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See Inside December 2007

Supersonic Pulse Power

Nazi-era weapons lead to fuel-stingy aircraft engines

For nearly a year after Nazi Germany unleashed its V-1 flying bomb in June 1944, the early, crude cruise missile plagued English and Belgian cities and countrysides with its terrifying buzzing sound before raining down random death and destruction. Engineers are now refining the pulse jet, the simple but noisy and fuel-wasting power plant that propelled the buzz bomb, into a lightweight and powerful engine that relies on repeated shock wave–driven combustion cycles to produce thrust efficiently. In a decade or two, such pulse detonation engines could power many types of aircraft.

Pulse jets rank among the simplest of engines, explains Narendra Joshi, leader of a research group at General Electric in Niskayuna, N.Y. In many ways, the device resembles a combustion cylinder in a standard automobile engine, though without a piston. The basic recipe: Take a short metal tube and place injector valves at one end that can rapidly meter pressurized fuel and air into the tube on command. Then squirt in small amounts of fuel and air to create a flammable mixture and light it off with a spark plug. The resulting conventional fuel burn will generate expanding combustion gases that whoosh out the other end of the pipe, creating thrust. Now do it again—and again, 50 times a second (a frequency that produces the engine’s infamous buzzing sound). Although the pulse jet is a simple and effective propulsion system, it consumes fuel comparatively slowly and incompletely, making the combustion process inefficient.

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