[bediger4000]

Liquid handling in microgravity has always been weird. Big gas bubbles in the fluid, surface tension effects causing liquid to float in balls in the ullage, stuff like that. Turbopumps break if they ingest a larger bubble.

There could be some odd failure modes I would think. Failure to pump the liquid, broken pumps, who really knows? My guess would be that a failure mode would be a big spill, a failure to pump, only partially refilling, or broken turbopumps before an explosion.

[MadnessASAP]

For something like a transfer between Starships you can resolve a lot of those problems by (very) gently spinning the 2 craft. It won't take much force for the liquids to settle at the bottom of their respective tanks where you would presumably put the intakes.

[pmontra]

A probably very naive question: why not pistons?

[labcomputer]

Because there’s a much, much simpler and easier way:

1. Connect the two ships

2. Connect the liquid valves from both cryo tanks together.

3. Spin the ships about the short axis

4. Open the vent valve for the cryo tank to receive liquid.

5. Lock closed the vent valve for the cryo tank to supple liquid.

Steps 2, 4 and 5 are how you normally transfer cryo fluids between dewars on earth. You just to create pseudo gravity / acceleration in the body frame of the ships to make it work in space.

[bediger4000]

NASA has used tanks with a bladder. Pressurizing gas on one side of the bladder, all fluid inside. Cold liquids (methane in SoaceX case) means materials are crucial. A big piston is heavy and could jam.

[idiotsecant]

Seems like you could use peristaltic pumps

[pants2]

That would take ages!

[idiotsecant]

Why?