
Prototype of a rotating detonation engine, previously thought impossible - elorant
https://newatlas.com/space/rotating-detonation-engine-ucf-hydrogen-oxygen/
======
Cross-Slip
I’ve personally worked in rotating detonation engine (RDE) research, so I’d
like to add some background on the field as well as my perspective on the
novelty of the results in this paper.

But first, I’ll link some cool videos of RDEs firing on Youtube that you might
enjoy.

Purdue University firing a RDE using LOX/Natural Gas (successful firing at ~45
second mark):
[https://www.youtube.com/watch?v=pHcxI-8GtZg](https://www.youtube.com/watch?v=pHcxI-8GtZg)

UT Arlington video from 2009 of RDE running H2/O2 (it looks like a successful
firing occurs at 3 minutes, 22 seconds).
[https://www.youtube.com/watch?v=QbJgOk2rDbI](https://www.youtube.com/watch?v=QbJgOk2rDbI)

I agree with another comment here that it can be hard to tell when an
advancement, such as the one claimed by the authors of the paper, really first
occurs. However, the work of the article linked is certainly not the first
time that a rotating detonation engine running H2/O2 has been demonstrated
successfully or for an extended period of time (read: dozens of minutes). Some
short tests using H2/O2 are shown in the UT Arlington video above. A wealth of
research on RDEs successfully running H2/O2 can be found on Google Scholar
going more than 7 years back.

AFRL East (specifically the Turbines and Combustion Branch at Wright-Patterson
AFB) has been running and publishing results of rotating detonation engines
using oxidizer/fuel combos of atmospheric air and standard liquid hydrocarbon
aviation fuels for several years now. AFRL West (in this case referring to
Edwards AFB, which was involved in the research in the linked article) is
interested in RDE applications where there is no atmosphere to use as an
oxidizer supply, so an oxidizer would need to be held in a tank. AFRL East is
instead more interested in terrestrial applications, so RDE research work
there has naturally focused on using air as the oxidizer.

There are several aspects of using liquid fuels and regular air that create
additional challenge compared to using gaseous fuel/oxidizer combos (including
H2/O2), not the least of which is ensuring proper atomization and mixing of
the liquid fuel in the vary narrow window of time that it can be fed into the
detonation annulus before a detonation front returns to consume them.

For a 6-inch diameter RDE, the detonation wave circles the annulus around 3000
times per second, depending on the fuel and oxidizer used and their
equivalence ratio (the ratio of fuel and oxidizer in the annular detonation
chamber). This creates an incredibly loud sound that is roughly 3 kHz, which
is directly related to the 3000 times/sec the wave circles the annulus.
Sometimes, two or more (in the paper they mention five) separate waves
(distinct detonation fronts) will emerge, usually distributed equi-radially.
This means that a detonation wave will pass a given point in the annulus five
times as frequently, so the noise heard by an observer is then 5*3kHz = 15kHz.

When using aviation fuels in air-breathing RDEs, it is typically necessary to
preheat the liquid fuel before mechanical atomization via injector, which is
easier in H2/02 configurations. Hydrogen and oxyen are stored in tanks as
liquid and may be quite cold, but between raising their temperature before
injection using waste heat [0] and their higher volatility, getting them to
atomize and mix is considerably simpler.

At my former employer, when we tested new parts for our research RDE rig that
were fresh from the machine shop, we would run H2/O2 or H2/N2O as an easier
cheat test to see if there were any glaring manufacturing issues before moving
on to the more involved air and liquid aviation fuels tests.

So yes, I’ve personally seen larger RDEs running on H2/O2 fire literally
hundreds of separate times. I can think of five or so other institutions doing
RDE research that have already accomplished (and published on) more than what
I see in this article. I love the RDE research space, but the linked article
has me scratching my head at their claim to be first.

[0]
[https://en.wikipedia.org/wiki/Regenerative_cooling_(rocket)](https://en.wikipedia.org/wiki/Regenerative_cooling_\(rocket\))

------
valuearb
How much higher ISP, and how much more engine weight/complexity? If it’s
15-20% higher but requires heavier engines that are harder and more expensive
to maintain or use, it’s not going to be that useful or revolutionary. If it
causes excess vibrations, it’s unusable. If it doesn’t scale up over a hundred
thousand pounds of thrust, it’s a not very useful toy.

The biggest cost in space travel isn’t fuel, fuel is so cheap it’s a rounding
error in today’s launch costs. The big costs are the manufacturing cost of
making expensive hardware.

We already know the solution to dramatically lowering the cost of space
access, it’s reusability. Burning up your entire stack every launch is
insanely expensive. SpaceX ‘s reuse of Falcon 9 first stages saves as much as
$30M per launch on the already cheapest stack due to mass manufacturing
techniques.

