The Coventry differed from the Surya by having an extended rear edge to the primary hull to house extended hangar decks. These hangars often carried Attack ships as well as the more usual shuttles. Some ships of this class served 50 years in the fleet.
The Sheffield Class was built in very small numbers as it was a victim of technical progress.
Intended to supplement the Surya and Coventry class Frigates, the USS Sheffield Commissioned in October 2266, and a class of 30 ships was envisaged to allow the replacement of the remaining Asia class ships. However while the Sheffield was being tested Starfleet started to bring Linear warp drives into the inventory. The Sheffield’s design was assessed and it was found that the changes from the standard Coventry class design made the ship less suitable for use with the linear drive nacelles and warp core.
As a result only 7 further Sheffields were completed, the remaining 22 were cancelled in favour of additional ships of the new Babcock class.
The Delta Class was an off shoot from the Perimeter Action Ship program, initially designed n response to the requirement that led to the building of the Kiaga class PA, the design was put into limited production as a light frigate. The Delta was a relatively small vessel, with a very flat side profile which was intended to increase the stealthiness of the design. Only 12 Deltas were built, but they were intended for gathering intelligence in locations were the use of a specialist intelligence gathering ship was not possible for political or tactical reasons.
The Surya class was designed as a Frigate counterpart for the the Constitution class. The ships had useful hanger capacity in the rear of the primary hull, and, along with the Miranda class, set the basics for classes to follow from the Coventry through to the Avenger/Reliant.
The Mercury Class frigate was based on the Asia class design. It was used almost exclusively as a deep space explorer, far beyond the Federation treaty zone. With the introduction of more capable ships such as the Constitution class the ships were later used on more short range missions. The last ships of the class decommissioned as late as 2279.