Rotating Detonation Engines

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bobbymike

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https://aerospaceamerica.aiaa.org/departments/increasing-engine-efficiency/


A turbine engine must generate a uniform flow of combustion gases to spin its blades without excessive wear or risk of damage. A conventional turbine engine avoids damaging spikes in temperature and pressure by allowing the volume of gas to expand, which encourages even deflagration, the term for the rapid burning of the fuel-air mixture as it’s sprayed or injected into the combustor. The resulting stream of combustion gases rushes through the engine, spinning turbine blades before it exits.

Aerojet Rocketdyne’s Advanced Programs-Rocket Shop in Alabama is working on a radically different combustor design, one that would release energy in a rapid, continuous succession of detonations set off by shockwaves rotating inside a cylindrical combustor. Less fuel would be burned to turn the blades at a given speed, but the engineers must avoid subjecting the turbine blades to fluctuations in temperature and pressure that could damage them or wear them out too soon.
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It would be interesting to see a diagram of the process.
 
So far, the research has remained strictly ground-based, performed at UCF’s Propulsion and Energy Research Laboratory. The work has been supported by funding from the U.S. Air Force Office of Scientific Research. But don’t expect it to stay Earthbound forever — although it may still be a while before a completed rocket with this propulsion system makes it to the stars.

“We are going through a strategic path forward for technological development,” Ahmed said. “The U.S. Air Force is targeting … a rocket launch flight test by 2025, which we are contributing to.”
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www.digitaltrends.com

‘Impossible’ propulsion system will allow rockets to travel farther than ever

Researchers at the University of Central Florida have demonstrated a breakthrough rocket propulsion system that has been eluding engineers since the 1960s.
www.digitaltrends.com www.digitaltrends.com
 
Heat and vibration are the enemies of all engines.
 
Looks like they need equivalent of 'cavity magnetron' strapping.,..
 
Rhinocrates said:
edwest said:
Heat and vibration are the enemies of all engines.
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Bill Sweetman guesses that the hypothetical 'Aurora' maybe used a PDE. If this is true, that may be what killed it.
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Aurora had a distinctive pulsed engine exhaust ... according to a witness who lived in California City during the 1980s. He watched Aurora depart Edwards AFB around midnight, climb to 50,000 feet and fly westwards at Mach 3 ... many times. He made his millions in nuclear medicine while dabbling in skydiving and flying as pass-times.
The USAF/CIA/NASA/DARPA had completed their test program by the time the press heard of it, so were able to honestly deny having an Aurora program.
 
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A bizarre choice for a hypersonic since PDEs lose their advantage over other cycles above Mach 3.5.
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I'm only speculating of course, but maybe the tradeoff might have been considered worthwhile. That is, a light and simple engine that promised to operate adequately throughout the whole flight regime appeared preferable to one (e.g., turbine-based combined cycle) that could perform optimally at (relatively) low and high speeds, but required heavy, bulky, and complicated plumbing. Or maybe the PDE was a lighter and simpler way than turbines to get the aircraft up to about Mach 3 where the ramjets would take over. It seems that successful engineering for extreme performance depends on gambling that you've made the right tradeoffs, paid costs you can afford, and haven't piled up too many risks.
 
riggerrob said:
Aurora had a distinctive pulsed engine exhaust ... according to a witness who lived in California City during the 1980s. He watched Aurora depart Edwards AFB around midnight, climb to 50,000 feet and fly westwards at Mach 3 ... many times. He made his millions in nuclear medicine while dabbling in skydiving and flying as pass-times.
The USAF/CIA/NASA/DARPA had completed their test program by the time the press heard of it, so were able to honestly deny having an Aurora program.
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Sweetman was pretty consistent in claiming "Aurora" to be hypersonic; that's exactly where you don't what a PDE.

But a Mach 3 VLO aircraft would actually make a tremendous amount of sense and is indeed one of the knees
in some of the decades old survivability curves that Lockheed has been showing.
 
