A t first glance, the engine bolted to the test stand looks like an unlikely candidate to lead an aerospace revolution . Its size is unimpressive: At about four feet long, its dwarfed by the machinery that feeds it air and fuel, machinery that fills a house-size structure at the China Lake Naval Air Warfare Center in California . And its appearance is unremarkable: This machine has none of the grace of the high bypass turbo-fans that power modern jetliners, with wide, sweeping inlets and delicate blades. From the outside, it’s simply a collection of metal tubes, one large cylinder feeding into five smaller ones terminated by convex, barnacle-shaped nozzles.
But Gary Lidstone and Tom Bussing have bet that this little aircraft engine—the most advanced expression yet of a revolutionary concept called pulse detonation— could absolutely bury all those that have come before it. Lidstone is the manager of propulsion programs for Pratt & Whitney’s Seattle Aerosciences Center, and Bus-sing is his boss and the creative force behind the device’s design. Here at China Lake, standing in the desert heat, the two survey their handiwork like proud papas, explaining how it has taken years to show that the concept behind this engine can open up an entirely new world of jet propulsion. ‘There’s a big payoff,” Lidstone says . “It’s a paradigm shift that could make other things obsolete.”
Indeed, Lidstone’s team is hardly alone in its quest. In the past 10 years, the promise of the technology—a promise of a propulsion system far simpler than today’s turbofans and capable of operating across a much wider velocity range, powering aircraft from takeoff to Mach 4 with ease— has touched off an explosion of interest at university, military and NASA research centers, and in labs as far away as Japan, France and Russia. In just the past three years, the two companies that now stand to gain or lose the most from the rise of a revolutionary, market-disrupt-ing jet engine technology have begun to invest heavily in pulse-detonation engine (PDE) research . In January 2001, Pratt & Whitney bought the company Bussing had created to develop his concepts . That same year, General Electric designated pulse detonation a top priority. Arriving late in the game but armed with a new approach that could trump Pratt & Whitney, GE began plowing resources into building a PDE development team at its Global Research Center in upstate New York. “We see pulse detonation throughout our entire product line,” says Harvey Maclin, manager for advanced technology, marketing and government programs at GE Aircraft Engines and one of the early sponsors of pulse-detonation research at GE. “That’s why we’re so interested in it.”
For decades, these two companies have been battling for supremacy in the global jet-engine arena, exploiting any advantage that might give them an edge in the struggle for civilian and military market share. But those advantages have grown smaller as conventional jet engine performance edges closer to the limits of thrust-to-weight ratios and fuel efficiency. Pulse-detonation technology offers a chance to escape from this spiral of diminishing gains and score a big win—not to mention the first lucrative corporate and military contracts. Those contracts could be for super-efficient engines for subsonic jetliners, which would chop fuel consumption by an amount that engineers would “kill their grandmothers” to get, Lidstone jokes, or for supersonic, unmanned aerial vehicles or manned fighters. We could also see a supersonic airliner that’s much cheaper and more practical than the recently grounded Concorde. Pulse detonation would also offer cheaper access to space, saving us tons of liquid oxygen and fuel by powering vehicles from the ground to high altitude and hypersonic velocity, where conventional rocket engines would take over to lift them into orbit.
“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 on 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 deto-nation, 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-I 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 new 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 tubé, 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-4os, 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.”
The turning point came in 1998 with a series of NASA and Air Force—funded performance demonstrations of a two-tube PDE at the Naval Postgraduate School. That rig did everything Adroit said it would—detonating each of its tubes 40 times per second, running for up to 30 seconds, and generating more than a hundred pounds of thrust—and after a little head-scratching, most of the naysayer came around. A little more than two years later, Pratt & Whitney showed what it then thought of the new technology when it bought Bussing’s 24-member team from Adroit lock, stock and intellectual property.
The engine at China Lake is several generations beyond the one that ran at Monterey. Standing in a tiny square of shade cast at noon by a canopy over the test stand, Bussing and test engineer May Lau go over the basic anatomy of the device. Like any other jet engine, it takes in air at its front end—in this case, air that has been heated and pressurized by the test facility to simulate flight at Mach 2.5 and 40,000 feet . If this were a conventional jet engine, that air would be driven by a fan through a multistage compressor and into a combustor, where fuel would be burned continuously. But in this engine, the airflow has to be switched between five tubes, in each of which an air-fuel mixture must detonate cleanly 80 times per second. Bussing solved this problem with two mechanisms: a patented disc, called a rotor valve, with specially designed holes in it, which alternately covers and opens tubes to the airflow as it spins at 2400 rpm; and a “predetonator” on each tube, which uses supplemental oxygen, ethylene fuel and a Ferrari spark plug to kick-start detonation in each main tube. The result is 400 detonations every second, producing an amount of thrust that neither Bussing nor Lidstone will disclose, but which is good enough for a supersonic cruise missile.
Giving a missile ‘supersonic capability at subsonic prices” has been a focus for the group from the very beginning. The military will help fund the project and has given the team a simple, small-scale test platform for the technology. Later, Lidstone sees a “supercharged” version of the pure PDE, followed by a conventional turbofan with pulse-detonation tubes mounted in the bypass duct around its compressor—a so-called duct burner. Finally, Lidstone’s road map ends—perhaps 15 or 20 years out—with “the real pot of gold at the end of the rainbow”: a hybrid engine in which sections of the central compressor and combustor of a gas turbine have been replaced by pulse-detonation tubes, combining the best features of a high - bypass turbofan and PDE. “That’s where the big market is,” Bussing says. He sits a few feet away from his engine and focuses on a monitor showing his team reassembling the engine . “But to do that right, you really have to build devices like this . You have to go through this to get there.”
When it comes to technological innovation, nobody has a monopoly on road maps. A continent away from China Lake, at General Electric’s vast research center near Schenectady, New York, engineers have a map of their own—one they think shows a faster, better way to get to a hybrid PDE “pot of gold.” Getting there means playing catch-up; Pratt & Whitney, after all, has a substantial head start.
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 jack hammer. 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—’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 PDF that will function without supplemental oxygen to initiate a DDT, and that maybe able to operate at far ...........!
September 2003. (Pgs.51-58)
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