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Soaring With the Eagles in the Sun!

Posted on May 9th, 2016 by in New Materials & Applications


On April 23 at 11:44 pm PDT a watershed event took place in California. The bang of this milestone was drowned in the cacophony of the season. The late hour of the night in the West Coast deprived most of the sleeping nation of watching the live landing of Solar Impulse 2. The feat accomplished: it took Solar Impulse 2, 62 hours and 29 minutes to fly non-stop from Kalaeloa, Hawaii to the Moffett Field in Mountain View, CA using only solar power. At the helm were pilots Borschberg and Piccard (homophone of Picard, Jean-Luc, Ă  la Star Trek: The Next Generation).

The pilots flew the Solar Impulse 2 in record time across the Pacific Ocean entirely on solar energy. And there was no fuel of any kind on board! They flew the plane not too high and not too low, at maximum altitude of 8,634 meters during daylight hours and just 1,524 meters at night flying on stored energy in batteries. The average speed was 65.4 km/h while Solar Impulse 2 covered a distance of 4,086 km. Never before had an airplane flown such a distance over water using only solar power without stop or even a place to stop! Of course neither the speed nor the accommodations are the stuff of the real world. But that is not the point….

Swiss engineer and businessman André Borschberg and Swiss psychiatrist and aeronaut Bertrand Piccard lead the privately financed project. They co-piloted Breitling Orbiter 3, the first balloon to circle the world non-stop. The Solar Impulse project began in 2004 aiming to circumnavigate the Earth (35,000 km) by a piloted fixed-wing aircraft using only solar energy (, a first. They built a prototype plane, which flew its first test flight in December 2009. In July 2010, it flew an entire diurnal solar cycle, including nearly nine hours of night flying, in a 26-hour flight. Piccard and Borschberg completed successful solar-powered flights from Switzerland to Spain and then to Morocco in 2012, and a multi-stage flight across the USA in 2013. They improved the design and construction of the second plane Solar Impulse 2 based on learning from the prototype (Figure 1), such as increasing the number of photovoltaic cells to 17,000.

Figure 1. Solar Impulse 2 travelling over the Golden Gate Bridge.

The wingspan of Solar Impulse 2 is 71.9 meters, slightly less than that of an Airbus A380 (80 meters), the world’s largest airliner, but unlike the 560-ton A380, the carbon fiber Solar Impulse weighs only 2.3 tons, little more than full size car (Figure 2). The cockpit is a non-pressurized box, 3.8 cubic meters in volume aptly nicknamed the coffin. It houses advanced avionics and an autopilot to accommodate multi-day transcontinental and trans-oceanic flights.  Supplemental oxygen and other environmental support systems allow the pilots to cruise up to an altitude of 12,000 meters.

Figure 2. Comparison of wingspans of Solar Impulse 2 and Airbus 380
(source:, ©Solar Impulse/EPFL Claudio Leonardi)

The trip began in Abu Dhabi on 9 March 2015 where the trip will also end (Figure 3). Solar Impulse 2 was originally scheduled to return to Abu Dhabi in August 2015. Twelve stops were originally planned to allow for the alternation of pilots and to await good weather conditions along each leg of the route. Crossing the Pacific and Atlantic oceans are the longest stages of the journey, each expected to take about five days. On multi-day flights, the pilots take 20-minute naps and use Yoga or other exercises to promote blood flow and maintain alertness.

Figure 3. Solar Impulse 2 flying over soared over the Sheikh Zayed Mosque in Abu Dhabi
(source: Daily Mail,, February 26, 2015)

By the end of May 2015, the plane had traversed Asia. It made an unscheduled stop in Japan to wait for favorable weather over the Pacific, increasing the expected number of legs of the journey to 13. The aircraft began the flight from Japan to Hawaii on June 28, 2015. It arrived in Hawaii on July 3, setting new records for the world’s longest solar-powered flight both by time (117 hours, 52 minutes) and distance (7,212 km). During that leg overheating damaged the plane’s batteries because of excessive insulation. New parts had to be ordered, and, as it was late in the season, the plane was grounded in Hawaii. New batteries were manufactured and installed in the plane in the early weeks of 2016. Test flights began in February to prepare for resumption of the circumnavigation. A favorable weather window opened in April 2016 allowing the journey around the world to continue to California.

So, why are Borschberg and Piccard doing this? The Project manifesto best describes the goals and hopes of the leaders and the team behind the Solar Impulse flights:

Our ambition for Solar Impulse is for the worlds of exploration and innovation to make a contribution to the cause of renewable energies. We want to demonstrate the importance of clean technologies for sustainable development; and to place dreams and emotions back at the heart of scientific adventure.

The public, which gets excited about great adventures, is ready to join the dreams of pioneers and explorers. Solar Impulse wants to mobilize this enthusiasm in favor of technologies that will allow decreased dependence on fossil fuels and induce positive emotions about renewable energies.

Public attention must be drawn towards the changes necessary to ensure our planet’s energy and ecological future. Also, a positive and stimulating image of environmental protection must demonstrate that the alternative energy sources, related to new technologies, can achieve what some consider impossible.

To borrow from the good wishes of Mission Control to John Glenn (on the first manned orbital mission of the USA on February 20, 1962): Godspeed André and Bertrand!

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