USS Challenger (NCC-1676-A)

USS Challenger (NCC-1676-A) was a Federation starship on active duty in Starfleet during the early 2290s and then briefly in 2370. (Star Trek: Challenger Chronicles)

In the early 2290s, USS suffered heavy damage to its primary hull during a battle.

Starfleet engineers, working with a Challenger engineering officer, constructed a new Challenger — given the registry NCC-1676-A — using the extant secondary hull and a primary hull originally scheduled for a  vessesl.

The resulting hybrid was called (but never formally designated) a pocket battleship, since it retained the photon torpedo launchers and phaser banks of an Enterprise subclass ship, combined with the phaser banks, photon torpedo banks and megaphaser cannon of a Miranda.

Although no engineering manual was done by the Starfleet International chapter, it seems possible to align the secondary hull’s warp power conduits to those in the primary hull; coupled with the use of megaphasers, this means that this was Starfleet’s only starship with more than one reactor, until the building of USS Prometheus (NX-59650). 

Officially, only one of these hybrid starships was constructed to "full-build"; a second, USS Potadeia, was constructed to 57 percent of completion before the project was canceled. Several reasons were behind its cancellation:

Although Enterprise-type secondary hulls would be commonplace following the class’ decommissioning in the late 2290s, Mirandas continued to prove their worth as they could be modified for a variety of mission roles at a cheaper cost;

The Khitomer Conference, and the warming of relations with the Klingons marked a cessation of building pure military ships, much like the ’s missions were changed from space control ship and battleship to heavy cruiser/explorer;

And the loss of Challenger-A herself, following an encounter with the USS. At that time, it was not known that the loss was not due to any engineering problems, but rather because the ship had been thrown forward in time to 2370.

In 2370, Challenger-A was decommissioned and most of her crew given the USS. The Challenger-A’s firepower was comparable to contemporary ships, and could have taken part in battles against the Dominion. However, many of the so-called Frankenstein fleet fared poorly. Though Challenger-A was based on better engineering principles, Starfleet decided not to recommission her.

The USS Potadeia — apparently taken from a Fleet depot and completed sometime in the past 65 years — was lost at the Battle of Wolf 359 in 2367.

The Potadeia'' was the Enterprise secondary hull seen. ''

Background
Challenger was also a Starfleet International chapter based in beginning in 1988.

The USS Challenger was originally designated NCC-1666 (probably according to the FASA RPG. The Starfleet International chapter decided against that number and went instead as NCC-1676.

It has been presumed, but not established (unlike the Challenger-A), that the B and C followed the lineage of the USS, that they were Excelsior and ships. Alternately, the Challenger-C could very well be the class ship of the USS Buran present at Wolf 359 in 2366-67.

As well, the Challenger-D being the fifth such named ship is true only for that NCC number, and does not take into account the cruiser, nor the  survey cruiser from Task Force Games/Star Fleet Battles nor the ship from DC Comics' "Thin Ice."