The plane has an ultra-light carbon-fiber frame that allows it to weigh 3,500 pounds — about the same as a mid-size car. It has the wingspan of a 747 and a slender fuselage, giving it the look of a giant high-tech dragonfly.
The plane’s power is drawn from the sun by 12,000 photovoltaic cells that form the top of its wings. The juice is collected in a series of batteries arrayed behind the craft’s four electric engines. It routinely reaches altitudes of up to 28,000 feet, about a mile below the thin air traversed by big commercial airliners zipping around at close to 500 miles per hour. On-board instruments alert the pilot if the plane banks even a degree too far.
For all of its innovations, at this stage of development, Solar Impulse is no more practical for commercial flight than was the single-engine Spirit of St. Louis that Charles Lindbergh piloted across the Atlantic Ocean in 1927.
The plane’s engines put out about 10 horsepower — roughly the same amount of power as the Wright brothers’ first planes. Solar Impulse cannot take off or land in windy conditions, nor can it fly through clouds. The lone pilot wears a parachute and is confined to an area the size of a “bad economy seat,” noted the project’s chief executive and co-founder Andre Borschberg, 60, an engineer and former fighter pilot.
The tiny cockpit is unheated and unpressurized, meaning the pilot must endure extreme heat and cold and wear an oxygen mask. On long flights, Borschberg practices meditation and advanced breathing techniques to stay energized. His co-founder and the plane’s other pilot, Bertrand Piccard, a psychiatrist, does self-hypnosis.
And as for bodily functions — the pilot relies on spent water bottles and eschews fibrous foods in the days before a flight to make sure that diapers do not have to be pressed into service.
But comfort is not the project’s goal. “The point of this is to underscore how far we’ve come and how far we need to go to develop alternative sources of power, particularly solar energy,” said Bob van der Linden, chairman of the aeronautics department at the Smithsonian National Air and Space Museum. “This will help push the technology along.”
He said there are some possible applications raised by Solar Impulse’s innovations, including high- and long-flying unmanned planes that could be used for mapping purposes. But any broad commercial uses, he said, lie beyond the horizon.
None of this dims the enthusiasm of the project’s founders.
The idea to build the plane started with Piccard, 55, a hang-gliding pioneer, who earned international acclaim in 1999 when he and co-pilot Brian Jones won a competition to fly around the world non-stop in a hot air balloon. Although they won, Piccard was struck that while they started the trip with 8,200 pounds of propane, they had only 88 pounds remaining when the balloon landed.
That steeled his determination to again circle the globe but this time without consuming any fossil fuels.
“For me, it was obvious that an airplane that could fly around the world without fuel was the next big adventure of the 21st century,” Piccard said.
He took his idea to the Swiss Federal Institute of Technology, which initiated a feasibility study headed by Borschberg. Design of the plane began in 2003, with Borschberg and Piccard assembling a team of 80 partners — which includes racing yacht maker Decision, the precision instruments firm Omega and the Bayer materials company — to build it after traditional airplane manufacturers told them it was impossible.
In all, the project has cost more than $140 million. The plane has performed well. Borschberg piloted the first solar-
powered night flight in 2010, spending 26 consecutive hours over Switzerland. In 2011, it flew from Switzerland to Belgium and then on to France, where it was a showpiece at the Paris-Bourget International Air Show. And last summer, the plane crossed the Mediterranean from Spain to Morocco.
After being transported to this California airfield aboard a 747 cargo plane, Solar Impulse has soared over the Golden Gate Bridge, raising curiosity across the Bay Area and drawing several thousand visitors to its temporary home inside a cavernous hangar that a couple of generations ago housed military dirigibles.
The plane will make five stops on its flight across the United States, spending up to a couple of weeks in each city it visits: Phoenix, Dallas, St. Louis and Washington, where it is slated to land at Dulles International Airport. From there, it is on to New York.
The U.S. tour is a prelude for a planned around-the-world flight in 2015. That journey will be taken by a second-generation solar plane, with accoutrements necessary for long-haul flights, including automatic pilot, a more ergonomic cockpit and a toilet beneath the pilot’s seat. That vessel should be ready for testing next year.
Although even Piccard and Borschberg do not expect to see development of a more practical solar-powered plane anytime soon, they are confident that some of the lessons from Solar Impulse — the use of lighter materials, more gradual descents to airports and more direct routes to destinations — can help make aviation more energy-efficient in the short run.
In the long run, they added, anything is possible.
“I think people have a tendency to forget [the limited capabilities of] the airplane of the Wright brothers. How was the airplane of Charles Lindbergh? How was the first one of Chuck Yeager? All these airplanes, they were very limited,” Piccard said. “They were just the maximum of what could be done in those days. So the people who criticize Solar Impulse are like those who looked at the Wright Brothers and said, ‘This has no future.’ ”