Pneumatic Link Secondary Power Systems for Military Aircraft

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.

This study examines the influences of these two options on the power density of pneumatic link start systems for military aircraft, and leads to the conclusion that higher bleed pressure ratios of up to 6.0 can provide more than a 10 percent savings in power/volume ratio dependent upon APU location. The utilization of bleed power augmentation can provide more than 25 percent reduction in weight and volume, but may conflict with increasing demands for higher avionics systems ground cooling capacity.