Three companies compete for a long-endurance UAV concept

might look at aircraft differently, said DARPA’s Dan Newman, the Vulture program manager. “We would no longer define an aircraft by the launch-recover-maintain-launch cycle,” Newman said.

The ability to keep flying after sundown is 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 in Virginia. “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.”

Lasting through a winter night up north is something Parks and Aurora plan to address early on. Bye said technology in the works to power Vulture was getting better every day. Boeing is similarly optimistic. “We believe DARPA’s goals are within reach, given continued progress in the subsystem technologies,” Boeing spokesman Chris Haddox said.

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 which is not 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 solar cells begin generating electricity again once the sun rises.

Hodhes writes that this is a lot of parts to keep working for five years. Engineers and scientists are still trying to improve the system so that it retains a higher percentage of the energy arriving from the sun. 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. Developers are wrestling with this trade-off to solve the nighttime flying issue.

Judging by the artist’s rendering that Boeing has released, its concept is much like that of British firm QinetiQ’s Zephyr, but larger and with solar panels on the wing. Although development of the Zephyr so far has been funded by the U.K. Ministry of Defence, the U.S. Department of Defense funded the record-breaking demonstration flight through its Joint Capability Technology Demonstration program designed to fast-track and field urgently needed technologies.

Zephyr’s 82-hour, 37 minute flight was unofficial because it was not certified by the Federation Aeronautique Internationale, the world air sports federation, as the focus was on demonstrating technologies and capabilities to the U.S. military. Still, 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.