Sustainable global flight

C&I Issue 6, 2014

‘This aeroplane could not exist without the chemical industry,’ said Bertrand Piccard, co-founder of Solar Impulse, at the public unveiling of Si2, the organisation’s second zero fuel aircraft, which is scheduled to fly 35,000km around the world using only solar energy in 2015. Piccard sees this challenge as a natural progression from his record non-stop balloon flight around the world in 1999.

‘The Solar Impulse initiative is both scientific and innovative,’ says Piccard. ‘It is also philosophical, by its goal of raising awareness in society about the need to save our planet’s energy resources.’

The new aircraft is derived from the earlier Si1, which in 2010 demonstrated the feasibility of flying by day and night using only solar power and, after a number of other flights, garnered eight world records, culminating with a flight across the US in 2013 (C&I, 2013, 8, 12). As Solar Impulse’s other co-founder, Andre Borschberg expressed it: ‘Si1 was a demonstrator, to show it is possible to fly day and night; Si2 has been designed to travel.’ Si1 flew for a maximum of 24 hours at a time, while the larger Si2 is capable of being airborne for up to five days to enable it to cross the Pacific and Atlantic Oceans.

The new aircraft has a larger wingspan, at 72m, than a Boeing 747 Jumbo jet, compared with the 64m of the earlier aircraft, determined by its higher weight of 2300kg, about the same as a small van, compared with 1600kg for Si1, due to the extra batteries, life support and safety equipment required for the longer flight stages. To supply the increased power demands, the number of individual solar cells on the surface of the aircraft’s wing has been increased by 50% to over 17,200, providing Si2 with around the average power of a small motorcycle.

The aircraft, which like Si1 is based on a lightweight carbon fibre structure, has also been refined incorporating technical developments by partners and supporters of the Solar Impulse project that have become available since the construction of Si1.

The chemical industry is well-represented in this list of organisations, as, for example, Piccard points out: ‘Without Bayer and its mission of conducting “Science for a better life”, this solar airplane would never have been light and efficient enough to be able to fly night and day without fuel.’ Bayer MaterialScience (BMS), which has been an official partner since 2010, is similarly committed to delivering sustainable products and solutions, as Richard Northcote, BMS executive committee member responsible for sustainability, points out: ‘The Solar Impulse project impressively demonstrates how our innovations can help preserve the planet and its natural resources, improve people’s lives and create value...[The project] uses technology that today only exists on Si2.’

BMS has contributed a lot of technical expertise to the Solar Impulse project. It was responsible for the complete design of the cockpit shell, for example. As Borschberg points out, a key component of BMS’s involvement is how it has been able to take this project from design concept through to manufacturing.

The shell itself is fabricated from rigid polyurethane foam supplied by BMS, however, the cockpit door has been produced using Baytherm Microcell, which is an extremely high-performance insulation foam, described as having as much as 10% greater insulating performance than the current standard. Such performance is especially important for this aircraft because it must withstand temperature fluctuations between -40°C at night and +40°C during the day.

Other BMS materials used include a polyurethane/carbon fibre composite for the door locks, and thin sheets of transparent polycarbonate for the window. Overall, despite the cockpit being larger overall than that of the original Si1 aircraft, it is only minimally heavier.

Rigid BMS polyurethane foam is also used to insulate the batteries, while raw materials have also been provided for the silvery coating covering large portions of the aircraft and the adhesives that hold the textile fabric in place on the underside of the wings.

The solar cells that cover the upper surfaces of the wings have been supplied by SunPower, a subsidiary of France’s Total, and are protected against water ingress by an ultra-thin Halar ECTFE polymer film supplied by Solvay, which has also provided Solstick adhesive tape to seal the gaps between the individual cells and allows them to move with the wings.

Solvay also provided the PVDF Solef and F1EC for use in the Li-ion batteries; Fomblin PFPE lubricant, while the main wing spar contains a honeycomb structure made from paper imprenated with Torlon PAI polymer, and numerous mechanical parts, including fasteners and screws, are made from various Solvay speciality polymers, including PEEK and SRP, and polymamide, which has been used for complex parts produced using 3d printing. Finally, the pilots’ underwear has been made from Solvay’s Emana polyamide 6.6 yarn, a smart fibre, which interacts with the body, stimulating micro-circulation and helping muscle performance.

Nutrition also makes a major contribution for the conditions that will be experienced by the pilots both before and during the long flight stages, and here NestleHealthScience has played a major role providing science-based personalised nutrition.

As well as a technical challenge, this planned flight also represents a human challenge for the solo pilots. Piccard and Borschberg will take it in turns flying the individual stages, but flying continuously for five days, compared with only 24 hours with the first aircraft, has meant that they have had to train themselves to operate with little sleep for up to 120 hours. Borschberg has been learning so-called polyphasic sleep, which involves sleeping for 20 minutes at a time, with a total of 2 hours/day, while Piccard practises yoga, breathing techniques and self-hypnosis to relax without sleeping.

Si2 has a larger but still unpressurised and unheated cockpit; ‘Business class compared with the economy cabin of Si1’, according to Borschberg. It has a fully reclining seat, developed by Swiss aircraft seating specialist Lantal that will enable the pilot to relax, but also a virtual co-pilot that can wake up the pilot if the aircraft and flight parameters start to move outside the normal flight conditions. The aircraft is also monitored in real-time from the ground.

As Borschberg pointed out, flying solo for such extended periods means reliability is essential: ‘I don’t want to have to go for swimming lessons,’ he said. ‘We need to be able to cope with equipment failures, and we need to be able to cope with the weather on the other side of the ocean.’

In December 2013, Piccard completed a 72 hour non-stop flight simulation, equivalent to crossing the Atlantic, with full physiological monitoring. Before the round-the-world flight takes place, a full simulation using real-time weather information will be conducted by the Solar Impulse ground team using computer models built by Altran.

The round-the-world flight is planned to begin in the Gulf States in March 2015. The eastwards flight will touchdown in India, Myanmar, China, crossing the Pacific Ocean to the US, from where it will continue on across the Atlantic to Southern Europe/North Africa, before returning to the Gulf. In all, the flight is expected to take three months, which will include 20 days of actual flight at an average speed of 100km/hour.

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