Solar Impulse 2: Around The World On Sunshine And Guts

By Terry Dunn

Despite its dragonfly-like appearance, SI2 is indeed a radical and ground-breaking machine.

With its gaunt skeletal frame and awkward, lanky proportions, the Solar Impulse 2 (SI2) is a far cry from the supersonic image one normally gets when discussing revolutionary, record-setting aircraft. Where is the pointy nose? What about the fire-belching rocket engines?

Despite its dragonfly-like appearance, SI2 is indeed a radical and ground-breaking machine. Swiss Pilots Bertrand Piccard and Andre' Borschberg recently completed a 23,000 mile journey around the globe in SI2 using only solar power. The words don't quite capture the enormity of this accomplishment. So let me say it again: This airplane flew completely around the world without using one single ounce of fossil fuel!

Solar Impulse 2 may look a little strange, but it is a high-tech machine that holds numerous world records. It recently completed an around-the-world journey using only solar energy.

If the feat accomplished by SI2 and her pilots does not leave you slack-jawed and perhaps drooling ever so slightly on your keyboard, you still don't get it. This is an enormous milestone for both aviation and solar power technology.

Most aeronautical achievements are as dependent on technological breakthroughs as they are piloting feats of derring-do. SI2's around-the-world success was no different. It required not only a cutting edge machine, but also pilots who were willing to risk everything to see what it could do.

Stop and Go

SI2's circumnavigation was not accomplished in a single flight. The team's original plan divided the route into 12 eastbound legs spanning a period of about 4 months. These things take time when your average flying speed is only 41 miles per hour. As it turned out, SI2 landed in 17 cities and required more than 16 months to complete the trip.

Solar Impulse 2 has only one seat. Swiss pilots Bertrand Piccard (left) and Andre' Borschberg alternated flying duties during the globetrotting trip.

The airplane is built to carry just one person, so Piccard and Borschberg traded off flying duties…each flying solo for specific legs of the flight. Throughout the journey, a support team travelled to each of the waypoints to receive and send off SI2. There was also a mission control center located in Monaco where, among other things, specialists kept an eye on the weather, and monitored SI2's myriad systems via satellite.

The shortest leg of the flight was an outlier lasting less than 5 hours between Lehigh Valley, Pennsylvania and New York City. The next shortest flight was just over 13 hours, with several other legs lasting less than 20 hours.

Six of the legs lasted more than a day. The longest and most challenging segment of the journey involved crossing the Pacific Ocean on a flight from Japan to Hawaii…a marathon hop lasting nearly 118 hours (almost 5 days). This leg alone set world records for the longest duration solo flight (in any type of airplane), the longest duration flight for a solar-powered airplane, and the furthest distance travelled by a solar-powered airplane (4481 mi/7212 km).

Imagine the technical challenges posed by flying a solar-powered airplane nonstop through five days and nights. Now factor in the demands placed on a human pilot at the controls for this same unbroken period of time. Both the technological and physiological obstacles were tackled in creative ways. In fact, explaining how SI2 and its pilots managed to stay aloft and in control for these overnight flights is probably the best way to summarize how innovative the team was forced to be.

This chart illustrates how SI2 uses solar power to charge its batteries and gain altitude during the day so that it can remain aloft in darkness.

On Through the Night

Trying to understand SI2 by comparing it to more common aircraft is a fool's errand. You have to pick and choose single elements to make any associations. Sure, it has about the same wingspan (236 ft/72 m) as a 525-passenger Airbus A380 airliner. But that is the only common ground between these two vastly different airplanes. Saying that SI2's 5,100 lb/2300 kg weight is comparable to a WW2-era T-6 Texan is meaningless unless you also make the point that it has roughly the same horsepower (70 hp) as a humble Piper Cub. The only conceivable airplane with multiple similarities is Solar Impulse 1, the proof of concept ship that preceded SI2.

The huge wings and parts of the SI2 fuselage are skinned with thousands of solar cells. During the day, the electrical power generated by these cells was sufficient to power SI2's electric brushless motors for an ascent up to 29,500 ft/9000 m. Some of the solar power was also utilized to charge the onboard Lithium-Polymer flight batteries.

With a selfie-stick protruding from the cockpit, Piccard provides a sense of scale to the airplane's unusual proportions. We also get a glimpse of two of the four brushless electric motors that power the airplane.

As the sun began to set and solar energy faded, SI2 changed modes and began to utilize its stored energy. The first resource it cashed in was the potential energy provided by its 5.6 mile altitude. All four motors were shut down and the airplane executed a 4-hour-long gliding descent to an altitude of 5,000 ft/1500 m.

