Rotating Detonation Engines

[sanman]

Another separate thing I wanted to ask about -- will the circular motion of the detonation wavefront in the RDE result in any net torque around the axis of thrust? ie. Will it result in any roll?

And if so, then is the solution to just use these engines in pairs or clusters, running them in opposite directions to cancel out the torques?

Or alternatively, since wave harmonics and multiple wavefronts are possible in the same annular chamber, would it be possible to have opposing / counter-directional detonation waves going around the chamber in opposite directions, for torque cancellation?

[sanman]

Another thing -- NASA's GRCop-42 alloy, which is touted for its ability to conduct away heat while withstanding the intense heat of the detonation combustion, was supposedly arrived at by simply halving the ratios of the original GRCop-84 alloy.

Isn't half a little too convenient as a multiplier?
Did they bother to test out GRCop-47, GRCop-38, etc to see if these might produce better results?

[sanman]

Lets you design an engine to only operate with a known supersonic flow inlet condition, rather than needing to operate all the way from standstill bootstrapping all the way to desired operating speed.

But detonation engines shouldn't care about inlet flow conditions, since their constant-volume combustion is not based on flow. They should be able to flexibly work at a wide range of operating speeds.

Zooming out to the whole engine-as-a-system, the evolution of appropriate (and by all accounts, finicky) conditions needed for detonation could certainly be influenced by flow.

Here's what an article in Scientific American said:

...GE Aerospace, one of the world’s biggest jet engine builders, sent supersonic air through a subscale lab rig that combined a Mach 2.5-class turbofan with a rotating detonation-enabled “dual-mode ramjet” thought to be capable of Mach 5 velocities. The RDE would supply the Mach 3 speeds needed to start up the ramjet in flight, velocities turbines have difficulty reaching....

So the RDE is a mach-independent propulsor that can help propel anything to the right flow conditions to get going.

[edzieba]

Another separate thing I wanted to ask about -- will the circular motion of the detonation wavefront in the RDE result in any net torque around the axis of thrust? ie. Will it result in any roll?

No, the engine is not rotating, and the working fluid is not rotating.

So the RDE is a mach-independent propulsor that can help propel anything to the right flow conditions to get going.

No, it just means one particular team thinks they can make an engine operate within an inlet pressure range from STP to whatever conditions they decide to switch over to the Ramjet at.

Regular Turbojets and Turbofans have demonstrated operating conditions from STP to high Mach numbers (e.g. with inlet cooling), but that does not mean they are "mach independent". Instead, it takes a monumental amount of engineering to make an engine that operates over a wide range of inlet conditions vs. a single inlet conditions. A gas turbine that will always be operating at STP (because its sat on the ground) will be a lot easier to design than one that need to go from STP to supersonic, even with the same shaft power, because it greatly simplifies design by removing a design constraint. If you take a gas turnbine designed to operate at STP and feed it Mach 1.5 inlet air, you;re not gojng to have a good time.
This applies to RDEs too - design and operation is greatly simplified if you can keep the inlet conditions in a small range rather than needing to operate over a large range, and RDEs are complex enough to stably operate as it is.

[sanman]

What advantages does the Ram-Rotor Detonation Engine (RRDE) offer over the regular Rotating Detonation Engine(RDE)?

From the articles posted, it's said to offer compression. But I thought that's what the detonation shockwave front is doing -- it's offering the compression. So what exactly is this rotating blade-duct setup doing? Is it somehow just shaping the detonation wavefront? To what benefit?

[edzieba]

So what exactly is this rotating blade-duct setup doing?

Controlling inlet conditions without a separate compressor stage.

[Asteroza]

Another separate thing I wanted to ask about -- will the circular motion of the detonation wavefront in the RDE result in any net torque around the axis of thrust? ie. Will it result in any roll?

No, the engine is not rotating, and the working fluid is not rotating.

So the RDE is a mach-independent propulsor that can help propel anything to the right flow conditions to get going.

No, it just means one particular team thinks they can make an engine operate within an inlet pressure range from STP to whatever conditions they decide to switch over to the Ramjet at.

