Solvay and NTPT’s preforms for the wing spar were manufactured by specialist Decision SA using an out-of-autoclave technique, which allowed the company to create large composite molds and parts at low cure temperatures.

“Obviously, autoclaves that you would normally use are really expensive,” says Claire Michel, global marketing communications manager at Solvay. “Being able to manufacture it out-of-autoclave means cheaper infrastructure and the ability to manufacture something a lot bigger.”

In addition to the two wing spars, NTPT’s thin ply epoxy prepregs were used for the aircraft’s fuselage and empennage. Solar Impulse says the thin plies for the fuselage reduced the weight of the carbon fiber sheets from 100 g/m² to 25 g/m².

Covestro contributed to the aircraft’s cockpit cladding, door hinges, engine cowling, wings and cockpit windows. The company used a polyurethane resin system with a biaxial carbon fiber fabric as the matrix. Rothe says the door hinge was made by high-pressure resin transfer molding (HP-RTM) and that Solar Impulse built and adapted a special mold during the optimization process. Covestro partnered with a mold manufacturer experienced with RTM molds and a design team to help with an optimum 3-D design. Rothe says the feedback and exchange with Solar Impulse composite experts helped to make the composite parts as lightweight as possible.

However, according to Claire Michel, there was more to the Solar Impulse’s composite construction than simply achieving light weight. “[With composites], you can manufacture very complex [parts] in one step,” she says. “You can integrate components and cut down the number of parts. That in turn cuts down on the cost because you need less labor, less equipment in your factory, less tooling. You will also have reduced your emissions.”

According to Solar Impulse, the company’s next major project will be joining Google, Airbus and Facebook in the solar-powered drone race. Solar Impulse’s proposed drone, known as High Altitude Pseudo-Satellite (HAPS), may also be constructed with composites.