Isegoria

The concept behind rotation detonation engines dates back to the 1950s:

In the United States, Arthur Nicholls, a professor emeritus of aerospace engineering at the University of Michigan, was among the first to attempt to develop a working RDE design.

In some ways, a Rotating Detonation Engine is an extension of the concept behind pulse detonation engines (PDEs), which are, in themselves, an extension of pulsejets. That might seem confusing (and maybe it is), but we’ll break it down.

Pulsejet engines work by mixing air and fuel within a combustion chamber and then igniting the mixture to fire out of a nozzle in rapid pulses, rather than under consistent combustion like you might find in other jet engines.

In pulsejet engines, as in nearly all combustion engines, igniting and burning the air/fuel mixture is called deflagration, which basically means heating a substance until it burns away rapidly, but at subsonic speeds.

A pulse detonation engine works similarly, but instead of leveraging deflagration, it uses detonation. At a fundamental level, detonation is a lot like it sounds: an explosion.

While deflagration speaks to the ignition and subsonic burning of the air/fuel mixture, detonation is supersonic. When the air and fuel are mixed in a pulse detonation engine, they’re ignited, creating deflagration like in any other combustion engine. However, within the longer exhaust tube, a powerful pressure wave compresses the unburnt fuel ahead of the ignition, heating it above ignition temperature in what is known as the deflagration-to-detonation transition (DDT). In other words, rather than burning through the fuel rapidly, it detonates, producing more thrust from the same amount of fuel; an explosion, rather than a rapid burn.

The detonations still occur in pulses, like in a pulsejet, but a pulse detonation engine is capable of propelling a vehicle to higher speeds, believed to be around Mach 5. Because detonation releases more energy than deflagration, detonation engines are more efficient — producing more thrust with less fuel, allowing for lighter loads and greater ranges.

The detonation shockwave travels significantly faster than the deflagration wave leveraged by today’s jet engines, Trimble explained: up to 2,000 meters per second (4,475 miles per hour) compared to 10 meters per second from deflagration.

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A rotating detonation engine takes this concept to the next level. Rather than having the detonation wave travel out the back of the aircraft as propulsion, it travels around a circular channel within the engine itself.

Fuel and oxidizers are added to the channel through small holes, which are then struck and ignited by the rapidly circling detonation wave. The result is an engine that produces continuous thrust, rather than thrust in pulses, while still offering the improved efficiency of a detonation engine. Many rotation detonation engines have more than one detonation wave circling the chamber at the same time.

As Trimble explains, RDEs see pressure increase during detonation, whereas traditional jet engines see a total pressure loss during combustion, offering greater efficiency. In fact, rotation detonation engines are even more efficient than pulse detonation engines, which need the combustion chamber to be purged and refilled for each pulse.

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According to the Air Force Research Lab, RDE technology could make high-speed weapons much more affordable, which is of particular import following a recent Defense Department analysis that indicated the hypersonic (Mach 5+) weapons in development for the Air Force may cost as much as $106 million each.

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