Sending NASA’s Space Shuttle into orbit required more than 3.5 million pounds of fuel, which is about 15 times heavier than a blue whale. 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. The engine, however, is too unpredictable to be used in an actual rocket.
Researchers have developed a mathematical model that describes how these engines work. With this information, engineers can develop tests to improve these engines and make them more stable.
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. 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 startup phase, a number of stable combustion pulses form that continue to consume available propellant. This produces high pressure and temperature that drive 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.
To describe how these engines work, the researchers first developed an experimental rotating detonation engine in which 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 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. From there, they developed a mathematical model to mimic what they saw in the videos. The model allowed them to determine whether an engine of this type would be stable or unstable and allowed them to assess how well a specific engine was performing.