Conceptual Design Studies of Fuel Cell Powered Unmanned Aircraft for Long Loiter

The conceptual design space is explored for liquid hydrogen, fuel cell powered, unmanned, aircraft for long loiter. Several tailless planforms are assessed, including arrowheads and a blended wing body. Hydrogen’s low volumetric energy density and low packing efficiency impose a volumetric constraint on the different planforms. Two trade studies attempt to mitigate the volumetric constraint by investigating hypothetical advances in fuel cell performance and liquid hydrogen storage. Improving hydrogen fuel cells is found to substantially improve mission endurance but does little to relieve the volumetric constraint because the fuel cell improvements free little volume compared to the hydrogen volume. Higher conformity liquid hydrogen tanks can mitigate the volumetric constraint by physically packing more fuel into available bays, but the overall mission performance is sensitive to the added weight. Takeoff is a secondary limitation common to these planforms driven by the low max propulsive power available compared to combustion. The takeoff constraint is examined through two trade studies with one investigating the number of engines and the second examining the addition of surge capacity. It is found that the one engine inoperative (OEI) takeoff requirement drives this architecture to 4 engines, but it’s an open question whether the OEI requirement is reasonable given the increased reliability of fuel cells and electric motors. The final trade study finds that surge capacity can alleviate constraining takeoffs. These studies show the challenges surrounding hydrogen fuel cell powered, unmanned aircraft can be overcome with further development of fuel cell and hydrogen storage. The enabled aircraft offer great potential for both military and commercial aviation when persistent coverage is essential.