In this video: Rocket Lab - In Focus Launch 10/28/2020, at 13:50, it is seen that the middle of the stage of Electron turns from black to white:
I have seen this in other videos as well. Why is this happening?
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It's frost! That part of the tank holds the liquid oxygen. Liquid oxygen is very cold, and as a result, moisture in the atmosphere freezes onto the cold wall of the tank. The white stuff is ice. You can tell by how much the liquid oxygen filling process has proceeded by seeing how high the ice goes up the tank.
All rockets that use cryogenic fuels without insulation show the same behavior. It's visible on the Falcon 9, the Atlas V, and the first stage of the Saturn V, for some examples.