NX class

The NX-class was a series of United Earth scout starships in Earth Starfleet service around the time of the 22nd century. The NX-class was commissioned as an exploratory vessel as a medium for the new warp 5 engine that would form the basis of Humans first reach into the galaxy. (Star Trek: Enterprise)

History
Enterprise was the first NX-class starship built, registry NX-01. Constructed in orbit above Earth, the Enterprise featured the first warp five engine, as well as one of the first transporters certified to transport biological material. The ship was launched in 2151 under the command of Captain Jonathan Archer to transport the Klingon courier Klaang back to Qo'noS. Due to the early launch of the Enterprise several systems—most notably the starship weapon systems—had not yet been completed. These weapon systems were completed in the field at a later date. 

With a series of successful early missions, Starfleet decided to proceed with building further NX class ships. In 2151, three more NX class ships were in the early design stages. Construction was proceeding on the unnamed NX-02 in 2153 when the Xindi attacked Earth. The ship, later named Columbia, was completed in November of 2154 and launched under the command of Captain Erika Hernandez. 

Specifications
In 2159, these vessels were equipped with phase banks and four torpedo launcher banks. 

Known vessels

 * Enterprise (NX-01)
 * Columbia (NX-02)
 * Challenger (NX-03)
 * Avenger (NX-09)
 * Currie (NX-16)

Mirror universe vessels

 * ISS
 * ISS
 * ISS
 * ISS
 * ISS
 * ISS
 * ISS

Background
No explanation is given for these vessels' sometimes Federation Starfleet-style "USS" name prefix, despite existing years before the inception of the Federation.

The Legacy game software included numerous other names for ships of this class that were not prepared to be used in gameplay, and are hidden in the game's source code, including USS Adams, USS Aurbach, USS Boston, USS Commonwealth, USS Concord, USS Fenway, USS Liberty and USS Paul Revere. Possibly, these vessels were intended for some feature of the game that was not completed. For example, none of these ships are coded with registry number graphics like those the included ships possess.