This article was taken from the April issue of Wired magazine. Be the first to read Wired's articles in print before they're posted online, and get your hands on loads of additional content by subscribing online
Around lunchtime on 3 December, 2009 two-dozen people gathered at an airport in Dübendorf, Switzerland, and watched nervously as test pilot Markus Scherdel fired up four ten-horsepower propeller engines and launched an aircraft along a runway.
When the plane reached 35kph, Scherdel slowly pulled back the control column and the machine took off. Twenty-eight seconds later, after flying just 350 metres, it landed back on the ground to thunderous applause.
It had taken six years of development for the two men behind the project, Bertrand Piccard and André Borschberg, to see in flight the aircraft that they hope will soon circumnavigate the Earth using only solar energy. And those 350 metres meant they were on track.
<img src="http://cdni.wired.co.uk/674x281/d_f/demange_004 copie.jpg" alt="Solar Impulse"/> "We didn't know for sure whether it would actually fly,"
Borschberg said immediately after the test flight. Piccard, the Swiss psychiatrist and balloonist behind the project, and Borschberg, its chief executive, have built a team of around 50 people to make Solar Impulse fly. They include physicists, computer scientists, structure and materials specialists, engineers who have worked in Formula 1, and Claude Nicollier, a former astronaut who has under taken several Space Shuttle missions.
Behind the core team is a large network of technology partners, consultants and academics. All this doesn't come cheap: of the €44m (£39m) budget, around three-quarters has come from sponsors such as the Belgian pharmaceuticals and chemical company Solvay, the watchmaker Omega and Deutsche Bank.
The prototype -- registration code HB-SIA -- was first revealed in June 2009 at a hangar in a disused Swiss Air Force base in Dübendorf, near Zurich. It takes time to get used to the aircraft's size and elegance: although it weighs as much as the average car (1,600kg), it has the wingspan of an Airbus A340 (63.4 metres), and in flight should consume about the same amount of energy as a Vespa scooter.
The test flight last December was one step in a process that will culminate, if all goes to plan, in this iteration of the plane taking off for a longer journey this spring or summer from Payerne, another military base in Switzerland.
<img src="http://cdni.wired.co.uk/674x281/d_f/demange_002 copie.jpg" alt="Solar Impulse"/>
This planned 36-hour flight is designed to address a couple of important issues: firstly the plane's controllability and behaviour in the air. Secondly, whether its lithium batteries, which will be recharged by the Sun during the day, will be able to store enough energy for it to fly continually at night. After further tests the team will construct the HB-SIB, a more advanced version of the plane, which will attempt to cross the Atlantic.
If it succeeds, Piccard and Borschberg will then try to circumnavigate the globe, on a west-east route, in five legs of five days each. They will take turns in the one-person cockpit (which means that each of them will fly non-stop for stretches of five days and four nights) and switch at prearranged destinations. "It's not just an adventure," Borschberg says, standing in the Solar Impulse hangar in Dübendorf. The team wants to use the plane as "a platform to test the development and exploitation of renewable energy and clean technology".
The project might appear fanciful, but the pair behind it have impressive credentials. Borschberg is a successful businessman with an engineering background who was a fighter pilot for the Swiss army. Piccard -- from a family of Swiss scientists and explorers -- is best known for completing the first non-stop circumnavigation of the globe in a balloon.
He is the latest in a line of pioneers stretching back to his grandfather, Auguste, who discovered the principle of the pressurised cabin -- which opened the way for modern aviation and the conquest of space. In 1931 Auguste was the first man to reach the stratosphere in a balloon -- climbing to an altitude of 51,772 feet -- and observe the curvature of the Earth.
Bertrand's father, Jacques, who died in November 2008, went down instead of up. In 1960, he and US Navy Lieutenant Don Walsh piloted the submersible Trieste to the deepest point of the ocean: 10,916 metres down into the Mariana Trench, which lies in the Pacific between Japan and New Guinea.
Bertrand Piccard trained as a psychologist and psychiatrist, but he was always fascinated by flight. After two failed attempts, Piccard and Englishman Brian Jones completed the first circumnavigation of the globe in a balloon, Orbiter 3, in 1999.
They took off from Château d'Oex, in the Swiss Alps, on 1 March, and landed in the Egyptian desert after travelling for almost 20 days and covering a distance of 40,805 kilometres. It was the longest recorded flight in aviation history, both in terms of distance and duration. The best-known photograph of the mission shows Piccard and Jones, smiling and exhausted, in the Egyptian desert after landing the balloon.
