Pulsejets (which could have been called pulse deflagration engines) rely on pulsed deflagration to generate energy and thrust. Their pulsed mode makes them hideously loud and rather inefficient, but they can be quite simple to build - they can even have no moving parts.

Pulse detonation engines work in a rather similar way, but rely on pulsed detonation instead of deflagration. It makes them much harder to build (and possibly even louder), but theoretically much more efficient.

Rotating detonation engines are a further development. The pulsed regime being source of inefficiencies, vibrations and noise (so much noise), it is replaced by continuous detonation fronts endlessly going around an annular chamber, with fuel and oxidiser injected behind each front, for the next one.

Is there such a thing as a rotating deflagration engine? Is it theoretically feasible?

The advantage appears obvious at first glance: avoid the inefficiency of pulsed mode from a pulsejet, while avoiding the complexity of using detonation for more manageable (if less efficient) deflagrations. One could also hope to keep some of the simplicity of pulsejets, with fewer moving parts and air being aspired in the chamber as part of the cycle.

However, I couldn't find any reference to such an engine, whether attempts to build it, studies, or even description of why it wouldn't actually work.

Note: this is not about turbines or rocket engines. Those are technically continuous deflagration engines, but there is continuous pumping of fuel and oxidiser (air from the compressor in the case of the turbine), and a stationary burn zone. A rotating deflagration engine would have moving deflagration fronts with fuel and oxidizer (like air) injected after a deflagration front moves away.

  • $\begingroup$ Isn't a common gas turbine a continuous deflagration engine? The flame fronts don't rotate, though, nor in a ramjet. $\endgroup$ – Zeiss Ikon Jul 17 '19 at 13:16
  • $\begingroup$ @ZeissIkon Like the rocket, it is a continuous deflagration engine, but not a rotating deflagration engine as meant here: you are pumping fuel and oxidizer (air from the turbine) and have them burn stationary, instead of having a front circling an annular chamber, with injection right after it has passed. Edited the question to add the precision. $\endgroup$ – Eth Jul 17 '19 at 13:25
  • $\begingroup$ How would you extract the energy from the hot gas without pistons or a turbine? $\endgroup$ – Zeiss Ikon Jul 17 '19 at 15:00
  • $\begingroup$ @ZeissIkon The pulsejet (especially valveless) is an interesting example for that. According to the explanations there, the air itself acts as a piston. That said, rotating detonation engines can use a compressor for air intake and be used in a jet engine (or alternatively as a rocket with both stored fuel and oxidizer), so it is not incompatible. Of course, it would be more interesting if it was possible to design a rotating deflagration engine without a turbine (or even with no moving part, while we're at it), but it may not be possible. $\endgroup$ – Eth Jul 17 '19 at 15:28

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