Design and development of cutting edge technology is an iterative process in which improvements in one generation are incorporated in the following generation. That is the scenario that Bertrand Piccard and Andre Borschberg followed in leading a team of 80 engineers and technicians that developed the Solar Impulse project in Payerne, Switzerland. For the past 12 years they went through a feasibility study, concept, design and construction of Solar Impulse 1 and now have unveiled an updated and improved Solar Impulse 2 (Fig. 1). During development of the solar-powered planes they worked with 80 technological partners and more than 100 advisers and suppliers.
Solar Impulse 2 is the second generation of Solar Impulse 1 that flew across the U.S. in 2013. In 2014 Solar Impulse 2 will undergo testing in Payerne, Switzerland where it was designed and Dübendorf, Switzerland where it was built. Next, Solar Impulse 2 will do what no one has ever done before: fly through consecutive days and nights using only its primary solar power as it crosses oceans from one continent to the next. Scheduled to start in March 2015 from the Persian Gulf area, its flight path will take it over the Arabian Sea, India, Burma, China, Pacific Ocean, Unites States, Atlantic Ocean and southern Europe or North Africa before returning to its departure point.
To make this around-the-world goal a reality, the engineers and technicians had to apply many innovative solutions to Solar Impulse 2. They had to make maximum use of every single watt supplied by the sun and store it in lithium-ion batteries. And, they had to track down every possible source of energy efficiency to ensure adequate flying time.
Spread across the top of the wing, fuselage and tail section of Solar Impulse 2 are 17,248 monocrystalline, 135 microns thick silicon solar cells that cover 269.5 m2. Solar Impulse 1 had about 12,000 solar cells that were 150 microns thick and rated at 45kW, peak power.
Energy collected by the solar cells is stored in lithium-polymer batteries, whose energy density is optimized to 260 Wh/kg. Solar Impulse 2 has batteries able to guarantee 2,000 flight hours, compared with only 500 for the Solar Impulse 1. Batteries are insulated by high density foam and mounted in four engine nacelles, with a system to control charging thresholds and temperature. Their total mass amounts to 633 kg, or just over a quarter of the aircraft’s weight.
For propulsion, the plane employs four brushless, sensorless motors, each generating 17.4 hp (13.5 k) mounted below the wings, and fitted with a reduction gear limiting the rotation speed of a 4m diameter, two-bladed propeller to 525 rpm. The entire system is 94% efficient - a record for energy efficiency. At sea level minimum speed is 36 km/h and maximum speed is 90 km/h (49 Kts). At its maximum altitude: from 57 km/h (31.5 Kts) to 140 km/h (49 Kts).
The plane will fly at a maximum 8500 m (28,000) ft during the day so it can store solar energy, then descend during the night using the drop in altitude to maintain air speed without drawing on the batteries until it reaches between 6000 and 9000 ft (1800 to 2700 m) and goes back to battery power .
Engineers designed the entire structure to be proportionately lighter than that of the best glider. Every gram added had to be deducted somewhere else, to make room for enough batteries on board, and provide a cockpit in which a pilot can live for a week. In the end, its weight is similar to a small van: 2300 kg.
The airframe is made of composite materials: carbon fiber and honeycomb construction. It employs sheets of carbon, which weigh only a third as much as sheets of printer paper (25 g/m2). To maximize aerodynamic performance the plane’s wingspan is 72 m (263 ft), compared with 68.5 m for a Boeing 747. The upper wing surface is covered by a skin consisting of encapsulated solar cells and the lower surface by a high-strength, flexible skin. There are 140 carbon-fiber ribs spaced at 50 cm intervals that give the wing its aerodynamic cross-section, and rigidity.
The plane is sensitive to turbulence, so takeoffs and landings are scheduled for the early morning and after dark. In addition, wings must be kept stable for takeoffs and landings, so the ground crew uses electric bicycles to hold the wings up on takeoff and catch them again on landing. They grab on to posts suspended from the plane, which are also equipped with small wheels, to accommodate emergency landings.
The plane’s 3.8 sq-m (40.9 sq-ft) cockpit was designed using computer-aided ergonomic simulations. It has the ability to convert into a bunk so the pilot can catch a nap during the five-day ocean crossings. To save weight, the cockpit isn't pressurized or heated. The pilot relies on an oxygen supply for high altitude flight. Both the pilot and batteries are protected against the subzero temperatures by a new insulating foam developed by Bayer.
In an emergency, an autopilot alerts the pilot via a wrist-mounted alert buzzer. In addition, the flight will be followed by a ground-based mission control. Communications are via a satellite communications system by Swisscom, which weighs less than 5 kg (11 lb), and uses a lightweight satellite antenna.