A solar-powered airplane performs the implausible, flying day and night without fuel.

In July, a lightweight airplane with a carbon fiber structure completed a two-month round trip journey between Switzerland and Morocco. At first glance, the voyage of more than 3,700 miles may not seem noteworthy. However, the flight was made without any fuel. The Solar Impulse HB-SIA traveled more than 131 hours in a series of eight flights powered exclusively by solar energy. It was the world’s first roundtrip solarpowered intercontinental flight.

“It’s been an extraordinary adventure not only for what we’ve achieved with this airplane – originally only designed to demonstrate the possibility of flying day and night with a purely solar energy – but also for what has resulted in a tightly fused team,” says André Borschberg, CEO and co-founder of the Solar Impulse program in Lausanne, Switzerland. For nearly a decade, the company’s team of 70 employees has worked with 80 partners to build and test the HB-SIA. Among those partners were composite material supplier Bayer MaterialScience and manufacturer Décision SA.

The skeleton of the wings, the fuselage, the horizontal stabilizer and the cockpit of the HB-SIA all feature composites made of a carbon fiber honeycomb sandwich structure. The top of the wing surface is covered with a skin of embedded solar cells, while the under sides are covered with a high-resistance flexible film. The wing achieves its aerodynamic profile and rigidity from 120 carbon fiber ribs placed at 50 centimeter intervals.

Deciding on materials and building the solar-powered plane was not a simple task. The company devoted one year of studies, four years of design work, two years of building and a year of testing to the project before the HB-SIA prototype took to the air.

The Project Takes Flight

Engineers test the cockpit structure of the HB-SIA.

Engineers test the cockpit structure of the HB-SIA.

Solar Impulse was born from the vision of two aviators – Borschberg and Bertrand Piccard. In 1999, Piccard made the first non-stop hot air balloon trip around the world. “He almost failed because of a lack of fuel,” says Martin Kreuter, project leader on the Solar Impulse program for Bayer MaterialScience. “Piccard made a promise to himself that the next time he would fly around the world, it would be with no fuel at all.”

Equally concerned with protecting the environment, Piccard hatched the idea of creating a solar aircraft to promote renewable energies and encourage energy savings. “Adventure in the 21st century consists of using creativity and the pioneering spirit to develop the quality of life to which present and future generations are entitled,” says Piccard, president of Solar Impulse. In 2003, he teamed up with Borschberg to launch the company and authorize a feasibility study for the project conducted by École Polytechnique Fédérale de Lausanne, a Swiss technical university.

The next several years were spent developing the concept, designing the airplane and building the HB-SIA. On April 7, 2010, the airplane made its maiden voyage, a two-and-a-half hour flight over Switzerland.

Materials Make the Difference

Bayer MaterialScience, based in Leverkusen, Germany, teamed with Solar Impulse prior to the first flight to supply high-tech materials for airplane components. Approximately 30 researchers work in the company’s laboratories on innovative ideas for lightweight construction and energy efficiency. The team is led by Kreuter and Bernd Rothe, leading engineer.