Light touch for solar plane construction

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Solar Impulse HB-SIA is the world’s first ‘fuel-less’ aircraft, using instead 12,000 solar cells that skin the upper side of its wings, which together power the plane’s four 10HP electric motors.

This carbon fibre aircraft – which has been seven years in development – has the wingspan of an Airbus A340 (63.4m) but weighs in at 1,600kg, the same as an average family car; an achievement made possible by plastics.

Amongst the main project partners is Bayer MaterialScience, which is extending its role in preparation for an around-the-world flight, scheduled for 2015. The entire trip will take some 20 days of flying time – all without recourse to traditional aircraft fuel. By day the solar cells recharge 400kg lithium batteries, which then enable the plane to fly through the night.

Among Bayer MaterialScience’s contributions will be an extremely high-performance insulating material – a polyurethane foam that will protect the pilot from the worst excesses of temperature fluctuations. The news of the development was announced by the Solar Impulse team and Bayer MaterialScience at a joint news conference in Payerne, Switzerland, that also revealed that the ungraded solar aircraft is expected to be completed in late 2013 with test flights the following year.

"We are now deepening our involvement as we go from materials supplier to system leader for the new cockpit," said Patrick Thomas, CEO of Bayer MaterialScience. "The Bayer Cross will be displayed on the aircraft in the future as a visible symbol of our commitment to this excellent partnership."

Bayer MaterialScience is contributing a variety of products to ensure that the second, larger model of the aircraft will be lightweight while retaining its rigidity. For example, because the cockpit cowl will be hinged for the first time, a section of it is being supported by carbon fibre-reinforcement.

In addition, Baytherm Microcell polyurethane foam will be used as insulation, developed in partnership with Solvay. The material offers significantly greater insulating performance than the current standard because Bayer researchers were able to shrink the pores in the foam by an additional 40%.

High efficiency insulation is particularly important for the aircraft because it must withstand temperature fluctuations between minus 50 degrees Celsius at night and plus 50 degrees during the day.

"The significantly larger size of the new cockpit shell and Solar Impulse’s tight weight budget meant that we had to further optimise the weight through design measures and targeted choice of materials," explained Martin Kreuter, Solar Impulse project manager at Bayer MaterialScience.

One of the innovations announced by Kreuter was the use of Baytubes carbon nanotubes in carbon fiber-reinforced structural components, which offers significant weight savings without compromising strength.

"This will allow us to enlarge the wings of the new aircraft and increase the number of solar cells mounted on them," said Kreuter.

Solar Impulse CEO and cofounder André Borschberg told attendees at the signing that work on the upgraded aircraft is already well advanced. "Eighty percent of the design phase and 50% of the construction phase have been completed," he said.

The first manned around-the-world flight in a fuel-less aircraft is expected to take 20 flight-days, with five or six needed just to cross the Pacific and two to three for the Atlantic crossing, according to Borschberg.

Including the necessary breaks, the solar-powered aircraft’s journey from west to east will take a total of three to four months.