An auxiliary power unit (APU) with integral firebox provides an integral
unit for aircraft to utilize an APU without the use of a separate
firebox. The present invention comprises a turbomachine intake air duct
secured to an enclosure, the enclosure having a bleed air port extending
from the combustor to outside of the enclosure, with an APU secured
therein. The APU can be retrofitted to existing aircraft, or installed in
newly-built aircraft.