Small electric unmanned air vehicles are often considered limited by the low endurance of several hours, mainly set by the energy of their battery energy storage system. The Naval Research Laboratory has been extending the duration of electric UAVs through the use of hydrogen fuel cells, which take advantage of both the high energy of H_2 fuel in combination with the high efficiency (～50%) of polymer fuel cells. In this paper, we describe a project to demonstrate the 3-day flight of NRL's 35-lb Ion Tiger UAV on liquid hydrogen (LH2) fuel. The use of LH2 on a small UAV is complicated because of the limited weight and volume budget, plus the extensive insulation required to keep the LH2 in its cryogenic state (20K) for multiple days, plus match the variable H_2 demand of the fuel cell under flight conditions. A compact, lightweight LH2 flight vessel was successfully designed and built with the aid of extensive thermal modeling. The system performs well in flight and is on track to deliver a 3-day flight. We also discuss issues related to LH2 handling and transportation. While LH2 systems are complex, they can provide unprecedented flight duration for small UAVs.
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