What you are referring to is an ullage burn. Apollo definitely had ullage burns that mitigated this problem before lighting up the rockets in earnest. https://

What you are referring to is an ullage burn. Apollo definitely had ullage burns that mitigated this problem before lighting up the rockets in earnest. https://en.wikipedia.org/wiki/Ullage_motor https://en.wikipedia.org/wiki/Ullage_motor Another note is that it wasn't just for the calculation of how the main burn would go. If the propellant was sufficiently distributed in the tank, not enough propellant would actually get to the valves at the back of the tank, and the engine could potentially not even fire.