University of Washington researchers are at work on a simpler fuel efficient rocket engine that could be cheaper and enable lighter spacecraft. The researchers have developed a mathematical model that describes how rotating detonation engines work.

It takes a lot of fuel to launch something into space. Sending NASA’s Space Shuttle into orbit required more than 3.5 million pounds of fuel.

But a new type of engine – called a rotating detonation engine – promises to make rockets not only more fuel-efficient but also more lightweight and less complicated to construct. There’s just one problem: Right now this engine is too unpredictable to be used in an actual rocket.

Section view of the rotating detonation engine (RDE) used for this study. The engine geometry is such that gaseous methane and oxygen are directed into a narrow annular gap through a set of propellant injectors. A spark plug ignites the mixture, which rapidly transitions to a number of circumferentially traveling detonation waves. Image Credit: Credit: Koch et al./Physical Review E. Click image for the largest view.

Researchers at the University of Washington have developed a mathematical model that describes how these engines work. With this information, engineers can, for the first time, develop tests to improve these engines and make them more stable. The team published these findings in Physical Review E.

Lead author James Koch, a UW doctoral student in aeronautics and astronautics starts us with, “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. 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 conventional rocket engine works by burning propellant and then pushing 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 explained. “Its 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 rocket 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.

Koch noted the situation with, “The combustion-driven shocks naturally compress the flow as they travel around the combustion chamber. 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 a 0.5 second to complete, but the researchers recorded these experiments at 240,000 frames per second so they could see what was happening in slow motion.

The researchers developed an experimental rotating detonation engine (shown here) where they could control different parameters, such as the size of the gap between the cylinders. The feed lines (right) direct the propellant flow into the engine. On the inside, there is another cylinder concentric to the outside piece. Sensors sticking out of the top of the engine (left) measure pressure along the length of the cylinder. The camera would be on the left-hand side, looking from the back end of the engine. Image Credit: James Koch/University of Washington. Click image for the largest view.

From there, the researchers developed a mathematical model to mimic what they saw in the videos.

“This is the only model in the literature currently capable of describing the diverse and complex dynamics of these rotating detonation engines that we observe in experiments,” said co-author J. Nathan Kutz, a UW professor of applied mathematics.

The model 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.

Co-author Carl Knowlen, a UW research associate professor in aeronautics and astronautics offers this view, “This new approach is different from conventional wisdom in the field, and its broad applications and new insights were a complete surprise to me.”

Right now the model is not quite ready for engineers to use.

Koch said, “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.”

Mitsuru Kurosaka, a UW professor of aeronautics and astronautics, is also a co-author on this paper. This research was funded by the U.S. Air Force Office of Scientific Research and the Office of Naval Research.

This promises to be quite an improvement in rocket engines, the vehicles and the payloads. One could expect some reduction in costs as well to lift materials into orbit. There will likely be some other applications and the limit might not apply to rocket engine combustion as well. This is going to be very interesting indeed.

Of other interest is the serendipity of the research team having the brain storm to have a different look instead of conventional thinking. For that, the Congratulations are more than earned, they are deserved.


2 Comments so far

  1. matt Musson on February 28, 2020 7:45 AM

    A semi-internal combustion rocket engine.

  2. Brian Westenhaus on February 28, 2020 7:50 AM

    One might think this research goes much further than orbital launches. Mil backed, suggests missiles, might be an answer to opponents speed efforts.

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