Make sense , then help me understand what role frigate in 2020s needs to fulfill ?
I am not sure I understand the the overlap in Anti air , Anti ship , Anti submarine capabilities between a modern destroyer and frigate.
It obvious that both are needed , but how the function together then ? Is this the same principle of economy of force ? meaning frigate could handle tasks that other wise would take a destroyer there by freeing it to perform other task ?
A2A, ASh and ASW are tactical missions a warship would undertake in combat. The tactical role it would take in say a formation would depend on other ships in the group.
Destroyers are larger and have more sensors and weapons with longer range. Frigates are smaller and so less expensive and ideally more numerous. But if you look at the frigates in the West, they are the same size as destroyers so the terms are becoming meaningless.
The PLAN, though, definitely follows the terms in the traditional sense. The Type 054A frigate is explicitly smaller than the 052C/D destroyers. So you would deploy them in different ways. Destroyers can provide wide area anti-air for a carrier task force while the frigates with shorter range weapons take up picket positions on the fringe for anti-sub.
When used individually, the frigates would be used in lower threat areas nearer to hone waters to match lower endurance and weapons fit. Destroyers would range farther and in more hostile territory.
For much of the middle part of the two decades that the 054As have been around, it was the most numerous blue-water ship in the PLAN (discounting the decidedly green-water 056A.) It undertook long trips to the Indian Ocean as well as patrolled the close-by China Seas. The 052D became so numerous in recent years that some of the previous dynamics have changed. The 054A could probably be replaced in the longer deployments by the 052D. It will certainly be replaced by the 054B for carrier escort and long range duty as that ship is larger with more endurance and speed.
But China definitely still likes the 054A for patrols around China. That is why it is still being built for the CG. In fact, it will likely go the way of the 056(maybe some A as well?) which are progressively transferred from Navy to CG as China gets more 052DLs, 055s and 054Bs.
PLAN is looking like it will get progressively bigger ships for its blue-water fleet while transferring the remaining 054As to greenwater missions. There are still around 50 056As in PLAN, we do not know if all of them will go to the CG. But it seems the Navy is holding onto the 054A for certain because unlike the 056 there have been no transfers. The CG literally have to order new 054A hulls for their cutters.
From the Navy's standpoint, they may want to keep huge numbers of 054As and 056As in green water not just to patrol but to saturate near seas for anti-sub warfare.
Also task forces will grow around new LDP/LHP/LHA's that are coming online.
It'll interesting to see how things shake out!