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NASA and General Electric have teamed up with French industry to revive a type of fuel-efficient aircraft engine shelved in the 1980s, in an effort to tackle aviation carbon emissions and high fuel costs.

The "open rotor" engines, despite their relatively green characteristics, will still be unpopular with many anti-aviation campaigners as they are significantly noisier than ordinary turbofans.

New open rotor engines, as envisaged by Rolls Royce

Sound and fury, signifying green?

Open rotor engines work in a fashion not dissimilar to today's high-bypass turbofans, which use a central gas-turbine core to drive a larger-diameter fan which rams a lot more air through the outer part of the engine ("bypassing" the central jet, hence the name). This makes for much better fuel efficiency than the turbine and its compressor alone.

But the fan on current engines is still enclosed inside the nacelle, which cuts down on noise but limits the area of air on which the blades can work. For true efficiency, it would be better to use larger fan blades still, ones so long that it would no longer be practical to fully enclose them and hang them beneath an airliner's wing. It's thought that two counter-rotating fans would give best results, saving as much as 25 per cent of the fuel an airliner now uses for a given journey - but at the cost of a lot more noise.

The result is an open rotor engine, something between a turboprop and a turbofan. Back in the 1980s, with the oil crisis of the 70s still fresh in everyone's memory, General Electric planned to put an open rotor engine on the market. The result was the GE36 design. By the time the GE36 was ready to go, however, fuel prices had fallen and there wasn't enough interest in a new efficient engine, so General Electric never actually launched it commercially.

But the soaring oil prices seen before the recent economic slump - now climbing again - combined with governmental plans for massive levies and crackdowns on carbon emissions have made aviation industry chiefs think again. GE and NASA announced jointly that they would restart wind-tunnel test work on open rotors last year, and yesterday they said that the necessary test rigs - last used for the GE36 work in the 1980s - have now been refurbished. Tunnel tests at NASA's Glenn research centre are expected to begin this summer.

"The tests mark a new journey for GE and NASA in the world of open rotor technology," said David Joyce, president of GE Aviation. "These tests will help to tell us how confident we are in meeting the technical challenges of an open-rotor architecture. It's a journey driven by a need to sharply reduce fuel consumption in future aircraft."

France's Snecma - collaborator with GE on the CFM56 joint engine series - will also participate.

GE, NASA and Snecma aren't the only ones looking for new engine technologies to cut fuel consumption on future airliners. Pratt & Whitney are pushing their "geared turbofan" concept, and British-headquartered Rolls-Royce has expressed its corporate view that open-rotor is the way ahead.

The big snag for open rotors is admitted by all concerned to be noise. A big part of the protest attendant on airport expansion efforts like those planned for London's Heathrow is actually driven by local dislike of aircraft noise rather than ecological concern as such. Many people don't really care about carbon emissions so long as planes stay quiet.

The conflict between reducing CO2 and reducing noise is indeed already apparent. According to Professor Ian Poll, Fellow of the Royal Aeronautical Society, noise concerns are already hurting the environment. Today's A380 double-decker superjumbo, he says, already has a "cruise burn fuel penalty" which is the result of design compromises made to make it comply with noise regulations.

Many would argue that everyone should simply fly less, of course, so reducing the amount of fuel burned and emissions given off without the need to have noisier greener planes. But a 25 per cent cut in flights - to deliver the same reductions you could get by going to open-rotor - would seem likely to be painful economically.

And with recent research indicating that railways can actually hurt the environment more per passenger mile than airliners, flying less could even be a retrograde step in ecological terms. ®

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