With jet fuel prices now three times as high as they were a decade ago, fuel economy is one of the chief motives behind the new interest in the pulse detonation engine. In theory, a PDE powering a next-generation aircraft could operate nearly as simply as the pulse jet, while being relatively cheap to build and vastly more efficient.
But simple on paper and simple in the real world are different things. The physics of this seemingly simple thermodynamic cycle prove to be exceedingly complex. Researchers at NASA, the California Institute of Technology, Pennsylvania State University, and Ohio State University have all struggled to accurately model the turbulent flow of air and fuel in a detonation chamber, the shock wave’s interaction with the chamber walls, and how those processes affect the speed of combustion.
There are engineering challenges too. Enabling just the right fuel-air mixture to appear at precisely the right time and at high frequencies—20, 40, or even 80 cycles per second—calls for control schemes and valves of the highest order of complexity. And don’t forget the most basic problem: Substituting detonations for the relatively gentle deflagration found in typical jet engines requires that the detonation tube be exceptionally strong. To be suitable for aircraft, the strong materials must also be lightweight, two characteristics that rarely go together. But the world’s leading engine makers think they’re up for the challenge to perfect the PDE.
IN THE 1970s, the annual contest between GE and Pratt & Whitney to win the lion’s share of U.S. Air Force fighter engine orders became known both inside the companies and at the Pentagon as the Great Engine War. In PDE research, their rivalry is no less fierce, and by many measures, P&W beat GE to the punch.
Pratt & Whitney jumped into the modern phase of PDE work relatively early, in the mid-1990s, when it worked on U.S. Navy projects as part of a Boeing team looking at new ways to power high-speed missiles launched from ships. The engine maker worked closely with a small company in Seattle widely acknowledged to be a modern-day pulse detonation pioneer, a startup called Adroit Systems. A few top Pratt & Whitney executives saw the future in the plucky company and its entrepreneurial founder, a former National Aerospace Plane program engineer named Tom Bussing. Bussing was a refugee from Boeing, and he explains the story of Adroit’s birth with an anecdote about Boeing’s Alan Mulally, then the president of Boeing Commerical Airplanes. (Mulally left Boeing last year to head Ford Motor Company.) When the NASP program was cancelled, Bussing says Mulally told him it would be more than a decade before Bussing would get a chance to manage a large-scale program. A short time later, Bussing left to form Adroit. A few years and several patents later, Pratt & Whitney bought it, rechristening it the Pratt & Whitney Seattle Aerosciences Center.
Pratt & Whitney has focused on valves as its approach to regulating the complex detonation sequence. The company’s five-tube test engine has a valve, patented by Bussing, that can rotate at 2,400 revolutions per minute to rapidly mix air and fuel, yielding 400 detonations per second. The valve isn’t the company’s only advance in PDE research. In 2003, Bussing and his colleagues fired an advanced pulse detonation engine on a rig at the Navy’s China Lake test center in California. Sim Austin, who heads Pratt & Whitney’s military engine special projects office, notes that last year “we demonstrated…how we could use PDE with fossil fuels”—military-grade JP8 or JP10 jet fuel—“without supplemental oxygen.” That was a key advance, made possible in part through work funded by NASA and the U.S. Office of Naval Research.
GE had a lot of catching up to do. Its answer to Pratt & Whitney’s advanced Seattle unit sits peacefully overlooking the Mohawk River in the bucolic upstate New York town of Niskayuna, between Schenectady and Albany. The campus luxuriates across 525 acres of rolling land, and, tucked in among comfortable homes and suburban cul-de-sacs, you can be forgiven for wondering whether the future really is taking shape anywhere near here. But it is.
“We are turbine-oriented, trying to go for the big prize, and doing it with liquid fuels,” says Anthony Dean, who heads the Propulsion Systems Laboratory at GE’s Global Research Center. An avid cross-country skier, Dean is the picture of a modern scientist who has left the pocket-protector stereotype behind.
Dean, like his company, came to the PDE party a bit late, but has worked hard to make up for lost time. GE, with financial interests in dozens of businesses ranging from appliances to locomotives and credit cards, has designated the pulse detonation engine as one of only six long-term technology areas meriting continued corporate research and development funding; it’s on a par with such hot areas as biotechnology and nanotechnology. With U.S. government money drying up for pulse detonation, that’s a good thing.
A lot of research programs in the field have occurred during the early part of this decade, Dean says, but “those government programs were four or five years, and they’re ending now.” The Navy’s Office of Naval Research, for example, had a multi-university research initiative which ended last year. Though there’s a lull in U.S. government funding, he says, “there’s still funding going on in Japan and a little bit in Russia.”
And there’s also funding at GE. “We’ve actually grown the program a little bit this year,” Dean says. He’s reluctant to give his competitors at Pratt & Whitney any hints of what that means in researchers and resources—“They might write that down and figure out what we’re doing”—but he’ll allow that GE’s PDE program is up “almost 50 percent” from 2006.
Those resources won’t be spent trying to plow the same ground already turned over in Seattle. “GE started eight years ago,” Dean explains, but “really actively started an advanced pulse detonation technology program” —the effort he now leads—“about five years ago. Most of our competition was focused on systems that use oxygen to begin detonation.” But because “you don’t want to carry extra oxygen,” he says, his team is focusing on avoiding that.