How exactly do spaceship fuel tanks even work?

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Capslock118
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How exactly do spaceship fuel tanks even work?

Postby Capslock118 » Fri May 02, 2014 6:21 pm UTC

Hi everyone,

I have a ....burning :roll: ... question just popped into my head and I figure xkcd forums is the best place to ask this question.

Let's say a spaceship is in space and it's fuel tank is 50% full. How exactly do our spacey engineers make sure that the fuel in the tank is always ready to be sent to the engine rather than a vacuum existing between the fuel line and the fuel itself? I'm thinking of a theoretical spaceship that has flipped around a couple times in space to mix the fuel tank (for fun or some other purpose).

Maybe we create some kind of compression in the fuel tank so that the fuel is always "at the bottom" of the tank? I assume this isn't necessary while you are in an active burn since the thrust would keep the fuel "at the bottom".

Does this question even make sense? I'm not sure how to ask google...

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gmalivuk
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Re: How exactly do spaceship fuel tanks even work?

Postby gmalivuk » Fri May 02, 2014 6:33 pm UTC

I imagine you'd just set it up so internal pressure forces the fuel out of the tank. A pressurized vessel doesn't care too much if there's gravity around.
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Re: How exactly do spaceship fuel tanks even work?

Postby Izawwlgood » Fri May 02, 2014 6:45 pm UTC

You're probably asking about liquid fuels, but for what it's worth, solid fuel tanks are literally just that, and the reaction is burning a core through solid matter by forcing a reactant along it.

Don't forget the liquid tanks are under pretty extreme pressure, and probably extremely cold. I wager there are a handful of tricks involving pumping cold liquid over warmer engine parts to both cool said parts and to cause the liquid to expand for further pumping. But yeah, these things aren't just giant cylinders filled with liquid and a burny part at the bottom; there's probably a lot of engineering involved to move liquid around.
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Re: How exactly do spaceship fuel tanks even work?

Postby brenok » Fri May 02, 2014 7:04 pm UTC

Doesn't the Up goer five comic say that the tank is filled with funny voice air (helium) as it empties? At least in Saturn V, anyway.

Capslock118
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Re: How exactly do spaceship fuel tanks even work?

Postby Capslock118 » Fri May 02, 2014 7:24 pm UTC

I was thinking of liquid fuels, thanks for the clarity.

you guys are great, thanks for the information. also thanks for the up goer five reference; i forgot about that comic.

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Re: How exactly do spaceship fuel tanks even work?

Postby PhoenixEnigma » Fri May 02, 2014 7:30 pm UTC

If I understand what you're asking, you want to know how rockets make sure to draw fuel from a liquid fuel tank instead of vacuum or a pressurizing gas or fumes when being started in a zero-g environment, since you couldn't rely on the denser fluid settling in any one place? If so, one answer is ullage motors, which are small rockets fired to force fuel to slosh to the correct end of the tank. Once the main engine(s) is started, that provides the acceleration needed to keep fuel where it's supposed to be, as you surmised.
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cjameshuff
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Re: How exactly do spaceship fuel tanks even work?

Postby cjameshuff » Sat May 03, 2014 4:33 am UTC

In addition to ullage motors, rockets intended to start in freefall can use internal meshes and other structures keep the propellant in place via surface tension.


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