The materials provided by Bayer MaterialScience include polyurethane foams in the cockpit cladding, motor cowling and wings. The airplane’s structure comprises carbon layers impregnated with epoxy resin that cover honeycomb structures or foam cores.

Bayer researchers are currently devising solutions for the HB-SIB, the next generation of the airplane slotted for an around-the-world flight in 2015. One area of focus is the cockpit windshield, currently made of two highly transparent films with a cushion of air between them. “Because condensation forms quickly when the temperature drops and vision is impaired, we are looking for alternatives,” says Kreuter. “One possible solution would be to use a compact panel made of polycarbonate, but this is slightly heavier than the existing solution.”

Nanotechnology is another focal point. Bayer MaterialScience currently provides carbon nanotubes (CNTs) for the wings and other composite parts. “Our CNTs have been specifically developed to function in an epoxy matrix since they can make carbon composite structures much lighter due to advanced mechanical properties,” says Rothe. “But there is always room for improvement and new ideas.”

The Structure Is Built

Swiss company Décision SA manufactured the composite structure of the airplane then shipped the pieces to Solar Impulse for assembly. This was the first airplane project for Décision SA, which primarily builds boats.

Décision SA engineers advised Solar Impulse on construction methods, which influenced the design of each part. Solar Impulse then designed the airplane, pushing the technological limits of each component to make them as light as possible. “If, at 100 percent of the intended load the part breaks, then it must be too fragile,” says Borschberg. “But if it doesn’t break, there’s a good chance it’s too heavy!” The HB-SIA airplane weighs only 3,527 pounds – about the same as the average car.

Décision SA manufactured the molds and tools for each piece, while Solar Impulse engineers calculated the thickness of the structure to determine the lamination plan of the carbon plies and sandwich material. The next-generation HB-SIB will use a new laminating process called thin ply technology (TPT) from North TPT. It allows for complex laminates using carbon/epoxy tapes as light as 25 g/m2, decreasing the weight of the airplane.

During manufacturing of the HB-SIA, Décision SA cut sheets of prepreg carbon fiber according to Solar Impulse’s technical drawing, glued the elements to shape each piece and baked those pieces in an oven to rigidify and fix them. Next, the company assembled all the main pieces of the plane, which were sent to Solar Impulse for final construction and testing.

Powering the Plane

Flying without fuel requires the combination of an aerodynamic design and energy optimization. The wing and horizontal stabilizer of the HB-SIA were covered with more than 11,600 monocrystalline silicon solar cells, each 150 microns thick. They were selected for their lightness, flexibility and energy output.

The biggest challenge was storing enough energy in the lithium polymer batteries, situated in four engine nacelles beneath the wings. Each nacelle contains a 10 HP motor, a battery set and a management system controlling charge/discharge and temperature. While the Solar Impulse team could have chosen a more energy-efficient solar solution, other options would have added weight. Engineers and designers continually evaluated the necessity for a lightweight airplane against the advantages of various solar energy capture and storage systems.

Such considerations led to numerous calculations, tests and simulations throughout the project. During construction, wing-loading and vibration tests allowed engineers to fine-tune their models. Prior to the first test flights, Solar Impulse developed a simulator to train pilots on the airplane’s unique handling characteristics: It has a very small cockpit, a large wingspan and low wingloading. The mission team conducted many simulated flights to study weather patterns, prepare for takeoff and landings and more.

All the work paid off on May 24, 2012, when the Solar Impulse airplane left Payerne, Switzerland, on its recordbreaking intercontinental flight. Piccard and Borschberg took turns in the single-seat cockpit navigating the HBSIA to Morocco and back. Now the focus is on the future – an around-the-world flight in 2015 – and beyond.

“Solar Impulse allows research on new material solutions beyond the daily boundaries, pushing technologies to the limit and paving the way for long-term business opportunities,” says Rothe. “It provides an innovation stimulus that goes far beyond the single applications that finally make it into the airplane.”