According to Figure 2-1 of the SA-503 flight manual the LH2 continuous vent was opened during the coast phase, and closed during the ullage stage of the S-IVB restart T_6 + 0:00:42.0 (but open for some of the initial firing's ullage burn T_5 + 0:00:59.0).

Given the Propellant Venting section on page 6-13 mentions its use for settling the propellant, why do we see it used primarily during the coast phase, and not for the restart? Additionally, why are they included if the Auxiliary Propulsion System already has ullage engines built-in?

  • $\begingroup$ LH2 continuous vent , please add the time information, T5 + 59 seconds? Please structure your question by inserting line breaks and blank lines to increase readability. $\endgroup$
    – Uwe
    Commented Jul 17, 2021 at 10:39

1 Answer 1


The Press Reference section on the third stage states

The continuous vent system is used to provide a thrust force required to position propellants at the aft end of each tank during coast.

So there was a requirement to settle the propellants between burns. The reason for this requirement is not given in this source. Speculation: to prevent the mass properties from changing during the coast phase.

  • $\begingroup$ The J-2 engine required the propellant be settled prior to restarting. To facilitate this, the Auxiliary Propulsion System had engines to provide the ullage. If we look at T_6 + 0:00:42.2 we see the LH2 vents were closed prior to this burn (yet they were open at T_5 + 0:00:59.0 for some of the ullage burn). $\endgroup$ Commented Jul 18, 2021 at 11:48

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