After Combustion: Detonation!

At this particular moment, the game of catch-up involves an almost intolerable amount of noise. The sound of a hydrogen-air mixture detonating 40 times a second in a 3-foot-long, 2-inch-diameter metal tube is a cross between a cruise-ship horn and a jackhammer. It seems to go right through your skull, even from behind the concrete and double-pane tempered glass of the control room. The noise stops after a seemingly endless five or six seconds, as the tube slides back along the thrust stand to its resting position; the roar of the compressors that feed the test cell is almost soothing in comparison.




"At a little bit lower frequency, we've run it for an hour straight," Tony Dean, the head of GE's pulse-detonation research effort, says proudly. His colleague Adam Rasheed, setting up the computers for another test run, has a somewhat more painful memory of the achievement. "I was in here and I had ear protection on, but after an hour I was just hearing this kind of . . . buzz," he says.




Behind all that sound and fury, Dean explains, is a carefully choreographed cycle in which a valve admits hydrogen gas into a stream of air flowing into the test rig, a spark plug ignites a DDT, and a shock wave blasts down the tube. High-pressure gas left in the tube by the detonation blows out, generating thrust.




Watching Dean explain the progression, you can see how much it fascinates him. Still boyish-looking despite his graying hair and mustache, Dean has a falconlike visage-small and thin, with sharp eyes behind round glasses. A Stanford Ph.D., he spent the '90s at GE working on the knotty problem of minimizing gas-turbine emissions-the company's jet engines power not only airplanes but also ground-based electrical generators. But when
Dean talks about pulse-detonation research, a field his team entered only in 1999, you get the feeling that he has found his true calling.




"It's amazing what you see in these flows, the insights you get," he says, his voice rising with enthusiasm. We've left the control room and descended to the floor of the test cell, and Dean is describing the output of his team's imaging system. Shooting through a transparent combustion chamber, the device uses the distortion of light paths in areas of varying air density to produce ghostly images of shock waves and turbulent flows inside the engine. It's a revealing glimpse of an otherwise invisible process. "I mean, it triggers ideas-â€Ah, we gotta do it that way!' It's all part of getting inside the process," Dean explains.




Getting inside the process-understanding the profoundly strange phenomena involved in pulse detonation-is critical because GE is preparing to leapfrog to a whole new level of PDE technology. Next year, it will begin building a hybrid PDE that will function without supplemental oxygen to initiate a DDT, and that may be able to operate at far higher ignition frequencies than other researchers' engines.