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Solar Impulse 2 Landing in Hawaii

A decade and a half ago, during my first visit to Hawaii (before I moved here), I took a ride in a way off road electric vehicle, the Atlantis IX submarine. Today I was part of the welcome for another visitor to Hawaii, namely the Solar Impulse 2 solar-powered airplane, and its pilot and support crew. This is an even-farther-off-road electric vehicle, which arrived from Nagoya, Japan, in the middle of a flight around the world!

Solar Impulse 2 on approach

Solar Impulse 2 left from Abu Dhabi in the UAE on March 9, 2015, and had made it to China by the end of the month. At that point serious weather delays began to plague the team; they made it from Chonqing to Nanjing during a 17-hour flight on April 20, but then had to sit on the ground for over five weeks before attempting the next and longest leg of the journey, across the Pacific Ocean to Hawaii. The trickiest part of the problem was that, before they set out on this multi-day flight, the team had to be confident that the predicted weather all the way to the end of the journey would allow the plane and pilot to get through. The wings and fuselage of the aircraft are covered by solar cells, which power it through the day and store energy in onboard batteries to power it through the night. The aircraft climbs to about 28,000 ft altitude during the day and descends during the night; clouds above this elevation will cut the amount of sunlight available to power the engines and charge the batteries, so those must be avoided along with dangerous turbulence, and of course precipitation.

They launched from Nanjing on May 30; however, about a day into the flight the forecast for the last couple of days of the journey turned sour, and so they had to turn around and make an unplanned landing in Nagoya, Japan. They thought they had a favorable forecast on June 24, but just minutes before takeoff the forecast for the final days again turned against them, so they had to abort on the runway. Finally on June 28 they took off again, and this time the weather held! Above you see Solar Impulse 2, with a helicopter chasing it to shoot some of the beautiful aerial photos on their website, flying toward Kalaeloa Airport at the west end of the island of Oahu, under a Hawaiian moon just before dawn on July 3.

Solar Impulse 2 flying over

The aircraft had actually been in a holding pattern off Oahu most of the night; I understand that this was to allow the pilot to rest a little more (he had to snatch sleep in 20-minute naps for the entire flight!) and to wait for calm pre-dawn wind conditions for the landing. However, neither I nor the assembled press corps was inclined to complain that this let the landing occur in bright twilight just before dawn rather than in darkness. The plane made a low circle over the airfield before coming in on final approach.

Solar Impulse 2 rolling out

A solar-powered vehicle, whether for the road or for the sky, has to be designed with extreme attention paid to light weight and low power use. It takes at least a few square meters of solar cells to provide a single horsepower worth of electricity even if the sun is striking the panels straight on, and power will be reduced when the sun lights them obliquely. A conventional battery-electric road vehicle (EV) can charge its batteries while parked, using sunlight collected by many square meters of solar cells mounted on the roof of a building, but that is of course not an option for a multi-day, multi-night flight like this one. Thus Solar Impulse 2 flies a lot more slowly than your typical trans-Pacific airliner, and with its light weight and high lift its landing roll-out is even slower. Here you can see a bicyclist under each wing keeping pace with the plane to grab hold of a stanchion below the wing, to keep it from dipping as lift is lost and taking a divot.

Solar Impulse 2 coming to the hangar

Here the plane is being brought to the hangar, and to the waiting press corps and welcoming committee, in the dawning sunlight. Solar Impulse 2 doubtless has the largest wingspan of any plane that has ever landed at this small airport: at 72 meters, it's longer than that of a Boeing 747, and I don't think the runway is big enough to handle jetliners! At the left edge of the photo you can see a fire truck ready in case of a crash; fire is not a big concern here, of course, without any combustible fuels on board.

