In the US, a rotating detonation rocket engine takes flight | Hacker News Reader

In the US, a rotating detonation rocket engine takes flight

arstechnica.com

115 pointsLorenDB73 comments

From the article:

"In contrast to a traditional rocket engine, in which a highly pressurized propellant and an oxidizer are injected into a combustion chamber where they burn and produce an energetic exhaust plume, a rotating detonation engine is different in that a wave of detonation travels around a circular channel. This is sustained by the injection of fuel and oxidizer and produces a shockwave that travels outward at supersonic speed."

Nope ... on my way to do web searches to try to figure out what this means.

First stop: https://en.wikipedia.org/wiki/Rotating_detonation_engine

"A rotating detonation engine (RDE) uses a form of pressure gain combustion, where one or more detonations continuously travel around an annular channel. ... In detonative combustion, the flame front expands at supersonic speed. It is theoretically up to 25% more efficient than conventional deflagrative combustion ... Disadvantages include instability and noise."

No images, no animations.

OK ... bookmarked, and I'll chase the references later.

Edit: OK, here's the best reference I've found so far:

https://www.sandboxx.us/news/what-is-a-rotating-detonation-e...

Now back to work.

If I’m understanding this correctly, if you set off one bomb the explosion travels at a certain speed, but if you put a bunch of bombs in a circle and set them off one at a time, the very last explosion will be going a lot faster than the first one?

From my understanding, it's more that the explosion phase of a detonation is more fuel-efficient than the burning phase, but can't last very long under normal circumstances because the flame front is moving so fast-- This is a trick to continue fueling the explosion to sustain it over an indefinite time period.

Think about it in terms of an old-fashioned gunpowder line fuse: If you lay it out in a ring and have some kind of mechanism to continuously place down new gunpowder on the ring in front of the flame, you can keep it going until you run out of fuel.

What i've never understood is how detonation can be more efficient than deflagration. What does that mean? Both types of engine take a mix of fuel and oxidiser and turn it into hot combustion products. The hot combustion products then expand through a nozzle to produce thrust. How is that process different between the two? Does a deflagration engine leave some fuel unburnt, that a detonation engine burns? Does the combustion of the same fuel somehow produce more heat? Or less heat but more pressure? Is it something about the expansion?

To put it another way, if you set up a deflagration engine and a detonation engine next to each other, and fed them fuel and oxidiser at the same rate, how would the streams of exhaust gas coming out of them look different? What other external differences would you see?

The energy produced by the combustion is the same but different fractions of it are converted into work. Just like efficiency differences in other types of heat engine.

η = 1 - ( T_c / T_h )

Carnot-Efficiency is the theoretical limit of the cycle’s ability to get work out of the thermal energy where T_c is the exhaust (coldest) temperature after expansion and T_h the hottest temperature (before expansion). Temperatures given in K (Kelvin), so 100% if you manage to get T_c to absolute zero temperature.

This is mentioned in the introduction here [1]. The version I'm looking at is an image, so I cannot easily copy-paste the relevant passage, but it seems to say that the efficiency gain comes from detonation causing the combustion to occur at a higher pressure than in the case of deflagration. Generally speaking, higher peak pressures increase the efficiency of heat engines, as this allows for greater expansion of the working fluid, and thus more work being done for the same fuel consumption.

With regard to your comparison, I guess this means that the detonation engine can have a higher pressure in the combustion chamber, together with a larger bell, a faster-moving exhaust, or some combination.

[1] https://arc.aiaa.org/doi/10.2514/6.2013-3971

Just think in terms of the P-V diagram, you want the pressure increase curve to be as vertical as possible to maximize the area enclosed by the cycle

This doesn't help at all. Diagrams are diagrams, what does it actually mean physically?

Explosions release more energy in a shorter time, which leads to a higher peak temperature and thus higher kinetic energy in the products. This directly results in more pressure/thrust.

