Last summer, Solar Impulse flew an entirely solar-powered aircraft across the U.S. Now the Swiss team is preparing to fly a larger and more capable aircraft around the world in 2015. The challenge goes well beyond the design and construction of a pioneering aircraft that pushes the limits of electric propulsion and lightweight materials.

The team is also entering new terrain in the human factors involved in coping with five-day-long flights. Moreover, Solar Impulse is establishing smart logistics, including unique weather forecasts, on the ground.

The aircraft, to be registered as HB-SIB, is being assembled in a hangar at Dubendorf air base, near Zurich. Construction of the 236-ft.-span wing is well underway, with the ribs and leading edge already fitted to the massive spar. Each of its carbon-fiber plies weighs just 25 grams per square meter (0.08 oz. per sq. ft.), the typical minimum to date being 90 grams per square meter. The upper surface is being covered with high-performance photovoltaic cells—their efficiency is close to 22%, while they are only 135 microns thick. The forward fuselage appears to be close to completion, too, as the cockpit is fitted with some flight controls, instrument panels and wiring.

The assembly team is under time pressure, because the official unveiling of HB-SIB is scheduled for April 9, and first flight is expected in May. Although the construction facility is located in Dubendorf, close to some German suppliers, the company is headquartered in Lausanne and has its flight-test base at Payerne Airport. HB-SIB will be trucked in subassemblies to Payerne, where traffic is limited enough to accommodate such trials.

The aircraft is scheduled to take off for its around-the-world flights in March 2015, from “somewhere between Morocco and the United Arab Emirates,” says Christian Le Liepvre, head of the Solar Impulse partnership and director of the Altran Foundation for Innovation. After 8-10 stopovers, Ireland is a possibility for the final landing. It will be critical to fly over China before the summer monsoon, Le Liepvre adds.

HB-SIB is larger than its predecessor, HB-SIA. It has to accommodate only one pilot but, as some flights will last up to five days, the cockpit is roomier, and more food and water will be carried on board. Several oxygen bottles will accompany an electric onboard oxygen generation system designed by Air Liquide. In total, payload capability has been doubled, according to Le Liepvre.

The aircraft's external dimensions have also changed: It is about one third heavier and the wing span is 18% greater. Because solar cells must cover a larger wing and additional surfaces, the number of cells has increased to around 17,000 from 12,000. This will give extra margin to recharge the batteries over a five-day flight during which the weather may not always be sunny. The lithium-polymer batteries will have better energy density, at 260 watt-hours per kilogram instead of 240.

To alleviate the pilot's workload, and even allow him to sleep, a stability augmentation system (SAS) has been designed. It can act on all control surfaces but, as the aircraft is already very stable on the pitch axis, most important are the electric actuators on the ailerons and the rudder, says CEO and pilot Andre Borschberg. “The SAS can hold a heading or an attitude but can't navigate,” he explains.

The upper-level system providing higher-level control is the monitoring and alerting system (MAS), which Borschberg describes as a “virtual copilot.” The MAS oversees the aircraft, SAS and pilot. Borschberg emphasizes that it uses other sources of data than does the SAS. In case of a deviation from usual attitudes, it warns the pilot early enough for him to react, even if he had been taking a nap.

Sleep is a major issue, but pilot Bertrand Piccard, who is also company chairman and founder, appears to be confident. He was due to spend 72 hr. in a mission simulator in Dubendorf to fly virtually from New York to Seville, Spain, last month. The idea is to take 10 naps of 20 min. each over a 24-hr. period. Piccard is working with sleep experts on sleep lead-in and wake-up transitions. Moreover, in case sleeping is ruled out because of tough flight conditions, methods for keeping awake will be tested. Caffeine pills will be used only in emergencies. “I want to surf on sleep waves when I feel them coming,” Piccard says. He was trained as a psychiatrist and thus wants to use hypnosis techniques. The impact of fatigue on pilot reaction time will be measured throughout the simulated flight.

On the ground, short- and mid-term weather forecasts will be available to the team to decide when to take off and possibly work out a diversion, if required. A notable level of optimization, although not actually a forecast, is to statistically determine the best route between two stopovers, finding a trade-off between sunny areas and tailwinds. In this regard, the flights across the U.S. were a rehearsal for those around-the-world, says Christophe Beesau, an Altran simulation expert. In 2003-15, the project's budget is 150 million Swiss francs ($170 million).