Son of a Buzz Bomb
An engine with a checkered past is the power of the future.
- By Jim Mathews
- Air & Space magazine, September 2007
GE Propulsion Systems Lab
(Page 2 of 5)
The PDE takes this idea one step further. The slow, gentle flicker of candles at dinner and the explosions inside the pulse jet share one characteristic: They burn subsonically, in a process scientists call deflagration. The process happens faster in an explosion than it does atop the candle, but nonetheless the chemistry takes place at speeds of, at most, tens of feet per second.
By contrast, detonation—the “D” in PDE—happens supersonically, with combustion occurring all at once. Detonation of a fuel-air mixture produces a high-pressure shock wave that speeds down the length of the tube at faster than five times the speed of sound. The shock wave itself creates a compression that instantly ignites fuel and air in the rest of the tube. As the exhaust gases exit from the tube, the pressure at the forward end drops. That pressure drop sets the stage for the cycle to repeat, dozens of times per second. The process is called pressure-rise combustion, and it’s a drastically different, higher-energy affair than simply burning a fuel-air mixture with a flame.
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.