Son of a Buzz Bomb

An engine with a checkered past is the power of the future.

A pulse detonation engine, fueled by ethylene and air, fires on a test stand at a General Electric research center. (GE Propulsion Systems Lab)
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“In addition, there was no work around what happens when you try to combine this type of combustion with a turbine engine,” he adds. “At GE, we felt the real end goal was engines that were more efficient than today’s,” a concept in which pulse detonation technology inserted into an airplane’s turbine engine might make more sense.

When GE started looking seriously at pulse detonation engines, there was a lot of good theoretical work, particularly from Caltech, but it was of the single-shot variety: Fill a chamber, detonate it, and see what happens. Dean readily credits those engineers with doing “good science, but it didn’t give you a sense of the engineering challenges.”
Working with NASA, GE combined its PDE test rig with a large axial turbine pulled from a locomotive. Why not an aircraft turbine? The train engine “had the size we wanted, the flow we wanted, and for cost reasons,” Dean says. He and his team ran several configurations, and operated the machine with a test sequence of more than five minutes rather than just a few seconds. The long runs enabled the engine to reach a steady state of operation, “and to my knowledge, certainly with a turbine connected, that’s the first time that’s been done,” Dean says. GE’s rig had eight tubes for pulse detonation, each of which ran at about 20 to 25 cycles per second. “We got a million pressure cycles,” he adds.

It’s a bit of a journey, however, from a PDE rig bolted to a railroad engine to a flyable propulsion system.

“In five years, you could have a flight-weight demonstrator,” Dean says, noting that if the government signaled its commitment with additional funding, “I think GE would pony up its resources, and the other guys would too.” At the beginning of the decade, some scientists said there could be a flying demonstrator by 2010, but “no way are we going to make 2010 at the current level of investment,” he says. “This type of stuff requires a lot of effort, a lot of money. Even GE, a big company, will only go just so far ahead of its customers. We’ll only go so far ahead of where the government is… In fact, [the government] is pulling back on its own internal effort at NASA labs.”
But is it? It’s not a stretch to believe that PDE technology is ready enough to submerge itself into the murky world of what the Pentagon calls “black programs.” These are programs that don’t exist—at least not publicly—and yet they do. The bat-wing B-2 stealth bomber, for example, was a black program, as was the stealthy F-117 Nighthawk. The closest near-term application for a pulse detonation engine was a proposed high-speed missile, and the missile remains the most likely place for a PDE to emerge first, military officials and researchers agree. The U.S. military budget shows money earmarked for such umbrella programs as “Propulsion Technology Initiatives,” and in-house funding continues for propulsion research led by the Office of Naval Research and the Air Force Research Laboratory. And the big players like GE and Pratt & Whitney still continue to put as much of their own money in as they can, without “getting ahead” of their government customers. That suggests that what the companies are spending on PDE research is in line with what their government sponsors expect it to be.

Gary Lidstone, a colleague of Austin’s at Pratt & Whitney’s Seattle unit and the division manager there, says that meanwhile, “all of us are still working independently to garner funds for the technology development.” Austin says he and others are “working that issue” with the officials at the government labs who write the checks, hoping to get some early funding later this year or early in 2008. A lot of smart people are betting that the money will come from the missile world, especially given the technology-readiness gap between the PDE for missiles and the more complex concept for a hybrid commercial aircraft engine.

NASA, the U.S. government, and lots of tech companies worldwide use a numeric scale to rank the risk or readiness of technologies. The scale starts at 1 for the lowest level of technology readiness and climbs to 9 for fully operational. Lidstone says the engineering community figures the readiness level for PDE hybrid commercial aircraft engines is “in the two or three range,” while for the “missile activity, it’s three or four.” In Pentagon parlance, “three or four” means you’ve tested all the pieces together in a lab to see if they work; five takes those tests to a more realistic, operational setting. Lidstone says that under his team’s development plan, the first use of PDE technology is probably a “small-scale, high-speed missile
The pace of current research and development points the way to three phases of pulse detonation engine technology, each a bit more complex than the one preceding it.

The first phase could be called the “pure PDE”: Essentially it focuses on developing the detonation tube, which would power a very-high-speed, air-breathing missile. In this application, engineers and scientists can punt on two of the biggest technology problems—life, or the durability of the system, and noise. The missile has to fly only once, so long life for the metals or components is not a concern. And at the high speeds—around Mach 6—and altitudes in which the missile would operate, less noise is also moot. This is the area in which Adroit Systems, and later Pratt & Whitney, made the most strides. It was their machine that would have been flown on NASA’s F-15B.

The next phase could involve using pulse detonation engines to address another pressing issue in combustion: afterburners for fighter aircraft. Today’s fighter engines simply spray aerosolized fuel into a long tube aft of the turbine section, literally dumping extra fuel-air mixture into the hot gas stream for a brief extra kick of speed. Engineers think that if they add pulse detonation technology to a low-bypass-ratio turbine engine—the modern fighter jet engine—they can get the efficiency benefit of pressurized, shockwave combustion. It’s relatively simple because the pulse detonation tube would be at the end of the engine and not in the middle of the turbo-machinery. Here again, life and noise are less of an issue than they might be in a commercial aircraft. Fighter pilots only fly on afterburner about five percent of the time, and anyone who has seen an airshow knows fighter jocks usually don’t worry about making a racket.

The third phase is where it gets most complicated, but is the one that may offer the biggest payoff: pulse detonation in the middle of the engine. Having a compressor upstream and a turbine downstream, says GE’s Dean, is a potential high-value payoff that keeps his company attracted to PDE development. A PDE-based combustor is one of the main areas of work for a young researcher on Dean’s team named Adam Rasheed. Rasheed is chronicling his work on a publicly available blog, “From Edison’s Desk” (Massachusetts Institute of Technology’s Technology Review magazine in 2005 named Rasheed one of the world’s top 35 researchers under the age of 35.

Like everyone else, Rasheed has his eyes on a jet engine that burns five percent less fuel—an enormous leap compared with today’s fuel-saving techniques. He suggests in his blog that after 50 years of tweaking, aeronautical engineers may be close to wringing out the very last ounce of performance from today’s jet aircraft engines. In a world in which efficiency improvements of even 0.2 percent are considered a major breakthrough, “PDEs represent a possible game-changing technology that could revolutionize aerospace propulsion,” Rasheed writes. Even a one percent improvement would save hundreds of millions of dollars in fuel. And by reducing the amount of fuel they burn, PDEs produce fewer emissions and gases, making for a greener propulsion technology (see “Fly Green,” September 2007).

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