Making it easy is not part of the latest Branson challenge.

Andrew Doyle/LONDON

WHEN VIRGIN AIRWAYS chairman Richard Branson and balloon manufacturer Per Lind- strand launch their attempt to circumnavigate the globe in a balloon, it will be more than a test of human endurance. The performance of the materials, components and systems which make up the most advanced aerostatic vehicle ever designed will be crucial if their assault on "the last great aviation record left on Earth" is to be a success.

The fundamental innovation which makes the attempt possible is the development of a combined hot-air and helium balloon. The hot-air balloons used by Branson and Lindstrand for previous flights across the Atlantic and Pacific oceans are not suitable for a global flight, because of the impossible amount of fuel which would be needed to maintain the required altitude.

To capture the 220kt (400km/h) jetstream winds which will carry them around the globe, the balloon will have to remain above a height of 30,000ft (9,000m).

The solution to this problem was first proposed in the eighteenth century by Jean Francois de Rozier, who evolved a combined hot-air and hydrogen balloon after his first manned flight in a Mongolfier hot-air balloon in 1783.

The Global Challenger relies on this principle, but with helium, a safer, inert, gas used instead. The helium will be kept warm during night by propane burners, thus preventing the balloon from descending from the jetstream as the gas cools.

Although the helium balloon has a much longer endurance than that of a hot-air balloon, it will also be more difficult to manoeuvre between the various jetstreams. In addition, no helium balloon has yet been subjected to sustained flight in a jetstream, and so a series of test balloons were built to investigate its flight characteristics.

The Global Challenger's balloon envelope contains some 25,000m3 (8,830ft3) of helium. It is made from polyamide fabric, coated with a polyurethane compound which acts as a gas barrier and filters out ultra-violet light.

To further improve ultra-violet protection and reduce porosity, a layer of aluminised melinex film was applied. This material has been specifically designed by ICI and is claimed to be one of the most efficient "gas barriers" available.

The envelope was formed using a thermal-welding process, which needed to be carried out with extreme precision as a single hole the size of a pin head (less than 1mm diameter) could lose enough gas to cut short the flight by three to four days.

realistic test conditions

As it would not be possible to test-fly the Global Challenger, because of the inevitability of damage occurring during landing, Lindstrand's design team devised a programme to test the fabric in simulated conditions as close as possible to those expected during actual flight.

The team used an Instron computer-controlled tensile testing machine, which incorporated an environmental chamber with a temperature range of -70¡C to +330¡C, to simulate the equivalent of two weeks of flight in the jetstream.

"We believe that this is the first time such testing facilities have been available for balloon manufacture, and it produced invaluable data for us," says Lindstrand.

Meanwhile, the Global Challenger's crew capsule has to be pressurised to permit sustained flight above 30,000ft. The choice facing the team was whether to opt for a complex system which involves pumping air into the cabin continuously, or to settle for a simpler, closed, system where cabin air is re-circulated, cleaned by chemical "scrubbers" and topped up with a liquid-oxygen supply.

The former system was selected because it will supply higher-quality air. A propane-powered piston engine drives a CompAir screw compressor, which pumps pressurised air into the cabin. Cabin pressure is regulated by an aneroid-controlled outflow valve.

As the system is safety-critical, two engines and compressors are run at half-speed, so that, in the event of failure, one is capable of carrying the load.

The capsule was developed using computer-aided-design (CAD) software, and the data downloaded into a finite-element-analysis (FEA) package to allow structural stresses to be calculated. With the capsule design frozen, its exact weight was calculated using the software, so that the envelope could be accurately sized for the lift requirement.

Using CAD and FEA software, the whole balloon was "inflated" and "flown" in various simulated environmental conditions before actual construction began.

"Since much of what happens in the jetstream, where the balloon will be flying, is unknown, it is essential that we can predict the likely stresses that the balloon will encounter so that we can prevent structural failure," says Lindstrand.

The flight itself is expected to take 18-21 days. "Paradoxically, although we believe that the latest technology has made this attempt possible, the concept of our Rozier balloon is over 200 years old and our flight has been a dream of people for almost as long," he says.

As Flight International went to press, the Global Challenger team was facing an anxious wait for suitable weather conditions to begin its flight. Virgin says that the chosen launch site at Marrakesh, Morocco, is experiencing "the most unusual weather conditions over North Africa for 45 years", and rates their chance of making the flight this year at "not more than 50%". The last available launch window is forecast as falling between 15 and 18 February.

Source: Flight International