Solar Impulse team's next challenge: To circle the globe

  • 31-Jul-2013 08:21 EDT

The Solar Impulse cruises low over a nighttime tarmac during the plane’s cross-country trip.

Now that we’ve followed Solar Impulse—the solar electric ultralight with the wingspan of an Airbus A340 airliner—make its stately way across North America, it’s good to remember that there is more to come, perhaps by 2015. For the Solar Impulse (HB-SIA) is just the prototype for an as-yet-unnamed plane that is being built and tested right now. It is with this very similar but somewhat larger aircraft, designated HB-SIB, that Bertrand Piccard and André Borschberg are to circle the Earth without burning fuel, the latest of a long line of classic aviation challenges.

The HB-SIB’s project engineers are, for example, currently testing the airworthiness of the plane's polyurethane foam cockpit shell at the RUAG Large Subsonic Wind Tunnel in Emmen, Switzerland, said Richard Northcote, head of sustainability for Bayer MaterialScience, which is a technical sponsor of the Solar Impulse project and is supplying the cockpit structure and the cabin door. Other team members are meanwhile simulating the in-flight behavior of the materials of the second aircraft, which will need to be even more mass-efficient than the first model.

Bayer technology additions

The new cockpit contains polyurethane insulating material that traps heat significantly better than standard versions of the material to better protect the pilot from in-flight temperature fluctuations that can range from -50 to +50°C. “This kind of microcellular polyurethane can be much thinner, as much as one-half as thick for same R value, which gives it applications in automobiles, appliances, and industrial operations,” Northcote said.

Another task for the NB-SIB development team is to lower the chances that condensation could form on the cockpit windshield and thus impair the pilot’s vision, which can happen if the temperature drops rapidly. The current configuration comprises two high-transparency polycarbonate (PC) films with an air gap between them. Under consideration is the use of an enlarged, more stable cockpit windshield that employs a Bayer PC sandwich structure. Similar transparent but scratch-proof PC (plus ABS) formulations are used for automotive glazing on the Mercedes SLK and Bugatti Veyron 16.4 Grand Sport.

Bayer researchers are likewise focusing on special coatings to improve the properties of the plane's high-strength fabric skins. Lightweight, weather-resistant coatings to protect the wing skin fabrics are particularly important, a potential hazard that became real several weeks ago when wing fabric tore off as Borschberg flew the Solar Impulse from Washington, D.C., to New York City. Happily, the composite wing structure held together despite the gaping hole.

Flying sustainability platform

Northcote said that Bayer has spent from $4 to 5 million developing seven products (mainly foams) for the Solar Impulse project by optimizing and tweaking existing material and processes to address the extreme, but not totally unique, problems thrown up by the ultralightweight airplane design.

The company’s CEO, Patrick Thomas, thinks the Solar Impulse serves as an ideal platform for Bayer to demonstrate how its materials can contribute to sustainable development, including energy-efficient transportation. In this vein, the company is working with catalyst experts at the University of Aachen to reduce carbon dioxide plant emissions and even extract the carbon for use as a basic feedstock for polymer manufacturing.

After Thomas heard their presentation in 2010, he was inspired by the passion of Piccard, the famous balloonist, and Borschberg, the Swiss Air Force pilot and engineer, both pioneers “who began with the notion that ‘the two of us need to fly around the world without burning any fuel,’” said Northcote. “It works as a symbol, to use science to achieve something impossible.”

Solar Impulse’s development path

The story goes that Piccard and Borschberg were told by several aerospace firms they had contacted that their dream flight “couldn’t be done,” eventually leading them to a high-tech boat builder, Decision of Ecublens, Switzerland, which specializes in polymer composite structures. The plane was reportedly designed at the École Polytechnique Fédérale de Lausanne, and then the technical partners competed to provide products that best satisfy the design’s tough specs. At its height, the design/engineering/build team had 40 people on it.

The Belgian chemical company Solvay reported that it had conducted some 50 research projects to address various issues in the HB-SIA design and that about half of them succeeded. “We worked closely with Solvay on the blowing agents for the foams,” Northcote said, “which parallels the high degree of industry collaboration that is necessary nowadays.”

As with the Solar Impulse, efforts to shave weight from the HB-SIB structure are reportedly being met by fabricating spars, ribs, and airframe members out of “thin-ply” carbon-fiber tapes from North Thin Ply Technology of Penthalaz, Switzerland. The thin layers of pre-preg fabric tape—one-fifth the thickness of standard tapes—are precisely positioned by robot, resulting in laminate structures with ultralow mass density. The featherweight wing spar, for example, for the HB-SIB is 70 to 80 m long, which will make the plane's wingspan approach that of the Airbus 380, the world’s largest passenger jet.

Bayer brought something special to the carbon fiber/epoxy mix as well with a contribution of carbon nanotubes (CNTs) to further reinforce the composite and boost strength and durability. The nanotubes, Northcote said, were manufactured by Bayer’s now-mothballed Baytubes pilot CNT plant, which the company shut down earlier this year for lack of a growing market.

Next year’s model

The new larger aircraft will weigh somewhat more than its 1600-kg (3500-lb) predecessor. It will share Solar Impulse’s general configuration with four electric motors that are powered by more than 10,000 thin-film silicon solar cells supplied by SunPower. Electrical storage is provided by a lightweight lithium-polymer battery pack that nonetheless accounts for about a quarter of the plane’s mass. And it will employ the proven daily flight plan, which is to “porpoise” through the atmosphere—to first climb to 8000 or 9000 m during the day and then slowly glide down at nighttime, losing 15 m per minute to altitudes as low as 1500 m, before rising again when the sun rises.

Borschberg and Piccard will take turns piloting the HB-SIB on its 20- to 25-day world-circling odyssey on three- to four-day jumps whose duration is limited by pilot physiology. But the slow-flying plane will need to be in the air far longer than were earlier such flights. Dick Rutan and Jeana Yeager, for example, stayed in the air for 9 days in the Rutan Voyager in 1986, whereas Steve Fossett flew for only 67 hours in the Virgin Atlantic GlobalFlyer in 2005. Even the first round-the-world balloon flight in 1999 with Piccard and Brian Jones in the Breitling Orbiter 3 took fewer than 20 days.

To complete the global marathon, the new airplane will feature a more spacious cockpit to allow the pilot to move around and sleep. The ergonomically optimized cockpit is said to include a seat designed by Lantal of Langenthal, Switzerland, which will allow the pilot to shift positions, including attending to calls of nature. The plane will need to carry sufficient food as well as oxygen and water, but efficient recycling is being investigated to minimize stores.

And, of course, to help them meet these design, structural, and operational challenges, Borschberg, Piccard, and the Solar Impulse project team are now looking for additional sponsors and backers, not to mention a low-mass battery technology.

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