Innovation
10 questions about Switzerland's Solar Impulse aircraft – answered
By Susan Misicka, swissinfo.ch
Mar 18, 2015 - 3:56 AM

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By now you may have heard about the Solar Impulse – the Swiss solar aircraft attempting to circle the globe without a smidgen of jet fuel. How is that possible? What’s the point? And how do the pilots go to the bathroom? Here’s the low-down on this high-flying project.

The Solar Impulse is a completely solar-powered plane. Unveiled in 2009, the Solar Impulse made history as the first solar aircraft to fly through the night in 2010. That was also the plane that flew across the continental United States in 2013. Engineers back at the base in Payerne, Switzerland, then fine-tuned the aircraft.

The current plane, presented in 2014 and dubbed Si2, is larger and more powerful than its predecessor. In March 2015, it kicked off its quest to fly around the world in several stages – a journey of about 40,000 kilometres with stops in 12 destinations.

How big is the plane, and how many people can it carry?

With a wingspan of 72 metres, it’s a bit wider than a Boeing 747Jumbo Jet. But instead of carrying 400 passengers, the Si2 only has room for a single pilot in its 3.8 m3 cockpit. It weighs 2,300 kilograms, about as much as an SUV.

Who are the pilots?

Two pilots take turns flying this plane, just as they did with the prototype. Initiator Bertrand Piccard, who comes from a family of explorers and scientists, is known for making the first round-the-world balloon flight. CEO André Borschberg is an engineer, a former Swiss Air Force pilot and graduate of the Massachusetts Institute of Technology. There’s also a test pilot, Markus Scherdel, who helps make sure the Si2 is airworthy.

How does it work?

The wings are covered in 17,248 solar cells, which feed the plane’s lithium polymer batteries. These in turn help the Si2 to generate and store enough energy to power its motors to fly for longer periods. Extremely lightweight materials also contribute to the aircraft’s energy efficiency. However, it can only fly in fair weather – and is sensitive to high winds. Meteorologists help the Solar Impulse team to find the best weather windows. Departures can be delayed for days if there’s rain, which was the case in India in mid-March.

Can it really fly in the dark?

Thanks to the batteries, yes. During the day, it climbs to its maximum cruising altitude of 8,500 metres to capture the most rays. To conserve energy, it works its way down to 1,500 metres in the evening, and stays there overnight. The maximum speed is 90km/hour. The flight from Abu Dhabi to Muscat, for example, took 13 hours – something a normal plane could do in about 90 minutes.

How long can the Si2 fly?

In theory, and under the right weather conditions, the Si2 could remain in perpetual flight, but that would be a bit much for the pilots and crew. So far, they’ve flown for as long as 26 hours. Distance-wise, the Si2’s longest flight was 1,468km – a world record for a solar plane with a pilot. Crossing the Pacific and Atlantic oceans is expected to take five consecutive days and nights.

Because the temperature can range from -40°C to 40°C, the pilots have several layers of clothing. They also carry a week’s supply of water and specially-prepared food, plus oxygen for use at the higher elevations. In an emergency, they can bail via parachute and float away on a life raft.

How do the pilots sleep or go to the bathroom?

Autopilot and a special seat make both activities possible. The pilot can recline and move around somewhat to keep his circulation going. If nature calls, he can convert the seat into a lofty portable potty. Except for 20-minute power naps, they’ll hardly get more than two or three hours of sleep per 24-hour period. Piccard, who is also a psychiatrist, uses self-hypnosis techniques to cope with that. Borschberg meditates and does yoga. Physicians as well as a yoga guru will be monitoring them from the ground.

How many other people are involved?

About 90 people are part of the Solar Impulse team – including 30 engineers, 25 technicians and 22 mission controllers. The folks at mission control, located in Monaco, are in constant contact with the pilot and the plane. Data, including hundreds of technical parameters, is transmitted via satellite. For every destination, a ground crew shows up in advance to prepare for the arrival as well as the next departure.

Who’s paying for all this?

This “idea born in Switzerland” has an overall budget of CHF150million (€140million). Its many partners include the Swiss government, which granted Solar Impulse the use of two airbases. The mission control centre is funded by Prince Albert of Monaco’s environmental protection foundation. In addition, numerous companies and institutions have contributed their expertise and high-tech materials. The names of the Si2’s main and official partners are written on the plane.

What’s the point?

Indeed, this technology could hardly replace commercial aviation. It’s not meant to.

“Solar Impulse was not built to carry passengers, but to carry messages. We want to demonstrate the importance of the pioneering spirit, to encourage people to question what they’ve always taken for granted. The world needs to find new ways of improving the quality of human life. Clean technologies and renewable forms of energy are part of the solution,” says Piccard.

The round-the-world route includes several destinations where the Solar Impulse team will take the time to introduce the plane and promote its message of the power of renewable energy.


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