The efficiency of solar cells, the power source for many high-altitude, long-endurance UAVs, has generally doubled over the last 10 years. Fuel cells, which convert hydrogen to water and can charge onboard batteries for flying at night, have continued to shed pounds. With no moving parts, fuel cells mean reliability.
O’Neil, the SolarEagle program manager at Boeing’s Phantom Works, calls the UAV a “pseudo-satellite,” which would fly at about 65,000 feet, circling combat zones or other targets. “The key technologies have now emerged enough that we can talk about designs that inhabit the upper atmosphere,” he says. “Ten years ago the technologies were not ripe.
“Your candidates are undoubtedly the composite family of materials,” O’Neil continues, “whether you’re talking soft skin over a composite frame or honeycomb, or very lightweight composite plies [sheets or surfaces], where you might have just a couple of plies in key areas just to keep the weight down.” Plastics and carbon fiber are attractive to engineers because of their strength and light weight.
But prolonged exposure to extreme altitude means dealing with ultraviolet radiation and even the occasional gamma particle from space. On the interior, some metals may be needed to shield avionics from these threats.
For early lessons learned on high-altitude, long-endurance flight, O’Neil points to a Boeing program, Condor, that he sees as a jumping-off point. In 1988, the all-composite Condor, with a wingspan greater than that of a B-52, set an altitude record for piston-powered airplanes—67,000 feet—and stayed aloft for two and a half days on its turbo-charged, liquid-cooled, six-cylinder engines. “Condor continues to serve as a strong technical foundation for a lot of what we’re doing today,” O’Neil says. “And that was an unmanned system. So you’re thinking about something 24 years ago—quite an accomplishment.”
NASA’s Nickol points to the late Paul MacCready, founder of AeroVironment, and his human-powered Gossamer Albatross of the late 1970s. “Basically, once they built these super-lightweight vehicles that were very efficient, the next obvious step, instead of having a guy pedal, was to put in solar power and electric motors. So I think the real breakthrough was Solar Challenger. That flew across the English Channel [in July 1981] and actually flew quite a bit farther. At that point they had proof that, Hey, this could work. But then the technology had to catch up.”
It has caught up with manned flight too. Solar Impulse, a piloted airplane, flew from Switzerland to Belgium last May, the first international flight of a manned solar vehicle. Pilot and company CEO André Borschberg reported that he gathered more energy in flight than he needed, and used no more energy than if he had made the trip on “a small scooter.” The goal of Solar Impulse is an around-the-world flight.
SolarEagle will have some catching up to do when it begins flying. Last summer a wispy airplane called Zephyr flew as high as 70,740 feet above the Arizona desert for two weeks, quadrupling the previous record for flight duration by a UAV. Built by the British military firm QinetiQ (pronounced “kinetic”), Zephyr is the descendant of a smaller solar airplane originally dreamed up to capture video of a record-setting balloon flight. The balloon flight never happened, but the British military got curious about Zephyr’s potential. More recently, Boeing has brought QinetiQ on board as a partner to develop SolarEagle.
Zephyr weighs about 110 pounds and carries a payload as heavy as a telephone book, yet its 74-foot wingspan provides so much lift that it can be launched by hand: Five of the company’s employees hoist it to shoulder height and run along until it lifts away. It lands on two four-inch launch handles on each wing, and comes to an almost immediate stop with no damage. Solar cells as large and thin as a sheet of loose-leaf paper blanket Zephyr’s wings, silently powering twin propellers at a cruise speed of about 14 mph at sea level and almost 70 mph at altitude. The cells also charge lithium-sulfur batteries, which supply more than twice the energy per weight of any other battery type. Zephyr descended a few thousand feet at night and relied on the batteries alone to continue, but future flights are intended to maintain altitude overnight. An earlier model of Zephyr bounced hand-held radio transmissions about 350 miles between Phoenix and San Diego, showing its worth as a communications relay. The UAV’s steady flight also makes for prime surveillance. “It doesn’t have the shake and shudder of a jet engine,” says chief designer and pilot Chris Kelleher. The narrow-field-of-view lenses used at extreme altitudes are often compared to looking through a soda straw, which requires that the camera not shake. “You can look down that drinking straw very stably,” he says of Zephyr.
Kelleher has always loved airplanes that push boundaries, and has won titles for aerobatic flying in England. He also spent 20 years planning launch trajectories and flight dynamics for military satellites. “It was all good fun,” he says now. “But you start to think, Is there another way of doing some of these things? ” High-altitude UAVs, he says, provide constant views for far less cost than satellites. “You do it with a telescope from orbit or a pair of binoculars from [a high-altitude aircraft].”