Preparing for pilot to stand up

Piloting duties on this round-the-world journey alternate between two men: Bertrand Piccard, in the orange jacket outside the aircraft, was waiting to greet André Borschberg, who had flown Solar Impulse 2 from Nagoya (and from Nanjing before that). Numerous dignitaries were also waiting to climb the ladder and greet him, including the Swiss ambassador to the U.S. (the pilots and much of the team are from Switzerland), the Governor of Hawaii, the state Senator for the airport's district, the Mayor of Honolulu, and the airport director. However, Borschberg had been in the air for almost 118 hours, shattering the record for duration of a solo flight, solar or otherwise (and also the records for distance and duration of a solar-powered flight, which he had previously set during the 44-hour flight from Nanjing to Nagoya), and first he needed to have his legs massaged so they'd hold him up as he returned to the ground. Anybody who's flown across the ocean in coach can certainly sympathize, even though that's only five hours or so rather than five days!

Emerging from the cockpit

Finally the pilot emerged from the cockpit, draped with the leis of a proper Hawaiian welcome. He also holds a rather large bottle that I presume is Moët Hennessy champagne (note the sponsor's name across the leading edge of the wing).

Hawaii Pa'u Riders

A further local greeting awaited him on the ground. Here the Hawaii Paʻu Riders present a lei; dancers from a hula hālau (I didn't catch which one) are at the left of the photo as well.

Pushing the plane into the hangar

As the pilots stepped off to talk to the assembled press, other crew members moved the plane into the hangar. Since the aircraft's wingspan is so large, I'm sure they measured very carefully in advance to be sure it would fit! It helps that, despite its wingspan exceeding that of a jetliner weighing hundreds of tons, Solar Impulse 2 weighs only as much as a large car, so that it could be lifted onto dollies and moved into the hangar by hand, sideways. No tug needed here.

Holding up the wing

Of course, since the designers dispensed with the weight of landing gear out along the wings, the plane continued to rely on muscle power to keep the wings from hitting the ground. Note also the very long propellor blades parked horizontally for the same reason. (Note added 5 July 2015: I later saw some video of earlier flights with fixed landing gear attached to the wing roots, with wheels at the end of long diagonal struts. I was told that these were detached from the plane before the trans-Pacific flight to reduce weight but that, even on overland flights when they were used, the great length of the wings still required that a crew member hold up each wing starting from the landing rollout, as was done by the bicyclists in the second photo at top.)

Piccard and Borschberg

The vision of Bertrand Piccard and André Borschberg, and of the Solar Impulse team, is not to fill the skies with solar-powered airliners; the power limitations of solar input on available wing and fuselage area just don't allow for much cargo and passenger weight or for high speeds. That limitation is due to physics, not technology. There are applications for which solar-powered day-and-night flight would be of commercial value: notably, an unmanned solar-powered aircraft would not need to return to the ground for fuel or for the needs of a human pilot, and if reliable enough electrically and mechanically such an aircraft could remain aloft for weeks, months, or years on end. Such a platform could be a comparatively very low cost replacement for space satellites for some purposes, like providing communications links or surveillance in remote or inaccessible areas; companies ranging in size from startups to Boeing and Google are actively developing such "atmospheric satellites."

However, the main purpose of the round-the-world flight of Solar Impulse 2 is to draw attention to the capabilities of clean solar energy itself: if it can enable a journey like this, think what else can be done with it! You can certainly move away from fossil fuels to power your house, to pick an easy example. I felt a degree of kinship to these explorers' purpose, which was the same as the motivation behind my own 1998 drive Clean Across America and Back in my natural-gas-powered van, or Kris Trexler's Charge Across America in a GM EV1 electric car, or other alternative-fueled long-distance road trips before or since: you wouldn't necessarily want to do this kind of journey routinely, but the fact that it can be done should give some credibility to more ordinary uses of these power sources. Judging from the volume of press coverage I've seen of the round-the-world flight of Solar Impulse 2, I'd say that they are succeeding very well in this effort.

To close, I'd like to thank the Solar Impulse 2 press team for allowing me to join the pros in the press viewing area, despite the fact that I'm no ABC or CNN! I had expected to watch the landing from the airport's public viewing areas, but they responded to my query about this instead with a very generous invitation to get up close and personal, as you can see from the photos above. Actually, July 3, 2015, marks seventeen years since the launch of this website, so merci beaucoup for the anniversary gift!

