Solar-powered flight

Last week, Solar Impulse, an aeroplane powered by the sun flew from Madrid in Spain to Rabat in Morocco in 19 hours. This was the world’s first solar-powered intercontinental flight, crossing the Strait of Gibraltar which separates Africa from Europe.

The aircraft was piloted by Swiss psychiatrist and balloonist Bertrand Piccard and compatriot Andre Borschberg, a businessman. The two founded the Solar Impulse project in 2003, in co-operation with the Ecole Polytechnique Fédérale de Lausanne and with technical help from the Swiss lift manufacturer Schindler and the Belgian chemicals company Solvay. They plan to fly the solar plane around the world next year.

Solar Impulse uses a lightweight carbon-fibre honeycomb sandwich for its wings, which have a span as great as that of an Airbus A-340 airliner. During daytime, the 11,628 solar cells on its upper surfaces power four 7.5 KW electric motors which drive the propellers and charge the lithium polymer batteries which run the motors at night.

Solar airplanes

The plane can fly for 36 hours at an average speed of 70 km/h. Of course, a modern jet airliner can do the same trip in one hour, carrying many hundred more passengers. However, even the Airbus A-380, a particularly economical aircraft, would burn about 24 litres of kerosene per passenger flying from Madrid to Rabat, releasing about 60 kg of carbon dioxide into the atmosphere. Whereas the Solar Impulse has hardly any carbon footprint.

Commercial flight

Nonetheless, commercial passenger or cargo transport by solar-powered aeroplanes remains a somewhat distant prospect. What is possible in the near future is commercial flight using solar-powered lighter-than-air airships, also known as dirigibles.

Airships are divided into three types: blimps (which use a pressure level greater than that of the surrounding air to retain their shape), semi-rigid airships (which require internal pressure to maintain their shape, but have extended internal keel frames) and rigid airships or zeppelins (which do not require internal pressure to maintain shape, having rigid internal frames containing several non pressurised gas cells).

Before the Second World War, airships were regular passenger carriers, competing with fixed-wing aircraft. Germany’s ‘Graf Zeppelin’ rigid airship flew over 1,700,000 km - including the first aerial circumnavigation of the earth - and carried over 13,000 passengers without a single accident.

However, a series of high-profile accidents, including the crash of the futuristic British R101, culminated in the Hindenburg disaster.

Reported live on radio and seen by a large crowd and later by millions of movie-goers, the image of the burning zeppelin put paid to commercial airship travel (the Graf Zeppelin ended its last flight from Brazil to Germany the next day).

Radar surveillance

During the Second World War the Americans used airships for anti-submarine duties, the Soviet Union for paratroop training, moving equipment, minesweeping and clearing wreckage. Post-war, airships were used for advertising, sightseeing, filming and surveillance.

In the 1980s, an airship revival began. Britain’s Airship Industries made ‘Skyship’ blimps, using modern materials: the envelope was made of the para-aramid synthetic fibre Kevlar and the gondola of composite materials.

In the 1990s, Germany began producing Zeppelin ‘New Technology’ airships (semi-rigid airships, not true Zeppelins). They had triangular graphite-reinforced-plastic trusses connected by longitudinal aluminium girders and tightened with aramid cords.

Laminated layers of poly-vinyl fluoride (to hold the gas) and polyester fabric (to give stability), wedded by a polyurethane layer, made up the envelope.

Conventional aeroplanes

The solar-powered airship saga began when, in 2009, the US military commissioned the Defence Advanced Research Projects Agency to develop a US $ 400 million remotely-piloted solar-powered dirigible for radar surveillance. The airship is expected to operate non-stop for ten years, floating 20,000 metres above the earth, cheaper than spy satellites to launch and operate.

Meanwhile, America’s SolarAirShip Company has come up with a design, which it is patenting, for a solar-powered cargo-carrying blimp which it calls the High Speed Solar Airship. It been tested on a 1:20 scale model. The full-sized airship is expected to have 2,400 square metres of thin-film solar cells integrated into the top of the envelope, giving 63 kW of power. At the designed operating altitude of 9,000 metres, the cold would boost the power by 30 percent.

The airship would be capable of carrying 55 tonnes of cargo at costs equivalent to rail transport, lower than for road-borne containers. At the designed altitude, it can get onto atmospheric jet streams and travel at up to 275 km/h. Unfortunately, in tropical areas such as Sri Lanka it will be difficult to find high-speed jet streams.

Meanwhile, the Canadian company Solar Ship has been tackling the problem differently. It has designed a hybrid airship/aeroplane, using advanced aerodynamics, synthetic textile laminates, smart electronics, lightweight batteries, and high-efficiency solar cells on the surfaces of the delta-wing. A test flight is expected to take place somewhere in Africa later this year.

The aircraft is filled with just sufficient helium buoyancy gas to neutralise its weight. The flow of air over the wind surface lifts it off the ground, as with conventional aeroplanes. The heavier-than-air design makes the airship less bulky, obviates the need for mooring and makes handling easier in wind.

The airships are expected to be able to take off from 50-200 metre strips, carrying up to 12 tonnes of cargo (depending on the model) at up to 84 km/h. While the smaller models will have ranges of 500-1,000 km per day, the largest is expected to be able to fly an unlimited distance.

The limiting factor on airships is that helium is a scarce resource, produced slowly by decaying uranium and thorium. The other options, ammonia, coal gas, hydrogen, and methane are worse greenhouse gases or are dangerously flammable. This need not be a problem for Sri Lanka, which can decay domestically-available thorium.

Sri Lanka should consider the airship option seriously as its roads gets clogged with more and more lorries and passenger vehicles. A low-cost aerial method of transportation certainly has to offer a lot, especially given the high cost of internal air travel.