Editing Constitution Class (section)

===Refit history===

From 2254, or earlier, to 2265 Constitution-class vessels featured a large deflector dish, a large bridge dome of semi-spherical shape, and an antenna spike protruding from the Bussard collector cap on each warp nacelle. The impulse drive had two exhaust vents in 2254, and as many as eight smaller vents in 2265. (TOS: "The Cage", "Where No Man Has Gone Before")

Sometime between 2265 and 2266 the vessels underwent a major overhaul: the old deflector dish was replaced by a significantly smaller model, the spikes on the Bussard collectors were removed, a smaller bridge dome of flatter curvature was installed, the aft caps on the warp nacelles were each equipped with a spherical attachment, and the impulse drive now had only two large exhausts. (TOS: "The Corbomite Maneuver")

The changes to the impulse drive section may indicate that completely new impulse engines were installed. It is unclear whether the warp nacelles were merely modified or completely replaced. 

The interior passageways, main bridge interior and briefing room were already redesigned sometime between 2254 and 2265, and new intercoms were installed.

In 2266, the interior passageways were again modified, the briefing room was completely redesigned, and the overhauled main bridge featured an enlarged main viewscreen and upgrades to the control interfaces and station arrangement, but the overall appearance of the bridge remained relatively unchanged as compared to 2265. (TOS: "The Corbomite Maneuver")

The crew quarters of the 2254 configuration had the capability of carrying slightly more than 200 crewmembers. (TOS: "The Cage") In the 2266 configuration, crew quarters could hold a crew complement of over 400. (TOS: "Charlie X")

In the late 2260s to early 2270s, the Constitution-class starships underwent their final major refit program. The actual refitting took eighteen months of work and essentially a new vessel was built onto the bones of the old, replacing virtually every major system. Thus, the Constitution-class continued in service for a further twenty years.

Essential upgrades were made to the Constitution-class' warp systems; the old cylindrical nacelles were replaced with new angular ones, and also the warp nacelle struts were new, as they were connected to the engineering hull much closer to the neck than before. The engineering hull roughly retained its original shape – while the original hull was essentially a conical cylinder, the refit was much more rounded.

The deflector dish was upgraded, doing away with the "satellite" dish architecture. As for the interior of the hull, the most obvious upgrades were the enlargement of the shuttle deck and landing bay, as well as the addition of a horizontal matter-antimatter reaction assembly and a vertical intermix chamber.

New also was the installment of the double photon torpedo launcher with its rectangular housing in the neck of the vessel. Also, the phaser configuration was changed to channel energy though the warp core. Furthermore, the saucer section was considerably extended (almost 20 meters), while the rest of the surface remained about the same.

Major changes were made to the interior of the Constitution-class starships; many new systems were added and existing ones upgraded. Summarizing, only the internal structure of the saucer and very little of the engineering hull and neck may have survived the 2270s refit. (Star Trek: The Motion Picture)

Some refit configurations had the warp nacelles rotated 90 degrees and included additional hatches along both sides of the saucer. (TNG: "Booby Trap")