Why are some ship designs unavailable until you get the tech?

[Osito]

For example, I can't build capital ships till have have the required tech. However, as far as I can see, I can put exactly the same components on a frigate as I can on a capital ship. So what is the point of deferring the availability of some ship types, when you can use any other ship type for the same role?

Another question, along the same lines, is this: is there any difference between escort, frigate, destroyer, cruiser, capital ship and carrier, other than the name, and the role you choose to assign to it?

PS I wondered whether there might be a minimum size for the later ships, but that doesn't seem to be it. Once I got capital ships I could build them at a size less than 200 if I wanted.

[Bingeling]

Carriers are special - they have specific size rules and requirements for fighter bays. Resupply ships also have special rules to them.

Apart from that, the ship classes only help the AI in assigning fleets. Pirates also "steal" the lower designs to build their ships.

The reason you can't build capitals at the start, is that the AI design is too large to build. The AI (in legends) tries to fit designs under the limit, but it will only go 'so far'. So for the AI capital ship to be buildable, you need several upgrades to yard size. There is however nothing stopping you from creating a capital design that is smaller than an escort, or an escort larger than an AI capital ship.

[Osito]

Ah, ok, that makes sense. Thanks!

[hfl13]

In my first game (human) I never could built carriers & resupply ships until the Shakturi were finished and the open sandbox game was officialy over and I got alle research from the Guardian.

In my second game (human) I decided against too much micro management and only built 1 cruiser (230) till I built my first 5 downgraded capital ships (300). Getting bigger ships is a main ingredient for military strength. At the moment my captal ships are
size 650 and quite powerful. The starting design for capital ship is 400. maybe you can downgrade that to 230 than you can start with capital ships early.

Watch for speed the original designs are quite good and I built some designs with extra power (2nd or 3rd reactors and more engines and trusters which made very slow designs. Adding extra fuel cells is always nice.

Is there a post somewhere with best designs at game start (human prefered) ?

The categories are only there to allow for different roles of ships.

[Plant]

HF13, carrier tech is tactical interceptors (third interceptor tech) and the tech needed for resupply ships is Accelerated construction; I think in your game, you only got the tech required after you gained the tech from the guardians.

As for the best designs at game start, it just depends on preferences and fulfilling the size limit as best that you can.
Remember to try to dedicate equal amounts of space to offence and defence if you want to maximise effectiveness, which is something the game designs fail spectacularly at.