I think there's a tendency to think of the Enterprise going on 5 years missions
Which is weird, if you think about it. We have evidence for exactly
one 5-year mission for
one ship, the
Enterprise -- and even that evidence is not from TOS itself (title narration aside), but strictly from TMP and
Voyager: "Q2" (and the latter two Kelvin movies). And yet fandom has always taken that one isolated example as "proof" that
every starship mission ever had to be five years, which is nonsense. With only one example, there's no way to know whether it's the norm or the exception. And it doesn't make sense that all mission profiles would be the exact same length, with no flexibility. Sometimes fandom takes really dumb ideas for granted.
, but for the sake of argument I think that in spirit TOS (and TAS?) are the 5 year mission; with others maybe started out and not necessarily finished. So, hypothetically Enterprise gets a little bit of a refurbishment after the end of the original mission, and goes out for a second, but doesn't finish that one. And then after TMP again starts a new 5YM and doesn't finish that one, either. Just a little bit of fun speculation.
What I've posited in a couple of my TOS novels is that five years was the
maximum recommended mission duration for a
Constitution-class ship before coming in for maintenance, refits, and crew rotation, so that there was room for shorter mission profiles.
From a story telling standpoint, I can see the gap between TOS and TMP as fertile ground for stories were new characters and situations can be introduced and played out, and avoiding contradicting and being contradicted by the movies. I think the idea of the pre-TMP 5YM is just in the minds of some of the authors. I'm not an expert, but the ones I've read so far don't overtly state and only very lightly and subtly hint.
Exactly. The whole "second 5-year-mission" thing is more of a fan interpretation than something clearly codified in the novels. As I said, the 5-year duration was mentioned only in the narration and never, ever mentioned within the actual story of any work of Trek fiction prior to TMP. So the writers of those novels may have simply disregarded the narration and decided to treat the mission as open-ended, without any fixed duration -- which, really, is probably how TOS itself would've handled it if it had gone on long enough. (See other examples from the era including
Run for Your Life, a 3-season show about a man with no more than 18 months to live, and
M*A*S*H, an 11-season show about a 3-year war.)
They do feel like they take place a year or two or three after TOS and/or the original 5YM, at least some of the individual ones do.
The largest gap specified, IIRC, is
The Romulan Way being 8 years after "The Enterprise Incident." Which just about works out to two 5-year missions if you assume "Incident" was near the start of the third year of the first mission.