Look closely at the image and you'll see that, attached to the capsule in which they had been living, there is an almost empty cylinder of liquid propane -- the adventurers had launched with 32. "We started off with 3.7 tonnes of propane," Piccard says. "And we landed with just 40 kilos. I realised how close we'd been to failure, and I promised myself that if I was going to fly around the world again, I was going to do it entirely independently of fossil fuels."
Piccard has become a popular public speaker at corporate events on the back of his adventures. In his presentations he spins an analogy derived from his 20 days in a pressurised capsule. "People talk of 'pioneer spirit', thinking that the pioneers are the ones who develop new ideas," Piccard says. "Not so. It's easy to have new ideas. The pioneers are those who have the courage to throw a lot of ballast overboard. Habits. Certainties. Convictions.
Dogmas. Paradigms. And when we do that, life is not just a line that goes in one direction and in one dimension: it becomes the meeting point of all possible lines that spin out in all directions, in three dimensions. A pioneer spirit is what allows us to explore all ways of doing, behaving, thinking, to find how to go in the direction we want. Just like the pilot of the balloon who is looking for the right wind."
But decision-making isn't always clear cut. At last July's TEDGlobal conference in Oxford, Piccard told a favourite anecdote. One day, when he and Jones were in Orbiter 3, meteorologists asked them to fly lower and to slow down. "But we disobeyed because we thought that at that speed we would never be able to travel around the world," Piccard said. "And so we climbed to a higher altitude and found a jet stream, which doubled our speed.
The meteorologists told us to come down immediately, but we refused because we were proud of our piloting. That was until one of the meteorologists said, 'There's a bank of low pressure to your left. If you keep going this way, in a couple of hours you will be pushed into a new trajectory and end up at the North Pole. Tell me: since you're such a good pilot, do you prefer to go quickly in the wrong direction, or to go slowly in the right direction?'" He took the advice.
<img src="http://cdni.wired.co.uk/674x281/d_f/demange_006 copie.jpg" alt="Solar Impulse"/>
Nothing like HB-SIA has been built before. Designed to reach an altitude of between 26,000 and 30,000 feet, it's intended to remain in the air for several days (and nights) powered solely by the energy of the Sun, which will be captured by the photovoltaic cells covering its huge wings.
During the day the solar panels will run the four propellers while charging the batteries. At night the plane will fly using thermal currents (along with the energy stored in its batteries), gliding down to an altitude of about 7,000 feet. The higher the HB-SIA rises before the Sun goes down, the longer it will be able to glide during hours of darkness.
The HB-SIA is necessarily large and fragile. It's also slow with a cruising speed of around 70kmph. Such a light and unwieldy structure will be highly sensitive to turbulence, making it difficult to control. Piccard and Borschberg will depend on good weather.
Strong winds, rain or hail will keep it grounded or require the team to find alternative routes tracking the Sun and avoiding hostile weather. The plane will also have to contend with headwinds during the night that may slow it down and consume battery life. To try to minimise the potentially damaging effects of bad weather, an international team of meteorologists is using computer simulations and known data to identify the optimum route.
Currently, Piccard and Borschberg are thinking of leaving from the United Arab Emirates and stopping in China, Hawaii, the continental US and probably Spain. This could change: new analysis of weather patterns may lead them to choose a more northerly route.
The route may be flexible, but Solar Impulse's engineering challenges are not. The aircraft has to withstand not just turbulence, but also cold, humidity, differences in air pressure, tension and torsion. But for the HB-SIA to be able to fly day and night using only the energy captured by its solar panels, the plane must also be as light as possible.
The engineering team has eliminated all unnecessary weight by using cutting-edge carbon-fibre technology. Some of the strips for the wings are 0.01mm thick yet extend for 20 metres.
Human-machine interfaces will monitor the pilots -- sensor-fitted suits will vibrate if they detect a problem either with the plane or one of the pilots. Information will be relayed to and from the plane constantly: voice data, video imaging and a satcom system will offer the pilots and the Solar Impulse team on the ground real-time data.
The HB-SIA is made of an incredibly lightweight composite material similar to that originally developed for the hull of Alinghi, the America's Cup-winning Swiss yacht. The interior structure of the fuselage looks a little like a sandwich: a carbon fibre exterior that's 0.2 mm thick and weighs 90 grams per square metre contains a honeycomb construction that has been bonded while in a vacuum and then heated repeatedly in an oven.
The 200-square-metre surface of the wings and the rear stabiliser are covered with 11,628 mono-crystalline silicon cells.
These are designed to capture the photons emitted from the Sun and transform them into to electricity that runs the engines and is stored in the batteries.