Think of it as similar to hi compression automobile engine (race car) being more efficient vs a low compression engine (tractor engine). Not exactly correct but a way of starting to think about it

You've got the speed at which a detonation progresses within the explosive material and the speed at which the shock wave travels through the air. Neither is going to change based on the bomb's arrangement. But if you detonate a string of bombs so that each goes as the shock front from the first reaches it you can get, through constructive and destructive interference, a shockwave going mostly in one direction. But that's mostly not related to how this engine works.

Integza made a YouTube video[1] where he visited a lab where they had a RDE, and includes some nice explanations, animations and slow-mos of the prototype in action.

His channel has a certain flavor, but at least the video is informative.

[1]: https://www.youtube.com/watch?v=fRMMSyCcTDI

> His channel has a certain flavor

It's kind of an anti-tomato flavor.

What's wrong with tomato degloving by rocket?

Scott Manley has a great video about them: https://www.youtube.com/watch?v=rG_Eh0J_4_s

My first thought when I read this: "I bet Scott Manley has a video about this."

Steve Mould has a "rotating flame" video which also helps visualize this: https://www.youtube.com/watch?v=SqhXQUzVMlQ

Those descriptions are actually both correct, but one is describing the elephant's trunk and another the elephant's ear.

To be a bit more clear, the detonation is racing around a circular track in one plane, but the net thrust is in the direction perpendicular to that plane. Fuel and oxidiser are continuously pumped in all along the track, dropping to ~0 after a detonation and rising afterwards until the detonation front arrives again. Since you don't have to pump into a continual high pressure deflagration like in a conventional rocket engine this looks like it should be easier in terms of pump power.

Rocket engine efficiency, like all heat engine efficiency[1], governed by the difference in temperature between hot and cold in the cycle. Because at any given point in a detonation engine the combustion engine only comes in pulses it can reach temperatures that would otherwise melt the rocket engine. That's true of regular rocket engines too, which use active cooling, but seems more true of a detonation engine. And I would guess that the exhaust, already being supersonic, needs less of a chock on the de Laval nozzle to ensure laminar flow.

[1] Strictly speaking this only applies to combustion rocket engines. An ion engine, for instance, is a rocket engine but not a heat engine.

By "chock" do you mean "impact"? In English a chock is a rubber wedge you put under a wheel to keep it from rolling, or various similar objects, or the act of using one to keep something from moving: https://en.wiktionary.org/wiki/chock

Oops, that should have been "choke".

The first reference in the wiki article has a nice animation

https://www.rtx.com/news/2025/03/04/more-power-no-moving-par...

I imagine an internal combustion engine with a ring of spark plugs firing rapidly in sequence into a nozzle that has fuel/air injected uniformly

That is my naive attempt to grok with an analogue, that is probably wrong.

Basically it's a turbine engine but the rotating blades are replaced with orbiting shockwaves instead.

That makes no sense at all. How do you think a turbine engine works?

This is a very low-information post. It would be more useful if you elaborate civilly on why you disagree with parent post.

> Turbine made out of shockwaves

That sounds like something out of sci-fi, amazing.

NASA tested one at Marshall about a year ago: https://www.youtube.com/watch?v=UShD03eG9IU

https://youtu.be/RVxgyz_avQM?si=S2gqMXGJfx0tkC7h

The most fundamental idea of a detonation engine is that, if you substitute fuel "burning" process with subsonic propagation used in every conventional engines with fuel "explosion" process with supersonic wavefront, the reactions will become more instantaneous and energetic and that improves efficiency. This is the detonation part of detonation engine concept.

The rotating part is a solution to the problem that forcing a continuous sustained explosion inside an engine can be complicated. By letting the explosion constantly happening, expanding, and traveling radially across a thin cylindrical gap, it solves such problems as sustained combus-^H^H^H detonation, fuel supply, etc. It's not the only possible type of an engine based on detonation principle, but so far the most promising. Detonating piston engines for cars, for example, are much less promising.

Engine part is just regular reaction rocket. It shoots the gas out the back. The faster and heavier the gas, the more reaction force it creates.

