If you're interested in learning more about this kind of thing, you want to look stuff related to "ullage". Fun fact: it doesn't actually take that much time w

If you're interested in learning more about this kind of thing, you want to look stuff related to "ullage". Fun fact: it doesn't actually take that much time without forwards acceleration for ullage to become an issue. For example, the Saturn V included solid-fuel rockets on the interstage between the first and second stages to provide enough forwards acceleration for the second stage engines to ignite, even though the gap between the first stage engines shutting down and the second stage engines igniting wasn't more than several seconds [0, page 14]. You can see the exhaust from those motors on the outside of this video of the Apollo 4 first stage separation [1]. Another fun fact: instead of using smaller motors to provide acceleration between stages, (some?) Soviet rockets ignited upper-stage engines while the lower-stage engines were still firing ("hot-staging"). This is why some soviet rockets had a lattice (or lattices) between stages instead of solid metal. Yet another fun fact: it doesn't take much acceleration to sufficiently settle the fuel for ignition. The Saturn V's S-IVB stage used its equivalent of its RCS system, which is normally used for attitude control, to settle its fuel prior to firing for trans-lunar injection. If you're interested in installing KSP mods, RealFuels includes ullage simulation, so you'll need to have some method of providing acceleration to get larger rocket motors to ignite reliably. [0]: http://web.mit.edu/digitalapollo/Documents/Chapter5/saturnas501.pdf http://web.mit.edu/digitalapollo/Documents/Chapter5/saturnas... [1]: https://youtu.be/iJIY4n5oXjc?t=8 https://youtu.be/iJIY4n5oXjc?t=8 Edit: Fix description of hot-staging for Soviet rockets. Some Soviet rockets hot-staged their third stage and not their second stage, making the description incorrect.