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After seeing all the Starship failures, having something with a membrane keep things in place seems like an obvious solution. Has it been considered or tested - or do we simply lack a material that is elastic at the (cryogenic) temperatures required? What would it theoretically take? What subjects would you need to study to develop such a material?

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    $\begingroup$ A similar idea is using moving diaphragms for the tank ends. No image link but this Youtube video shows an example (link is to point in video where diagram is shown). $\endgroup$ – Alex Hajnal Apr 21 at 8:44
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    $\begingroup$ @BenCrowell I presume that's a reference to the various Starship prototypes. SN1, SN2, ... $\endgroup$ – Alex Hajnal Apr 21 at 18:46
  • $\begingroup$ @BenCrowell ...with SN standing for Serial Number. $\endgroup$ – Alex Hajnal Apr 21 at 18:54
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They do!

Many propellant tank, especially those required to work in zero-g environments, do use just such a bladder-inside-a-tank for the fuel. Typically monopropellants for thrusters. enter image description here

It completely removes the requirement for Ullage of the propellants, but adds complexity, cost, mass and failure modes.

Additionally, flexible bags are a bit hard to make at deep cryogenic temperatures.
Additionally, flexible bags that can contain 200 tons of cryogenic liquid sloshing around under 5 g of acceleration are.... simply impossible to manufacture.
I don't think we will ever have the technology to make a soft bag that can hold 1000tonnes (200 tonnes under 5g) of cryogenic liquid that is sloshing around.

For interest: take a peek at this. Work on developing a bladder that is cryogenic compatible. But max size very much small.
https://www.geekwire.com/2020/ancient-art-origami-provides-pathway-building-better-tank-rocket-fuel/

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  • $\begingroup$ It's worth noting that the ones that operate long term aren't cryogenic fuels, so... $\endgroup$ – PearsonArtPhoto Apr 20 at 20:32
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    $\begingroup$ Do any boosters use these? $\endgroup$ – Organic Marble Apr 20 at 21:08
  • $\begingroup$ @OrganicMarble From the first link: "Our bladder tanks are used on the 400 N Attitude and Orbital Control System (AOCS) of all versions of Ariane 5 and in the VEGA 200 N Roll and Attitude Control System (RACS)." $\endgroup$ – Dan Mašek Apr 21 at 8:05
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    $\begingroup$ Those are RCS systems. Not the booster tankage. $\endgroup$ – Organic Marble Apr 21 at 11:09
  • $\begingroup$ Aahh... it's the math again :-D $\endgroup$ – Dagelf Apr 23 at 20:30

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