Improving small unmanned aerial vehicle flight endurance with cryo-compressed hydrogen vessels

Abstract

Detailed analysis indicates that substantial increases (22–43%) in flight endurance of small unmanned aerial vehicles (UAVs) powered by liquid hydrogen (LH2) are possible by increasing the maximum allowable pressure of the cryogenic storage vessel beyond the critical point to contain evaporated hydrogen (H2) and mitigate vent losses to the environment. Taking an existing UAV (US Naval Research Laboratory's “Ion Tiger”) as a baseline, we consider the effect of increasing Dewar maximum allowable pressure on flight endurance, under two different scenarios. In Case 1, the weight of the H2 storage system (including H2) is kept equal to the baseline design to maintain flight conditions unchanged. In Case 2, the external volume of the Dewar is kept equal to the baseline design, and the weight of the Dewar (and UAV) increases when the maximum allowable pressure increases, with the result that the propulsion power for the heavier UAV increases.The results are favorable. Although the modified Dewars have smaller inner volume (Case 1) or greater weight (Case 2) than the original Ion Tiger, flight endurance increases by 22% (Case 1) and 43% (Case 2), because the large H2 vent losses (39%) of the original design are reduced to only 1.6% (Case 1) and 1% (Case 2). The much higher utilization efficiency of the H2 stored in these modified Dewars compensates for their volume and weight disadvantages, resulting in UAVs with superior endurance.