Solar Impulse Concept Plane Unveiled

Illustration credit: Solar Impulse  |  Artist's conception of the HB-SIA in flight

Today, Swiss engineers unveil the Solar Impulse HB-SIA prototype plane. As the name suggests, the plane is designed to fly solely on solar power, and its next trick will be a 36-hour test flight early next year. That’s one compete day and one complete night fueled only by rechargeable batteries and sunlight. Its successor is slated for a trans-Atlantic trip sometime in 2012. The final goal is a complete trip around the world.

The two men at the heart of the enterprise—mechanical engineer Andre Borschberg and CEO Bertrand Piccard, who was with the first group that circumnavigated the world in a balloon—have been working since 2003 to make this zero net energy concept vehicle.

Until now, the plane’s design was a closely guarded secret. The vast expanse of carbon composite and silicon has been under construction since 2007, hidden in a hangar on a ghost-town of a Swiss Air Force base in Dübendorf, about a 15-minute train ride north of Zürich.

I got a chance to take a very brief peek at it last summer when I was in town. Rachel Bros de Puechredon, the adorable blond Frenchwoman who is Solar Impulse’s sole press rep, charged ahead of me in sky-high, pointy-toed white stilettos and ordered me to leave my camera behind, and of course swore me to secrecy.

Super secret hangar

Photo credit: Sally Adee | The inside of the hangar, which was divided into two partitions; the first (shown) was where reporters were allowed. The second (semi-visible through far door) was where reporters were not allowed.


The Solar Impulse plane is one sleek monster. Its 61-meter wingspan is equal to that of an Airbus A340, one of the transatlantic behemoths that ferries passengers between London and New York (228 of them). Unfortunately for the solitary pilot of the HB-SIA, all that width is in the service of a tiny sarcophagus suspended from the middle of the mammoth wing.

The tiny cockpit holds the pilot, two batteries and the flight electronics and not an ounce more. It weighs just 1500 kilograms. Compare that to the A340, which weighs 260,000 kg without cargo.

Why the insane ratio of width to weight? It turns out that only these disproportionate dimensions will let the plane coast at the proper cruising speed, which is about 45 kilometers per hour. That’s slow enough for the motors to operate four propellers that can subsist on what's supplied by the solar panels encrusting every inch of the wings (at most 10 HP per motor).

These monocrystalline silicon solar cells convert about 20 percent of incoming solar energy into electricity. The cells, which are not the most state-of-the-art in terms of performance, were chosen more for their weight than their efficiency. The high-grade solar cells on satellites, for example, are made from compounds like gallium indium phosphide and gallium indium arsenide and would weigh down the delicate plane. The plane’s heaviest components are its four 100-kg lithium polymer batteries that store excess incoming power. The long wingspan means less power needs to be produced by the motors to keep the plane perfectly horizontal. It also means greater surface area to hold all 10,748 of those solar cells. (FYI, you can adopt one. I tend to hate cutesy marketing stuff like this, but I admit this one got me.)

The engineers solved the weight challenge, but in the process created another problem. The featherweight vehicle needs to be in complete control at all times; tilt the plane more than 5 degrees and the pilot will likely lose control. Even at the comparatively low altitude of 8500 meters (the max you can go without pressurizing the plane if you don’t want to pass out), this would be bad news for plane and pilot alike.

So it would take something like a trained fighter pilot to keep the massive plane within five degrees of horizontal for 36 hours solid.

As luck would have it, Borschberg was a fighter pilot with the Swiss Air Force. He likes to fly and he’s not afraid of a little risk, as evidenced by the grayish-green tint of Rachel’s face when she picks me up from the train station. On this unusually humid and sweltery day, Switzerland is in the throes of rainstorms extending from Zürich down to Lausanne, 200 km south and home to the research arm of the Solar Impulse enterprise. I’ve opted for the three-hour train ride. Rachel, on the other hand, left Lausanne in the early hours with Borschberg by helicopter. They got about halfway, tumbling around the opaque black sky, before Rachel, moments from vomiting, screamed at him to turn back. He pressed on just a little further (she tells me he’s masterful at pushing his luck just to the limit and not an inch further) before acquiescing, and turning back. They ended up driving.

Photo credit: Sally Adee  | Andre Borschberg in a mockup of the plane's cramped cockpit, training on a flight simulator.


But it’s not just cojones that will get Borschberg through 36 straight hours in the air. Research has shown that sleep deprivation, even just 24 hours, has the effect of a 0.10 blood alcohol level, illegal in Canada, the United Kingdom and the United States. But they also found that 20 minute-catnaps, spaced out properly, can stall these effects. That’s because a full complement of sleep includes 90-minute cycles of four stages of sleep, each more deep than the previous one, and finally REM sleep, in which the sleeper dreams. The average person has four or five of these 90 minute cycles per night. If you’ve ever been forcibly awakened 40 minutes into a night’s sleep you know how disoriented and groggy you feel. The magic bullet, research has shown, is the 20 minute nap—you’ve entered only the lightest stage of sleep, and on waking you feel completely alert. But how does the team make sure the far-away pilot never stays asleep for more than 20 minutes?

At the Lausanne campus of the Solar Impulse enterprise, they’ve cooked up a special shirt that will make sure he only sleeps in increments of 20 minutes.

You’ll have to read tomorrow’s post for the sexy details.

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