A hybrid propshaft/jet propulsion system is implemented with a novel piston engine that can provide either or both shaft and gas power regardless of atmospheric density. Utilizing a facultative adiabatic internal-combustion engine (FAICE) experimentally demonstrated to be capable of delivering shaft power with and without aspiration, a controllable hot gas generator for producing useful reaction propulsion at high altitudes and low airspeeds is shown. Fueled by direct-injected nitromethane and backpressured with a variable area expansion turbine, the engine combustion power available is divided between shaft power produced by the expansion of the combustion products against a piston and by expansion in a turbine driving a ducted-fan-powered jet to produce reaction on the vehicle. Maximum subsonic motive power is found to occur when the high enthalpy, low mass flow engine exhaust products are used to augment the total mass flow ejected and to moderate the ejection velocity by aspirating, compressing and mixing with available ram air. Additional impulse energy is shown to be available by liberating the residual chemical energy of the underreacted exhaust gases through catalytic ignition and combustion in the duct with the inducted air prior to final expansion to ambient pressure.
