After Combustion: Detonation!

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.