The Shuttle cost was around $50k per pound of payload to orbit. Before SpaceX
commercial launchers were $5k to $10k per pound of payload. A expendable
Falcon 9 sells for around $1,500 per payload pound, a reusable Falcon Heavy
launch sells for near $1,000 lb.

A fully reusable launch system would be dramatically cheaper. Each Starship
launch is expected to put over 200,000 lbs of payload into orbit for about
$500,000 in fuel. Keep refurbishment costs to $10M per launch and amortize the
build cost over dozens of launches, and your cost per pound of payload is down
to roughly $100. And it would be the largest, most capable lift vehicle ever
made.

And it will do it with engines 20% less efficient than the Shuttle. Efficiency
is only useful if it’s application cost is low.

~~~
wahern
According to Wikipedia it can increase efficiency by 25%. By the time this
method becomes practical (if ever), that 25% might provide a competitive
advantage. In fact, we should _hope_ it would, as it would mean that other
launch companies met or exceeded the efficiency of SpaceX.

~~~
valuearb
A 25% more efficient engine sounds like a lot, but doesn’t necessarily mean
squat for increasing total system efficiency.

The RS-25 for the Shuttle and SLS has an ISP of 366 at sea level and 451 in
Space. The first stage F1 for the Saturn V had an ISP of 261. SpaceX Merlin
has an ISP of 282 at sea level, the vacuum Merlin achieves 311 in space, but
require a different bell. That makes the RS-25 more than 30% more efficient
than Merlin or F1. Yet this is the result.

Payload fractions: SLS: 3.4%, Saturn V: 4.3%, Falcon 9: 4.1%, Falcon Heavy:
4.5%

Thrust to weight ratio:

RS-25: 54

F-1: 82

Merlin: 194

The big difference is the SLS choice of hydrogen as fuel. That provides the
higher ISP, but also requires significantly larger and heavier tanks. It’s
efficiency requires more dead weight in engines and structure that more than
negates the ISP advantage. So much weight that the SLS and the Shuttle
required the help of massively expensive solid rocket boosters just to reach
orbit.

If a detonation engine is heavier, that undercuts its efficiency. If it cokes
up, it’s not going to be usable in deep space, if not easily reusable it
useless.

It requires hydrogen, which means it needs a sea level ISP of 450 to be worth
the heavier structure in a launch system. But if it can do 550 in vacuum, it
will make a great deep space engine.

~~~
Animats
_Payload fractions: SLS: 3.4%_

That money drain needs to be cancelled. Building RS-25 engines like the ones
from the Space Shuttle to use once and throw away is a pure pork program. And
they get a crap payload fraction on top of that.

~~~
cryptonector
What, the Senate Launch System?

------
avmich
Reading news about this research over years, hard to be sure when the "world-
first" event actually happens. For example, a couple of years ago (?) similar
achievement was claimed by Energomash in Russia.

In any case this seems to be wonderful technology, when made working. Kudos to
scientists and engineers involved, it's hard research.

------
entropyjump
This page [1] provides a useful overview of the fundamentals of detonation
engines (pulse and rotating), including some more background on why detonation
is more efficient than deflagration (see Figure 1 in the linked document for
the relevant pressure-volume diagrams, followed by the expressions for their
efficiency).

[1] [https://www.intechopen.com/online-first/a-theoretical-
review...](https://www.intechopen.com/online-first/a-theoretical-review-of-
rotating-detonation-engines)

------
ashtonkem
Do we have any idea about what kind of ISP these rotating detonation engines
can achieve?

And secondly, want to bet that the KSP2 devs are rushing to add this engine
ASAP?

~~~
montenegrohugo
If I understood this table [1] correctly, theoretically these kind of engines
could increase thermal efficiency by a lot , almost doubling them. Which of
course would be incredible and result in a much higher payload/rocket
percentage [2].

In terms of Isp, the same paper [1] quotes 1000-1200!!! Isp for small rotating
detonation engines. Which is insane (state of the art rocket engines are
around ~400)

[1] [https://www.intechopen.com/online-first/a-theoretical-
review...](https://www.intechopen.com/online-first/a-theoretical-review-of-
rotating-detonation-engines#tab1)

[2]
[https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation](https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation)

~~~
baybal2
I think you are not reading it right. I think they are putting it for an air
breathing version.

~~~
montenegrohugo
Yeah, that seems more plausible. Pity, it was the only source I found of ISP
values of Rotating detonation engines.

Now I still have no idea what kind of ISP increase we could see if RDE's were
to be practical.

------
sgt101
Anyone else think about the Aurora spy plane fables from the 1990's?

"strange, loud pulsating roar... unique... a deep pulsating rumble that
vibrated the house and made the windows shake... similar to rocket engine
noise, but deeper, with evenly timed pulses."

[https://en.wikipedia.org/wiki/Aurora_(aircraft)](https://en.wikipedia.org/wiki/Aurora_\(aircraft\))

~~~
na85
From TFA, the shockwaves released that are used to ignite new detonations
travel at Mach 5 within the combustion can. At that speed, with the minute
distances involved, I very much doubt this would sound like the Aurora was
fabled to sound like.