Regular Turbojets and Turbofans have demonstrated operating conditions from STP to high Mach numbers (e.g. with inlet cooling), but that does not mean they are "mach independent". Instead, it takes a monumental amount of engineering to make an engine that operates over a wide range of inlet conditions vs. a single inlet conditions. A gas turbine that will always be operating at STP (because its sat on the ground) will be a lot easier to design than one that need to go from STP to supersonic, even with the same shaft power, because it greatly simplifies design by removing a design constraint. If you take a gas turnbine designed to operate at STP and feed it Mach 1.5 inlet air, you;re not gojng to have a good time.
This applies to RDEs too - design and operation is greatly simplified if you can keep the inlet conditions in a small range rather than needing to operate over a large range, and RDEs are complex enough to stably operate as it is.

I think a lot of misconceptualization here is due to a lot of RDE work using direct injection of oxidizer in an otherwise closed (exit only) combustion chamber. Which means it's a rocket essentially. Which means it can nominally be used anywhere a rocket is used. Such as an ejector nozzle (operating as a gas generator system for inlet entrainment) in ejector nozzle ramjets, thus allowing for a zero speed ramjet start.

We haven't seen much about ingestion of inlet air into the RDE chamber without using injectors to control things.

What advantages does the Ram-Rotor Detonation Engine (RRDE) offer over the regular Rotating Detonation Engine(RDE)?

From the articles posted, it's said to offer compression. But I thought that's what the detonation shockwave front is doing -- it's offering the compression. So what exactly is this rotating blade-duct setup doing? Is it somehow just shaping the detonation wavefront? To what benefit?

The RRDE has the ability to directly extract shaft horsepower, whereas a typical RDE must have a turbine grafted on somehow.

[sanman]

I think a lot of misconceptualization here is due to a lot of RDE work using direct injection of oxidizer in an otherwise closed (exit only) combustion chamber. Which means it's a rocket essentially. Which means it can nominally be used anywhere a rocket is used. Such as an ejector nozzle (operating as a gas generator system for inlet entrainment) in ejector nozzle ramjets, thus allowing for a zero speed ramjet start.

We haven't seen much about ingestion of inlet air into the RDE chamber without using injectors to control things.

So are there any limits/constraints on the size of the RDE?
Regular combustion chambers suffer from combustion instability problems as they get too big.
That seems less likely to be a problem with annular chambers.
Is there any such thing as an overly large RDE?
What about overly small? Could I fit one on the soles of my rocket boots, for example?

Also, I wonder if we could have concentrically nested RDEs that would efficiently pack onto the bottom cross section of a rocket?

What advantages does the Ram-Rotor Detonation Engine (RRDE) offer over the regular Rotating Detonation Engine(RDE)?

From the articles posted, it's said to offer compression. But I thought that's what the detonation shockwave front is doing -- it's offering the compression. So what exactly is this rotating blade-duct setup doing? Is it somehow just shaping the detonation wavefront? To what benefit?

The RRDE has the ability to directly extract shaft horsepower, whereas a typical RDE must have a turbine grafted on somehow.

Ahh, so that's interesting. So can that make RRDE the detonation counterpart to the turboshaft engine?
Can RRDE then be used in applications wherever turboshaft engines are used?
I'm thinking helicopters, Auxiliary Power Units (APUs), even the M1 Abrams tank famously uses a turbine powerplant.

[Asteroza]

I think a lot of misconceptualization here is due to a lot of RDE work using direct injection of oxidizer in an otherwise closed (exit only) combustion chamber. Which means it's a rocket essentially. Which means it can nominally be used anywhere a rocket is used. Such as an ejector nozzle (operating as a gas generator system for inlet entrainment) in ejector nozzle ramjets, thus allowing for a zero speed ramjet start.

We haven't seen much about ingestion of inlet air into the RDE chamber without using injectors to control things.

So are there any limits/constraints on the size of the RDE?
Regular combustion chambers suffer from combustion instability problems as they get too big.
That seems less likely to be a problem with annular chambers.
Is there any such thing as an overly large RDE?
What about overly small? Could I fit one on the soles of my rocket boots, for example?

Also, I wonder if we could have concentrically nested RDEs that would efficiently pack onto the bottom cross section of a rocket?

What advantages does the Ram-Rotor Detonation Engine (RRDE) offer over the regular Rotating Detonation Engine(RDE)?

