly inertia. Significant weight savings are therefore feasible with rotating assemblies using lower density materials such as ceramics and composites. This paper discusses the results of analytical studies supported by current development efforts and research paths to implement fast start technology for small gas turbine APU's.

This paper discusses the results of analytical studies supported by current development efforts and research paths to implement fast start technology for small gas turbine APU's, using non-metallic rotor components. APU start system weights can be a significant fraction of the total installed APU weight, especially if fast starts are demanded under sub-Arctic environments. It is shown that both start system weight, and rotor containment armor weight are proportional to the product of rotational speed squared and rotating assembly inertia. Significant weight savings are therefore feasible with rotating assemblies using lower density materials such as ceramics and composites. Recent tests are described at the authors affiliation wherein a modified T20 small gas turbine was accelerated from zero to 100% speed in 2 1/2 seconds.

Emergency standby generator systems require rapid start characteristics capable of providing on-line operation within 10 seconds under a wide range of environmental conditions. This paper describes the design and development of a fast start, air impingement system for a 200-kW, single-shaft, gas turbine generator set. The conventional electric start system was replaced by a high pressure, air start system which reduced the total start-to-load time from 21 to 9 seconds.

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.