Design space exploration of distributed propulsion HALE UAVs burning liquid hydrogen.

Abstract

High altitude long endurance (HALE) unmanned aerial vehicles (UAV) could serve as a platform to promote disruptive aircraft technologies in addition to set the stage to sustain week-long flights with electronic equipment. Hydrogen fuel is essential to meet the long-endurance requirement of low-speed HALE UAVs due to its high energy content per unit mass—2.8 times greater than that of kerosene. Hydrogen fuel could also be used to cryogenically cool the electric transmission system in a turbo-electric and/or hybrid-electric distributed propulsion system. This advanced propulsion system has the potential to affect all the aspects of a HALE UAV, from how much power is required to sustain flight to how power is produced, managed and distributed. However, in the literature there are no indications or design rules about how an integrated airframe/distributed propulsion system should be designed to maximise the integration synergies. The aim of this research was to identify a multi-disciplinary and multi-fidelity methodology for design space exploration studies of distributed propulsion low-speed HALE UAVs burning liquid hydrogen. The purpose of this methodology was to assess how the aircraft power requirement, production, management, and distribution are affected by the airframe selection, the distributed propulsion system and the energy management system. The results indicate that the slipstream-wing interaction of distributed propellers could increase the maximum endurance by nearly 60% on a tube-and-wing airframe for a given engine cycle. Superconductivity was assumed for the hydrogen-cooled electric transmission system that links the core engine to the distributed propulsors. These endurance benefits were three to four times greater than that of the series hybrid energy management strategy and of the wave rotor hybrid cycles. As such, the distributed propellers technology should be furthered investigated for both low-speed HALE UAVs and other low-Mach applications.