Rotary ramjet is an engine that uses ramjets attached to a shaft to allow them to operate at the supersonic speeds they require without the engine itself being in translational motion. The huge advantege of such an engine is that it essentially contains a single moving part instead of thousands. I've seen patents for using deflectors at the nozzle to direct some of the exhaust along the shaft for propulsion. Why is that they are not currently replacing turbojets?
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$\begingroup$ To whoever VTC as "opinion based", there is nothing opinion-based about the fact that this is unworkable as a mechanical engineering design - in my opinion, of course :) $\endgroup$– alephzeroCommented Feb 24, 2019 at 1:49
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2 Answers
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The reason you haven't seen one yet is because any mechanical engineer who has worked on rotating structures would take one look at this design and fall about laughing.
Werner J.A. Dahm, Andrew P. Lapsa and Peter E. Hamlington, "Inside-Out Rotary Ramjet Turbogenerator," AIAA 2006-4169, claims
The inside-out design has the advantage of placing the ramjet flowpath material on the inside rim of the rotor under purely compressive centrifugal loading.
Sorry, but that claim is pure fantasy, and the paper doesn't even attempt to explain why the authors think it is correct. If the authors don't understand the most elementary ideas about stress fields in rotating structures, it's not too surprising if their ideas aren't taken very seriously as a practical design.
If somebody wants to construct an "inside-out" rotor with a supersonic rim speed, well, good luck with that. Maybe they could try making the entire machine out of a single artificial diamond, or something similar. Even then, they would have to run the rotor inside a vacuum tank, or the external windage would absorb most of the generated power producing a vortex trail extending many miles behind the aircraft, but zero thrust.
Any conventional structural material would simply explode before it got up to its working rotation speed. Even if it did work, any structural failure would be catastrophic, not only for the engine but for the entire plane as well. It would be impossible to design a containment system for such a rotor that was light enough to be useful in practice.
To be fair to the authors, after a few decades of working on aircraft engine design, this sort of thing doesn't really surprise me, though. I've seen dozens of "fantasy engines" invented by aerodynamicists which were no more impractical than this one.
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In addition to alephzero's points, here is another:
We take the case of a fan with ramjets on its blade tips. By some clever contrivance we feed fuel and ignition out to each ramjet, spin it up to speed and ignite the ramjets.
A brief flow perturbation induces a flameout on one of the ramjets. The system is now severely out of dynamic balance and we need a method of restarting the dead jet before the entire device tears itself to pieces. This is a nontrivial problem.
answered Feb 24, 2019 at 6:29