NX class

The NX-class was a type of Explorer starship in the United Earth Starfleet in service during the mid-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)

The first six ships of the class were named after the NASA space shuttle orbiters of the late 20th century. 

History
Enterprise was the first NX-class starship built, registry NX-01. The ship was launched ahead of schedule 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. By 2151, three more NX class ships were in the early design stages. Construction of the unnamed NX-02 had begun by the time the Xindi attacked Earth in 2153. The ship, later named Columbia, was completed in November of 2154 and launched under the command of Captain Erika Hernandez. 

Sometime between the Enterprise's return to Earth after the mission to prevent the Xindi from destroying Earth and the formation of the United Federation of Planets, the Enterprise and her sister ships underwent a major overhaul that took 180 days. (ST calendar: Ships of the Line 2011)

In August 2155, the Challenger and Discovery were completed and commissioned. Several months later, the Atlantis and Endeavour entered service. Between mid-2155 and mid-2156, the Columbia, Challenger, Discovery, and Atlantis were destroyed or lost as a result of Romulan attacks. The Endeavour, equipped with countermeasures to the Romulan arrenhe'hwiua telecapture technology, battled with the Romulans at Altair VI. 

NX-class production would later be restarted by 2159. Captain Picard remembered 15 or 16 ships of this class having been built. Some of them would be Intrepid,  Avenger, and Currie. 

Specifications
When first launched, the NX-class was meagerly equipped with plasma cannons and spatial torpedoes. Instead of a tractor beam, it used special grappling cables. The first of the NX-class, the Enterprise had three phase cannons in storage, but it wasn't until midway through their first year that they were equipped. During the Xindi superweapon crisis, the Enterprise got retrofitted with twelve phase cannons and the newly developed Photonic torpedoes. With these armanents, as well as improved hull plating, it was able to defeat three Klingon birds-of-prey. 

After saving Earth from the Xindi superweapon, the NX-class received another upgrade including an upgraded navigational deflector, an improved computer interface, pulsed phase cannons, dorsal and ventral torpedo launchers, and polarised hull plating that had 12 percent greater performance than previous versions. 

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

Mirror universe
In the mirror universe, these were known as NX class battleships. There were at least 9 ships in service by 2155. Most of these were refitted with advanced weapons from the USS Defiant (NCC-1764) in the same year.