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Comment by obituary_latte | Hacker News Reader

Is there an ELI5 breakdown of this anywhere?

IANA Rocket Scientist, but I'll try.

Rather than have one big continuous burn, the rocket engine rotates to create many small burns, by mixing and igniting small amounts of two gases at just the right frequency and amounts. This apparently leads to a more efficient conversion of chemical to kinetic energy.

This sounds similar to how a car engine uses a fuel injector to inject then burn small amounts of fuel to make the pistons go.

What I understood from the article: it's not the engine that rotates.

Instead, the fuel mixture ignites in one spot, and the explosion spreads at Mach 5 around the engine but once it reaches the end of the combustible fuel, more is injected behind it so the explosion can just keep traveling around in circles.

Judging by the article they managed to get that working, and also came up with a better way to take videos of this happening (high speed camera, tracer chemicals in the hydrogen, better signal strength).

How does it eject this energy to propulse through space, if it's just spinning in circles inside the engine?

There used to be one, does anyone remember Rotary Rocket? They had a small vehicle they claimed could SSTO.

That was very different: conventional rockets for ground-to-orbit, but landing with folding helicopter-like rotors. I think the original concept was to use the helicopter rotors to get to high altitude before throttling up, which was supposed to save a lot of fuel, but that was dropped pretty early.

I think they actually wanted to use some rotating set of engines and fuel tanks early on with the rotation taking care of propelant flow.

IIRC they dropped that early on as well.

Thanks, I feel enlightened. You could write press releases :)

Here is a video from the Technical University of Berlin where the transition from ignition to the continuous circular detonation was also filmed among other things.

I'll link directly to the transition part:

https://youtu.be/ERTei7D8LJs?t=28

Thanks, but I feel there's more to the story. For instance, in car engines, there's a lot of attention paid to preventing detonation. And the article doesn't seem to provide any information about possible performance improvements involving numbers. This seems like a pattern with short articles about some supposed leap forward and I still find it annoying and frustrating although I expect it. I'd like to ask the author - why do you think it's groundbreaking?

Uncontrolled detonation or pre-ignition is a problem in ICE not because it creates less force, but rather because it creates more force and at the wrong time. If engines could be made strong enough to remain reliable while operating on detonation, and the pulses timed right, I imagine you would get more energy out of a given volume of fuel. This whole problem is avoided in a rocket combustion chamber without all the rotating parts

according to the article, it’s the _explosion_ that rotates around the engine, not the engine

« rotating detonation rocket engine»

« The rotating detonations are continuous, Mach 5 explosions that rotate around the inside of a rocket engine »

maybe like a wankel engine without the rotor?

My layman's attempt at understanding and explaining: a conventional rocket burns where the two fuels meet, whereas this one creates an unignited mix and then an ignition wave travels through that mix, apparently that's something that happens at mach 5. They build a circular explosive gas cloud that they replenish before the ignition wave comes through again. The big engineering achievement in this is to master precise high speed injection to the point where they always have a fresh cloud of pre-mixed explosive gas in place right before the explosion wave comes through.

I'm certainly no "Combustion and Flame" reader but if this new method is such a big deal and has been theorized since the 1960ies then I guess a burn of pre-mixed gas must be known to be even more violent than combustion at the mixing boundary.

Rather than deflagration combustion, this engine uses detonation combustion (supersonic flame front). Theeoretically this is much more efficient, but is extremely difficult to control.

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