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Quest is on for UAVs that stay up for years

When a solar-powered Zephyr unmanned aerial vehicle, made by the British firm QinetiQ, completed a flight over Arizona on July 28 that lasted three and a half days, it not only made history but also gave credibility to a quest by a number of companies and agencies to develop ultralong-endurance drones for reconnaissance or communications relay.

No longer does the idea of a UAV flying at altitudes as high as 90,000 feet for five continuous years sound like science fiction. But it’s only recently that such a notion has been taken seriously.

Derek Bye, who designs airplanes for Lockheed Martin, remembers the titter that ran through the audience when the U.S. Defense Advanced Research Projects Agency held an industry day in Arlington, Va., to announce Vulture, an unmanned plane that would fly for five years carrying a half-ton of payload and drawing just 5 kilowatts of power.

But such a thing wasn’t completely out of the blue. Seven years earlier, a strange-looking, unmanned solar-powered plane called Helios set an altitude record for propeller-driven craft of 96,863 feet. The Helios flying wing eventually broke apart off Hawaii, but U.S. defense officials saw potential in the idea.

So DARPA hatched the Vulture program. In April, the agency awarded $4 million design contracts to Boeing and Lockheed Martin, and to the specialty-UAV company Aurora Flight Sciences. They will study competing Vulture designs under an initial 12-month analytical effort.

The winner or winners will advance to a second phase, in which they will attempt to keep a subscale aircraft aloft for three months. A third phase would extend that goal to a year.

A Vulture flying at 60,000 feet could produce a continuous high-resolution image of a battlefield within a 750-mile-diameter viewing footprint, which could take some of the pressure off traditional military UAVs.

Such aircraft might eventually do some jobs now handled by satellites, but more cheaply, flexibly, repairably and without creating orbiting space junk. The ability to keep flying after sundown is the key for Vulture, but experts disagree on how soon the underlying power-storage technologies will be ready.

There is “technology that will store the energy, but the question is, how good is it?” said Craig Nickol, an engineer at NASA’s Langley Research Center, Va. “If you’re flying near the summer solstice, when the days are really long and the nights are short, then it looks pretty good, but if you’re trying to fly missions at northern latitudes during the winter, when the days are very short and the nights are very long, at that point, it becomes a challenge.”

Power is the most critical subsystem, and it would come from one of two technologies: regenerative fuel cells or batteries.

In the fuel cell option, electricity that isn’t spent to power the airplane and payload during the day is directed to an electrolyzer, a sponge-like membrane containing water. The electricity splits the water into hydrogen and oxygen, which are stored, then at night they join to become water again, releasing electricity.

The efficiency of fuel cells is 40 percent to 50 percent, compared with 95 percent for lithium batteries, Parks said. The drawback to batteries is that they are heavier, which would mean sacrificing payload capacity or burning more energy to stay aloft.

Judging by the artist’s rendering from Boeing, its concept is much like that of QinetiQ’s Zephyr, but larger and with solar panels on the wing.

Zephyr’s 82-hour, 37-minute flight was unofficial but the carbon-fiber aircraft — flying on solar power and carrying a U.S. government communications test payload weighing 2 kilograms — proved that an ultra-long endurance UAV can no longer be laughed away.