This isn't the first RDE ever to have been built, fired, or fired in a freefall, but it's finicky and experimental enough that it warrants a news story like this.

>...if you substitute fuel "burning" process with subsonic propagation used in every conventional engines with fuel "explosion" process with supersonic wavefront...

This fragment confused me, because it looks like there are three substitutions. There aren't; there's only one. Read it as:

If you substitute fuel burning (which has subsonic propagation, and is used in every conventional engine) with fuel explosion (which has a supersonic wavefront)...

The first and third "with" link a noun (the respective process) with a property (how fast it shoots gas out the back). The second "with" is the substitution.

English is hard! I'm a native speaker, and I had to take a look at a few webpages to understand just this part! And I'm still left with questions, like why subsonic is described as having "propagation", but supersonic is described as having a "wavefront". Is this a distinction with a difference? I don't know.

I must admit that I'm in fact not a native English speaker and my sentence construction is still jarring at times... sorry for that.

That distinction was not intentional, but IIUC, chemical reactions propagating faster than the speed of sound(of surrounding material, not necessarily of air(which idk how is it even defined, ductile limit or something(of air!?))) has effect of consolidating what would normally be multiple consecutive waves of density changes following one another, into a single shockwave with higher peak pressure, and higher peak pressure in an engine is generally thought to lead to higher exhaust gas velocities and better efficiencies so long that the engine holds. I think this is what some comments mentioning P-V diagrams and Carnot efficiency is meaning to say, except mine is in uneducated terms.

Yeah, that bit about the shockwave in the first quote is a weird thing to say; I suspect LLM use here. All shockwaves "travel outward at supersonic speed."

>Sassie Duggleby and her husband, Andrew Duggleby, founded Venus Aerospace nearly five years ago with the long-term goal of developing powerful rotating detonation engines and building a hypersonic aircraft that could carry perhaps a dozen passengers and travel at astonishingly fast speeds worldwide.

As an engineering project it sounds really cool, but is the application really commercial travel? I would have thought the military would be far more interested and much better funded.

From the bottom of the article:

"We're actively engaged with US defense and national security agencies as well as commercial partners exploring hypersonic applications in logistics, aerospace, and future mobility—including large primes," she said. "The enthusiasm we’re seeing reflects a broad recognition that Venus’s technology can unlock new operational and economic possibilities across multiple markets."

What are ‘large primes’ here?

Primary defense contractors. Lockheed, Boeing, Raytheon, etc.

M136,279,841 is pretty big..

It's a smokescreen for hypersonic missile development. It's a common tactic. I knew someone who worked on a novel satellite surveillance system. Their pitch was that it would allow them to engage in futures trading because the resolution of their satellites would allow unique insights into global trade for certain markets using realtime optical imaging.

The guy was former Lockheed. One night at dinner I told him to cut the BS and that their real goal was to target military/government contracts. He said "off the record" this is correct. This an unspoken goal by everyone involved - management, investors, etc.

My understanding is it can be hard to position yourself as a defense company as a start up if you don't already have a foothold in the space.

>Their pitch was that it would allow them to engage in futures trading because the resolution of their satellites would allow unique insights into global trade for certain markets using realtime optical imaging.

I mean, that pitch is an accurate claim if the insights in mind are "We just told the pentagon to blow up these pixels". That tends to affect global trade and futures markets.

Just like all the start-ups building autonomous drones for "search and rescue" missions. There might be more of those companies than people who go missing in national parks each year.

Tbh military surveillance satellites are used for economic espionage, not just military. The US used to, and still does I'm sure, keep track on agricultural yields and similar, because that information can inform strategic decisions and negotiations.

The amount of times the word "hypersonics" was mentioned should be pretty clear sign they are aiming for DOD money.

This is one of the few hypersonic technologies which could actually be enabling. This is because detonation speeds things up and increases range compared to deflagration (burning) without necessarily needing air-breathing added. This means the craft can remain a simple tapered cone traditional rocket and avoid shockwave imgingement sites on the craft air intakes and the thermal load of that that is so far infeasible to maintain for more than a minute or two.

