> with the plastic fuselage supplying up to one-fifth of the total propellant used.
> 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.
> So it doesn't matter if it makes up 10% of the mass consumed - what matters is how much impulse it provides.
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.
Yes, the cool thing about this approach is that you can reach extreme vehicle mass ratios without staging. So for example, it could make a single stage to orbit vehicle feasible, on re-entry you replace the cheap consumable "body" and you are ready for another launch.
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.
It’s been said that there’s no new ideas in rocketry. Even the most cutting edge stuff was theorized and probably even tested in some rudimentary way by some crazy aerospace engineers in the 50s. They just watched it explode and gave up because rocketry was as much art as science back then - literally throwing stuff against the wall and seeing what explodes (and what explodes a little longer and more usefully).
You also need something that can transfer the thrust to the payload. If you’re burning this thing as an SSTO, it needs to survive being shoved forward at rather more than 1g with payload attached, presumably at the end opposite the nozzle.
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 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 :)
I would note the question isn’t should you add more rocket fuel instead of burning the structural mass, it’s can you convert the necessary structural mass into impulse vs being dead mass. You need the structural mass one way or another, and adding more fuel will add structural mass. By burning the structural mass as it becomes less necessary instead of simply ejecting it you maximize delta-v.
There have been several small engines that use acrylic housing as combustion chamber and fuel, combined with liquid or gaseous oxygen to form a hybrid rocket engine. None of this sounds new, but I haven't looked deeply into what they're doing here.
What are the exhaust products of HDPE? Any larger molecules will reduce the exhaust velocity, which will lower the specific impulse compared to methane even if they are producing the same amount of energy.
Wow, fascinating! From the title I was envisioning something akin to a solid rocket motor where the fuel gets consumed from the inside out and wondering what the point of that was, since at the end of the burn you'd still need to have enough fuselage left to avoid compromising structural integrity. (And at that point why not just build the fuselage like that from the start and use the extra mass for more liquid fuel?) But from the video it looks like this engine actually burns the hull back to front, meaning the fuselage actually gets shorter throughout the course of the flight until there's nothing left. That's really cool!
Yes I was absolutely not expecting a hot glue gun with a jet of flame coming out of the nozzle instead of hot glue, but that’s essentially what it looked like.
Is the idea that in a vertical version of this the motive force pushing the stack into the nozzle would be provided from the rocket's own acceleration?
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).
> The concept of a self-eating rocket engine was first proposed and patented in 1938. However, no autophage engine designs were fired in a controlled manner until a research partnership between Glasgow University and Dnipro National University, Ukraine achieved this milestone in 2018.
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.
Right, but there you're either putting the acrylic in an extra structural tube, or you're using an acrylic tube which is hugely thicker than necessary so you can leave a substantial unburned shell there as the structure.
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.
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.
>The Ouroborous-3 uses high-density polyethylene plastic tubing as its autophagic fuel source, burning it alongside the rocket’s mix of gaseous oxygen and liquid propane.
Promin Aerospace, a Ukrainian company founded by a friend of mine, has been working on the same technology. Interesting to see this take on it though, it's an idea that seems almost too fantastical to work.
Weirdly there are solid fuel rockets that can be turned on and off.
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.
I wonder if it'd be possible to design the body in such a way that the nozzle shape changed as the rocket rose through the atmosphere, so that the nozzle was continuously optimized for ambient air pressure.
Hm, you normally need more nozzle (/bell) as you ascend. I'm not sure how to square this with autophagy but if you figure it out I will subscribe to your newsletter.
Burning plastic sounds like an environmental hazard which is completely unaddressed in the article. At full scale, the communities near the launch site will never accept it.
There's a massive difference between HDPE (which is a hydrocarbon) and PVC (which contains chlorine and is awful to burn). Might be some plasticizers in the mix as well, but mostly they're just H, C, and O. Cl makes the real nasty stuff.
Rocket fuels can have some pretty nasty stuff in them as well, of course...
Most fuel mixes for rockets are just Kerosene and Liquid Oxygen, at least in the USA. Seems like most of the next generation are aiming for Methane and LOX.
Burning plastic incompletely would most likely produce more noxious stuff than Kerolox or Methalox engines. Maybe on par with existing Solid Rockets or Hypergolics.
> What kind of world do you live in where you equate ESG with being harmful?
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:
The discussion in the comments indicates that the test included a run outside of it's operational range for pulsed firing, with the intent to see how it would react.
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.
Indeed many (most?) technology breakthroughs and great products start as "garage" experiments.
- 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:
> 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.
As I said, it reduces problems of SSTO (Single Stage To Orbit). The problem of SSTO is low payload mass to the size of the rocket and fuel necessary to transport it. The main solution to this is having multiple stages. Which comes with its own problems -- multiple engines, complexity, more points of failure, etc.
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.
> 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...
[2] https://www.mdpi.com/2226-4310/6/4/45