After Combustion: Detonation!
The race heats up to replace the jet turbine with a more efficient source of Mach-breaking airpower: the pulse-detonation engine.
At 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, it’s 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 turbofans 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 Bussing 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 stand to gain or lose the most from the rise of a revolutionary, market-disrupting 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 superefficient 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 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 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.”
The turning point came in 1998 with a series of NASA and Air Forceâ€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 naysayers came around. A little more than two years later, Pratt & Whitney showed what it 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 2,400 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 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.
The vehicle Dean plans to use to reach his goal looms over the rest of the test cell, looking for all the world like a chunk of water main. The GE research group calls it simply “the big rig”-a heavily instrumented length of pipe roughly 16 inches in diameter. The one part of the test cell that’s off-limits to visitors is the area just in front of the big rig’s mouth-the only place that would give a view down its throat, presumably revealing details that GE would prefer to keep to itself. Those details, one guesses, have something to do with what Dean refers to as “valveless” operations, which could be the key to generating detonation frequencies as high as hundreds or even thousands of cycles per second in a single tube.
Dean is reserved on this subject, noting in a later e-mail that he’s “not ready to say much about this.” But Maclin, GE’s marketing manager, is more expansive. “We’re looking at an order of magnitude higher frequency than anybody else in the industry,” Maclin says. “I like to think of it as an aerodynamic valve as opposed to mechanical valves, and that’s what allows us to get to the much higher frequency, because there’s a limit to what you can do with mechanical valving.” In such a design, the air-fuel mixture and timing would be controlled by aerodynamic forces created by the shape of the detonation chamber itself. This “aerodynamic valve,” Maclin says, would “allow air in before detonation, but the pressure from detonation will be high enough to prevent the second charge of air and fuel from entering until the detonation wave moves downstream.”
When asked what kind of results he’s getting so far with the big rig, Dean again speaks carefully. “We’re getting fast flames and all sorts of interesting behaviors, but I would not characterize them at the moment as detonations,” he says. “I do think we’ll be successful-we’ve got the right measurements, the right people, the right computational tools-but I can’t claim that we’ve gotten there yet.”
It’s an odd mix-a healthy skepticism combined with faith unswayed by the presence of so many unknowns. In a way, the skepticism is part of Dean’s job description. Maclin, by contrast, is unreservedly bullish on GE’s prospects for taking full advantage of PDEs. For him, as for his counterparts at Pratt & Whitney, focusing only on incremental improvements to conventional gas turbines at the dawn of the 21st century runs the risk of going the way of a buggy whip manufacturer at the beginning of the 20th. “People are always discovering new things,” he says. “You can’t be fixated on the buggy whip, and you can’t be fixated on the turbine engine.”
Jim Kelly lives in Galveston, Texas, where he writes about
science for the University of Texas Medical Branch.
RACING TO PULSE DETONATION
A division of General Electric and Pratt & Whitney-two titans of jet-engine design-are battling to develop the first operational pulse-detonation engine.
GE: $11.1 billion
P&W: $7.6 billion
GE: 1-A: First U.S. jet engine (1941); J93: First Mach 3 engine (1957); GE90-115B: World record for single-engine thrust: 127,900 lb. (2003)
P&W: J57: Powered first supersonic production aircraft, the F-100 Sabre fighter (1953); PW2000: First engine to use digital controls for maximum fuel efficiency (1984); F119-PW-100: Allowed supersonic cruise without afterburner, in F-22 Raptor (1997)
GE: Bench-scale experiments ($8 million invested since 1999)
P&W: Full-scale multi-chamber test engine ($20 million invested since 1993)
GE: Aerodynamic valve prevents airflow to the chamber at the appropriate time,
P&W: High-speed rotary valve cuts off airflow to each of five combustion chambers; a pre-detonator initiates the transition.
GE: Hybrid PDE prototype by 2005
P&W: Pure PDE missile prototype by 2005
GE: Hybrid engines for subsonic and supersonic aircraft
P&W: Pure PDE missiles, then hybrids in military and passenger jets
by Courtesy Pratt & Whitney