Back in 2009, the US Naval Research Laboratory (NRL) set an endurance record for electric unmanned aerial vehicles (UAVs) when its fuel cell-powered Ion Tiger aircraft managed to stay aloft for 26 hours and 1 minute. Now, NRL has announced that the same aircraft has trounced that record, by remaining in flight for 48 hours and 1 minute.

In its previous record-breaking flight, the UAV’s Polymer Electrolyte Membrane (PEM) fuel cells were running on compressed gaseous hydrogen stored at 5,000 psi. On its latest flight, which took place from April 16th to 18th, cryogenic liquid hydrogen was used instead. Liquid hydrogen has three times the density of gaseous hydrogen, thus its ability to power a much longer flight.

One challenge lay in developing a lightweight dewar (a double-walled insulating vessel) for carrying as much of the fuel as possible, while keeping it sufficiently cold. If the liquid hydrogen were to get too warm, it would boil off at a rate faster than the UAV could use it.

NRL is particularly interested in using fuel cells for such drone aircraft, as internal combustion engines are inefficient and loud (and thus not particularly stealthy), while battery electric UAVs have a very limited range. The researchers are hoping to develop a system in which an onboard electrolyzer and energy source (such as solar or wind) could be used to electrolyze, compress, and refrigerate liquid or gaseous hydrogen from water.

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