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I'll be the first to admit I know little to nothing about fluid dynamics, especially in a pressure vessel. I was wondering how pressurized rocket fuel tanks don't cause weight-distribution problems when they begin to tilt, or do they?

This is what I was thinking through basic observations:

  • I have a water bottle in my hand, it is full.
    • If I flip this bottle in the air it will flip uniformly.
  • I drink half the water in the bottle and flip it again.
    • On the first half rotation, the liquid sloshes to the other side.
    • This force causes the rotation to not be uniform.

However, with a pressurized can of aerosol or liquid:

  • I flip the can when it's full.
    • It flips uniformly.
  • I flip the can when it's half-empty.
    • It flips uniformly. - According to @Uwe my assumption is false, however at a small scale like this the "slosh" may not be obvious.

This may be more of a physics question at this point, but why does it work like that? Does it?

I guess the main questions after my random rambling would be:

Can someone explain the potential problems with sloshing for different types of fuels, 1 pressurized example and 1 non-pressurized example would be great to show how a pressurized vessel would "slosh"? Maybe a diagram would help me understand better.

Magic Octopus Urn
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    The pressure inside the tanks could not prevent sloshing of the liquid. The pressure is needed by the pumps to prevent gas bubbles and cavitation. – Uwe Nov 30 '18 at 15:55
  • @Uwe so I'm entirely off-base in my assumption that pressurization causes the propellant to be more-evenly distributed? Cool. That actually works better for the question, how in the world do they manage and minimize that slosh then. – Magic Octopus Urn Nov 30 '18 at 15:56
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    the acceleration of the rocket keeps the fluid at the bottom of the tanks, baffles stop sloshing. –  Nov 30 '18 at 15:59
  • @JCRM at the bottom, but not technically... uniform. It would be the lower triangle of the tank if it was tilted, correct? – Magic Octopus Urn Nov 30 '18 at 16:47
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    Our simple slosh model of the propellant tanks in the External Tank assumed that the top 25% of the liquid volume sloshed (may not have been exactly 25%, it's been a long time, but only the upper portion). Pressurization has no effect. The model shut off at MECO because then the residual propellant was free to drift around in the tank volume (ignoring capillary/wetting effects). – Organic Marble Nov 30 '18 at 16:49
  • No, both the tank and fuel are being accelerated by gravity, so it's effect cancels out -- effectively they are in freefall. Only the tank is being accelerated by the engine, so the tank accelerates the fuel in the direction it is being accelerated, which is in line with the engines - so the fuel goes to the bottom of the tank - bodylift may cause a slight lateral force, causing the fuel to go a little off centre, but pretty level spacex second stage fuel tank –  Nov 30 '18 at 21:12
  • @JCRM shuttle had a pretty good tilt angle of the thrust vector during second stage. It was far from level. – Organic Marble Nov 30 '18 at 21:31
  • yup, @OrganicMarble, but that level would be determined by the thrust vector, not the attitude. –  Nov 30 '18 at 21:42
  • Don't see any contradiction with what I said. – Organic Marble Nov 30 '18 at 21:50
  • Some papers about anti slosh baffles in tanks, 1, 2, 3, 4, 5. – Uwe Dec 02 '18 at 17:38

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