There's some parts there that are rather difficult to parse. As far as formatting goes, always use a space after a comma or full stop.
"The Raynor is one of the most powerful warships in the current service, [space here] designated [or designed?] in 2397 after the battle of Ikeya to replace [the] older Leander destroyer."
If you mean that the Raynor is supposed to replace the Leander as an entire ship class, then you need the definite article there, otherwise it might seem that the GTVA designated (or designed?) the Raynor (as in a single ship) to replace a single Leander-class destroyer...
Named battles are generally treated as proper nouns, as in the Battle of Stalingrad or the Battle of Britain. Similarly here it should probably be "the Battle of Ikeya", if you refer to a specific, historically decisive engagement.
"During the battle, [the or a?] fatal error [flaw?] of the [a?] Leander-class destroyer lent [led?] to destruction of half the fleet."
Does this mean that there was a fatal flaw in the Leander-class design in which case you would say "...the fatal flaw of the Leander-class destroyer..."?
Or that a single Leander-class destroyer committed a fatal error in the course of the battle, in which case it should be "During the battle, a fatal error of a Leander-class destroyer..."
But if it was an error by the ship's captain, that wouldn't require a commission of a new ship class, so I'm assuming that the former is the intended case.
Moving on, did you intend to use the word "lent" as in "contributed to"? Or did you mean to use the word "led" as in there was a direct causal relationship between the error, and the heavy losses suffered?
Also, what does the destruction of "half the fleet" refer to? Did GTVA suddenly lose the entire half of all its deployed ships? Or did it lose all the Leander-class destroyers?
Or did you mean that the GTVA lost half the ships deployed in that particular engagement, the Battle of Ikeya? That makes more sense but it may be better to just say "...contributed to heavy losses in the battle".
"[The] Raynor is an improvement of the Leander's hull, with better armor and weaponry."
This actually sounds more like a refit than a complete new ship class to me. Did they scrap or decommission the existing Leanders, or refit them to the new Raynor-specification? Was there a substantial change in the hull configuration?
In that case, new ships could be referred to as the Raynor class, while the Leander-class ships with upgraded armour and weapons could be referred to as Leander refits, for example.
There are a lot of examples of this in both real life as well as fiction - one of the most famous in latter category being the USS Constitution refit class of the Star Trek fame (the NCC 1701-A was originally a Constitution-class starship, and was later upgraded to Constitution-class refit configuration).
"These mammoth ships [plural] serve as an all-around support ship [singular]"
should be
"These mammoth ships serve as all-around support ships" or "This mammoth ship class serves as an all-around support ship".
"They are an indispensable asset to long-range battlegroups that are [can be omitted] expected to undergo long-term operations far from friendly outposts."
You clearly have a vision of what you want to say, but in many of these cases, imprecise use of language leads to either confusion about what exactly you mean, or just makes the text longer than it needs to be. The versions The E posted have a much smoother flow about them, and you'll note that they have also fixed the ambiguity in the places I pointed out.
This is something that will only get better by constant practice, both writing and reading. So, keep at it, and don't get discouraged. Always revise your text a few times after you've written it. Right after you've written something, it may seem right to you as your brain still remembers the "mode of thought" it had when you wrote it, but if you wait a few hours or days even, problems in the text may become obvious.
If you can find someone to go through your text and point out the problematic parts, you can figure out better ways of saying what you mean. Just don't rely too much on them posting correction suggestions - you should come up with solutions yourself, otherwise you will keep making similar mistakes.
What I and The E have posted here shouldn't necessarily be taken as direct suggestions, but rather examples to compare your text to. You should write with your own flair, just make sure the text has correct grammar, spelling, and it's not ambiguous or confusing to read.
EDIT: Formatting and stuff about the Raynors, Leanders and refits
