[MarkusWandel]

The (early) Atlas ICBM was even less substantial than a soda can. There used to be one parked on the front lawn of our local science museum. It had to be kept inflated with pressurized air. When the time came to retire it and they let the air out...

https://ottawacitizen.com/news/local-news/museum-shrugs-as-a...

In service, of course, the necessary pressure would have been maintained by propellant boil-off.

[chipsa]

In service, it was pressured with nitrogen, just like it was while not in service. RP-1 is not an especially volatile fuel.