I found "Technical Information Summary AS-501: Apollo Saturn V Flight Vehicle" when looking for info about the Saturn V staging sequence. It contains a nice diagram on pages 14-16 showing the various events from liftoff to TLI, with approximate times after liftoff for each event. The ordering of events for the TLI burn caught my eye, though. Here are the events prior to the reignition of the S-IVB, with approximate time after liftoff:

  • S-IVB Restart preparations ~11,235
  • S-IVB Ullage engines on ~11,235
  • S-IVB LH$_2$ and Lox vent valves close ~11,235
  • S-IVB LH$_2$ and Lox chilldown pump off ~11,561
  • S-IVB Ullage engines off ~11,565
  • 2nd ignition S-IVB engine ~11,570

Specifically, the ullage engines shut down prior to the S-IVB igniting. This seems strange to me, as I thought that the acceleration was needed to ensure the fuel was settled against the bottom of the tank. Cutting the ullage engines prior to ignition seems like it'd give the fuel an opportunity to drift away from the fuel inlets, potentially causing issues.

Why do the ullage motors cut out before S-IVB ignition? Is it an error in the diagram? If not, why did this not problematic for the Apollo missions?

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    $\begingroup$ It was only for 5 seconds. $\endgroup$ Oct 25, 2018 at 18:24
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    $\begingroup$ @OrganicMarble So was the assumption that there just wasn't enough time for the fuel to drift away? I don't know what rate the fuel drifts away in cases like this $\endgroup$
    – awksp
    Oct 25, 2018 at 18:52
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    $\begingroup$ Looking at pg 40 it takes almost 5 seconds from the start command to when the J-2 reaches 90% thrust. Maybe this is just a definition of terms thing. Anyway, that's a nice document, thanks for posting. $\endgroup$ Oct 25, 2018 at 19:01
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    $\begingroup$ If they start the fuel and oxidiser pumps before ignition that flow of cold mass should produce some thrust so that the fuel and oxidiser does not drift away from the pump inlets. $\endgroup$
    – lijat
    Oct 26, 2018 at 6:21
  • $\begingroup$ @OrganicMarble I guess that would depend on whether "2nd ignition S-IVB engine" corresponds to the 0 second mark on page 40. At least to me, it seems to imply that the ignition command is given 5 seconds after ullage shutdown, but as you said it may be a definition thing. $\endgroup$
    – awksp
    Oct 26, 2018 at 16:30

2 Answers 2


This document states (sorry for image, I couldn't copy the text) that after the ullage engines cut off, the LH2 Continuous Propulsive Vent System kicks in and continues to settle the propellants.

enter image description here

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(Diagram from here - emphasis mine)

  • $\begingroup$ This doesn't seem to answer the question, which is about restart for TLI, which uses two (70 lb thrust each) ACS thrusters, since the solid APS ullage thrusters have already been used (and jettisoned). See the answer to space.stackexchange.com/questions/31639/… $\endgroup$
    – amI
    Jun 20, 2019 at 5:08

The S-IVB had ACS thrusters, one of which was used for ullage but not part of the "ullage thrusters" usually listed.

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    $\begingroup$ Interesting. Do you have a source for that? $\endgroup$
    – Hobbes
    Jun 18, 2019 at 19:31
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    $\begingroup$ I'm a tad late to the party, but the ACS thrusters in question are the Auxillary Propulsion System. You can find more detailed information in the SA-503 flight manual, beginning on page 6-20. $\endgroup$ Jul 16, 2021 at 16:17

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