We covered the rotating detonation engine on Orbital Index a few months back. The main conclusion was that, while more fuel efficient, the RDE is currently way too unpredictable for real use. Here's a cool video of the circular detonations though: https://www.youtube.com/watch?v=gEYFy3qRNdo
I was thinking that a solid state hydrogen/oxygen gas generator might be a safe way to store propellant, as it could exist in the inert, uncompressed form.
It looks like AlGalCo makes an aluminum-gallium alloy that produces hydrogen + aluminum oxide when mixed with water. This seems like it would be useless in space, because the oxygen is locked away until you run the waste through an aluminum smelter.
Hmm, I wasn't thinking of re-usability for the aluminum. Just that it would keep the propellants in a safe state. water+alloy might be better than nasty toxic fluid + pressurized oxidizer. I am also not a rocket scientist.
It isn’t a question of reusable type - you need to have fuel and oxidizer to burn things. A truck can use atmospheric oxygen for the oxidizer; a rocket can’t.
I also imagine that the extra weight of the aluminum pucks would tank your Isp (though I suppose you might be able to make back some tank weight).
What is the maximum increase in fuel efficiency (e.g., ISP) with such a design? I was under the impression that modern chemical rocket engines operated near their thermodynamic limits. If that's the wrong question, what's the right one?
I'm not sure what's the news here are (H_2 + O_2 instead of H_2 + air?) but if you want to see experimental continuous detonation thrusters in action, just search for videos on Youtube.
I posted this elsewhere in the comments already but to save your time looking for it, here it is again: a video of ignition and transition to continuous, circular moving detonation in such a thruster: https://youtu.be/ERTei7D8LJs?t=28
One thought: this was done in a laboratory setting where they were able to statically fine-tune the flow rates. In an actual rocket, it seems plausible that external forces such as the acceleration of the rocket, changing force from gravity, etc. could significantly affect the propagation of the detonation waves, so the flow rates would need to be dynamically adjusted in real-time (a non-trivial problem). Still, this is definitely progress!
Interesting. As I understand it, deflagration is usually used in rocket engines because detonation is hard to control, but they’ve managed to control it using very precise engineering.
I guess one question is, can they maintain that precise control over a long enough period of time, without the explosions degrading the injection apertures, to really use the technology.
At the fuel flows they are using, the explosions aren't that much of an issue. These look like wisps of hydrogen and oxygen in a vacuum chamber, not a big jet of fire steaming out of a nozzle. The detonation wave doesn't look like it is contacting much of anything.
They have a sustained rotating ring, a detonation chasing itself around a circle. That's great, its more efficient, but it is a long way from a working engine. They need a properly shaped chamber, a nozzle, and a few other things before they can test for the expected increase in ISP.
Well, minus the pulse, which would be great for air-breathing engines too.
If you made a ring of pulse-detonation engines that fired one after another in sequence, you'd almost have such a thing. Connect the tubes side-to-side if you can, merging them into a coaxial detonation chamber with a helical detonation wave that rotates. (looking almost like an annular aerospike engine but leaving a helical exhaust trail) You probably need to run with more than one helix in the engine to avoid disastrous vibration.
There is a comment at the bottom of the article from an expert in the Air Force:
> William Hargus, lead of the Air Force Research Laboratory's Rotating Detonation Rocket Engine Program, is a co-author of the study and began working with Ahmed on the project last summer.
> "As an advanced propulsion spectroscopist, I recognized some of the unique challenges in the observation of hydrogen-detonation structures," Hargus said. "After consulting with Professor Ahmed, we were able to formulate a slightly modified experimental apparatus that significantly increased the relevant signal strength."
> "These research results already are having repercussions across the international research community," Hargus said. "Several projects are now re-examining hydrogen detonation combustion within rotating detonation rocket engines because of these results. I am very proud to be associated with this high-quality research."
It's a 'brief communication' of three pages, so not many details. Most interesting thing is the citations, one of which is to the _much_ more enlightening 'Rotating Detonation Rocket Engine' tech readiness report from the Air Force in 2018:
The question I have after reading this press release is, "Why was it previously thought to be impossible?" as stated in the beginning of the article.
