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?