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Solar power: A flight to remember

The dream of perpetual flight without fuel has inspired pilots to take to the skies in solar-powered planes. Vicki Cleave looks at a mission to fly a solar plane through the night - and around the world.

Credit: SOLAR IMPULSE/EPFL CLAUDIO LEONARDI

When the Wright brothers made their maiden flight in a powered aircraft on a windswept beach in 1903, it was a short hop, skip and jump into the record books. More than a century later another single-seater aircraft is on its way to making its own record-breaking hops, skips and jumps around the globe. Each of Solar Impulse's wings will cover more distance than Orville Wright's first flight; but the plane's 80-metre wingspan is not what's truly impressive about it. The remarkable thing is where it will get its power — and how little it will need. Driven solely by energy from the Sun, the plane will be carried aloft by solar cells that generate a total of around 9 kilowatts — roughly the same power available to the Wright Flyer from its single engine.

In the Wright era, aircraft were dubbed 'heavier-than-air machines', reflecting the disbelief that they could leave the ground, let alone be successfully piloted. The history of manned solar aviation fosters similar scepticism (see 'Solar aviation highs and lows'). Most solar planes move so slowly through the air, their ungainly frames buffeted by weather, that they challenge our expectations of modern-day flight. Yet the pilots who wish to fly Solar Impulse around the world plan on staying aloft for up to five days at a time, and flying through the Sun-starved night.

The US$91-million Solar Impulse project is the vision of Bertrand Piccard, a Swiss aeronaut already in the record books as one of the pilots of the first non-stop round-the-world balloon flight in the Breitling Orbiter 3 in 1999. Piccard, who will also be one of the pilots on Solar Impulse's trip around the world, comes from a family of adventurers. His grandfather Auguste made a record-breaking balloon ascent to 23 kilometres in the 1930s and his father Jacques was one of two people to have reached the Challenger Deep in the Mariana Trench, the deepest surveyed point in Earth's oceans.

Piccard says he first thought of the solar project after he stepped out of the Orbiter. "The press was saying that my balloon flight around the world in 1999 was the last adventure that was still possible because everything else had been done," he says. "I was 41 years old at that time, and I thought 'it's a pity if everything has been done.'"

“I thought it would be great to have a vehicle that would fly day and night with no fuel. Bertrand Piccard ”

He was also disappointed that the round-the-world balloon trip had used such massive amounts of fuel: having taken off with almost four tonnes of liquid propane, it landed with just 40 kilograms. "We were really limited in fuel, in duration, and if the wind had been slower on the Atlantic we would have ditched and not made it," he says. "I thought it would be great to have a vehicle that would fly day and night with no fuel, with no limit of duration."

Piccard was not the first to be attracted by the notion of fuel-free flight. More than 25 years ago, US aeronautical engineer Paul MacCready from AeroVironment in Monrovia, California, built a plane light enough and slow enough to fly on the low power output of solar cells. His Gossamer Penguin weighed less than 31 kilograms without a pilot and its speed over ground was slower than a bicycle. The planned Solar Impulse will have a maximum weight of 2,000 kilograms, including the pilot, almost one-quarter of which will be from batteries for storing energy to fly through the night. If all goes well, the plane will fly at speeds of 50–100 kilometres per hour on its round-the-world trip, landing five times along the way to swap pilots.

The trip will be a no-frills experience for the solitary pilot. Inside his snug cockpit, which will protect him from temperature extremes of 80 ° C to −60 °C, the pilot will endure the same cramped position for up to five days at a time. "We are not sure how we will sleep or cope with maintaining alertness," says André Borschberg, the other pilot and chief executive of the project.

They will need to be at their most alert when flying the slow-moving craft through the hours of darkness. Although unmanned solar aircraft have made night flights before, no piloted solar plane has stayed aloft for more than 6 hours at a time. The Solar Impulse team plans to get through the nights with a mixture of gliding down to lower altitudes and using batteries for power. After dusk, the plane will descend from its maximum daytime altitude of 12 kilometres to just 1 kilometre. The air is denser at lower altitudes, slowing down the plane and reducing the amount of power consumed.

