Spacecrafts powered by thunder

Thunderous sound waves could one day propel spacecraft to the edge of the solar system, say engineers who have developed a new type of acoustic engine.

Current long-range spacecraft - like the US-European Cassini probe now orbiting Saturn - roam too far from the Sun to use solar power so instead carry plutonium bricks to fuel their engines. As the radioactive plutonium decays, it generates heat that produces an electric current between two different types of metal.

This system uses no moving parts - an advantage since these can fail - but the bricks are large, heavy, and difficult to produce. And these engines yield efficiencies of just 7%.

So NASA is funding research into Stirling engines, which use temperature differentials between reservoirs of gas to create electricity. Conventional Stirling engines are an old technology, invented in 1816 as a safer alternative to steam engines.


Reliability issues


The modern nuclear Stirling engines developed by NASA boast efficiencies between 25% and 30%. So, if used in a spacecraft like Cassini, they would require fewer plutonium bricks. But there is a reliability issue as they use two pistons - one to move the gas back and forth and one to extract electricity.

Now, a team of engineers at Los Alamos National Laboratory in New Mexico and Northrop Grumman Space Technology in Redondo Beach, California, have built a Stirling engine with just one piston.

"It's more reliable and more easily scaled to very large sizes," says team member Mike Petach of Northrop Grumman.

The engine consists of a 0.3-metre-long tube filled with helium gas and about 1000 closely-spaced metal screens. Decaying plutonium heats one end of the tube to 650 �C, causing the gas around it to expand. That gas transfers its heat to the next screen, then contracts again. This process repeats in a domino effect all the way down the tube.


Burst eardrums


The expanding and contracting gas produces sound waves - a deafening roar - that oscillate at a frequency of 120 Hertz and drive a piston, generating electricity.

"Inside the engine, the acoustic pressure is high enough to pop your eardrums," Petach told New Scientist. "It's louder than a thunderclap."

He adds that the sound does not escape the engine, so the device could be used to produce electricity for submarines, which must glide undetected beneath the ocean's surface.

Team member Scott Backhaus of Los Alamos has worked on the cooling applications of the acoustic engine for years, such as the liquefaction of natural gas. But the new laboratory model is the first to generate electricity.

Their model runs at 18% efficiency - more than double that of today's space engines - but Petach says that within two years the engine could be tweaked to match the 25% efficiency of two-piston Stirling engines.

Higher efficiency "saves you weight and gives you longer missions", he adds.

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www.newscientist.com...

Thanks for the info. Sounds pretty cool. It seems like its use in submarines may be the most signifigant benefit it could produce.