Rhinocrates said:
I'm only speculating of course, but maybe the tradeoff might have been considered worthwhile. That is, a light and simple engine that promised to operate adequately throughout the whole flight regime appeared preferable to one (e.g., turbine-based combined cycle) that could perform optimally at (relatively) low and high speeds, but required heavy, bulky, and complicated plumbing. Or maybe the PDE was a lighter and simpler way than turbines to get the aircraft up to about Mach 3 where the ramjets would take over. It seems that successful engineering for extreme performance depends on gambling that you've made the right tradeoffs, paid costs you can afford, and haven't piled up too many risks.
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A PDE-based combined cycle engine? Sure. There are even patents on it: https://patents.google.com/patent/US6442930B1/en

But IIUC, the hard part of "combined cycle" is managing the transition between of two (reasonably) well known cycles.
Relying on a cycle with a lot of unknown unknowns would be a huge gamble and, IMHO, unlikely
given that Lockheed seems to been largely focused on ejector ramjets (and ejector thrust augmentation in general) since
the 80's to the present day including the SR-72.
 
Weird idea. The PDE and turbofan are simply collocated without any synergistic usage (the core is windmilled during PDE phases).
 
Thanks, very enlightening. I'm certainly not going to try the handwaving characteristic of fanfic, since I already threw a lot of ifs and maybes in there. If Aurora existed, it seems that whatever it was, it was cancelled anyway, due to insoluble technical problems, performance shortfalls, or simply not offering any advantage worth the cost. If there's cryptozoology, which is devoted to the study of the Loch Ness Monster and the like, perhaps there's cryptoaeronautics...
 
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https://twitter.com/x/status/1517100511467606017
View: https://twitter.com/TheDEWLine/status/1517100511467606017

Rotating Detonation Engine Powers Secret Tactical Missile Program | Aviation Week Network

After spending decades in the laboratory, a new propulsion technology enters testing on weapon system.
aviationweek.com aviationweek.com

Pratt And U.S. Air Force Accelerate Rotating Detonation Engine Plan | Aviation Week Network

Air Force speeds studies with Pratt on potential pressure gain combustion benefits for low-cost, high-performance supersonic propulsion.
aviationweek.com aviationweek.com
 
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Obviously, they don’t understand a lot of thermodynamic. The PV diagram shown at min (8:00) is copied from the NASA paper (min. 15:20), but the interpretation is totally wrong.

There is one thing I need to explain in advance, Gas turbines and combustion engines are not truly circular closed processes, but to make calculation easier an (not truly) equivalent circular process is choosen. In the equivalent process, the exhaust gases are cooled down at atmospheric pressure and reintroduced in the process. In this simplification, the mass doesn’t change (so no fuel is added, but heat) and the gas usually is described as ideal gas. The typical gas turbine process is called Joule or Brayton cycle (https://en.wikipedia.org/wiki/Brayton_cycle).

The NASA diagram seen in 15:20 is showing a closed Brayton cycle like it is used to estimate the thermodynamic potential.

Now going back to the Diagram min 8:00:

the point 1 in this diagram (usually this would be point 4) is the gas at the exhaust of the turbine and not at the entrance of the compressor. We get from 1 to 2 not by a magical compressor which reduces the volume without increasing the pressure, but by cooling (see the diagram at Wikipedia for the closed Brayton cycle).

From point 2 to point 3 they mention a magic pressure rise by moving the air to the combustor, this is of course bullshit. Here the compressor is doing its job and compresses air by reducing the volume.

From 3 to 4 they are right.

From 4 to one they are right, but now we have the hot exhaust gases right in front of the compressor because they forgott the cooling...

This is very typical for “Real Engineering” (and also “Engineering Explained”), they might impress people without engineering skills but never fully understand what they are talking about…

Edit:
An air breathing rotational detonation engine is working to the Atkinson cycle, which is a variant of the Otto cycle (with elongated expansion). In the ideal Otto cycle, the heat is added at constant volume (which is not practical at all…) just as in the rotational detonation engine. This pressure gain doesn’t require compressor work and is the main reason why piston engines are more efficient than gas turbines.
 
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That video "How NASA Reinvented the Rocket Engine" is about using rotating detonation in rocket engines.
The Brayton cycle does not apply to rocket engines.

In any case, when I see a "scientist" use a PV diagram to prove his point, I know that he does not really understand what he is talking about. PV diagrams are typically found in textbooks but are pretty useless in the real world.

Unfortunately we now have multiple parallel topics discussing rotating detonation.
Recently we had some discussion about the lack of thermodynamic knowledge of those RD "scientists" when it comes to rocket engines. Starting reading here: https://www.secretprojects.co.uk/threads/pulse-detonation-wave-engines-pde.6372/post-577662 and continue till the end of that topic.
 
Well, PV diagrams are good enough for those who don”t understand the hs diagrams;)

Since Nasa used the PV diagram for a closed Brayton cycle (15:20) this is not only rocket related, but I do agree this was not clear in the video…

The detonation engine approach might also be used in rocket engines, but here it would have lesser efficiency gains than in a "turbine cycle" (Brayton doesnt fit here....).
 
