On March 7, Solar Impulse 2 landed in Honolulu, Hawaii after a 7,202-kilometer (4,475 miles) five-day and five-night flight across the Pacific from Nagoya, Japan. This record-breaking flight, piloted by André Borschberg, was the longest and most challenging leg of the solar-powered airplane’s celebrated round-the-world trip. The next leg, this one piloted by Bertrand Piccard, will take Solar Impulse all the way to Phoenix, Arizona. Air & Space writer Dennis Hollier interviewed Piccard and Borschberg at their hotel in Waikiki, while the airplane was being prepared for another trans-Pacific flight.
Bertrand, let’s start with you. You’ve been working on this project for a long time. In your mind, when did it actually begin?
Piccard: I had the project in mind since the landing of my balloon on my flight around the world, in 1999. That was 16 years ago. You have to understand that I flew for 20 days, 45,000 kilometers—which, at that time, was the longest flight ever in aviation—but every day I was afraid of falling short of fuel. I had propane gas to heat up the envelop of the balloon, and when I finished the flight, there was only 80 pounds left out of the 8,000 pounds that I started with. I tell you, we were really scared that we would run out of fuel. It was a miracle.
This is when I made myself a promise that the next time I flew around the world, it would be without fuel, to try to reach unlimited endurance. When I started to speak about that, I was taken as crazy dreamer. Except by two people. One was Paul MacCready [who invented the Gossamer Condor, which is currently on display at the National Air and Space Museum]. I went to see him in Pasadena and he said, “You have to try, because I think you can do it.” The other person was the head of research at the Swiss Federal Institute of Technology, which is like the Swiss MIT. He said, “I want to organize a feasibility study for the project.” This how I met André, because he was appointed to lead that feasibility study.
How did you and André divvy up responsibilities for the project?
Piccard: André and I are very different. He’s an engineer/entrepreneur and a jet pilot, and I’m the psychiatrist/explorer and my background is as a medical doctor. So, we took two different parts of the project. André took the lead of the technical team to build the airplane, and I went to find the money. I raised $150 million. And I made all the relationships—with governments, with NGOs, with the media—in order to promote all these clean technologies. This is what is so important to me: Use these clean technologies to their ultimate application—flying with unlimited endurance with an airplane—to show that we can achieve incredible things with clean tech.
This clearly comes from the tradition of my family. My grandfather, Auguste Piccard, invented the pressurized capsule. He made the first flight ever into the stratosphere, in 1931. This was made to prove that it was possible to fly in thinner air with lower fuel consumption. In that sense, it was the beginning of modern aviation, and it was a way to protect the environment. Then, my father Jacques, together with Don Walsh, made the deepest dive ever, in the Marianas Trench, with the bathyscaphe that he had built with my grandfather. That was in 1960. In those days, there were a lot of governments that wanted to throw their radioactive waste in the bottom of the ocean. They cannot do that anymore because my father and Don Walsh found fish living down there, proving that it was completely crazy to dump the waste in these trenches. That was a big, big milestone in the protection of the environment.
So, you know, I was born and raised in the philosophy of scientific adventure to protect the environment. Solar Impulse is exactly from that.
What about you, André? You’ve just completed a 4,475-mile flight over the Pacific Ocean on Solar Impulse. What’s it like for you to fly this airplane?
Borschberg: First of all, it’s a fantastic airplane. It’s an airplane that I really love, because it’s been my home for five days and five nights. It brought me to Hawaii safely. It’s really fun and great to fly. So, for me, it was extraordinary.
If you’re asking about the flying qualities, you could say the airplane has two personalities. In calm air, you fly it with two fingers. You need very little bank. You need very little input in the flight controls. It’s a marvel to fly. But when there starts to be turbulence—and we never go in heavy turbulence because the airplane cannot handle it—the workload starts to be very difficult and very heavy. You really have to make sure that you keep the yaw under control. You need to keep the airplane wings level, and for this you need full authority in the flight controls. You need to use quite a lot of force. And you need to pay a lot of attention. That’s why we try to avoid turbulence.
Did you have any of those kinds of challenges on this flight?
Borschberg: On the last night, I flew through clouds, and, of course, clouds are always slightly turbulent. These were not thunderstorm clouds. They were stratocumulus. But that night it was a good, heavy workload. At night, you have to really focus on the instruments, because you fly by instruments. So, that was a moment—I don’t want to say it was dangerous—but it was very active, and I had to fight quite hard until I came out of the clouds.
That raises the question of safety. What precautions do you take for such a dangerous flight?
Borschberg: There are different aspects to that. The first is that you don’t fly alone. We have a control center in Monaco that is made up of engineers, weathermen and air traffic control specialists. It’s a little bit like what Johnson Control Center in Houston does for NASA when they go into space. These guys can simulate the flight of the airplane in advance. They can check what kind of weather there will be in two days. They can help me find the right trajectory, the right flight path—for example, if I need to hold somewhere to let bad weather pass. This is something we worked on for seven years, so we really have a trained group of people.
