Researchers at the U.S. Naval Research Laboratory say they've set a record by flying a liquid-hydrogen powered unmanned aerial vehicle for 48 hours straight.

The flight breaks the previous record of 26 hours set by the same Ion Tiger electric fuel cell drone when it was powered with gaseous hydrogen stored at 5,000 psi, they said.

The cryogenic liquid hydrogen was stored in a lightweight tank, allowing more hydrogen to be carried on board to increase flight endurance, NRL scientist Karen Swider-Lyons said.

"Liquid hydrogen coupled with fuel-cell technology has the potential to expand the utility of small unmanned systems by greatly increasing endurance while still affording all the benefits of electric propulsion," she said in an NRL release.

While long endurance is possible with conventional gasoline-engine vehicles, these are usually loud, inefficient and unreliable in this aircraft class, the researchers said, while electric vehicles powered by batteries are limited to endurances of only several hours.

The electric fuel cell propulsion system gives the Ion Tiger the low noise and signature of a battery-powered UAV while taking advantage of high-energy hydrogen fuel and the high electric efficiency of fuel cells, they said.