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Soaring With the Sun

Although solar energy is often seen as a technology tied either to spacecraft or terra firma, a new generation of engineers and adventurers is crafting solar-powered aircraft.
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This Friday, the first manned, solar-powered aircraft to attempt crossing the Atlantic and subsequently circling the globe will leave its top-secret concealment inside a Zurich hangar for the world to view. The Solar Impulse will have the wingspan of an Airbus yet weigh no more than an average car. It will run on an engine normally fit to power a scooter and demand no more electricity than needed to light a shop window. Almost 12,000 solar cells cover the top of its 262-foot wing.

The visionary behind global solar flight, Bertrand Piccard, comes from a long line of adventurers. His grandfather, Auguste, became the first human to reach the stratosphere. Jacques, his father, "ballooned" in the other direction inside a bathyscaphe, going deeper than anyone in his time. Bertrand, though trained as a psychiatrist, followed in his grandfather's footsteps. In 1999 he achieved fame as the first to fly nonstop around the world in a balloon.

Despite the cheers from an adoring crowd when Bertrand landed, he had a chilling awakening. After checking the amount of fuel still in the tanks, he discovered that out of the four tons of liquid propellant the flight started with, only 40 kilograms remained.

"We could have easily failed," he realized, having to ditch the balloon in the Atlantic "had the winds been weaker." It quickly dawned on him the dependency of the entire journey on fuel. The insight led Piccard to promise himself that "the next time I would fly around the world it would be with no fuel, independent of fuel."

At that moment, Piccard didn't have a clue on how to accomplish the challenge he had set for himself. So he turned to Paul MacCready, the father of solar flight.

MacCready, who died in 2007, first gained fame in aeronautics for realizing the dream that eluded dreamers and visionaries over the millennia — building the first flying machine run solely by human power. His ultra-light Gossamer Albatross flew across the English Channel with its propeller spinning by pedal power. As an encore, MacCready attached solar panels to a sister aircraft, the Gossamer Penguin. On May 18, 1980, the Gossamer Penguin left the ground with his 12-year-old son Marshall at the controls, inaugurating the era of piloted solar flight.

Next, MacCready decided to fly a true aircraft, like a Cessna 150, with solar energy. To this end, his team built the Solar Challenger, which, on July 7, 1981, crossed the English Channel, traveling 156 miles at 50 miles per hour and reaching an altitude of 11,500 feet.

An aviation expert described the Challenger's flight as "one of the last great adventures in the world of aeronautics." The American intelligence community became very interested. If pilotless and guided by remote control, one group saw the potential of such aircraft hovering high in the stratosphere, listening to and watching activities of interest below and relaying the information back for analysis. MacCready's team came up with a flying wing fully covered by solar cells that ran a bank of propellers in front. Though traveling only 23 mph, the plane set an altitude record for non-rocket powered winged aircraft when it ascended over 96,000 feet in 2001, beating out by more than 16,000 feet the previous holder, the SR-171 Blackbird, which flies at speeds greater than 2,000 mph.

Another solar aircraft, the Zephyr-6, set a world endurance record for unmanned flight by staying in the air for longer than 82 hours last fall.

The altitude and endurance records of these solar aircraft, called HALE UAVs (High Altitude Long Endurance Aerial Vehicles), have proven their worth for many applications. Flying above weather and air traffic and staying up for extended periods makes them a cost-effective alternative to satellites for many tasks. Unlike satellites that provide only two views of the same area per day, such aircraft can provide the military with real-time 24 hour surveillance, intelligence and reconnaissance. Furthermore, they can provide more localized visual imagery to monitor and manage large agricultural and timbered areas, assess crisis situations as floods, forest fires and other catastrophic events, as well as support rescue operations. They could also provide an economic alternative to satellites for future telecommunications in the developed and developing world. Requiring a fuel source onboard would severely hamper the length of the mission and the effectiveness of such airborne platforms.

Three revolutionary technologies have made solar HALE UAVs possible. Carbon fiber gives the body the required ruggedness while maintaining the required lightness to minimize the power necessary to keep afloat yet withstand the vicissitudes of turbulence. Extremely thin solar cells also help hold their weight down. High-capacity lithium batteries allow for nighttime flight.

These technologies have also proven essential for building the Solar Impulse. Piccard's goal though differs. He has no illusions that his plane will soon lead to solar-powered passenger planes. Instead, he hopes to inspire a whole generation to turn to sun for their energy source on earth "by saying what we are doing in the air, you can do on the ground."

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