Composites Develop Dramatically with Solar Impulse

The most advanced project for lightweight aircraft production may be a Swiss project to develop a manned solar-powered plane.

"We will take composite technology two to three times beyond where it is today," says Andre Borschberg, a former Swiss air force pilot and CEO of the project, called Solar Impulse. The wing span on the final model will be 80 meters, the same as the Airbus A380, which weighs 560 metric tons. The craft Borschberg will help pilot around the globe will weigh just two tons. That's about the same as a typical car.

The craft will require about 12 hp to provide daylight power and to charge onboard lithium batteries, or about the same amount of engine power on the Wright Brothers' flight at Kitty Hawk. The density of the developmental batteries must approximate 200 wh/kg (watt hours per kilogram), despite temperatures ranging from +80C to -60C.

The solar cells will be made of monocrystalline silicon about 150 microns thick, with about 20 percent efficiency. They will require 200 square meters of wing and tail surface, creating an aircraft that Borschberg says will be challenging to control in flight, particularly considering its very low speeds, about 70 km/hr (43 mph).

The photovoltaic units will be encapsulated in a composite "sandwich". The structures will be carbon-fiber-reinforced composite skins bonded to a low-density core, such as balsa, polymer foam or honeycomb. They provide high mechanical strength but very low weight. Stretching carbon sheet just a few tenths of a millimeter thick over lengths of 20 meters is one of the major tests of the project.

The official composites supplier for the Solar Impulse is a Swiss company called Decision SA, which provided materials technology for Americas Cup-winning yacht Alinghi.

The Alinghi composites have a weight of 10 kg/m2, but the materials in the Solar Impulse must weigh less than one-fifth of that. Borschberg says the solution will be very high-modulus carbon fibers with optimized integration, starting with small-scale prototypes. New structures will be tested at the The Composite and Polymer Technology Laboratory of the Ecole Polytechnique Federale de Lausanne in Switzerland.

A small prototype was recently shown at a Swiss airfield. The first day flight will take place later this year, and the first night flight will be attempted in two years. The first flight will take place just meters above the runway at Duebendorf airport in Switzerland to confirm, as safely as possible, nearly two years of computer simulations.

An initial transatlantic night flight will be attempted in 2010, or later. One of the biggest problems will be pilot sleep management since the Solar Impulse will have no autopilot capabilities. Four to five days is considered the maximum sustainable period for a single pilot.

One of the primary sponsors of the close-to-$100 million project is Solvay, which presented Borschberg at its K 2007 stand in Dusseldorf, Germany. Solvay Advanced Polymers is developing lightweight thermoplastic applications for the Solar Impulse, including a shielded throttle housing that will be made in Radel R polyphenylsulfone.