High-speed cameras help model new rocket type engine

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Researchers in the USA have tested an experimental rotating detonation engine in an effort to make them more stable and usable for space launches.

This new type of rocket engine could make rockets more fuel efficient, lightweight and less complicated to construct. now this engine is too unpredictable to be used in an actual rocket, claim the researchers.

The project at the at the University of Washington (UW), built an experimental rotating detonation engine and recorded the combustion inside the engine using a high-speed camera. Researchers then analyzed these images to develop a mathematical model that describes how the engine works and will be used to make more stable versions of it.

The research team published its findings in the journal Physical Review E last month.

“The rotating detonation engine field is still in its infancy. We have tons of data about these engines, but we don’t understand what is going on,” said lead author of the study, James Koch, a UW doctoral student in aeronautics and astronautics at UW. “I tried to recast our results by looking at pattern formations instead of asking an engineering question — such as how to get the highest performing engine — and then boom, it turned out that it works.

“A rotating detonation engine takes a different approach to how it combusts propellant,” Koch said. “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 that is moving faster than the speed of sound.

“This combustion process is literally a detonation — an explosion — but behind this initial start-up 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 so that it generates the work needed to propel the engine. But in a rotating detonation engine, the shock wave naturally does everything without needing additional help from engine parts.

“The combustion-driven shocks naturally compress the flow as they travel around the combustion chamber,” Koch said. “The downside of that is that these detonations have a mind of their own. Once you detonate something, it just goes. It’s so violent.”

To try to be able to describe how these engines work, the researchers first developed an experimental rotating detonation engine where they could control different parameters, such as the size of the gap between the cylinders. Then they recorded the combustion processes with a high-speed camera. Each experiment took only 0.5 seconds to complete, but the researchers recorded these experiments at 240,000 frames per second so they could see what was happening in slow motion.

Rotating detonation engine

The experimental rotating detonation engine has feed lines on the right directing the propellant flow into the engine. Sensors sticking out of the top of the engine on the left measure pressure along the length of the cylinder, while the camera is on the left-hand side, looking from the back end of the engine.
(Image: James Koch/University of Washington)

From there, researchers developed a mathematical model to mimic what they saw in the videos which has allowed the researchers to determine for the first time whether an engine of this type would be stable or unstable. It also allowed them to assess how well a specific engine was performing.

“My goal here was solely to reproduce the behavior of the pulses we saw — to make sure that the model output is similar to our experimental results,” Koch said. “I have identified the dominant physics and how they interplay. Now I can take what I’ve done here and make it quantitative. From there we can talk about how to make a better engine.”

 

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Ben has worked as a journalist and editor, covering almost all aspects of technology, engineering and industry for the last 20 years. Initially writing about subjects from nuclear submarines to autonomous cars to future design and manufacturing technologies, he was editor of a leading UK-based engineering magazine before eventually becoming editor of Aerospace Testing in 2017.

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