After Combustion: Detonation!

"Pulse detonation is a hot topic in combustion research," says Gabriel Roy of the Office of Naval Research. "Compared with gas turbines, the PDE has a much simpler configuration. It has the capability of going from subsonic to supersonic using less fuel, and it's thermodynamically more efficient. But there are big engineering issues-thermal fatigue, noise. It's very challenging research."

The concept behind the PDE is deceptively simple. In short, there are two kinds of combustion: the old, familiar, slow kind of burning, called deflagration, and another, much more energetic process called detonation, which is a different animal entirely. Imagine a tube, closed at one end and filled with a mixture of fuel and air. A spark ignites the fuel at the closed end, and a combustion reaction propagates down the tube. In deflagration-even in "fast flame" situations ordinarily called explosions-that reaction moves at tens of meters per second at most. But in detonation, a supersonic shock wave slams down the tube at thousands of meters per second, close to Mach 5, compressing and igniting fuel and air almost instantaneously in a narrow, high-pressure, heat-release zone.




That zone is where the highly efficient combustion that the Pratt & Whitney and General Electric engineers hope to harness takes place. To bring it into existence, one must precisely coordinate fuel input, airflow and the ignition spark to create a "deflagration-to-detonation transition," or DDT, the process by which an ordinary flame suddenly accelerates into an immensely more powerful detonation. And one detonation is only the beginning, because while it generates more thrust for the amount of fuel combusted than a deflagration, it also combusts only a tiny amount of fuel. To make a PDE work-to get any practical thrust out of it-one needs dozens of detonations every second, a detonation wave.




The first scientists to recognize that rapidly pulsed detonations might be used to create thrust were probably the Germans, who developed the V-1 "buzz bomb" in the 1930s. "The Germans attempted a detonation with the V-1 but never got it," says Chris Brophy, a propulsion research professor at the Naval Postgraduate School in Monterey, California. "The V-1 was a pulse-jet, more of a high-speed deflagration." Some theoretical and experimental work followed at universities in the '50s and '60s, but conventional jet-engine and rocket performance was improving so rapidly at the time that few people saw any reason to experiment with a phenomenon so difficult to create and measure in the lab. But in the early '90s, several factors generated a sudden renaissance in pulse-detonation research: the need for significantly higher performance, the availability of new diagnostic tools and high-speed modeling computers, and a small but critical supply of federal money to university professors and research entrepreneurs.




One of those entrepreneurs was Bussing, who in 1992 founded the company that Brophy calls "the real commercial thrust, no pun intended," behind PDE research in the '90s. Hired by Boeing just after receiving his doctorate from MIT, Bussing had labored for years on the never-to-fly hypersonic National Aerospace Plane before realizing that he wasn't going to get what he wanted-the chance to run a revolutionary technology project-inside the giant company. He started thinking seriously about pulse detonation. He left Boeing and gathered three colleagues to form a pulse-detonation research group for Adroit Systems, a high-tech research company.




Bussing's group at first struggled just to achieve a single detonation in a single tube, but quickly progressed to building a twin-tube test rig capable of firing each of its tubes 22 times a second, yielding a total frequency of 44 cycles per second. Despite their successes and those of other researchers, however, pulse detonation still wasn't taken seriously by much of the mainstream propulsion establishment. Skeptics pointed out that most of the work was confined to university labs or private companies, which regarded their methods and results as proprietary and made what many outsiders thought were unrealistic performance claims.




"There were a lot of times when the beating from the naysayers was fairly daunting," says Bussing, standing amid a warehouse full of PDE spare parts at the China Lake test site. He speaks quickly and rather quietly, punctuating his words with rapid hand movements. Tall, in his mid-40s, he has the athletic build appropriate for someone who climbs mountains in his spare time-though he hasn't had much since he left Boeing. "They said you can't operate the device in an unsteady manner, you can't isolate the inlet from the combustion process, you can't generate thrust, it's gonna fall apart. If you look at a textbook of all the physical phenomena that you can envision, every one of those became a question."