StackExchange 5/2/2020
Rockets are basically horizontally or vertically assembled and integrated. SpaceX selected horizontal integration for its current Falcon rockets, essentially for reasons of lower cost. This orientation was significantly different from the choices made by the vast majority of the other rocket operators.
All evidences show that the new vehicle, Starship is vertically assembled with all sorts of cranes, and SpaceX renderings show vertical integration of Starship and the booster. So for what reasons did SpaceX choose vertical integration instead of -- allegedly simpler and cheaper -- horizontal integration for Starship?
There are two separate operations mentioned in the OP: Manufacture of the stages that make up the stack, and Integration of the stack prior to launch.
Integration
The integration of a 2 stage rocket consists of 2 separate operations: integration of the payload to the second stage, and integration of the second stage to the first stage.
Integration of the first stage to the second stage is traditionally done inside a building. For smaller rockets such as the Falcon 9 and Soyuz, it makes sense to do this inside a standard building. These tend to be too low to stand a rocket up in. You then have the issue of standing the rocket up afterwards. For very large rockets such as the Saturn V and the Space Shuttle integration of the first and second stage is done vertically. This requires an extremely large specialized building and access equipment, but avoids the complications and risks of erecting the complete stack once it is integrated. The larger the rocket is, the more effort will have been made to save weight on the structural design, and the more challenging horizontal integration and subsequent erection will be.
In the case of Starship, the booster is designed to take off vertically, land vertically, and be ready to reuse quickly. The booster needs no heat shield as it only operates at suborbital velocity and most of the parts requiring servicing (in particular the engines) are at the bottom. The booster should never need to be laid down horizontally during normal operations. Hence vertical integration of the upper stage to the booster is the only option. And we know that it will be done outside, not in a building.
To my knowledge we have no information on how the payload will be integrated into the upper stage. It may well be done horizontally for some payloads. The upper stage will require a greater range of maintenance tasks than the booster, and I think it likely that it will need to be laid down horizontally at some point for work on the upper fins and heatshield. After that it would need to be stood up vertically. Here's a video of the space shuttle being stood up vertically and mounted to the external tank. Payloads could be integrated with the orbiter either horizontally or vertically, see section 7.3.3 of the shuttle user guide.
More:
https://space.stackexchange.com/questions/43840/why-did-spacex-choose-vertical-integration-for-starship