From the articles posted, it's said to offer compression. But I thought that's what the detonation shockwave front is doing -- it's offering the compression. So what exactly is this rotating blade-duct setup doing? Is it somehow just shaping the detonation wavefront? To what benefit?

The RRDE has the ability to directly extract shaft horsepower, whereas a typical RDE must have a turbine grafted on somehow.

Ahh, so that's interesting. So can that make RRDE the detonation counterpart to the turboshaft engine?
Can RRDE then be used in applications wherever turboshaft engines are used?
I'm thinking helicopters, Auxiliary Power Units (APUs), even the M1 Abrams tank famously uses a turbine powerplant.

You would probably face wall stiffness issues with concentric annular RDE chambers, even though that does increase packing density. The pulse nature of it means you also have vibration mode issues too. There's also scaling sizing, in so much as the circumference is a multiple of what it takes to reset the space with fresh oxidizer and fuel before the wavefront passes by again (you can have two or more waves running behind each other in a single ring, thus increasing the annulus size). In experimental setups they have had racetrack oval setups having long straight sections (to ease observation via flat plate quartz windows), so for practical purposes there is some flexibility to the loop/annulus design. I do wonder if tight radius circular annulus types that can only accommodate a single detonation wave may have some underlying properties specific to that.

The RRDE seems better suited for a single rotor/shaft setup, which may be not great for some turboshaft applications for other reasons (but may be good for APU/generator use). Substituting conventional RDE combustor cans for conventional can burners in normal gas turbines is the simpler solution in the short term though.

[Asteroza]

Looks like work is progressing on GE's RDE DMRJ enough that they are putting together a flight test engine.

https://www.geaerospace.com/news/articles/need-speed-ge-aerospace-using-ramjet-technology-ante-hypersonic-flight

[sanman]

Another separate thing I wanted to ask about -- will the circular motion of the detonation wavefront in the RDE result in any net torque around the axis of thrust? ie. Will it result in any roll?

No, the engine is not rotating, and the working fluid is not rotating.

Ed, are you sure? It's a wavefront going around in circles -- how is that not rotation? It's even there in the name.
So according to you, that wavefront and its combustion products (working fluid) are only exclusively shooting out along the axis of thrust?
Yet we can see that it's propagating along the circular route -- that's rotation -- and the radius of the annulus should ensure it produces torque around the central axis.
That torque may not be a problem for aircraft, just as torque from propellers/turbofans/etc isn't a problem, since there are wings and ailerons to oppose it. But for a spacecraft in space it could be an issue.

[sanman]

The RRDE seems better suited for a single rotor/shaft setup, which may be not great for some turboshaft applications for other reasons (but may be good for APU/generator use). Substituting conventional RDE combustor cans for conventional can burners in normal gas turbines is the simpler solution in the short term though.

So I'd like to know -- whether as a propulsor or as some other power generator, is detonation combustion the ultimate holy grail for extraction of energy from chemical fuels?
Without switching to something else like nuclear, and just staying within the chemical realm, are detonation engines pretty much the ideal?

[Asteroza]

The RRDE seems better suited for a single rotor/shaft setup, which may be not great for some turboshaft applications for other reasons (but may be good for APU/generator use). Substituting conventional RDE combustor cans for conventional can burners in normal gas turbines is the simpler solution in the short term though.

So I'd like to know -- whether as a propulsor or as some other power generator, is detonation combustion the ultimate holy grail for extraction of energy from chemical fuels?
Without switching to something else like nuclear, and just staying within the chemical realm, are detonation engines pretty much the ideal?

I think there have been proposals for photodetonation allowing combustion across a combustion chamber nearly simultaneously/instantaneously, but I don't think that can be practically implemented as that's ultimately a pulsed function rather than "continuous".

I think in practical terms, the typical single annulus style RDE combustion chamber we see may be the best, but the annulus limitation makes creating a high power density monolithic engine difficult. The chamber has to tolerate pulses so you can't make the chamber walls too thin, and concentric annuli make that problem harder. Maybe an array of annuli?

[sanman]

[Asteroza]

RTX (P&W) moving along in the their RDE work for DARPA Gambit

https://www.rtx.com/news/news-center/2025/03/04/rtxs-pratt-whitney-completes-series-of-rotating-detonation-engine-testing

[catdlr]

Venus Aerospace completes first US flight test of Rotating Detonation Rocket Engine