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Is ullage control a thing only engine startup? Would a rocket like the Saturn V or Falcon 9 leave an engine running for a second or two longer just for ullage control?

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  • $\begingroup$ Run an engine longer than what? $\endgroup$
    – Russell Borogove
    Commented Apr 5, 2021 at 21:04
  • $\begingroup$ Longer than the other engines: shut down all but one engine, say, then kill that last engine after a second or two. No clue if this makes sense. I ask because I think I've read something like this is done, maybe in the context of the space shuttle, but I never paid close attention to ullage, so all I have is a vague memory that may be pure BS. Just checking if something like this is done, I guess, so I can carry on without misconceptions :D $\endgroup$
    – user39728
    Commented Apr 5, 2021 at 21:13
  • $\begingroup$ You could do it the Soviet way, and ignite the upper stage before shutting down the first stage. This completely removes the ullage need, but adds a bit of complexity in interstage design. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Commented Jun 9, 2021 at 7:53

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Yes, it is only done on engine startup. Remember, ullage is done so that liquid propellant is fed to the engines, instead of gas bubbles. When a tank is allowed to sit in microgravity for a significant amount of time, there is a risk of some of the gas bubbles in a partially-empty tank migrating to the tank's outlet at the bottom. An ullage burn at the end of engine firing would do nothing to stop this process. Instead, you do the ullage burn at the start of engine firing, to let the liquid in the tanks settle to the bottom.

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  • $\begingroup$ Thanks! Would 15s between MECO and stage-2 engine startup be too much? Or would stage the engine maybe be able to fire right away without ullage control? $\endgroup$
    – user39728
    Commented Apr 5, 2021 at 21:27
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    $\begingroup$ @user39728: Best to use ullage in that case. I have numbers handy for Saturn IB. First stage engine cutoff at T+140.65, second stage engine start T+143.35. That's less than 3 seconds between stages, and they still used ullage motors! $\endgroup$
    – DrSheldon
    Commented Apr 5, 2021 at 22:23