(covers information from several alternate timelines)
Delta-shaped hull-class Edit
This type of starship was used by Starfleet during the mid-22nd century. The primary hull was triangular in shape, and had two nacelles mounted port and starboard angled upward from and slightly above the hull.
Background information Edit
Robert Bonchune described the inspiration of this design as "a kind of Romulan/Federation hybrid."  The model originally started out as the "back end" of the warp ship model seen flying above Luna during the opening credits of the Star Trek: Enterprise.
In the final draft script of "The Expanse", where these ships appeared, the script called for "warp two ships." According to Bonchune, "The next season when we needed some "older" starships (than NX-01), John Eaves went ahead and truly finalized the complete shape as was seen in the episodes. However, since this was considered a more advanced production version, the "back end" old style rocket exhausts were deleted and replaced with more "advanced" 'nurnies' and 'greebling'." 
Detached primary and secondary hull-class Edit
This type of starship was used by Starfleet during the mid-22nd century. The primary and secondary hull of this class were connected via the nacelles, which were directly attached to the primary hull and via pylons to the secondary hull. (ENT: "Storm Front, Part II")