Handle with care

But the biggest challenge will be the weather. Because of its light weight and slow speed, the craft can't handle strong winds or turbulence. "This aircraft is the size of the largest transport aircraft, but it follows any gust that you have," says Borschberg. And despite having a wingspan slightly bigger than that of an Airbus A380, he expects Solar Impulse to fly a bit like a hang glider — rather like the Wright Flyer.

When taking off, the pilot can choose the best weather window, but during the crucial overnight descent the plane will be at the mercy of winds and turbulence. "If we have headwinds at night, the night gets longer. If the night is longer the batteries might not be sufficient any more," says Borschberg. Each dawn will look sweeter than the last, as pilot and plane run low on energy.

Fellow solar-aviation experts — many of them pilots themselves — are upbeat about the project's chances. Piccard discussed his plans with MacCready before beginning the project, and his verdict was: "It will take an elegantly crafted vehicle, flown in meteorological conditions that are hard to find, but it's doable." "I'm just very sad he died before we could make our first flight," says Piccard.

MacCready's view is echoed by Chris Kelleher, technical director of the Zephyr project, the record-holder for the longest unmanned solar flight. "It's doable," he agrees. "The issues are the altitudes and the speeds that it would fly at, and the weather conditions." As the on-the-ground pilot for the much smaller Zephyr, Kelleher explains that handling isn't a problem once Zephyr is high enough, above about 18 kilometres. "At altitude, it handles like a big, commercial airliner because the gust sizes tend to be big features, and the aeroplane flies very slowly into the parcels of air."

Because it will be manned, Solar Impulse won't be able to fly as high as Zephyr. So Kelleher thinks that weather forecasting will be crucial for achieving the mission. "With stable conditions it's possible to predict the weather and come lower," he says.

André Borschberg (left) and Bertrand Piccard will pilot Solar Impulse on its round-the-world attempt. Credit: SOLAR IMPULSE/STÉPHANE GROS

But how often will the team be able to count on stable conditions? The planned flight path will follow the Tropic of Cancer, which maximizes the plane's exposure to daylight while hopefully avoiding the worst tropical weather. Borschberg agrees that weather prediction is going to be one of the most important aspects of mission planning. "At take-off it's not too difficult, because you decide when to take off. For landing it's more difficult because you cannot always plan exactly what happens."

To test its weather-prediction systems, the Solar Impulse team has been conducting virtual flights since last May. The researchers used a simulator that mimics the performance of the aircraft and allows them to introduce meteorological data. "You can have this aircraft basically flying in real conditions," says Borschberg.

They have learnt some valuable lessons from the simulations. "We learned that the flight could be longer than expected," says Piccard, and "that we cannot just take off with the absolute certainty that the next five days will be OK." Avoiding bad weather systems means more unplanned diversions. "When we made simulations from Hawaii to Miami, we had to land in Phoenix, Arizona, because there was a big thunderstorm on the Gulf of Mexico," says Piccard. "So we learned to be more flexible."

Fair-weather flyer

Earlier solar planes also faced turbulent weather — with mixed results. Gossamer Penguin's successor, Solar Challenger, completed its flight across the English Channel in 1981 on a sunny day with white puffy clouds. But Bob Curtin, who has worked at AeroVironment since the 1980s, recalls that "it was fairly turbulent actually, there were lots of clouds in the sky". However, Solar Challenger handled more like a small, light aircraft compared with the giant Solar Impulse.

The ultralight unmanned Helios craft, built by AeroVironment and NASA, didn't handle turbulence so well on its final flight. Its huge 'flying wing' structure was designed to flex into a curved shape when flying, and was able to handle moderate turbulence. As part of NASA's mission to build high-altitude and long-endurance craft it flew at altitudes above 29 kilometres. But during an attempt to set a longer flight record in 2003, the curved wing started oscillating uncontrollably, and the structure broke up over the Pacific Ocean. Unlike Helios, Solar Impulse's design follows a classic rigid-wing structure, so rather than oscillate, the craft will get knocked around by the wind. Piccard says that the main reason for choosing this design, however, was the need to incorporate the cockpit.

