Circumnavigating the world in an aeroplane powered by solar energy – a simple enough task? Perhaps not taking the comparison of the solar powered car. Even without the power burden of having to take-off, the cars undertaking the 3000km Darwin to Adelaide World Solar Challenge are allowed to start off with 10% stored energy to help them on their journey that can take up to two days. In fact, of this year’s 29 starters, nine did not finish.
The indignity of finishing the journey on a trailer is not an option for Solar Impulse - reliability is paramount. Additionally, each stage of Solar Impulse’s journey would have to be up to five days long, continuously flying through day and night, far longer than its land-based equivalents. So is it mission impossible, or just mission very difficult?
The Solar Impulse team are half way to proving it to be the latter as they have already flown in eight stages from Abu Dhabi to Hawaii, where the aircraft waits until the summer of 2016 to bring longer days of sunshine. Having reached such a stage it is clear that the fundamental technical challenges have been met, but it has already been a long journey.
It started in 1999 when Swiss adventurer Bertrand Piccard, along with Englishman Brian Jones, circumnavigated the globe in a hot air balloon. It was the first time that anyone had flown around the globe without using any fuel for forward propulsion. Allegedly he proclaimed on successful completion of the journey, with only a few drops left in the hot air burner fuel tank, that the next time he would do it would be without using any ‘traditional energy’. And in 2003 he launched the Solar Impulse programme to do just that.
To put the challenge into perspective, a jet airliner uses about four litres of fuel every second – or about 50,000kg of fuel for a transatlantic flight. Transferring to fuel-less flight has meant losing 50 tonnes in fuel weight, a big step in the ‘lightweighting’ effort crucial to the success of the project.
Eureka has been following the journey, most recently in the May 2015 issue (The strange phenomena of solar flight), just a few weeks after the first leg from Abu Dhabi, United Arab Emirates, to Muscat, Oman – a 772km journey which took 13 hours.
That article outlined the technology behind the solar flight; the use of over 17,000 solar cells covering the top surface of wings and fuselage to trickle charge the batteries, providing power day and night to the four 17.4hp motors.
Light-weight flight
Providing enough power meant having sufficient solar panels, and the more solar panels you have the more weight and the more power you need... classic chicken and egg scenario. The design settled on an optimum wing span of 72m, 3.5m more than a Boeing 747-8 or Airbus A380. The critical factor in making an aircraft with such a broad wing span was to make it as light as possible. That is where Bayer MaterialScience, since renamed as Covestro, has played a major role.
Dr Hubert Ehbing, director of processing and applications technology at Covestro, explained the relevance of the project. He said: “We want to inspire people. We want to encourage people to think about traditional ways of travelling, to think about having this technology and using it in a different way. To create some pioneering spirit with people.”
The technology he referred to was not necessarily new, just applied in different ways to push the available limits. Covestro’s main concern was to design the cockpit, a space of only 3.8m 3. This only gives enough room for one pilot, enough food (2.4kg a day) for five days, 2.5 litres of water a day and oxygen as the daytime flight peaks at the oxygen-rare altitude of 8500m. Adding a second pilot would add too much weight and for the same reason there is no autopilot, so the pilots are trained to live on 20 minute sleep segments throughout the day.
This is not sustainable in the long term, which is the main factor limiting flights to five days. There are two pilots, both with the same sleep training, Bertrand Piccard sharing piloting duties with his fellow co-founder Andre Borschberg.
Even though the cabin is small, it was imperative that weight was kept to a minimum, with total weight of the aeroplane being kept down to a little over two tonnes. But, alongside the material being light, it also had to have excellent thermal properties as, once again, heating inside the cabin was a weight luxury that couldn’t be afforded. The temperature at 8500m, which is just under the height of Mount Everest, can fall to -50°C and to make it liveable, if not exactly comfortable, for the pilot it must not fall below -20°C.
Covestro supplied foam materials for the cabin, polycarbonate sheet for the windows and various glues and coatings, but the company had to prove it offered the best solution in all cases.
