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 4 of 5)
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).
That five percent in fuel savings is the tantalizing prospect that drives Dean and his colleagues, as well as his rivals in Seattle and at Pratt & Whitney’s home base in East Hartford, Connecticut. The two engine makers, along with the third major manufacturer, British-based Rolls-Royce, all see the end of the jet engine as we know it. To be sure, there’s debate about the pace of that change. Pratt & Whitney’s Austin cautions that no one should be ready to count out the modern aircraft gas turbine engine just yet. He believes that there are still efficiencies to be gained in how the engines are fitted to and optimized for air vehicles, and in how the components go together.
But most engineers agree that while it may not happen today, or tomorrow, or even next year, it’s coming as surely as the automobile put the horse-drawn buggy out of business. And that’s why they’re all working so hard to find ways to bring pulse detonation technology out of the realm of scientific papers and into the working world.