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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 on trailer bed

Delta 4 stage, courtesy spacenews

Proton M stage on railroad car Proton M stage, courtesy americaspace and Roscosmos

Falcon 9 stage on trailer bed 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?

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    $\begingroup$ I believe the old Atlas required pressurisation. The newer ones are designed with transport in mind so a engineered appropriately. $\endgroup$ – tl8 Aug 17 '15 at 1:38
  • $\begingroup$ Remember, a rocket is mostly fuel. Those rockets are empty while being transported like that. $\endgroup$ – Loren Pechtel Aug 17 '15 at 1:41
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    $\begingroup$ I think Centaur upper stages still must be kept in tension or pressurized. Wikipedia seems to agree: en.wikipedia.org/wiki/Centaur_%28rocket_stage%29 $\endgroup$ – Organic Marble Aug 17 '15 at 2:48
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    $\begingroup$ Some rockets, like Ariane 5 are never transported horizontally since they are assembled vertically at the launch site. As for the Russians, their payload fractions (to LEO) are in the 2-3.5% range, so they have a lot more mass to work with. $\endgroup$ – Rikki-Tikki-Tavi May 10 '17 at 14:44
  • $\begingroup$ Ariane 5 is never transported horizontally while complete, but the first stage is transported horizontally from France to Kourou. $\endgroup$ – Hobbes Jun 15 '18 at 7:47
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It is true that some of the rockets, especially the initial Atlas family of launchers, are to be pressurized even during storage or they will collapse under their own weight.

Basically, these are simply steel balloons that will retain their shape and rigidity only under constant pressurization.

This 'steel balloon' design was essentially adopted from the ICBMs on which the first launch vehicles were developed. The first thermonuclear weapons were heavy and had to reach Soviet Union when launched from continental US.

enter image description here

Photo Courtesy: www.alternatewars.com

This meant that the launch vehicle should have a number of stages or be exceptionally light. The designers used a balloon tank to achieve this in Atlas ICBM.

In case of the ICBMs, they were constantly in a pressurized state, using Nitrogen. In case of Atlas ICBM (and launchers), the tanks should have Nitrogen at 34 kPa for rigidity.

However, with advances in materials and manufacturing techniques, the tanks no longer need pressurization (Centaur stage is an exception).

During storage and transport, these tanks were filled with Nitrogen. However, present day tanks can be stored and transported without pressurization, and most of them use the isogrid design.

enter image description here

Photo Courtesy: SpaceX

There are some launchers somewhere in between these two cases. For example, Falcon 1 first stage can be transported without pressurization; however, it gains additional strength when pressurized for flight.

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