I think more likely it would sound like an amped-up version of the V1 Flying
Bomb.

~~~
skykooler
Indeed. If the engine had a ring with a diameter of 1 foot, which seems
reasonable compared to other rocket engine combustion chambers, a mach 5
detonation shockwave would circle the combustion chamber at 1.7 kilohertz,
which is quite a high-pitched tone (though not as high as the whine of a jet
engine).

------
rapjr9
This is speculation on my part, but it seems like this might be difficult to
sustain as the external air pressure changes as a rocket climbs. While the gas
supply pressure can be controlled, changes in external pressure seem likely to
affect operation, e.g., as air density gets lower it exhibits less resistance
to the exhaust, changing the backpressure. Maybe that can be sensed and
compensated for? That may add additional complexity and reduce reliability,
especially if achieving stability requires very fine tuning. On land or in
space there may be no such issues, but air pressure and temperature changing
as a rocket climbs may cause issues. I'd want to see it fly before I'd believe
it could be used in rockets.

~~~
jacobush
Still, it never occurred to me that it can have other applications:

" In 2012, the Naval Research Laboratory estimated that rotating detonation
engines could save the Navy 15-20 percent off a ~US$2-billion annual fuel bill
if they were retrofitted in place of the gas turbine engines that run over 100
of its large ships. "

~~~
cryptonector
Well, only if they make it work with natural gas. Right now they're using
hydrogen.

~~~
jacobush
I don’t see any reason hydrogen is special

~~~
cryptonector
It's so lightweight that it seems into materials and makes them brittle.

Also, it's so light that you need bigger tanks, which adds weight. Of course,
with a big enough rocket the effect of this is minimal.

~~~
jacobush
I mean, I don't see any reason why explosion turbines can't work with other
gases than hydrogen.

------
cryptonector
The first question that comes to mind is: can you still have a pre-burner and
turbopump? I think the answer may well be "yes", but it could be complicated.
What if having any exhaust contents (water, in this case) mixed with the fuel
and oxidizer prevents detonation? Then you'd have to exhaust pre-burners
separately, but now it's not a pre-burner but a small-burner for running the
pump.

Maybe that would be a simplification: you wouldn't need high pressure pipes
conducting pre-burner outputs into the main combustion chamber,. Then the only
high-pressure components would be the small combustion chamber for the
turbopump, the turbine, and the main combustion chamber. And there'd be no
fuel rich/lean/full-cycle pre-burners, just a single separate and small burner
burning just enough fuel and oxidizer to power the pump into the main chamber.

Another thing is that hydrogen is a terrible fuel because it's so damned
light. It damages pipes and pumps and everything by making materials brittle
as it invariably seeps through. That makes hydrogen/lox engines less reusable
than methane/lox engines. Ideally the detonation engine could work with
methane and lox.

------
mikewarot
This could be used as a combustor in a turbine engine, increasing efficiency
by a few percent... which could save huge amounts of fuel.

------
monadic2
They specified the engine in terms of lbf. Why don't they use metric? I
realize that imperial units are used colloquially in the US but surely that
doesn't extend to actual engineering!

~~~
riversflow
Disclaimer, not in the mechanical engineering field. That's said, its ironic
that you use "actual engineering" as that "actual" bit is why, as I understand
it, engineers primarily use imperial units in the US—most people who do the
making know imperial. I can't speak for every industry, in fact I know
machinists make plenty of things in metric, but that is my understanding from
talking with Engineers here.

~~~
monadic2
My point is none of the engineering classes I ever took allowed the use of
imperial units because they wanted their students to be employable. Is this a
new paradigm or did I just have a very unusual experience?

------
enriquto
> We can even inspect the value of $PATH, here:

> (...)

> Your output is going to be different from mine because of the hashes in the
> store paths.

Does this mean that a nix install is not reproducible? If the same
installation is performed twice, the file names and contents will be different
due to some randomness?

~~~
sbmthakur
You posted on the wrong thread.

NixOS discussion:
[https://news.ycombinator.com/item?id=23087197](https://news.ycombinator.com/item?id=23087197)

~~~
enriquto
Thanks; too late for me, I cannot delete it.

------
sloucher
>The team has published their results in the journal "Combustion and Flame".

Sounds like a rip-roaring read!

~~~
WantonQuantum
It's a blast!

~~~
growlist
Hot off the presses

