I was also thinking about this and am working on an extended tech tree that would push most branches back by two nodes, adding a "Post Apollo" (tier 9) and "Pre-Modern" (tier 10) nodes to the rocketry branch. I also moved the Mercury capsule back as mentioned, moving the node to tier 4 and adding aviation3 as a requirement to the node. As for the other parts, I'm going to structure them based on balance and chronology, trying to keep the Delta IV and Atlas V at "Modern" (now tier 11,) Apollo in the same spot (tier 8,) and the rest of the rockets somewhere in-between. I was also going to extend the tree backwards for better integration of sounding rocket mods (Wacapella, etc.) adding a "tier 0" with three nodes (aircraft, rockets, engineering.)
So far, this is roughly how I was planning on structuring the progression:
Lvl start - WWII aircraft
lvl 0 - Jet engines, basic sounding rockets (can be merged into lvl 1 or lvl start, removing this tier)
lvl 1 - Rocket planes (X-1), large sounding rockets
lvl 2 - Explorer 1/Juno I, Vanguard, Atlas A/B/SCORE
lvl 3 - Atlas C, Thor, Able, Agena A, X-15, Juno II
lvl 4 - Atlas D, Agena B, Ablestar, Delta B/C/D, Titan I, Mercury
lvl 5 - Atlas E/F, Agena D, Titan II, Gemini, Thor Upgrades, Juno IV, Delta E (TAID)
lvl 6 - Saturn I, Atlas SLV-3, Gemini Lunar Flyby, Apollo Boilerplate, Delta ELT & HOSS, Vega, Centaur, Titan IIIA/B
lvl 7 - Saturn IB, Gemini Lander, Titan IIIC, Apollo CSM, Atlas SLV-3K
lvl 8 - Saturn V, Big Gemini, Apollo Lunar Lander, Atlas CLEV
lvl 9 - Atlas II, Titan IV, Delta K upper stage, Post Apollo Saturn (ETS), Apollo Block III, LM Base modules, Skylab, Venus Flyby (might be moved up one more. Thoughts?)
Lvl 10 - Atlas III, Delta P upper stage, Space Shuttle (benjee10), Skylab Extensions, Centaur II, Titan LDC
Lvl 11 - Atlas V, Delta III/IV, SLS (benjee10)
Lvl 12 - Near Future Rocketry
Lvl 13 - Mastery
Thoughts on how this would be balanced? Think this would make the tech tree too long? I would (try to) balance the costs of the tree as well to prevent it from being too difficult.
As of now I have the tree structured, just need to add titles and descriptions, and create/update the configs to sort the parts into each node. I am also trying to learn how to create/edit B9 part switch configs to update the hypergolic configs, adding the liquid fluorine parts to the mix, though I am new to modding so that will likely take time.
