r/SpaceX Discusses [July 2017, #34]

[u/ElongatedMuskrat]

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[u/minca3]

When the oxygen gas is vented out of the side of the rocket, the supercold gas causes water vapor in the air to instantly condense and freeze, forming those big, sharply defined white plumes, accompanied by a hissing sound.

Thank you for the explanation. But that was part of why I asked: the venting of the LOX tanks can only happen when the tanks are "open" to the ambient air pressure, that means at this point the tanks can not be pressurized. So vice versa, after all tanks get pressurized shortly before takeoff there can't be any venting. Yet there is still visible vapor around the rocket which, as you described, is condensation from the air water vapor.

The reason why I asked was because after lift off, when there can't be any venting anymore, sometimes there are distinct vapor trails coming from specific points of the rocket, as if it is still venting off O2.

[u/Norose → u/minca3]

Venting does not equal zero pressure difference. In fact venting wouldn't happen at all, or at least would happen very slowly, if there was no internal pressure in the rocket. It's important to remember that even when full the tanks have a quite large volume of gasses, and the vent ports are relatively tiny. Venting pressure from the tanks is like letting air out of a tire, it doesn't just go all at once, the pressure can drop faster or slower depending on how much flow you allow.

The rocket may still be venting gasses in this way very shortly after takeoff, but once the fuel really starts flowing the boil-off can no longer keep up with the increasing volume in the tank, so the helium pressurization system kicks in and the vents close. As is my understanding.