Dr Ehbing said: “We have other projects where we tested our materials, but they were not chosen simply because other partners supplied better material, mainly as it always has to be lightweight. The most important factor was, ‘can you supply some material that contributes most to reduction of the weight?’”
Determining the best shape to house the pilots required several cycles with both mechanical and heat simulation. Then the physical build was started and looped through many design cycles using a wind tunnel and other tests. “It roughly took three years to optimise the cabin structure,” said Dr Ehbing.
The final material used to make the fairing (the external structure) was an ultra low density polyurethane rigid foam, which provided the outstanding insulation and mechanical properties required to protect both pilot and equipment. It was also easy to make, maintain and repair. This was the critical area. The only sources of heat in the cockpit were a bit from the electronics equipment, but it was mainly from the pilot himself. The only way of keeping this heat in at night was through the insulation properties of the foam and the polycarbonate, which actually has worse thermal conductivity. The foam developed actually allowed the target minimum temperature of -20°C to be beaten, as it will not allow the temperature to go below -18°C.
Higher mechanical strength was required for the canopy opening system in case it was needed for a bail out. The required strength was delivered through a polyurethane carbon-fibre composite produced using a resin-transfer process.
The canopy was made of a special thermoformed multi-layer (aka double –glazed) polycarbonate sheet. This provided a glass-like appearance but with mechanical properties that were better than glass. It also added some safety functions such as anti-fogging.
This whole fairing was coated in a specially developed film that improved mechanical performance and provided weather resistance.
Dr Ehbing commented: “We had to develop a special foam here, which had a good combination of mechanical stiffness and little thermo-conductivity. We developed a new micro cell technology by special modification of the foam structure, in doing so we were able to reduce the thermal conductivity of the foam by 40%. That was what was required for the project here.”
However, Dr Ehbing claimed this has opened up new application areas. He said: “Think about building insulation for example, or fridges. If you have a fridge that has an installation material, which makes sure that you have less heat loss, up to minus 40%, you can reduce the thickness of the walls of the fridge, put more food inside and at the same time have a higher rating of your fridge. We are in discussion with fridge manufacturers now.”
This foam has a density of less than 30kg/ 3, which Dr Ehbing describes as ‘virtually nothing’ and as a consequence the total weight of the cockpit area, targeted to weigh in at 35kg, does in fact weigh only 24kg.
Beyond the structural insulating foam, other new materials were developed as well – notably several composite materials for mechanical parts and a special flame resistant foam that was used to house the batteries.
Although the main idea behind Solar Impulse is to promote the use of cleaner technologies, there have been both tangible and anecdotal benefits for Covestro in getting involved, as Dr Ehbing described. “We found new materials and new technologies,” he said. “But we also involved lots of people here, 30 in total. For us it was very inspiring as it allowed us to get hands on with new technology, bring people together for different functions, share new ideas and push the limits.”
Built to 'convey messages' The plane making this epic journey is in fact Solar Impulse II, which was largely completed in 2013 although design modifications have continued through the testing and trialling period. The first Solar Impulse was essentially the prototype and work on that was started on in 2003 when teh founders, Piccard and André Borschberg, started the project. At the time Picccard said: “Solar Impulse was not built to carry passengers, but to convey messages. We do not plan to revolutionise the aviation industry but instead to demonstrate that the actual alternative energy sources and new technologies can achieve what some consider impossible.” |
The journey so far The journey from Abu Dhabi to Hawaii has taken on eight legs with stops at Muscat (Oman), Ahnedabad (India), Varanasi (India), Mandalay (Myanmar), Chongqing (China), Nanjing (China), and Nagoya (Japan). The last flight from Japan to Hawaii was easily the longest taking four days, 21 hours and 52 minutes to cover 7212km at an average speed of 86.34km/hr. Maximum altitude was 8874m. The remaining five flights will first take it to Phoenix and then another US city before travelling on to New York. From there is a choice of a Southern route via North Africa or a Northern route via Western Europe, before a final five day stint back to Abu Dhabi.
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