And then we really worked on the reliability of the airplane. And that’s what’s fascinating about this renewable energy and clean tech. It lasts. Take solar cells: You can have them on your roof for 25 years and they’ll still perform as well. And you can keep them even longer. On an airplane, it’s a great source of energy. And the electric motors are normally extremely reliable. These are DC brushless motors. We designed them and had a Swiss company, ETEL manufacture them. We pushed the efficiency on these motors to 97 percent, which is really high. This is the really the high end of the curve. So, reliability is the second key to flying.
And then, of course, we had to develop strategies to rest while flying. So, sleeping for 30 minutes. I developed a cockpit in which I can do yoga. I do meditation. I use breathing techniques to relax and to energize myself. These are things that keep the body functioning. Even after five days flying, I was fine. I had no pain, no joint or muscle aching. And this keeps you in the right spirits. Yoga allows you to be an observer; and if you can observe yourself in stressful situations, you start to manage that stress in a different way. You understand: that’s what you do and the way you do it. And you can see it and appreciate it, and then be in another mindset very quickly. For me, this was a fascinating way, first, to train, and then to use it in flight during the five days and five nights of crossing.
Bertrand, the airplane is really quite large. What is the glide ratio of Solar Impulse?
Piccard: The glide ratio of the plane is 37 to 1. But it’s not so much a question of glide ratio that we’re looking for. What we’re looking for is a very, very small sink rate. That’s because the energy you need to keep the plane in the air does not depend on the glide ratio, but on the sink rate. Glide ratio is how far you glide; sink rate is how fast you go down. They’re not necessarily related to one another. If you take this spoon and drop it, it will have no glide ratio and a high sink rate. But if you take the seed from a maple tree—you know, the ones that spin like a helicopter—they also have a zero glide ratio, but they have a very small sink rate. So slightest wind will keep them up. What we’ve been working on is a very, very small sink rate—as few feet per minute as possible, so we need less energy to keep the airplane in the air.
How did it feel to you to fly Solar Impulse compared to other airplanes?
Piccard: It’s completely different, because the more you fly, the more energy you have. You see, because I come from the world of exploration and pioneering, for me, what’s fantastic is to fly on an airplane that is completely unique. There’s not another plane like that in the world. It’s an experimental prototype, and we’re opening new dimensions. So, this is really pioneering and exploration.
But it’s a difficult plane to fly because it’s very sensitive to turbulence. It’s very big. It’s very light. It has a lot of inertia. So, you really have to learn first on the flight simulator. Especially for me, because I have less airplane experience than André, so I needed more time to be able to fly with it. But now it’s fine.
Can you give an example of how you have to adjust as a pilot?
Piccard: It’s an airplane that has a lot of pilot-induced oscillations, a lot of adverse yaw. For example, if you have turbulence that lifts the left wing, you need to put the yoke to the left and the foot to the right. When the left wing lifts, that initiates sideslip on the right. If you want to compensate, you have put the yoke to the left, but you have to put the rudder to the right in order to get the airflow straight again.
Are there any special tactics you use to extend the range of the airplane?
Piccard: When you have enough energy in your batteries, the only way to store more energy is to fly higher. You store it as potential energy during the day, and at night you come down. During the day, we fly up to about 28,000 feet. How low we descend at night depends on the altitude of the clouds. Most of the time, it’s about 8,000 feet. The last night of André’s flight, I think it was about 3,000 feet. Without climbing during the day, you have about 10 hours of battery. This way, you get about four hours of energy for free.
André, you mentioned sleep. How do you manage that as a solo pilot on a five-day flight?
Borschberg: As a pilot, you are trained, of course, not to sleep. So, you have to build the right state of mind, the right confidence in the airplane, the trust that you can leave the flight controls, go back and lay down and close your eyes. You don’t go far away, but it’s still an action of leaving the flight controls, and that trust comes slowly. So, I would lay down and close my eyes, but when I close my eyes, I start to hear all the noises this airplane makes, which I don’t hear normally when I fly. Because you’re watching the instruments, and everything is normal, so you’re not so much aware of all these things. But, when you close your eyes at night, you start to hear these things and wonder, “What is this? What is that?” You also start to feel the airplane moving, and you ask yourself, “The airplane is moving in this direction; is it recovering? Yes, it is. It’s fine.” So, instead of lying down and going to sleep, in the beginning you’re suddenly more aware about everything that’s happening around you. So, it takes time to really get into a relaxed mode and attitude to sleep.
Of course, you have instruments to wake you up if something doesn’t go right, then you can quickly take control of the airplane. You have a few seconds in which you can do something, and if you don’t do it, you may end up in a situation where the recovery starts to be either difficult or impossible. You also develop, I guess, a sixth sense, an intuition that it’s maybe not the right moment to sleep. Many times, in fact, I didn’t feel that it was stable enough for the airplane to fly and I went back and took over the controls. But you cannot rely on that because we don’t have a “certified” instinct. But that’s part of the way you do it. It’s part of human nature.