The pilot then switched on the motors. Without sunlight, the motors were fed entirely by the batteries. Although each motor is capable of generating up to 17.5 horsepower, the pilot used only enough power to maintain the airplane's 5,000 ft. altitude. This effort was aided by the extreme efficiency of SI2's propulsion system. Gear reduction drives allow the motors to spin huge 13.1 ft/4 m-diameter propellers. The overall efficiency of the motor system is an astonishing 94%.

Battery power was consumed through the remainder of the night. As the sun emerged, the solar cells came to life and gradually took over from the batteries. Before long, solar power was adequate to power the motors and charge the batteries. The process then repeated.

Mind Over Matter

The SI2 cockpit appears quite roomy until you realize that the pilot was often cocooned in there for days at a time. The seat reclined to become a bed. A few alternate twists and turns made it a low-tech, gravity-driven toilet.

The pilot seat also served as a bed and toilet. Both pilots utilized unique methods to sleep and exercise during multi-day flights.

Even with SI2's autopilot system and the constant oversight provided by mission control, the pilot had to be available to deal with any situation that could pop up over a multiple-day flight. A normal day/night sleep schedule just wouldn't do.

Piccard and Borschberg took a somewhat spiritual approach to the sleep problem. Bertrand practiced hypnosis. Sometimes his hypnosis was self-induced. In other instances, a ground-based hypnotist did the job over the radio. Borschberg preferred yoga and meditation. By utilizing these techniques, they were able to take 20-minute power naps distributed throughout the entire day in lieu of a single, sustained sleep period. Although there was space and equipment for limited exercises, the aviators also claim that their mental strategies allowed them to promote improved blood circulation in spite of their forced sedentary condition.

With an unpressurized cockpit, Piccard and Borschberg had to breathe bottled oxygen whenever they flew above 12,000 ft.

The SI2 cockpit is not pressurized. Any time the airplane was flying above 12,000 ft /3700 m, the pilot had to don a mask and breathe bottled oxygen. On those multiple-day legs, they would have been wearing the mask a good portion of the day. The pilots used specialized creams to prevent facial hair growth, which could make the mask less comfortable and less effective.

The tall altitude profile of SI2 had it flying through air of widely varying temperatures. The passive methods for keeping the pilot relatively comfortable included specialized nylon flight suits and foam insulation around the cockpit.

There are obvious dangers that go along with crossing a few thousand miles of ocean alone in a kite-like airplane. Safety nets are few…although there was a parachute and a life raft on board if worse had come to worst. The pilots appear to have accepted the risks with a good-natured, almost fatalistic sense of humor. Their attitude is exemplified in a series of cartoons on the Solar Impulse website which depict a shark named Joe eagerly following SI2's ocean crossings and hoping for a malfunction that would result in a Swiss snack.

The dangers posed by circumnavigating the globe in a solar airplane were met with good humor. Several cartoons depict Joe the shark hoping for technical problems over the ocean.

Although Joe went hungry, the Solar Impulse team had to contend with several lesser setbacks. The cornerstone of SI2's airframe, a wing spar made of multiple carbon fiber laminations, broke during structural testing in 2012. It took 10 months to fabricate an improved replacement. During the June 2015 Pacific crossing, the batteries sustained thermal damage due to excessive insulation around them. Repairs in Hawaii took several months to complete. The team then had to wait for springtime in order to have sufficiently long days for the remaining legs of the journey. All of this resulted in a further 10 month delay.

Weather also played a significant role in the SI2 journey. The Pacific crossing was originally planned to originate from Nanjing in China. Two days after departing Nanjing, weather over the Pacific looked spotty. The team had Borschberg divert to Nagoya, Japan. It was nearly a month before a sufficient window of favorable weather appeared and allowed the team to continue on to Hawaii.

What Next?

Although SI2 was built with the singular goal of circumnavigating Earth, it can still be utilized for other purposes. Only 700 hours of the airframe's 2000 hour lifespan have been utilized thus far. There is talk of making the airplane fully autonomous to function as a prototype for unmanned solar-powered drones. Such high-altitude aircraft could potentially perform some of the duties that are currently accomplished with orbiting satellites. The advantage of the drones would be lower cost and much easier retrieval for maintenance and repairs.

The success of Solar Impulse 2 demonstrates that solar power can (and should) be pushed further for more widespread use.

One of the primary goals of the Solar Impulse mission was to demonstrate the viability of electric power for aviation as well as ground-based applications. Showing that solar cells and batteries have advanced sufficiently to attain long-term sustainable flight is a real eye-opener. The success of Solar Impulse will hopefully be a catalyst for further utilization and continued R&D of solar power technologies.

The information I have presented here merely scratches the surface of what went into this project. If you'd like to find out more about the development, flight, and future of Solar Impulse 2, the team's website is a splendid resource.

Terry is a freelance writer living in Lubbock, Texas. Visit his website at and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.

All images courtesy of Solar Impulse