I noticed that many rockets come out of the assembly building already with their fuelling umbilicals attached (I mean, with the entire umbilical tower - like Saturn 5, Ariane 5). Once in the launchpad, the fuelling of cryo propellants starts. My question is if there is feasible to attach manually these fuelling lines on the pad (ie, the rocket goes alone to launch table (like CZ-2FG, Soyuz, Zenit, etc) and only then the LH2 lines are attached. (CZ-2FG, Soyuz, Zenit doesnt uses LH2 - and LH2 fuelling is my main concern). Is there any safety issues for ground crews to operate attaching procedures for the LH2 fuelling lines on the pad, often in open air?
For the SpaceX Falcon 9 the fuel lines are 12.5 cm/5" (link: youtube video, SpaceX factory tour by Elon Musk) in diameter, so those are pretty big hoses to attach manually. LH2 is under pressure, making for a heavier hose. So you probably need hoisting tools to get the hose into place.
The attachment points for upper stages are fairly high up, meaning you'd need an access bridge at that point, and stairs or a lift to get up to them. These (and hoisting tools) are available in the assembly building.
If you wanted to add these items to the launch tower, you'd have to protect them from the extreme heat of the launch. The Ariane launch tower for example is a simple column. Adding all this stuff would make it more complicated for no apparent benefit. Why would you want to leave attaching the umbilicals until the rocket is at the launch platform?
LOX and LH2 are super cold. This makes the connection points much more complicated. It also means a more complicated leak check. By attaching them in the building, they can be connected under controlled conditions (no contamination or moisture to complicate things) and can be tested using sniffers. Attaching them at the pad would be much more difficult. Also remember, the F9R is assembled horizontal, not vertical like the Saturn and other rockets. This makes it much easier to access areas in the barn.