Still trying to wrap my head around this. Doesn’t this cause insane stresses with basically all explosions being off axis? Or are there multiple sets of explosions moving at same the on opposites sides?

The main drawback is that RDEs need substantially stronger structure to smooth out the forces. It's still worth it, the force of the detonation (it's not separate explosions, it's a single continuous detonation front moving in a circular channel) is small enough to be containable.

There have been tests with multiple detonation fronts, but controlling them is much harder. It's very hard to have them go around the channel at exactly the same speed, so their positions relative to each other remain fixed.

Plus if the rocket carries a warhead it doesn’t burn for long.

https://youtu.be/QR6iFVj_xHw?feature=shared Here's a good webinar with some visualization if you're interested in learning more.

JAXA flew one in 2021, to give a sense of developmental pace.

How do you restart after flameout?

Watch the Integza video (linked above): The lab interviewed starts the process using a very small pulse detonation engine, which injects a new explosion into the ring.

2 hours? What's the point? It could be 0 minutes with a 0% explosion rate and the total travel time would probably be longer.

The point is reduced fuel use and faster response times (either emergencies or warfare).

Maintaining a detonation (as opposed to deflagration) is quite a technical challenge. If nothing else, it pushes mechanical engineering and material science forward.

At supersonic speeds, the efficiency of an engine like this can be very high, and at high altitudes, travel is very efficient. People don’t seem to get that there is potential for more efficient transport at supersonic speeds/ hypersonic speeds. The ultimate example of this a satellite. How many times does it circle the world every 45 min, all on that initial fuel it took to get it up there? It seems wasteful, but it’s actually the most efficient form of transportation in existence.

Of course, hypersonic transport would not be as efficient as a satellite, but still, at 100k feet, for example, you can go 5000km/h while only consuming the same amount of fuel that you would use to go 300km/h at sea level. The potential for efficiency gains are huge, and the fuel cost of getting to altitude is mostly returned in the descent.

Of course, the technical challenges are also huge, so I don’t expect to see hypersonic airliners any time soon, but when we do get them, they could easily be 6-7x as efficient as existing transports. And it would be nice to have long haul flights be 3x as fast or faster.

People mistakenly assume that this kind of travel only has applications for the military or the elites, and, like air travel in general, that will probably be initially true. But after the rich and the government foots the bill in lives and research to make it safe and economical, it should hold huge gains for society at large.

Science and engineering is one of the only cases where trickle down economics actually kind of works. In knowledge, a rising tide really does lift all boats.

Presumably, you have to be travelling quite a distance before hypersonic/high-altitude flight becomes more efficient than current commercial airliners.

Certainly not for short hops, but if regular aviation is any indication, it’s more efficient even if you don’t even reach cruising altitude before beginning to descend, so I’d imagine any flight that currently is around 4 hours or more would potentially benefit. (Given full technology maturity) that said, we are a long, long way from having efficient engines for that wide of a speed envelope.

What about noise? So far all we have gotten is companies trying to build planes that still are very loud and produce audible booms all the way along their travel path. Pitched to the empathetic heros that are billionaires. And to sway public opinion they fly test planes at slightly over mach one in specific weather and altitude setting that avoids having the sonic booms reach the ground. While marketing and selling the idea of not mach 1.1 but mach 2-3 supersonics. Which is AFAIK completely impossible without the boom issue, even if made less bad - but arguably still bad - by fancy wing shapes. So all I see is selfish people trying to build something at expense of everyone else, including the environment.

Flying it the high limits of the atmosphere reduces sonic boom intensity due to both the extremely low density of the air and the distance from the ground. Ultimately, sonic boom is a sign of wasted energy, so if (big if) we can make aircraft that conform to the required aerodynamic ideals for efficient hypersonic flight, they will be relatively quiet.

