MOUNTAIN VIEW, CALIF. — Creators of a plane powered only by solar energy plan to fly it around the world, with help from Solvay SA and Bayer MaterialScience AG.
Still in the experimental stages, the Solar Impulse will soon begin a four-leg flight across America. Trial runs are helping developers make upgrades that will be used in a second aircraft for the attempted global flight in 2015.
The Solar Impulse team assembles components in a hangar at its base in Dübendorf, Switzerland, near Zürich.
Polymer-matrix-composite specialist and boat builder Décision SA of Ecublens, Switzerland, is building the structural components after major airframe makers passed on the opportunity. Décision developed a honeycomb structure, sandwiched between layers of carbon fiber, that encapsulates photovoltaic cells.
Schütz GmbH & Co. KGaA of Selters, Germany, provides Cormaster-brand honeycomb materials, and Toray Industries Inc. of Tokyo supplies the carbon fiber.
Kokam Co. Ltd. of Siheung, South Korea, manufactures lithium polymer batteries that need protection from minus 40° F temperature. Weighing a total of 882 pounds, the batteries are positioned in each of the engine gondolas, or nacelles, and in nearby wing structures to reduce the length of connecting cables.
Global solar innovator SunPower Corp. of San Jose, Calif., supplied 11,628 photovoltaic cells with 10,748 on the wing and 880 on the horizontal stabilizer. Each cell has a thickness of 150 microns. Solar Impulse strings the solar cells together and, via a layering process using ultrathin resins, protects them from ultraviolet rays.
The single-seat aircraft has a wing span of 208 feet, comparable to a Boeing 747, weighs 3,527 pounds and is designed for an average flying speed of 43 miles per hour using four 10-horsepower engines. About 90 percent of the structure is made with polymer-matrix-composite materials.
The seed for the Solar Impulse project was planted 14 years ago.
Bertrand Piccard, a visionary adventurer, pilot and psychiatrist, imagined the concept after his March 1999 completion of the first non-stop around-the-world balloon flight with British co-pilot Brian Jones.
Piccard, now 65 and chairman of Solar Impulse SA, recruited André Borschberg, 60, a mechanical and thermodynamics engineer, business entrepreneur and pilot with experience flying Swiss Air Force fighter jets, and helicopters.
Borschberg, who is CEO, flew the first plane, known as HB-SIA, for more than 26 hours in July 2010, exceeding an elevation of 27,000 feet and recording the first solar-powered night flight.
Piccard wants the project to be an ambassador for renewable energy. Solar Impulse has about 90 team members now. To move the plane to the U.S., the crew disassembled aircraft.
On Feb. 22, a Boeing 747 from Cargolux Airlines International SA airlifted all of the plane's components from the Swiss military's Payerne Airport to Moffett Field, landing on a 9,200-foot-long concrete runway near Mountain View, Calif., and the south end of San Francisco Bay.
The crew reassembled the plane over three weeks and ran technical tests at Moffett Field including flights on April 2, 19 and 23.
The plane was displayed in a massive wooden hanger at Moffett Field that the U.S. built decades ago to house lighter-than-air blimps. Currently, the joint civil-military airport operates under the auspices of the National Aeronautics and Space Administration.
Organizers plan to start successive legs for the across-America flights at Moffett Field in early May. The plane then will leave Phoenix in mid-May, Dallas in late May or early June, St. Louis or Atlanta in mid-June, Washington's Dulles International Airport in early July and New York's John F. Kennedy International Airport in mid-July. No leg will exceed 24 hours in flight duration.
The next generation
The second Solar Impulse plane will have a pressurized cabin, molded carbon-fiber-reinforced engine gondolas, a wing span of about 238 feet, improved battery cells, as many as 17,000 solar cells and an improved cabin environment for the pilot's likely flight duration of five days and five nights.
The total trip will involve about 20 days of flight, with the longest stint — non-stop across the Pacific Ocean — taking about five days.
Unlike the current demonstrator, plane two will be able to fly through clouds, thanks to the upgrades. Solar Impulse designers also want the second aircraft to be waterproof.
New, lighter batteries will incorporate Solvay-invented electrolytes, allowing for higher energy density. Bayer is allowing the project to use its nanotechnology.
"We need more development in the batteries," Borschberg said. "I expect a breakthrough [in battery technology] in about five years."
Improved thin-film technology reduced the per-square-meter weight to 25 grams for the second plane, from 80 grams.
Setback to success
A failure during a July 5 torsion test in Dübendorf pushed back the around-the-world effort to 2015 from 2014.
The team had built the main spar for the second airplane. Construction took nine months.
What happened next to the central section of the main spar — the second airplane's spinal column — was the first structural test failure of the program.
"We tested the loads," Borschberg said. "Seven to eight tonnes were applied.
"We had four weeks of testing. On the last test, it exploded. It opened like a can."
The team reflected on what went wrong. Too many members of the team were risk takers rather than pragmatists, the leaders determined.
"We went too far" in pushing the technology, he said.