Defense Technology International - September 2007 - (Page 18) DISPATCHES NICK GALANTE/PMRF GLOBAL AeroVironment’s solar-powered Helios crashed in a later flight, but certified technology that might wind up in Vulture. that operates at latitudes that do not support continuous solar-powered operation. A lot of the advanced technologies employed in the company-funded Global Observer project were developed for the Helios UAV that crashed in June 2003 during a test flight, but not before setting a UAV altitude record two years earlier of 96,000 ft. Helios was designed for long-endurance missions, using solar arrays by day and supplemental fuel-cell-based electrical energy systems at night to drive 10 electric motors. A regenerative energy system would have allowed Helios to fly nonstop at lower altitudes for up to six months. As designed, during daytime the system would have used excess power from the UAV’s 62,000 silicon solar cells, mounted across the upper surface of its 247-ft. wing, to power an electrolyzer that breaks down water into hydrogen and oxygen, which would have been then stored in pressurized tanks. At night, the process was to have reversed, with fuel cell stacks combining the gases DEFENSE ADVANCED RESEARCH PROJECTS AGENCY of operation and conduct design trade studies. They will also perform reliability and mission success analyses and conduct a system requirements review for a subscale demonstrator. A decision to continue Vulture into Phase II will be based on Darpa’s determination that one or more contractors successfully completed the Phase I exit criteria, and the availability of funding. The second phase includes laboratory and field demonstrations of major subsystems and at least 90 days of subscale demonstrator flight tests. Depending on the outcome of Phase II, one or more contractors will begin the third and final phase. “The primary Phase III objective is to conduct a full-scale Vulture flight test that validates the system’s capability of meeting program reliability and mission success criteria,” Darpa states. A continuous demonstration of at least one year is envisioned. Pulliam sees two major hurdles in getting Vulture off the ground: energy and reliability. The vehicle will either collect its fuel on station or be refueled periodically. Reliability requires redundant systems. Several options are envisioned. One has a single air vehicle aloft, powered by solar power and/or hydrogen fuel cells. Another requires in-flight servicing during which tankers dock with Vulture, replenishing fuel and swapping mission payloads. NASA this year published a study of various high-altitude, long-endurance (HALE) concepts. None of NASA’s aircraft configurations came close to achieving the six-month endurance goal. While solar-powered vehicles showed promise, they were limited to the long days of summer months. Darpa, however, has demonstrated technology that might support in-flight servicing of robotic aircraft. Last year at Edwards AFB, Calif., Darpa and NASA Dryden Flight Research Center performed the 18 first-ever autonomous probe-and-drogue airborne refueling. The demonstration was conducted with a NASA F/A-18 configured to operate as an unmanned testbed. The Autonomous Airborne Refueling Demonstration used GPS-based relative navigation, coupled with an optical tracker, to provide the positioning required to put a refueling probe into the center of a 32-in. basket dangling in the air stream behind a tanker. The tanker was equipped with a small relative navigation pallet, but production refueling equipment was not modified. Pilots were on board the F/A18 for safety. Autonomous inflight refueling is a critical enabler for affordable, persistent, unmanned systems. “This flight is a milestone—it demonstrates that autonomous systems can Vulture might require automated air-to-air employ the benefits refueling, which Darpa demonstrated with an of air-refueling,” says F/A-18 surrogate and a hands-off crew. USAF Lt. Col. Jim Mcinto water, producing electricity as a byCormick, Darpa program manager. AeroVironment, which fabricated product to power the aircraft. On the other hand, a non-regenerative the Helios solar-powered Hale UAV for NASA, is a likely contender for a develop- energy storage system would have allowed ment contract. The fi rm is flight-testing Helios to fly up to two weeks at all latitudes its Global Observer, a liquid-hydrogen- any time of the year. This system would powered UAV. The fuselage houses a fuel- have combined stored hydrogen with oxycell-based propulsion system that drives gen collected from the atmosphere. Another solar-electric-powered aircraft electric motors mounted along the wings. Electricity is generated in the fuel cell by that might inspire Darpa officials is the chemically combining hydrogen and oxy- Zephyr from Qinetiq, which is designed to operate continuously for three months at gen. The by-product is water. A 10,000-lb. full-scale Global Observer up to 65,000 ft. The Hale UAV has a 12could carry a 1,000-lb. payload at 65,000 meter (39.4-ft.) wingspan, but weighs just ft. for well over a week, according to com- 27 kg. (59.5 lb.). Zephyr uses solar power pany officials. Using two aircraft in rota- to fly during the day while storing energy tion, Global Observer would provide 24/7 in its rechargeable batteries to power it coverage, they add. Solar power was ruled overnight. In July 2006, Zephyr flew for out because customers desire a platform 18 hr., including 7 hr. in the dark. I DEFENSE TECHNOLOGY INTERNATIONAL SEPTEMBER 2007 www.aviationweek.com/dti http://www.aviationweek.com/dti
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