5 ms·
> 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 o
by dmurray 3y ago
> 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.
[0] https://en.wikipedia.org/wiki/Heat_of_combustion https://en.wikipedia.org/wiki/Heat_of_combustion
[1] https://www.researchgate.net/publication/336848986_Production_of_Fuels_From_HDPE_and_LDPE_Plastic_Waste_via_Pyrolysis_Methods https://www.researchgate.net/publication/336848986_Productio...
[2] https://www.mdpi.com/2226-4310/6/4/45 https://www.mdpi.com/2226-4310/6/4/45
- userbinator 3y agoParaffin wax is chemically similar to HDPE and that table puts it at 46MJ/kg, which seems about right.
- MPSimmons 3y agoThere 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.
- wcoenen 3y ago> 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.
- pletnes 3y agoThis, a thousand times this - there’s a logarithm in there, folks.
- IshKebab 3y agoDid you forget about the payload?
- metabagel 3y agoIsn't the payload pretty inconsequential in comparison with the total weight of fuel at launch?
- IshKebab 3y agoIt's not inconsequential in comparison with the total weight of the rocket at the end. That's what we're talking about.
- db48x 3y agoThe logarithm is far more troublesome than the payload.
- cornholio 3y agoYes, 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.
- romwell 3y ago>Yes, the cool thing about this approach is that you can reach extreme vehicle mass ratios without staging. 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.
- fnordpiglet 3y agoI 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.
- skykooler 3y agoWhat 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.
- userbinator 3y agoPerfect combustion yields H2O and CO2, same as methane.
- marcosdumay 3y agoWith the correct mixing, water and CO2. 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.