Given the unpredictable nature of the weather, the multinational team building Solar Impulse is perhaps wisely sticking to known technologies for the final design. A feasibility study done in 2003 predicted the technology improvements that were likely to be available in 3–4 years time, but didn't plan on any technological breakthroughs.

“We are not sure how we will sleep or cope with maintaining alertness. André Borschberg ”

For instance, the study predicted that monocrystalline solar cells would have efficiencies of 20% — they now provide around 22%. At just 130 micrometres thick, the solar cells are flexible enough that they can be integrated into the upper surface of the wings without shattering. The team also correctly predicted that the energy storage density of rechargeable lithium batteries would reach 200 watt hours per kilogram.

The design is now frozen with these technologies, so the challenge is one of engineering rather than science. "The technology is given, so you have to reduce energy consumption," says Borschberg.

Testing times

Now that the project has three of four major sponsors in place, and two-thirds of the funding it needs, the team has started to build a smaller prototype. Test flights with the 61-metre prototype, scheduled for later this year, should give a better idea of how feasible overnight flights are. Once the lessons learned from the prototype have been fed back into the overall design, the team plans to build the full-size plane during 2009–10, with the round-the-world mission slated for 2011 if all goes well.

When Solar Impulse finally gets airborne, its progress will be watched carefully. "We're very interested in Solar Impulse," says Kelleher, who, as a stunt pilot himself, would love a chance to fly the plane. In its quest for energy efficiency and low weight, he sees Solar Impulse as "the art of the possible". But his company is more interested in unmanned solar planes as an alternative to satellite technology for the communications industry. Because of the cost of transmitting data from Earth to satellites, for example, Zephyr could provide a cheaper way to relay information. "We don't expect it to replace satellites, but it may be able to do many of the jobs that satellites can't do so well, or do them much more cost effectively," Kelleher says.

“The real future is probably in an unmanned vehicle. Bob Curtin ”

Curtin, now vice-president of business development at AeroVironment, also sees a future for solar-powered planes, although the company's solar research has been on hold since the Helios crash. He says that everything the firm learned about low-powered flight is being applied to its Global Observer project, an unmanned high-altitude aircraft for communications relay and observation. Global Observer will be fuelled by liquid hydrogen in an internal combustion engine, and will fly for a week at a time.

The reason the company decided to go for hydrogen and not solar on this project, says Curtin, is that the payload of the Global Observer is large — around 181 kilograms — much bigger than the 30-kilogram Zephyr. Another limitation of solar-powered flight is that current technology requires long days and short nights — restricting the range of the craft to lower latitudes.

But Curtin doesn't rule out a return to solar power as solar cells and battery technologies improve. "The real future is probably in an unmanned vehicle," he says, "because you're trying to make something fly perpetually, and if you do that, it doesn't make sense to have a human on board."

Piccard is optimistic that his dreams of fuel-free flight, like MacCready's before him, will inspire others to change their thinking about energy consumption. When asked why they pursue such missions, both Piccard and MacCready cited the example of Charles Lindbergh's solo Atlantic crossing in 1927. "Lindbergh was alone because the rest of the payload had to be gasoline," notes Piccard, yet 35 years later aircraft crossing the Atlantic were able to carry 300 passengers. This solo pilot is gambling he won't be alone forever.

Authors

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Vicki Cleave is a senior editor for Nature Materials.

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Nature Insight: Materials for clean energy

Solar Impulse's website

QinetiQ's Zephyr project website

AeroVironment's website

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Cleave, V. Solar power: A flight to remember. Nature 451, 884–886 (2008). https://doi.org/10.1038/451884a

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