What is the solution to stop fuel inside starship collecting in center of the tank during zero G flight? Here is a rough sketch of starship with rudimentary diaphragm system to control flow of fuel while in zero G:


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    $\begingroup$ Please type the text directly into to the question instead of having text in the image. $\endgroup$
    – AJN
    Sep 24 at 18:59
  • $\begingroup$ If posting a new question - suggest looking up ullage, thermal expansion (I think the scheme needs negative expansion co-efficient which is rare but not impossible?) and possible have a look at youtu.be/Ehp2Y_JWVO8?si=BaGeas_USaaBiupT&t=352 for another way to do this. Then in the question link to things where relevant to show what you already understand. $\endgroup$ Sep 26 at 9:05
  • $\begingroup$ Hi levis - welcome. Please read our tour and How to Ask pages to understand the site rules before posting - your downvotes are likely to be: because you have used hard to read text in an image, you have made a lot of spelling mistakes, which makes that text even harder to read, and you don't seem to have read up on ullage, which is an essential part of liquid fueled rockets. $\endgroup$
    – Rory Alsop
    Sep 26 at 11:03
  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Space Exploration Meta, or in Space Exploration Chat. Comments continuing discussion may be removed. $\endgroup$
    – Rory Alsop
    Sep 26 at 11:04

1 Answer 1


In practice it's not worth preventing in large fuel tanks. The fuel is simply assumed to have floated away from the engines and any attempt to light them will at best fail (and at worst blow up your rocket due to uneven feeding.)

Rather, you use a much smaller rocket (could be the reaction control system) which actually does have bladders to prevent that from happening. You burn just enough to get the fuel in the main tanks to go to the engines, then you light the main engines. Alternately, you have some "small" (they can actually be substantial--I'm not finding the size of the ones on the second stage of the Saturn V, but the third stage ones were 3' tall) solid rocket motors to do the same thing.

The term you're looking for is an ullage burn.

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    $\begingroup$ "which actually does have bladders": or which doesn't use liquids in the first place. Cold gas thrusters could use reservoir tanks pressurized the last time the vehicle was under thrust, or some kind of centrifugal separator/boiler to keep any entrained liquid from reaching the thrusters. $\endgroup$ Sep 25 at 4:32
  • $\begingroup$ I apologise for my spelling and grammatical errors but as a dyslexic adhd/ asd suffer I struggle in communicating effectively this was the best I could do in trying explain my thought process my apologises if my poor grasp on rocket science is lacking $\endgroup$ Sep 25 at 16:52
  • $\begingroup$ @levisspringer I understood what you were after, I was just explaining that that's not how it's actually handled. $\endgroup$ Sep 25 at 23:11

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