The choice of cells was dictated by their weight: depending on temperature -- the cells become more efficient at lower temperatures -- they have an efficiency of 20-22 percent. There are cells that are slightly more efficient, such as those used on satellites, but they're much thicker, which makes them heavier. The Solar Impulse cells are just 150 microns thick and extremely light.
<img src="http://cdni.wired.co.uk/674x281/d_f/demange_005 copie.jpg" alt="Solar Impulse"/>
To power the four propeller engines -- which run at only 200rpm -- energy is stored in four large lithium batteries that account for a quarter of the plane's total weight. Optimised, they have a density (power thrust) of 220kW per kilogram, despite the extremely difficult external conditions and temperature range (the plane will fly between +50°C and -40°C). "We have to find a way to extract maximum power from minimum energy," says Borschberg, "and fly using as little of it as possible."Weight is everything, and the Solar Impulse team is obsessed by it. Borschberg, who is several centimetres taller than Piccard, has been dieting to lose a few kilos so that he can fit in to the 1.3-cubic-metre cockpit more easily.
Although the use of electricity in aviation dates back to 1884 -- when the French army piloted the first battery-powered airship on an 8km round-trip -- it wasn't until 1980 that a custom-built solar aircraft, the Gossamer Penguin, designed by American aeronautical engineer Paul MacCready, flew a few metres above the ground in the Houston Astrodome. A year later, a more sophisticated model, the Solar Challenger, crossed the English Channel.
In 1990, scientist Eric Raymond (who has worked with the Solar Impulse team) spent two months flying a solar-powered plane, Sunseeker, across the US in 21 stages, the longest of which was 400 kilometres. Last year, Raymond crossed the Alps, from central Switzerland to the Italian city of Turin, in six hours, flying a new version of his craft.
Nasa has developed several unmanned drones, one of which, Helios, set the altitude record for solar-powered flight by reaching nearly 10,000 feet in 2001. The same aircraft came down in the Pacific near Hawaii two years later, when air turbulence caused structural damage. But all the substantive solar-powered flights thus far share the same limitation: they have occurred during daytime, meaning that they haven't had to store energy for use during the night. "With 200 square metres of solar panels on the wings, we can produce the same energy consumed by 200 small bulbs," Piccard says. "Carrying a pilot around the world with an airplane that uses the same amount of energy that lights up a large Christmas tree? People will tell you that it is impossible."
The machine's engines produce around eight horsepower or six kilowatts, which is a similar amount of energy to that available to the Wright Brothers in 1903 when they made the first powered flight. But if the machine is a gem of technology and innovation, the pilots are two 50-year-old men who will have to spend five days and nights alone at 26,000 feet crammed into a cockpit barely big enough to fit a seat.
The pilots' welfare -- keeping a constant supply of oxygen and designing ways for Piccard and Borschberg to rest and eat -- has been as important for the Solar Impulse team as designing the technology. Although the HB-SIA prototype will remain a "raw" aircraft, the future version that will circumnavigate the globe will need to be adapted for the elevated altitude and will be fitted with systems to circulate oxygen and eliminate CO2 and moisture.
To limit weight, the current cockpit is neither pressurised nor heated, and at higher altitudes the pilots will have to wear oxygen masks. Flight controls are manual but the onboard electronics play a key role in controlling the plane's stability. An aircraft of this size flying at low speeds will stall if it tilts more than five degrees from level while making turns (a commercial airliner typically banks at 25° from level).
To deal with this, the Solar Impulse team has developed a new type of monitoring system: the sensors inside the pilots' suits will vibrate if the plane tilts excessively. Both pilots are following a fitness programme to get them in top physical and psychological condition and are undergoing sessions in a hyperbaric chamber to help them adapt to changes in oxygen levels, and many hours of flight simulation to assess the mechanisms of fatigue.
Each of them has developed an individual method of managing concentration, effort and energy: Borschberg prefers meditation, and Piccard uses self-hypnosis. During the mission Borschberg will force himself to eat little or nothing, and will come close to complete fasting; Piccard plans to consume small amounts of high energy food at regular intervals. "We need to show people that renewable energy is not a step backwards for quality of life, rather a leap into the future,"
Piccard says. "If we can travel around the world in a solar plane, in the future no one will be able to say that you can't do the same with cars, heating systems, air conditioning and computers."
But the flight remains theoretical until Piccard, Borschberg and their colleagues have proven otherwise. Which may be soon.
Borschberg says if all goes according to plan, the Solar Impulse's next significant test will be this spring or summer: staying in the air for 36 hours. Both Piccard and Borschberg are determined that this will not be the last of their solar-powered craft's achievements.