I like these ideas, but where is the 5 year mission (deployment 60 months), or was that only an unique experiment for the TOS era? Anyway, we see few crew members come and go, so if there was a 24-mo. crew cycle, then it must be staggered with a lot of re-upping to explain seeing the same characters year after year on the shows (except for VOY, they got the shaft on that deployment).Every Ship/Fleet is on that cycle for a variable amount of time in my Head Cannon based on their situation.
- 24 month Deployment cycles for entire Ships/Fleets
- Flexibility in which part of the phase a Ship/Fleet is in based on real world conditions & circumstances
- Operational Phases:
- Maintenance : 01-08 months
- Training : 04-08 months
- Deployments: 08-19 months
Like you indicate, ship upgrades/maintenance seems to be on a schedule in TOS:
- The Enterprise got a significant upgrade (new engines/nacelles, new bridge, smaller dish) between WNMHGB and the next episode.
- In my head canon, it got the Photon Torpedo System right after Balance of Terror after weaknesses were exposed with the Enterprise's weapon suite. They probably upgraded its Phaser system, too, based on its failure in combat.
- Between season 1 and 2 it got the Engineering room upgrade (M/AM reactor or power system upgrade?).
- It may have gotten either new engines or just upgrades to the engines a little later since the ship was hitting warp 8 routinely in season 2.
- I think it got an engine or m/AM reactor upgrade again at the start of Season 3. After that, they we hitting warp 9 routinely in season 3.
- Then after the 5 year mission, the ship was completely refitted for the TOS Movies era.