In 1999, pilot Bertrand Piccard had a radical idea – build a plane capable of flying night and day without using any fuel, propelled solely by solar energy. Four years later, after Piccard conducted a feasibility study with mechanical engineer and pilot André Borschberg, he and Borschberg began the Solar Impulse project.
The first plane in the project, Solar Impulse 1, completed successful solar-powered flights across Europe in 2012 and a multi-stage flight across the U.S. in 2013. Looking to improve the prototype, the company developed its successor, the single-seater Solar Impulse 2 (Si2), made of 17,248 solar cells that supply the plane with renewable energy. The goal for the Si2: complete a trip around the world.
While the idea behind the Si2 seemed exciting, many major players in the aerospace industry were skeptical, according to Bernd Rothe, the former Solar Impulse project manager who now serves as Covestro’s plant manager: “[When Solar] Impulse asked Airbus, Boeing and all the big airplane manufacturers if they wanted to also cooperate in the project, they all said, ‘That’s an idea which will not work, and we are not interested.’”
However, after years of designing and testing the aircraft, those OEMs were proven wrong on July 26, 2016, when the Si2 returned to Abu Dhabi on the final leg of its 16.5-month, round-the-world journey. According to Solar Impulse, composite parts from Solvay, Covestro and North Thin Ply Technology (NTPT) played a major role in the success of the Si2. Approximately 83 percent of the aircraft’s structure is made with composites. The structure incorporates carbon fiber sheets that are three times lighter than paper.
Rothe says that making the aircraft that light was possible because its composite material suppliers did not have the usual constraints of an Airbus- or Boeing-level aerospace project. “Let’s be honest, price and also production time are not really major factors because [Solar Impulse] just needs to produce one airplane,” Rothe says. “If the price is little bit higher than normal, that’s also OK, because the goal is to make it as light as possible.”
Claude Michel, the head of Solvay’s partnership with Solar Impulse, described that goal as “an obsession.” Overall, the plane is roughly the same size as a Boeing 747 and weighs just 2,300 kilograms (5,070 pounds) – which according to Solar Impulse is the largest aircraft ever built with such a low weight.
The aircraft’s wing spar was divided into three parts: middle, left and right. According to Michel, Solvay made the middle part entirely of carbon fiber Nomex® honeycomb sandwich panels impregnated with Solvay’s Torlon® PAI (polyamide-imide) thermoset resin. North Thin Ply Technology worked on the other two parts of the wing spar. NTPT spread M46J 12K ultra high-modulus PAN carbon tow from Toray to make unidirectional tapes. The tapes were resin-impregnated during the process and then converted into ±45° preforms up to 27 x 1.2 meters using NTPT’s automated tape laying machine.