(covers information from several alternate timelines)
This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The Crossfield class was a type of Federation starship in service during the mid-23rd century. Its design consisted of a saucer section, a wide secondary hull, and two elongated nacelles. At least two ships of the class, the Discovery and the Glenn, were equipped with an experimental spore drive propulsion system.
To facilitate the operation of the spore drive, the saucer section was divided into three concentric sections separated by gaps. When engaging the spore drive, the ship went into black alert, which alerted the crew to the drive's activation. The outer and middle sections housed rings that could rotate clockwise and counter-clockwise respectively, so as to provide excess energy cavitation. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")
Ships of this class also included a regular warp drive for faster-than-light propulsion.
Crossfield-class starships were also equipped with tractor beams for towing objects in space.
Two known ships of this class were utilized by Starfleet to test a theoretical spore drive, following the outbreak of war with the Klingons in 2256. Shortly after achieving a breakthrough, the USS Glenn was crippled and all crew killed in an accident. She was then scuttled by her sister ship, the USS Discovery. (DIS: "Context Is for Kings")
Ships commissioned Edit
Background information Edit
According to Star Trek: Discovery The Official Starships Collection, the Crossfield-class is 750.5 meters.
The Crossfield-class bears some resemblance to the B-24-CLN, which was seen at the Surplus Depot Z15 in TNG: "Unification I". Both models are based off a concept USS Enterprise study model designed by Ken Adam and Ralph McQuarrie for Star Trek: Planet of the Titans.
According to technical data prepared for the San Diego Comic Con in 2018, the ISS Discovery had 17 decks, a crew complement of 180, a maximum warp of 8, two impulse engines (approximate warp factor 0.3), and a displacement-activated spore hub drive (approximate warp factor 1431). 
The Crossfield class is a playable ship in Star Trek Online, and has a small chance of appearing when a player purchases an add-on called the “Discovery Lock Box”. The playable Crossfield comes equipped with the ability to use its spore drive to teleport the ship forward, unleashing a burst of mycelial energy that affects enemies' engines and leaving a payload of torpedoes to detonate in its wake, before teleporting back to its original position—evocative of the tactic employed by Discovery at Corvan II in "The Butcher's Knife Cares Not for the Lamb's Cry". The ability is referred to in-game as the "Lorca Manuever".