Autophage rocket engine consumes plastic fuselage for fuel
theengineer.co.uk
theengineer.co.uk
> A conventional rocket’s structure makes up between five and 12 per cent of its total mass. Our tests show that the Ouroborous-3 can burn a very similar amount of its own structural mass as propellant.
It seems like they are measuring the wrong thing.
Rocket fuselage isn't as good a fuel as, well, rocket fuel. So it doesn't matter if it makes up 10% of the mass consumed - what matters is how much impulse it provides.
However! HDPE is reasonably competitive with methane. While it doesn't make Wikipedia's list [0] of hundreds of potential fuels by calorific value, which puts methane at 55 MJ/kg, research in municipal waste incineration puts polyethylene around 43 MJ/kg [1]. And in fact it's already been studied in Japan as a rocket fuel [2] using N2O as an oxidiser. So this seems completely plausible.
[0] https://en.wikipedia.org/wiki/Heat_of_combustion
[1] https://www.researchgate.net/publication/336848986_Productio...
Minimizing the final dry mass at the end of the burn is also very important to maximize delta-v, according to the Tsiolkovsky rocket equation. So it's not just about the impulse provided by the fuel.
But it's only an improvement if the fuselage is actually effective in containing the propellant, or is the propellant (imagine a long stick of rocket candy burning at the end, mechanically drawn by a closed nozzle). Otherwise, what you gain in mass ratio you use lose in useful impulse because you need to lift this large mass of mostly inert plastic at launch.
Autophage is, effectively, continuous staging.
Except instead of discarding fuel tanks, they are burned.
>But it's only an improvement if the fuselage is actually effective in containing the propellant, or is the propellant
Well, of course. Which is why it's not something that has been implemented yet, even though people thought of it since the 1930s.
I suppose one could attempt a truly exotic design in which the fuel tank gets gasified, piped past the payload, and then burned, thus allowing the tank to be on top, but these seems unlikely to work very well.
I've been thinking of this design for a long time, but so far it sounds a bit preposterous even for a sci-fi enthusiast with no rocketry knowledge :)
The real question is what is the rate of heavy molecules, or equivalently "how correct is the mixing"?
I imagine those first prototypes weren't that good.
Would that negate some of the controlability characteristics?
And would it need to be carefully tuned to the launch profile? (I assume deviations wouldn't be self-correcting eg. if acceleration is reduced there's less injection force presumably leading to less thrust).
Is this true? I've seen many hobby hybrid rockets built in this fashion over the years, for example acrylic tube + oxygen gas. Maybe none of them were ever launched.
https://www.youtube.com/watch?v=VI8gdmjEwKc
It's pretty much the easiest way to do a hybrid rocket.
The outer part which is acting as structure cannot be burned as fuel because it's necessary as structure, and if you let the burn continue too long, it would cause the rocket to fail. When the burn ends, there's a lot of unburned acrylic still there.
This design allows a given piece of tube to first serve as structure, and then later serve as fuel, ostensibly ending the burn having consumed the whole tube with no structure left behind except perhaps a bit stuck in the feed mechanism. That's novel.
https://en.wikipedia.org/wiki/RocketMotorTwo
Also, I wonder how complex this is going to make the aerodynamic controls. You are losing moment arms between engine and the forward surfaces, if they exist.
Bonus points for naming.
That’s usually the major drawback to solid rockets.
(Apologies if that’s obvious)
Electric solid propellants (ESPs) is the term apparently. I saw youtube video where some guy was fooling around with the some brand or another of this type.
see https://ntrs.nasa.gov/api/citations/20190030421/downloads/20...
Rocket fuels can have some pretty nasty stuff in them as well, of course...
The whole "don't burn plastic" only applies if you don't know what type the plastic is.
Pure HDPE is just H and C, basically like candle wax with a much higher molecular weight. Products of combustion are H2O and CO2.
Chlorinated and fluorinated plastics are the ones that produce lots of toxic emissions when burned.
Burning plastic incompletely would most likely produce more noxious stuff than Kerolox or Methalox engines. Maybe on par with existing Solid Rockets or Hypergolics.
No one is allowed to be near the launch path or within 2-3 miles of the launch.
Go read the rest of the thread where they discuss the very technical reasons this is cool.
What kind of world do you live in where you equate ESG with being harmful? Sounds like "conservative uncle who doesn't know anything about the topic".
ESG in principle isn't harmful, but it does seem to attract grifters that game one or two of the E/S/G letters to make their firms appear more socially responsible:
https://www.bloomberg.com/opinion/articles/2023-07-12/it-s-easy-to-make-oil-companies-esg
https://www.offshore-technology.com/features/revealed-the-oil-gas-companies-leading-the-way-in-esg-2/
https://www.ey.com/en_us/oil-gas/what-if-oil-and-gas-operators-view-esg-as-a-catalyst-for-innovationSee the video.
At the very best it could stop SSTO to orbit suck so much. But it seems SpaceX is far enough that it is not possible to get close to the efficiency with an autophage rocket. It maybe was possible as long as first stage was always destined to crash, but when it can land safely the entire efficiency equation got turned on its head.
Would that kind of rocket be usable for deep space (basically -- improve thrust to weight by eliminating the weight of the fuselage as you thrust)? I also don't think so. For space, much higher specific impulse engines exist than any kind of fuselage material that can burn itself.
So I just don't see any use of this.
The delta-V per rocket stage is governed by the Tsiolkovsky equation:
dv = Isp * g0 * ln(m_start/m_end)
So, if m_end goes to zero, you get a significant dv gain.
It has absolutely nothing to do with the "efficiency" of returning a stage or not.
This is really neat. It doesn't have to be groundbreaking to be really cool, but who knows maybe one day it will be.
- Aviation didn't start with someone rolling out a 747, it started with the Wright Flyer - something that barely flew with just the right winds.
- Apple, Inc. literally started in a garage with an 8-bit computer with 8k ram and you had to provide your own case and power supply, monitor, and keyboard.
- The early stages of modern rocket technology looked not much different than this experiment: Here's a video of Goddard testing at his Aunt's farm:
https://upload.wikimedia.org/wikipedia/commons/transcoded/a/...
> "The neighbors complained"
> This both increases the maximum payload mass of a launch system and allows for the miniaturization of launch vehicles so they may be used as nano-launchers for the rapid access of small satellites to low-earth orbit.
Re-usabilitity is "not needed".
But in the meantime SpaceX learned to reuse the stages so suddenly the problems are no longer as much of a problem as they were before.
There isn't much point. I don't know an application where other approaches wouldn't be superior.