this is how I have been building all of my big rockets in SFS for missions to Venus and Mercury. Since I don't have infinite build space I simply launch multiple rockets and dock them in space. This is the same design that I used for the previous Venus Mission (   • Landing on Venus | Complete SFS Mission  . I am pretty sure all the players out there can make their own tweaks to this design to make it much better! Links for all the rockets that you see in this video are listed below:

Stage 1: https://sharing.spaceflightsimulator....

Stage 2: https://sharing.spaceflightsimulator....

Stage 3: https://sharing.spaceflightsimulator....

Side Booster: https://sharing.spaceflightsimulator....

Refueling: https://sharing.spaceflightsimulator....