http://www.newscientist.com/news/news.jsp?id=ns99996419

   Spacecrafts powered by thunder

   15:46 20 September 04

   NewScientist.com news service

   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 øC, 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.
   

... Einstein was good at 'riting and stuff.
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