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.
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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.
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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.