Further down, the release seems to indicate lots of people thought it was possible and have been studying it, "The technology has been studied since the 1960s but had not been successful due to the chemical propellants used or the ways they were mixed."
So just an engineering problem, which are seldom impossible.
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).
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.
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.
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
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.
That's about 2-2.5 km/s. Compare this with the exhaust velocity of the main engines of the Space Shuttle, 4.4 km/s. It's not clear why this is considered to be groundbreaking.
> the exhaust velocity of the main engines of the Space Shuttle, 4.4 km/s. It's not clear why this is considered to be groundbreaking.
And ion thrusters have an exhaust velocity in the 20-50 km/s range. Things can be groundbreaking for different reasons.
But in the case of this article, I dont think the velocity mentioned is the exhaust velocity. It says the "rotating detonations are continuous, Mach 5 explosions". This is likely the flame front speed inside the combustion chamber (a traditional rocket engine has subsonic flame front, ie conflagration not detonation). The exhaust velocity is a different measurement, after the nozzle/bell (converts thermal to kinetic energy). Here [1] for more details.
I'm interested to hear more about the possibilities this combustion chamber design brings up.
> And ion thrusters have an exhaust velocity in the 20-50 km/s range
I compared with the Space Shuttle main engine because they are both hydrogen-oxygen engines.
As for the speed they mention, it's not clear why they focus on that one. At the end of the day, there are only 3 things that matter for a rocket engine: exhaust velocity, how fast it can burn the fuel, and the mass of the engine. You want the highest exhaust velocity, and the lowest mass of the engine. For the upper stages, you don't need to be able to burn fuel that fast, but you wouldn't mind that either.
All other things are implementation details. Now, the Space Shuttle main engine achieved 86% of the maximum theoretical exhaust velocity [1]. If this innovation here managed to increase this to 90% or more, that would be quite interesting, but hardly groundbreaking if it adds substantial mass to the engine.
> I'm interested to hear more about the possibilities this combustion chamber design brings up.
I would be interested too. This article was quite unilluminating unfortunately.
> At the end of the day, there are only 3 things that matter for a rocket engine: exhaust velocity, how fast it can burn the fuel, and the mass of the engine.
While those are primary factors, and a rocket won't go up if they aren't good enough, they aren't the only ones (or else every rocket engine would be hydrolox staged combustion, like the SSMEs). Other key factors are engine cost (SSME does terrible here), fuel type (hydrolox needs way larger diameter boosters, isn't worth it for first stage), combustion stability (allows engines to throttle, key for max-Q and reducing max g as stage empties, especially on upper stage), reusability (important for some systems), controllability (how quickly it can throttle)... While their importance gradually diminishes, there is a list hundreds long of different trade-offs made in engine development. The reason I said I'd like to know more is indeed because that article wasn't informative. I certainly am not writing it off because it can't be plopped in a rocket right now
Thermodynamic efficiency matters. And detonation is highly efficient, because it's an (almost) combustion isochoric process. The shockwave/detonation front travels at far higher speeds than the movement/speed-of-sound in both the unburnt input mix, as well as the combustion products.
My understanding is that it's due to the combustion performing all it's work before any work/energy is extracted from the hot combustion products. This results in a higher peak temperature, and the higher the temperature difference, the higher the theoretical efficiency limit of converting the thermal energy into mechanical (or equivalent) energy. It's why diesel engines have a lower fuel consumption than gasoline engines: they burn hotter.
Maybe the speed mentioned here - ~5000 mph - is the speed of detonation wave, not the speed of exhaust gases? Theoretically the latter should surpass the Shuttle's Isp.
> This system improves rocket-engine efficiency so that more power is generated while using less fuel than traditional rocket energies, thus lightening the rocket's load and reducing its costs and emissions.
well, I was testing something on s
a clone version of a game and end up publishing that version to the app store. When I uploaded the game on my phone, my progress ware gone, all my saves. the colone version created a new database (thank God without deleting the old one). my game only had like 1k installations, and I found out my mess quick enough. just published a new update with the original database name and was lost.
And the actual paper: https://journals.aps.org/pre/abstract/10.1103/PhysRevE.101.0...
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