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Futuristic engine proves its mettle

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9:44pm, September 18, 2001
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Proponents of exotic aircraft engines called scramjets say the technology might eventually make it possible to reach any spot on Earth in a 2-hour flight (SN: 9/19/98, p. 182: http://sciencenews.org/sn_arc98/9_19_98/fob5.htm ). Yet, in more than 40 years of development, no scramjet-driven craft has ever made even a short hop under its own power.

Now one has—but just barely.

On Aug. 27, the Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., announced that DARPA-funded researchers at the company GASL, in Ronkonkoma, N.Y., had achieved the milestone. A small scramjet-equipped projectile, somewhat like an artillery shell and fired from a cannon, generated enough thrust to overcome air resistance, the agency reported. Instead of slowing down, measurements showed, it maintained a speed of more than Mach 7—seven times the speed of sound.

By definition, scramjets begin working at about Mach 5. They can exploit shock waves to compress air flowing through the engine, which boosts fuel-burning efficiency.

Previous scramjet tests, which have taken place mostly in wind tunnels or have required external rockets to get them up to the Mach 5 threshold, haven't clearly shown that scramjets actually propel themselves in free flight.

A NASA test in June of the 3.7-meter-long Hyper-X aircraft (also known as X-43A) was to demonstrate scramjet propulsion, but it failed when a rocket malfunctioned.

In the DARPA-sponsored experiment, the titanium, 10-centimeter-diameter scale model of a scramjet missile flew nearly the length of a football field in 30 milliseconds.

"We flew about twice the distance that the Wright brothers did," says DARPA project manager Preston H. Carter, noting that a short hop is sometimes all it takes to demonstrate that a novel type of flight is possible.

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