An operational version of Zephyr is about a year away. While satellite instruments may take years to test because they become untouchable after launch, “we can have a problem in the morning and be flying a solution on a subscale test Zephyr in the afternoon,” Kelleher says. “Our design cycle is very short, and that means advances can be very rapid.”
Boeing and DARPA are counting on that for SolarEagle. While Zephyr set its records above sunny Arizona during the long days of midsummer, DARPA has more ambitious expectations for SolarEagle: The operational version will be designed to fly for months on end at latitudes closer to those of Chicago through the winter solstice, when darkness reigns nearly two-thirds of every day.
“Most aeronautical engineers would tear their hair out over this, and I haven’t got much left,” Kelleher says. SolarEagle’s wings will stretch about 400 feet, with solar panels on almost every exposed surface—even the undersides of the wingtips, which are canted upward, to catch extra rays from the low winter sun.
The blessing and the curse of huge wingspans is that they’re much more at home high above most wind and weather than in the lower atmosphere or on the ground, where they’re vulnerable to winds and ground handling. Because of their delicate, lightweight construction they flex in flight, sometimes changing their aerodynamics in unpredictable ways that suddenly alter the behavior of the airplane.
In June 2003, when AeroVironment’s solar-powered Helios, a 247-foot-span flying wing, took off from the Navy’s Pacific Missile Range Facility on the Hawaiian island of Kauai, engineers knew its vulnerabilities. Like today’s designs, Helios was light and flexible and most at risk in the thick air and winds during its initial climb. It maneuvered between clouds to keep its solar panels illuminated as it rose to about 2,800 feet in half an hour. Weather balloons launched preflight provided data that enabled ground controllers to steer the aircraft away from wind shear zones. Then the view from a wingtip camera went haywire. Unknown to the operators, the wiry airframe was bending in unexpected turbulence like a crossbow with the string pulled too far back. Helios began to wobble, and surged to about two and a half times its maximum design airspeed, about 20 mph, for that altitude. Skin and solar cells began to rip away, and Helios disintegrated into the sea.
“This class of vehicle is orders of magnitude more complex than it appears,” concluded the NASA Mishap Investigation Board. Conventional design techniques “did not provide the proper level of complexity to understand” how Helios would handle turbulence. On top of that, NASA’s decision to curtail funding had unexpected fallout: Engineers hoping to attract commercial funding switched to new fuel cells that were 50 percent heavier, and the airframe could not handle the weight.
But there was optimism too. The Helios team “had identified and solved the toughest technical problems,” which would include weight, flexibility of composites, and electric propulsion in high-altitude, long-endurance UAVs, according to the report, giving the United States a strategic advantage in the field. While the board called for better analysis and modeling, the program had affirmed that high-altitude, long-endurance technologies such as solar cells, electric motors, and structural designs had reached new benchmarks that could be applied to future airplanes.
Now, those airplanes are in the air, with more on the drawing board. This year, Boeing hopes to make the first flight of Phantom Eye, an airplane powered by two Ford truck engines that run on hydrogen. Aurora Flight Sciences late last year rolled out Orion, a diesel burner. And AeroVironment had begun testing of Global Observer last year, a hydrogen-powered airplane able to carry 400 pounds for a week at 65,000 feet. Hydrogen supplies about 2.7 times the energy per pound that gasoline does, making it among the most weight-efficient fuels. Global Observer’s four propellers look remarkably small, thanks to their efficiency and the aircraft’s aerodynamics. An accident did occur last April, and is still being investigated, but a second airplane was nearly complete at the time.
Global Observer doesn’t carry solar cells, because most of the higher-latitude regions where the aircraft will operate remain beyond the useful limits of solar power, says Steven Gitlin of AeroVironment. While the company has not developed any solar airplanes since Helios, Gitlin says Helios and other early solar craft such as Gossamer Condor, another human-powered creation of Paul MacCready, prepared the way for Global Observer’s highly efficient design.
One more endurance strategy is to make aircraft lighter than air: Lockheed Martin in 2009 won a $400 million DARPA contract to build a solar-powered airship that would cruise the stratosphere for up to 10 years. Housing a highly sensitive radar system, the airship would supplant lower-altitude Advanced Early Warning and Control airplanes such as the E-3 Sentry and E-2 Hawkeye. But blimps, like satellites, lack the maneuverability and flexibility of airplanes; they cannot go exactly where controllers want, or adjust to new mission demands on the fly.