Changing the World of Flights

Whilst Solar Impulse is not the first solar aircraft project, it’s certainly the most ambitious one

Issue: 04-2015By SP’s CorrespondentPhoto(s): By Solar Impulse

With each of their great ‘firsts,’ the adventurers of the last century constantly pushed back the limits of the impossible. Today, the drive to make new discoveries must go on, with the aim of improving the quality of life on our planet. By writing the next pages in aviation history with solar energy, and voyaging around the world without fuel or pollution, Solar Impulse’s ambition is for the world of exploration and innovation to contribute to the cause of renewable energies, to demonstrate the importance of clean technologies for sustainable development; and to place dreams and emotions back at the heart of scientific adventure.

By going beyond the question of energy, Solar Impulse would also like to encourage each and every one of us to become pioneers in our own lives, in our ways of thinking and behaving.

As was the case for all the great ‘firsts’ in history, doubts and question marks have been accepted from the outset, and transformed into creativity and innovative solutions.

Bertrand Piccard’s vision, coupled with André Borschberg’s managerial experience and the skills of a multidisciplinary team have enabled an idea to be transformed into reality that no aviation specialists, apart from Dassault, believed in: “Too big, too light and impossible to control in flight”, they all said.

Quick Facts

Solar Cells: More than 17,000 solar cells, collecting up to 340 kWh of solar energy per day and representing 269.5 m²!

More precisely 17,248 monocrystalline silicon cells each 135 microns thick mounted on the wings, fuselage and horizontal tailplane, providing the best compromise between lightness, flexibility and efficiency (23 per cent).

In order to maximise the aerodynamical performance, the plane is built with a wingspan of 72 m: wider than that of a Boeing 747 Jumbo Jet!

Batteries: The energy collected by the solar cells is stored in lithium polymer batteries, whose energy density is optimised to 260 Wh/kg. The batteries are insulated by high-density foam and mounted in the four engine nacelles, with a system to control charging thresholds and temperature. Their total mass amounts to 633 kg, or just over a quarter of the aircraft’s all-up weight.

In order to save energy, the aircraft climbs to 8,500 m during the day and descents to 1,500 m at night.

Motors: Average power over 24-hour of a small motorbike (15 hp) with a maximum power of 70 hp (four 17.5 hp engines).

Four brushless, sensorless motors, each generating 17.4 hp (13.5 k), mounted below the wings, and fitted with a reduction gear limiting the rotation speed of a 4 m diameter, twobladed propeller to 525 rev/min. The entire system is 94 per cent efficient, setting a record for energy efficiency.

Speed: Solar Impulse can fly at the same speed than a car, between 36 kmph (20 kts) and 140 kmph (77 kts).

At sea level: minimum speed of 45 kmph (20 kts) and maximum speed of 90 kmph (49 kts).

At maximum altitude: from 57 kmph (31,5 kts) to 140 kmph (77 kts).

Lightness: Prowess of the engineers led by André Borschberg who managed to build the entire structure proportionately 10 times lighter than that of the best glider. Every gram added had to be deducted somewhere else, to make room for enough batteries on board, and provide a cockpit in which a pilot can live for a week. In the end, it is of the weight of a small van: 2,300 kg!

Stimulating innovation in the field of sheets of carbon, which now weigh only a third as much as sheets of printer paper (25 g/m²).

Robustness: The airframe is made of composite materials: carbon fibre and honeycomb sandwich. The upper wing surface is covered by a skin consisting of encapsulated solar cells, and the lower surface by a high-strength, flexible skin. 140 carbonfiber ribs spaced at 50 cm intervals give the wing its aerodynamic cross-section, and also maintain its rigidity.  

First Round-The-World Solar Flight

After the Solar Impulse prototype’s eight world records, when it became the first solar airplane ever to fly through the night, between two continents, and across the United States, it is time for Bertrand Piccard and André Borschberg to move on to the final phase of the adventure: the 2015 round-the-world flight.

What better way to demonstrate the importance of the pioneering, innovatory spirit than by achieving ‘impossible’ things with renewable energy and highlighting new solutions for environmental problems?

solar impulse 2

Construction of the second Solar Impulse aircraft, carrying the Swiss registration HB-SIB, started in 2011. Completion was initially planned for 2013, with a 25-day circumnavigation of the globe planned for 2014. However, a structural failure of the aircraft’s main spar occurred during static tests in July 2012, leading to delays in the flight testing schedule to allow for repairs. HB-SIB’s first flight occurred at Payerne aerodrome on June 2, 2014.

On March 9, 2015, Si2 took off for its first flight from Abu Dhabi to Muscat. André Borschberg landed after 13 hours and 1 minute of solar flight, flying over a distance of 441 km. On March 10, Si2 took off from Muscat (Sultanate of Oman), to Ahmedabad, India. Bertrand Piccard landed at 11.25 p.m. local time after 15 hours and 20 minutes of flight covering 1,485 km. On March 18, 2015, eight days after its arrival in India, Si2 took off for its third flight from Ahmedabad to Varanasi. On March 19, Si2 took off after an overnight “pit stop”, for its fourth flight from Varanasi, to Mandalay (Myanmar). Bertrand Piccard flew the solar aircraft for 13 hours and 29 minutes, travelling a distance of 1,398 km. On Sunday March 29, Si2 took off for its fifth flight from Mandalay to Chongqing (China). Si2 landed in China and the landing was challenging and delayed not only because of strong winds but also because of the intense traffic of Chongqing International Airport. From Chongquing, Si2 will fly to Nanjing and then via Hawaii it will reach Phoenix in the United States. From there the solar plane will go to Middletown, United States, and then to New York and Europe. The plane will then fly back to Abu Dhabi in its final lap.

This is a great historic first. For such an adventure, as for any premiere, there are no references. The team therefore, and will be, faced with a number of challenges, leading them to push the limits of technological, human and piloting performance.

Living up in the sky, for days

For the trip’s ‘long haul flights’ the pilot will be living in the 3.8 m³ cockpit for five or six days and nights in a row. The cockpit volume provides enough space on board for oxygen supplies, food and survival equipment, whilst also meeting the optimal ergonomic requirements for flights lasting several days.

Multipurpose seat: A multi-purpose seat functions both as reclining berth and toilet. A parachute and a life-raft are packed into the seat-back. When fully reclined, it allows the pilot to perform physical exercises.

Extreme temperatures: In the absence of any heating, the cockpit and the pilot will be facing extreme temperatures: from +40°C to –40°C! Bertrand Piccard and André Borschberg are protected against the ambient cold or heat by high-density thermal insulation in the cockpit structure.

Focus and vigilance techniques: Self-hypnosis and meditation techniques allow the pilot to maintain his powers of concentration and vigilance.

Diet and medicine: Science-based personalised nutrition has been developed by Nestlé Health Science, to meet the need of 2.4 kg (5.2 lb) of food, 2.5 litres (84.5 oz) of water, and 11 litres (33.8 oz) of sports drink per day, during the long legs of the #RTW solar flight. Physicians and specialists of high altitude medicine provide medical advice prior and during flights.

Real time flight monitoring: In contact with the MCC – Mission Control Centre – in Payerne the pilot receives support and flight indications.Continuous transmission of hundreds of technical parameters via satellite data link to the MCC. All possible eventualities are simulated by a multidisciplinary team to find the right combination of weather patterns, and pave the way for the solar airplane to enter controlled airspace and prepare for landings at international airports.