C.E. Evans wrote:
Actually, we don't know that at all. All we know is that Excelsior was the first. A second Exclesior-class ship could have been waiting in the wings in the final stages of construction at the time of Star Trek IV, and that ship became the new Yorktown.
So Starfleet is building multiple Excelsior
-class starships based on a technology that hasn't been proven in the field? I really don't think that's happening. They were just beginning trial runs in Star Trek III
and we have no idea how much the tampering done by Scott set the project back. Star Trek IV
takes place three-months after The Search for Spock
and the ship is still sitting in Spacedock.
There is a huge gap in the history of the ship between 2286-90. In 2293, Sulu states they've completed a three-year survey mission and in The Undiscovered Country
is using traditional warp drive by all accounts I've seen. She may have been spaceworthy prior to 2290, but I imagine quite a bit of time was spent swapping out transwarp drive for traditional equipment which also had to be tested in the new design.