Addendum 5 July 2015: A couple of days after landing, the Solar Impulse team held a public open house at the hangar, so that more people could have a look at this amazing vehicle before it flies on to Phoenix, Arizona and points east. We were also able to get closer to the aircraft than we could right after the landing, especially those who weren't invited to the press area. I brought my family, and a lot of others did too; the photo below shows only a small fraction of the visitors who were there when we were. There were around a dozen crew members available to answer questions, as well as video and static illustrations explaining the project.

Solar Impulse 2 in hangar

It was a hot, sunny day, so even inside the shade of the hangar each engine nacelle (housing power-handling electronics for the solar cells, a battery pack, and a motor) was being cooled by a portable air conditioning unit. You'll also note that the propellors on the near wing have had their spinner domes removed for maintenance, and those on the far wing had their blades off as well.

Solar Impulse 2 cockpit

As I said, we were able to get closer to the aircraft than on landing day; here is a good look into the space where André Borschberg spent nearly five days. I ended up addressing most of my questions to Marion, one of the ground crew who were there to help the project's educational mission (and who remembered me from the landing). In particular, I asked of her a question that I'm sure was on the mind of every kid who's seen pictures of this cockpit: what's up with the stuffed toy meerkat on a shelf above the pilot's seat? She told me that some members of the ground crew had thought that their leader looked a bit like a meerkat, and since these animals are known for vigilance somebody had the idea of sending a mascot along on each flight to watch over the pilots as a proxy for the whole ground crew. If you've been following the progress of the journey around the world on their website, you've seen images of the mission control center, where they track the weather and the health of the plane and pilot; that meerkat has a lot of backup. (Note added 15 July 2015: I learned from this webpage that he is named Nilsy, after the ground crew chief.) Many thanks to Marion for talking with me, and to the entire project team for opening up the hangar so that all the people who saw the reports on the local news could come see the aircraft in person and ask their own questions!

Addendum 15 July 2015: Well, it looks like this will not be the brief layover that the Solar Impulse team had planned before heading on to Phoenix, Arizona. Production vehicles are "wrung out" under all sorts of weather and environmental conditions during development to ensure that they can handle them; however, the Solar Impulse 2 aircraft is a one-off model, essentially its own prototype (though the smaller Solar Impulse (1) tried out some of its design elements starting in 2009), and so testing is done "on the job" for many circumstances. Unfortunately, the flight across the Pacific from Nagoya to Hawaii demonstrated that the pre-flight modeling of the heating and cooling of the engine nacelles in the warmth of the tropics was not completely accurate, and the batteries were damaged by overheating on the first day of that flight.

Because the solar panels of course generate no power at night, the plane has to fly on stored energy. One obvious way to store energy is as chemical energy in the batteries; however, as part of the mission design the aircraft also "stores" energy as gravitational potential energy, ascending to about 28,000 ft during the day and then taking some of the demand off of the batteries by reducing power to the motors and allowing the plane to descend slowly to about 8,000 feet overnight. Without staying pretty close to the original flight plan, including power and altitude profiles, the aircraft simply wouldn't be able to make it to Hawaii, and so the batteries continued to be stressed each day. The flight engineering team kept a close eye on them to ensure pilot safety, but upon inspection after landing it was determined that some parts of the batteries were damaged beyond repair, requiring replacement (along with redesign of the thermal control system).

This might be a matter of several weeks to a couple of months; however, the team was also counting on long summer days to provide energy for the aircraft in flight, and winter weather is also a concern. (They can't simply shift to the southern hemisphere during its summer season, while it's winter in the north, if only because the south Pacific is much wider than the planned traverse from Hawaii to the U.S. mainland.) Thus they have decided simply to park the aircraft on Hawaii for the winter, continuing to work on it while preparing for resumption of the round-the-world journey next spring, and as I write, much of the support team has already packed up and headed home to Europe.

Well, I will try to post another report when the project "comes out of hibernation" early next spring. A possible silver lining for those of us in Hawaii is that schools are starting to be back in session soon, and the Solar Impulse project has always made a great effort to reach out to schoolchildren in particular, wherever they have landed; this included the unplanned stop in Nagoya. Thus I hope that, before the aircraft leaves next spring, both of my kids will be able to share with their classmates, and many others, what they saw a couple of days after the landing!

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new 3 July 2015, updated 30 March 2016