Just like with regular air travel, though, I would expect a few decades of practical application for specialized service before it was commodified to be really useful for large amounts of passengers.

what about noise is you're supersonic noise and sound is about a mile back

I think the concern is for the frothing masses of earthbound unfortunates, not the passengers hurtling gloriously towards destiny miles above the troublesome surface of the planet.

> the fuel cost of getting to altitude is mostly returned in the descent

In energetic terms this is clearly true, but that leaves you at the end of the descent with very high speed / lots of kinetic energy. Great for missiles, because you dump the energy into the target. But I'm struggling to see how it's useful for any other application?

I would assume that any efficient hypersonic airliner would need to use variable geometry or powered manipulation of airflows to attain a reasonable efficiency in the transition zones of its flight envelope, so while you obviously would not get all of the acceleration energy back, it would transition into efficient lower velocity flight and reduction in engine thrust needed during the deceleration phase. But if we really knew how to do all of this we would already be flying transatlantic in an hour.

For the same reasons that theoretically possible doesn’t mean necessarily practical, theoretically difficult doesn’t mean practically impossible. We just won’t know until we significantly advance the state of the art. We might find out that atmospheric flight is a waste of time and that exoatmopheric ballistic trajectories are where it’s at. Or we might find out that zeppelins were the way all along.

We are still a long way off from having engines that can operate any where near theoretical efficiencies except within very narrow parameters and altitudes. Until we sort that we will have difficulty traversing a wide speed envelope without waste.

Fuel efficiency is the answer for "why would you do this?" That's really important for rockets, since to launch each gram of fuel you have to have some more fuel which you also have to lift.

Press release videos - https://drive.google.com/drive/folders/194_lxE-vy_ha7-5V_wFj...

Their YouTube is insanely technical, I think, lots of videos with big equations anyway - https://www.youtube.com/@venusaerospace/videos

> hypersonic

This is a meaningless term. There's just varying degrees supersonic. "hyper" implies a completely different thing.

EDIT: apparently the military has bastardized the term

Hypersonic begins at 5x speed of sound.

It's not a new term, and it exists because there are unique practical engineering problems associated with high mach numbers so having a term for this regime is useful even if the distinction seems arbitrary from a pure theoretically perspective.

Sure, but "hyper"? Is there any reason other than that it sounds cooler than "super"?

What are we going to do when we hit mach 11, start talking about "hyperhypersonic"? "superhypersonic"?

It's a distinct regime, so useful to give it a distinc name. Super and hyper are roughly synonyms, but super was already taken.

There was also some precedence from wave research back in 1938 which suggested hypersonic and ultrasonic for high frequency waves[1].

At this point it's a well-established term, with 60 years of history[2].

[1]: https://www.ias.ac.in/public/Volumes/seca/007/03/0163-0176.p...

[2]: https://secwww.jhuapl.edu/techdigest/Content/techdigest/pdf/...

> Super and hyper are roughly synonyms

Perhaps you're right in terms of how they're leveraged in today's society. I'll drop my pedantry.

They roughly have the same meaning in the languages they were borrowed from, Latin[1] and Greek[2] respectively.

Yeah it's random, but established at this point. The radio bands aren't much better[3] with their Very High Frequency (VHF), Ultra High Frequency (UHF) and Super High Frequency (SHF) and Extremely High Frequency (EHF).

[1]: https://www.etymonline.com/word/super-

[2]: https://www.etymonline.com/word/hyper-

[3]: https://en.wikipedia.org/wiki/Radio_spectrum#ITU

The term has been in use since the 1940s, pretty much as soon as it was sensible to think about making something that can go that fast. Does your modern perception of "hyper" being a cool word reflect how the word was perceived 80 years ago, or does your perception of the word being cool come from the way the word has since been used to describe such extreme things?

Maybe it was considered cool back then, but I wouldn't be sure of that. Maybe it was just an obscure latin term nerds used. I guess you could search comic book archives for clues. In any case, really fast things are cool, but only figuratively speaking.

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