Quote from recent answer: "Some rockets [can't be transported] horizontally unless their tanks are pressurized".
Oh? How is transportation handled then, and which rockets are like this?
Rockets are made only just strong enough to withstand the forces they are subjected to, otherwise the 1% to 3% or so that is available for payload vanishes to nothing. This I understand. I went looking for examples of a few different rocket stages being transported:
Delta 4 stage, courtesy spacenews
Proton M stage, courtesy americaspace and Roscosmos
Falcon 9, from reddit
Falcon 9s, Delta 4s, and also Atlas stages are supported the same way - at two points, near the top of the stage, and just above the engines. I'm not sure what is happening with the Proton. Maybe that belongs in a separate question.
So, those are pretty long unsupported spans. Are they cases where the tanks are pressurized,thus adding rigidity? What do they put in the tanks? How are they placed on the trailer beds, and taken off them?