Experimental rotating detonation engine. Feed lines (right) direct the propellant flow; inside is another concentric cylinder. Sensors on top (left) measure pressure along the length of the cylinder.
James Koch/University of Washington

A rotating detonation engine could make rockets more fuel-efficient, lightweight, and less complicated to construct. However, this engine technology is too unpredictable to be used in an actual rocket. A team of University of Washington (UW) researchers hopes to overcome that challenge with their newly developed mathematical model that could give engineers the ability to develop tests to improve these engines and make them more stable. The team published its findings Jan. 10, 2020, in Physical Review E.

“We have tons of data about these engines, but we don’t understand what is going on,” says lead author James Koch, a UW doctoral student in aeronautics and astronautics.

A conventional rocket engine burns propellant and then pushes it out of the back of the engine to create thrust.

“A rotating detonation engine takes a different approach to how it combusts propellant,” Koch says. “It’s made of concentric cylinders. Propellant flows in the gap between the cylinders, and, after ignition, the rapid heat release forms a shock wave, a strong pulse of gas with significantly higher pressure and temperature moving faster than the speed of sound.

“This combustion process is literally a detonation – an explosion – but behind this initial startup phase, we see a number of stable combustion pulses form that continue to consume available propellant. This produces high pressure and temperature that drives exhaust out the back of the engine at high speeds, which can generate thrust.”

Conventional engines use a lot of machinery to direct and control the combustion reaction. In a rotating detonation engine, the shock wave does everything naturally without additional help from engine parts.

“The combustion-driven shocks naturally compress the flow as they travel around the combustion chamber,” Koch says. “The downside is these detonations have a mind of their own.”

To describe how these engines work, researchers developed an experimental rotating detonation engine where they could control different parameters, such as the size of the gap between the cylinders. They recorded the combustion processes with a high-speed camera, recording 240,000 frames per second for each experiment that took only 0.5 seconds to complete. After examining what was happening with each combustion, researchers developed a mathematical model to mimic what they saw in the videos.

Koch says, “I have identified the dominant physics and how they interplay. Now I can make it quantitative. From there we can talk about how to make a better engine.”

The research was funded by the U.S. Air Force Office of Scientific Research and the Office of Naval Research.

University of Washington