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A particular spacecraft engine is designed to work with a propellant made up of very dense dust. If this engine was on a ship that was designed for constant acceleration through space, would there be any problems with storing the propellant in a giant exposed funnel, as opposed to an enclosed tank? It should work, as solids can generally survive in a vacuum, especially if they have a high melting point, the constant acceleration will keep the dust from floating away, and it would be far easier to refuel. Are there any problems that I've missed?

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  • $\begingroup$ Very dense - it serves some purpose? Regular density is not acceptable for some reason? Which reason it is? - it may be important for an answer. Generally looks workable, lid helps for convinience but not a necessity. $\endgroup$
    – MolbOrg
    Sep 2, 2021 at 10:53
  • $\begingroup$ @MolbOrg A funnel that is designed to hold a certain mass of dust will be smaller and lighter if the dust takes up less volume (i.e. is denser) $\endgroup$ Sep 2, 2021 at 11:02
  • $\begingroup$ Eh? Lol, okay. You can have no funnel if you store your reactive mass properly in blocks, and make the dust when the fuel is actually used, that if you care that much about mass of funnel. It itself can be made of that solid, to be crushed for dust, etc. You also underestimate (maybe) the amount of reactive mass you need for constant acceleration for significant time. But it is up to you, but I would recomend to make some simple visualisation of your design(paint picture) and figure out some numbers and add to this q or make another one for purpose to refine design, looks u may need it. $\endgroup$
    – MolbOrg
    Sep 2, 2021 at 11:14
  • $\begingroup$ What you have missed is to explain why would the propellant propel the ship insted of just propelling itself out of the funnel. $\endgroup$
    – AlexP
    Sep 2, 2021 at 11:29

2 Answers 2

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Exposed solid-dust fuel in space is probably not a good idea, for several reasons:

  • No protection from radiation. This is probably minimal, but many materials degrade or decompose when exposed to high amounts of radiation, as you'd get outside of a magnetosphere. Even a thin layer of shielding can make a big difference against particularly energetic particles.
  • Inability to maneuver/stop. If the only force holding your propellant powder in the funnel is the constant acceleration of the craft, then all the fuel will drift away when you stop or when the spacecraft turns.
  • No protection from micrometorites. If you were on the ISS and put your ear against one of the bulkheads, you would be able to hear the constant pinging as random space particles collide with the station. Similarly, your fuel would be bombarded with debris and other particulate matter, particularly if you're working at high speed. Worst case, one of these debris-grains could be just the right thing to light your solid-fuel storage.

A good analogy would be a bucket of sand in a car. Sure, most of the time, it wouldn't be a problem, but maybe you open the windows and it blows everywhere or you turn a bit too hard and suddenly you've doomed yourself to several hours of vacuuming. Although it wouldn't be used >99% of the time, the amount of investment that a simple lid would be is so small that it'd be silly not to get one.

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  • $\begingroup$ @MolbOrg for fuel to be useful as fuel, it needs to have a high energy density and a (presumably) very specific chemistry. In contrast, the stuff that you can find naturally in space and that gets bombarded in radiation all the time (asteroids, planets, etc.), tends to be very low energy as all the reactions that could've happened already did. Lunar regolith, for example, mostly consists of oxidized (burned) material. $\endgroup$
    – Dragongeek
    Sep 2, 2021 at 11:39
  • $\begingroup$ Hm, seems due the "dust" thing I automatically discarded the possibility of the thing to be a fuel. So I retract that comment. Still think you put too much focus on that decomposing, and not enough on conditions, as if it such a big deal immediately(and just one word about that it is not so), but yeah it being fuel it possible, even if it shielding for itself as well(outside layers). // @LiveInAmbeR this is rather what q-OP has in mind, MolbOrg just misread and misunderstood things, lol $\endgroup$
    – MolbOrg
    Sep 3, 2021 at 9:08
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Stuff can hang up in a funnel.

funnel flow

https://www.powderprocess.net/Powder_Flow/Arching_Bridging.html

And it might be tricky for your spacefarers to poke it with a long stick or jiggle it. Solid particles interact with each other according to size and surface properties. If your particles are milled to facilitate smooth flow that might be no problem. If they are less uniformly shaped with edges that can hang up you might have no flow. I am sure you can recall playing with a funnel and sand at the beach. Sometimes the sand stops flowing.

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