What was the most difficult part of this flight?
Borschberg: The most difficult part was the decision to leave Japan when I reached the point of no return. You have a moment when you know, “If I pass this point, I cannot come back” because the weather is getting too windy going east or the weather in Japan is so bad that I cannot go back because it starts to rain and get stormy. This moment is extraordinary. You have a choice. And, of course, the airplane wasn’t in perfect condition. With experimental airplanes, you always have something that does not work the way you would like them to work. And I had systems that failed—systems which I considered were not totally essential to me, which were not putting me at risk. And, with the weather window in front of me, I decided—Bertrand was of the same opinion, but because I was in the cockpit, I decided—that this was the right moment to go. Despite the fact that, of course, the engineers were against it. They don’t work with instincts; they work with facts. That’s their job. They saw that some of the equipment didn’t work and they said the airplane is not in the status it should be, so you should not continue. But, looking at the entire picture, looking at the difficulty we had to exit Japan because of the air traffic control and all the traffic problems they have there, looking at the weather window, looking at my status as pilot, looking at the way I could manage the airplane despite some technical difficulties, I decided to go. But it was a big emotion. I thought, “Am I right to do this? Am I taking too much risk, with respect to my family, especially my wife?” That’s an incredible kind of moment that I guess you only have in these exploration kinds of situations.
How much of the technology on Solar Impulse is new?
Borschberg: It depends on how you look at it. The technology we used does not exist in the aviation world. It exists somewhere, but it does not come from the aviation industry. The aviation industry does not use the light materials that we do. It does not use electric motors. It does not use solar cells. It does not use this type of batteries. So, from this perspective, the technology is new for the aviation world, but it’s not new for other industries. We did not invent the technologies; we chose technologies which were already available. We improved them sometimes. And, in fact, we did smart design, integrating these technologies to achieve a goal: to have an airplane of a size bigger than a 747, but with a weight not exceeding that of a family car, simply said. The choice of these technologies was such that we could build an airplane that was extremely energy efficient. You have to realize that, if you want to fly with the sun as the only source of energy, the only way to fly day and night is to use this energy in an efficient way. So, the airplane can be big, because it’s efficient. It’s light, of course, because the lighter it is, the less energy you use.
Of course, that lightness is what makes it difficult to fly in strong winds.
Piccard: Actually, it helps you if you have tailwinds. If you get in a jet stream, it takes less time. Normally we travel very slowly; our indicated air speed is around 25 knots. If we have a tailwind, though, we can easily multiply our ground speed by five. I flew 215 kilometers per hour going from India to Myanmar because I had tailwinds. But I actually flew faster with my balloon than with Solar Impulse. With my balloon, I once flew 234 kilometers per hour. With no engine, but in the core of the jet stream.
How does Solar Impulse II differ from Solar Impulse I?
Piccard: Solar Impulse I was a prototype for us to learn how to construct it. It wasn’t an airplane traveling. There were no redundancies. The pilot could not lie down. There was no autopilot. No toilets. So, Solar Impulse II allows us to fly for a long time. There now is a lot of redundancy in the systems.
Borschberg: There were no major differences, but there were a lot of improvements. We have more efficient motors. We have a lighter structure. To give you an example, the lightest sheet of carbon material—one layer of that fiber today weighs 25 grams per square meter. The piece of paper that you get from your printer is 80 grams per square meter. So, we’re using something that’s three times lighter than the piece of paper that you use to print at home. That tells you that today, we’re able to implement these materials in a very economical way where weight really matters, which is the case for transportation.
What kind of batteries does the plane have?
Piccard: We have lithium polymer batteries, but the battery is still the bottleneck. They’re a little bit more than a quarter of the total weight. So, we have 630 kilos of batteries, and the entire weight of the plane, empty, is 2,300 kilos. Everybody tries to make more efficient batteries. I think whoever invents more efficient batteries will be the king of clean technologies.
What we are the most proud of on this airplane are the motors, which operate with 97 percent efficiency. With the engine in your car, you get 27 percent efficiency. So, we have the best engines in the world. In addition to the motors, what we’re most proud of are the light material structures.
What’s been the hardest part of this project?
Piccard: It’s everything. When you think of exploration, it’s not only to be a hero waving your flag of success. Exploration is about everything that leads to the success. That means intelligent failures, frustration, delays, disappointments, conflicts, problems with administrations, human problems. This is exploration. And, if you manage to cope with all this, then maybe you can be successful.
Even before someone like Shackleton can go to the South Pole, they need to have a boat, they need to have a team, they need to have the equipment, the logistics, the funding. It’s so difficult to have everything working. So, explorations is not only when you push the throttle and take off with Solar Impulse. Exploration is how do you bring enough people together to make this crazy dream a reality.