Special report: Visionary design

For a little over a month – except for a few brief stopovers – Bertand Piccard or fellow pilot André Borschberg, will be confined to a cockpit measuring only 3.8m3. From here they hope to steer the plane into the history books and become the first to fly around the globe using only solar energy.

The new cockpit is significantly larger than that used in the original plane, redesigned to resist the rigours of flight at altitudes of 8,500m, as well as extreme temperatures, for up to five consecutive days at a time.

Such a test of endurance has been made a little easier thanks to the materials and technologies that have been carefully selected and refined over the past few years.

Bayer MaterialScience – an official partner of Solar Impulse since 2010 – designed the entire cockpit shell, at times dedicating a 30-strong team to its construction from May through December 2012. Although most of the materials had been successfully trialled on the prototype, every element was optimised and enhanced, says Bernd Rothe, Solar Impulse project lead, Bayer MaterialScience.

Solar Impulse The cockpit has been given a thorough overhaul for the new record attempt

Although the volume of the cabin increased by a factor of three to 25kg, only 2kg was added to the overall weight.

The Solar Impulse team had initially stipulated the entire cockpit fairing be restricted to only 20kg, he adds – but this proved a little too ambitious: “We made our calculations and told them that wasn’t possible using current materials. It was a pragmatic approach and needed a lot of discussions and adjustments; this project certainly had a lot of optimisation loops. It may be heavier but given the extra surface area and the increased functionality, on a percentage scale it’s a good result.”

Beneath the special silvery finish that coats the cockpit lies the inner workings of the Solar Impulse control hub. From here, Piccard will be at the controls and despite the limited cabin space can recline the chair fully to sleep.

With external temperatures falling as low as -40oC when airborne, ultra-low density polyurethane rigid foam provides insulation and protects the pilot.

“We’ve used Solar Impulse as an internal catalyst for our business and a lot of other projects were started because of it,” says Rothe. “Making a lightweight foam with high mechanical properties led us to other projects. The team working on SI2 had the chance to have a different attack angle – normally we work in specific areas and adhere to certain restrictions. For SI2 where the main thing was lightweighting, we had to have a new way of thinking – it wasn’t about processes, it was about making the material as light as possible. We learned a lot. Before long, fridges will be using these microcellular foams.”

STRICT REQUIREMENTS

Although some materials did not make it through the selection process, Rothe insists their development is being pursued within the company. One such example were inflammable rigid foams but Solar Impulse discounted them because their density was above targeted levels. Nevertheless, says Rothe, Bayer MaterialScience continues to work on them. Similarly, the team gained indepth knowledge of carbon fibre composites and is now working closely with other companies that are focused on composite production and is planning a series of projects.

“This was a real gain from the SI project because we could test these products in a short amount of time,” he says. “That’s the thing with a flying laboratory – you can put new materials inside and carry out some fast-lane tests. Not all of them will be positive but you learn just as much about the things that didn’t work.”

Another innovation is the door to the cabin, made entirely from Baytherm Microcell foam. The improved lightweight polyurethane (PU) foam boasts high insulation and mechanical properties, with low heat transfer and minimal thickness. Bayer researchers succeeded in shrinking the pores in the foam by an additional 40%, making its insulating capacity 10% better than the current standard. As a result, its density was only 27.5kg/m3, far below the requirements in most other end-use sectors.

Although a comparatively small component, the hinge attaching the door had great significance for Bayer MaterialScience too. Made from carbon fibre composite and reinforced by PU resin, its design broke new ground for the company, says Rothe. “The hinge may be a small part fixed to the door but we learned so much from Solar Impulse about on lightweight composites and that’s been really helpful for us to make further developments in this area.”

The automotive industry is increasingly turning to polycarbonate (PC) as a replacement for glass, the lightweight alternative reducing fuel consumption and CO2 emissions. This technology proved ideal for Solar Impulse. PC has a glass-like appearance and is scratch resistant with the same transmission rate and comparable mechanical properties. However, it is also far lighter: glass has a density of 2.6 g/cm3 and PC, around 1.2 g/cm3.

A sandwich of 1mm thermoformed polycarbonate (PC) sheets is used for the cockpit window, with a 5mm gap between the plates offering additional insulation. A special coating limits fogging during flight and ensures optimal visibility.

• Hinge

The specially-made hinge that attaches the door to the cockpit is produced using polyurethane carbon fibre composite – lightweight but extremely strong.

• Insulation

The plane’s 3.8m3 cockpit is designed to accommodate a single pilot for up to five consecutive days between legs – enough to travel across the Atlantic and Pacific Oceans. To protect the pilot, a unique foam was specified to insulate the cabin – and at a fraction of the weight of conventional materials.

• Door

The door to access the cockpit is made from a special polyurethane rigid foam with a 40% smaller cell size, reducing thermal conductivity by up to 10%.

• Windscreen

Polycarbonate sheets were specified for the window, boasting a glass-like appearance but with better properties and at a lower weight. A special coating prevents the windscreen from fogging up and obscuring the pilot’s view.