Solar-powered Impulse aircraft: something new under the sun

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Jean Revillard/Rezo The Solar Impulse aircraft flies over San Francisco Bay at sunset. The sun-powered plane is able to fly at night.

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.

Getting started

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.

Other suppliers to the project

Solvay Specialty Polymers became the Switzerland-based project's first main partner in 2004, and Bayer joined the program as an official partner in 2010.
Solvay provides Solar Impulse with 11 polymer products for about 20 applications in more than 6,000 parts.
Claude Michel, senior vice president, is Solvay's full-time project manager for the partnership with Solar Impulse.
Solvay provides film; Halar ethylene and chlorotrifluoroethylene copolymer for the thin top layer of the wing; and Solef polyvinylidene difluoride tape for the back of the wing, which is encapsulated with Halar.
Solvay's Radel polyphenylsulfone is used in the instrument-panel housing and, with Torlon polyamideimide and other polymers, in the throttle housing. The firm's Fomblim perfluoro¬ether lubricates the plane's rotating parts.
Bayer MaterialScience in Leverkusen, Germany, has 30 people in polycarbonate and polyure¬thane technologies working exclusively on the Solar Impulse project. Bayer developed high-performance insulating PU materials that help shield the cockpit from radical temperature changes.
Puren GmbH in Überlingen, Germany, mixes the Bayer PU, which is based on Desmodur 44V70 and Baymer VPPU27HB92, and Solvay's Solkane 365mfc liquid hydrofluorocarbon foaming agent, into large blocks of rigid foam.
Carving of the blocks produces the entire cockpit structure, the engine gondolas/nacelles, battery protection and the wing tips.
Dual layers of Bayer's Makrofol DE 1-1 PC will replace glass for the wind canopy.
Each of the layers has a thickness of 350 micrometers; an air barrier with two spacers separates the layers.
Bayer produces the material in Leverkusen, and Solar Impulse manufactures the wind canopy in Dübendorf.
"Construction of a dual layer is unique," said James Chrise, senior vice president for North American commercial operations of the Pittsburgh-based Bayer PC business unit.
Bayer produces a special coating based on Impranil DLC-F resin in Dormagen, Germany, which is processed by La¬nitz-Prena Folien Factory GmbH of Leipzig, Germany.
For plane two, Solar Impulse designers are considering other Bayer materials, including special PU-based insulation materials, coatings and adhesives. The design selections also include polymer materials from Solvay and other suppliers.

Bayer MaterialScience rep lives with, blogs about team

Among the Solar Impulse team, Bayer MaterialScience LLC's Ian Nash is working during April, May and June as a member of the ground crew in the U.S.
Nash joined the Pittsburgh-based Bayer AG unit in 2010 as a polyurethane advocacy and sustainability representative. He files blog messages about the project for other Bayer employees.
During the three-month assignment, Nash is living with the Solar Impulse team, initially in a lodge on the Moffett Field grounds. "There are some inevitable language barriers, as we have a mixture of native French, German and English speakers," he said in a posting.
Another blog item commented on physically moving the plane: "A few ground-crew members hold each wing, a few guide the tail and a few are responsible for pulling the plane by holding on to its main landing gear.
"I helped guide the tail to ensure that the plane did not hit any damaging bumps," he said.
Nash received a bachelor's degree in history from the University of Pennsylvania.