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Nicknick said:
Well, PV diagrams are good enough for those who don”t understand the hs diagrams;)
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A "scientist" who uses a p-v diagram because he does not understand an h-s diagram (or T-s or p-h diagram) should be fired.

I'm sure you know this, but others may not realise it:
The thermodynamic limits of a rocket engine are totally different from that of a turbofan.
In a rocket engine the thrust is from the exhaust velocity only. The maximum possible Isp of a rocket engine is limited by the energy balance of the engine: the sum of Enthalpy (function of temperature) and Kinetic Energy (function of velocity) can never exceed the energy content (Enthalpy plus Lower Heating Value) of the propellants. So there is a limit to the maximum kinetic energy of the exhaust jet, and consequently the maximum possible exhaust velocity which gives the Isp.
However in a turbofan the thrust is mainly from the fan (basically a special type of propeller) and only a small portion from the exhaust jet. A fan/propeller is a much more efficient way of driving a device than simply a jet, although not at high speeds.
The Isp of a rocket engine (no matter what type) using liquid propellants can't exceed 500 s due to thermodynamic limits, but a turbofan Isp can be a factor 10 or more higher.
 
For rocket engines, the rotational detonation engine will reduce the work load on the fuel/oxidizer pumps which would increase the efficiency somewhat. The amount varies on which kind of cycle is used for the fuel pumps (open/close…). It might also be advantageous in flow losses and make a nice combination with the aerospike nozzle, but I can’t judge it.

What I suspect is, that NASA might use this for an air breathing rocket, here it would really be very helpful, because you could built something in between a rocket and a Lorin or scram jet with it. This would also explain, why there is no distinction between rockets and turbines in this video….

h,s diagrams are only for those who can’t handle an analog computer…
 
This is the second video about it, but I still dont know is it air breathing or not? Does it have a compressor or is ram air sufficient to feed it?

P.S. Mazda allredy sells engine with a controlled detonation (spark supported CAI/HCCI combustion).
 
Nicknick said:
This is the second video about it, but I still dont know is it air breathing or not? Does it have a compressor or is ram air sufficient to feed it?

P.S. Mazda allredy sells engine with a controlled detonation (spark supported CAI/HCCI combustion).
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Theoretically it should be able to operate as either with the correct intake setup (or without). It's just fuel and oxidiser after all.
 
This was also my thought, but I doubt that it could start from the ground like that, it would need a conciderable amount of speed to produce a fair amount of intake pressure.

To finally "destroy" rocket technology it should be able to start from the ground and funktion in vacuum too....

I Youtubers pretend to be super clever, they should at least undestand and present the full system!
 
Nicknick said:
This was also my thought, but I doubt that it could start from the ground like that, it would need a conciderable amount of speed to produce a fair amount of intake pressure.

To finally "destroy" rocket technology it should be able to start from the ground and funktion in vacuum too....

I Youtubers pretend to be super clever, they should at least undestand and present the full system!
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Yes, it would have the same limitations as a ramjet in that respect, unless it carries its own oxidiser. So theoretically, for ground-to-orbit, you would need fuel+oxidiser T-O, fuel+ram-air intermediate operation and then back to fuel+oxidiser for exiting the atmosphere.
 
Nicknick said:
This is the second video about it, but I still dont know is it air breathing or not?
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On their website NASA calls the engine in the video a Rotating Detonating Rocket Engine, RDRE:

"While operating at full throttle, the RDRE produced over 4,000 pounds of thrust for nearly a minute at an average chamber pressure of 622 pounds per square inch, the highest pressure rating for this design on record."

For this test I would expect that the fuel and oxygen simply came from high pressure cylinders.
 
I'm suprised, that the NASA is using psi instead of MPa...

With that pressure, I guess it is more a new type of rocket and not an air breathing kind of jet porpulsion.
 
Dagger said:
The Isp of a rocket engine (no matter what type) using liquid propellants can't exceed 500 s due to thermodynamic limits,
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Just a tiny, pointless nitpick: there are chemical rocket engines that can exceed 500 Isp, it just requires using chemistries that no-one sane wants to touch, like the 60's Rocketdyne Li-F-H tripropellant rocket that achieved ~540s. It just also produced HF gas in the exhaust.

Chemical rockets you can get permits to fire outside in this day and age are limited to below 500s.
 

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