Starfleet Cruisers for the early 23rd Century.

The NCC-1009 USS Equinox was originally launched in 2236 as part of the programme to replace the near obstellete Moskova class heavy cruisers and stop gap effective but slow Baton Rouge class heavy cruisers. 

Based on the Confederation class cruiser hull, which utilised the Baton Rouge saucer, it was proposed to try out a new design saucer intended as part of Starfleet's plans to introduce a modular based starship building programme to cover all classes of starships throughout the fleet well into the 23rd century.

The Baton Rouge type saucer was unable to accept the latest heavy phaser armament and its associated support systems, which the new saucer design already had incorporated from the start.

On trials, the large Impulse Drive engine caused minor Warp field fluctuations at speeds of over Warp 7, despite giving excelent sublight maneuverability and power yields into the ship's power grid. The second starship under construction, the NCC-1010 USS Horizon, was fitted with a smaller Impulse Drive engine already being employed on frigates and destroyers to solve this problem.

The USS Horizon lacked the USS Equinox's warp field problems and the subsequent cruisers were all built to the Horizon specification. In 2341 the USS Equinox was also converted to the Horizon Class during a refit at the end of its first 5 year exploration mission.

The NCC-1009 USS Equinox and the NCC-1010 USS Horizon were both lost during the Klingon - Federation war of 2345. All surviving starships in the class were converted and upgraded to heavy cruisers of the Constitution Class from 2346. This involved the replacement of the secondary hull to a newer design developed for the Archon and Constitution classes.

The NCC-1017 USS Constellation was lost, as a Constitution Class heavy cruiser, during the Doomsday Machine incident in 2368.

