You're correct - the Atlas (ICBM as well as the current Centaur second stage) design that requires constant pressurization is known as a balloon tank.[0] SpaceX

You're correct - the Atlas (ICBM as well as the current Centaur second stage) design that requires constant pressurization is known as a balloon tank.[0] SpaceX doesn't use balloon tanks, but it does pressurize tanks with nitrogen during transport to keep the tank walls rigid and prevent damage. [0] https://en.wikipedia.org/wiki/Balloon_tank https://en.wikipedia.org/wiki/Balloon_tank