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The Falcon Heavy test flight included a demonstration of capability to coast for several hours and then re-light the second stage for a final burn. During the coast, the cryo propellants are absorbing heat and slowly boiling off. The pressure in the tanks would be managed by venting. My question is: venting what? Only gas (ideally), or a potential mixture of gas and liquid? If gas only, how would that be accomplished in zero gravity? In other words, how would the liquid phase be conserved and only "boiled" gas released?

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    $\begingroup$ There is no top of the tank in zero gravity. The third stage of Saturn V had solid fueld ullage motors as well as liquid hypergolic fueled ullage engines. See here. $\endgroup$ – Uwe Feb 11 '18 at 18:38
  • $\begingroup$ @Uwe You're quite right. My comment was based on inaccurate memory; I've deleted it. $\endgroup$ – Wayne Conrad Feb 12 '18 at 2:24
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Different stages use different means; Centaur uses(used) a thermodynamic vent mixer system for its LH2 tank.

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This device served to keep the bulk propellant well mixed, and ensured that only gas was vented.

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Theoretically - A small ullage thrust could be applied before opening gaseous vents on the opposite end from where the propellant settles.

In practice I'm not sure this is even necessary, the quantities should be fairly trivial.

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