Japan has successfully tested a rotary detonation engine in space

The basic concept of the rotating detonation engine (RDE) is so simple that it was born in the 1950s. RDE as a concept was first conceived at the University of Michigan in the 1950s, and is based on the idea that, unlike conventional internal combustion engines, the fuel is not burned but exploded in a circular chamber where the shock wave from the explosion travels along, providing propulsion while triggering another explosion – in other words, the process is self-sustaining until the fuel is used up.

The advantage of the idea is that much more thrust can be squeezed out of much less fuel (we’re talking about savings of around 25 per cent), while the disadvantage is that the explosion is a much more difficult process to control than combustion. The concept is also reminiscent of the pulse detonation engine (PDE), but better in that the combustion chamber has to be cleaned before each explosion, whereas with RDE it is a self-sustaining, constantly running process.

Unfortunately, however, there is sometimes a painfully wide gap between theory and practice, and despite decades of attempts to build such engines, it turns out that sustaining self-sustaining, continuous detonations is so difficult that around 2020, US rocket scientists publicly declared that the practical implementation of similar engines based on hydrogen-oxygen detonation was impossible. It didn’t take much longer than a few months for scientists at the University of Central Florida’s Air Force Research Laboratory to achieve the impossible: they built a prototype that worked by continuously detonating hydrogen and oxygen.

The prototype study also meant a renewed interest in this “impossible engine”, according to scientists who spoke to New Atlas. And the development of the RDE has reached another milestone after the 2020 milestone, with the Japan Aerospace Exploration Agency (JAXA) announcing on 19 August that it had successfully tested such an engine, not just in the lab but in space. One of the most important applications of the RDE would be in space travel/exploration, where it is important to provide the most thrust with the least amount of fuel.

JAXA has installed the system on an S-520-31 rocket capable of delivering 100 kg of cargo to an altitude of 300 km. The launch took place at the Uchinoura Space Centre on 27 July. The rocket launched the test after the first stage separation, when the RDE operated for 6 seconds, and after the device was lifted out of the ocean after re-entry, it was found that the RDE had developed a thrust of 500 Newtons. The researchers say they hope to see practical results from this experiment within 5 years. The US Air Force is also considering the practical use of the RDE in the same timeframe, and hopes to conduct a missile test with the engine by 2025.

The main use of RDE is therefore in space research, but it can also be used on the ground. In 2012, the Naval Research Laboratory estimated that replacing gas turbines on the largest ships with RDE could save 15-20% of fuel costs per year. But RDE could also be used for hypersonic and supersonic flight, and even power generation.