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Does anyone know how the AI determines how many freighters of each kind it maintains in its fleet?
I've noticed the number of freighters and miners fluctuates up and down quite a bit as the game proceeds.
So the AI is retiring freighters and miners from time to time.

Also wondering about freighter design. Is there any point in giving them different cargo sizes?
I get the impression the different size names may only be cosmetic and dont actually affect how the AI assigns transport and delivery orders. I see freighters of each type carrying sometimes tiny loads of a couple of hundred units of a single resource; and also fully loaded; and everything in between. So I get the impression the AI does not optimize orders, like a human transport manager might, but assigns individual orders to specific freighters.
All this leads me to think there may be no need to have different cargo sizes in the designs. It might be best to focus instead on optimizing speed and range and turning rate and adjust cargo space accordingly as research allows one to increase ship size.

ORIGINAL: Siddham

Does anyone know how the AI determines how many freighters of each kind it maintains in its fleet?
I've noticed the number of freighters and miners fluctuates up and down quite a bit as the game proceeds.
So the AI is retiring freighters and miners from time to time.

If I recall correctly, the computer builds a set number of small, medium, and large freighters each time a colony is founded, getting you the baseline fleet. Overall demand (and demand fulfillment) for resources seems to be the driving factor in long-term fluctuations while pirate activity and war are the major drivers of short-term fluctuations in the size of the freighter fleet. I don't know how that translates to the mix of freighters that the computer builds, however.

Also wondering about freighter design. Is there any point in giving them different cargo sizes?

In the sense of whether or not the computer will use them where they're most efficient? I don't think so; I believe that the computer simply selects the closest freighter to point A that lacks a mission when it generates a mission to transport cargo from point A to point B.

Giving different freighters different cargo sizes is one way to vary the resource and time costs of building freighters; freighters with higher cargo capacities tend to have better fuel economy and lower maintenance per unit of cargo capacity, but also require more resources to build and tie up construction yards longer whereas freighters with lower cargo capacities and thus lower resource costs and shorter construction times get into operation sooner. It's usually not that much of a concern, but keeping the average freighter resource cost down might help avoid or mitigate resource shortages or clear them up faster. Note, however, that a ship's design is set when ordered and cannot be changed until after completion; keeping the average resource costs of your freighters down can work as a preventative measure against construction delays due to resource shortages, but changing freighter designs to reduce their resource costs while construction is already delayed by resource shortages is unlikely to go very far towards mitigating the problem, as you will often have a relatively large number of ships already in the construction queue when you first notice a resource shortage.

There is also a consideration of absolute fuel cost. If reducing a freighter's cargo capacity means you can reduce its static energy cost, then it will use less fuel overall even if its fuel economy per unit of cargo capacity is worse than that of a larger freighter. Probably not a lot less unless the reduction in static energy cost is significant, but sometimes a little bit less fuel consumption is more useful than higher overall efficiency, especially if the higher overall efficiency is largely theoretical because the freighters frequently run mostly-empty.

Variable freighter size gives you some influence over the overall resource cost, fuel consumption, construction time, and efficiency of your freighter fleet. You can get the same effect with a uniform freighter fleet, though.

A possibly more significant draw, though, is simply that a varied freighter fleet is, at least in theory, more visually-interesting than a uniform freighter fleet, because you have a mix of little shuttles and big bulk freighters flying around rather than only one or the other.

ORIGINAL: Aeson
I believe that the computer simply selects the closest freighter to point A that lacks a mission when it generates a mission to transport cargo from point A to point B.

That is the conclusion I came to also

I became interested in the AI freighter fleet when I noticed in the early part of a recent game that the AI had reduced its fleet of freighters by almost 50% over a couple of years. I was alarmed and thought perhaps the Private sector economy was somehow being damaged or suffering. I thought maybe it had something to do with the balance between cash flow and GDP. I tested that and it wasn't.

This lead me to speculate that perhaps when Empire demand and planetary stocks reach some specific quotient the AI will begin retiring freighters and miners and maintain a reduced fleet. Then when demand increases, such as a new colony coming online, the AI will build new freighters to meet the demand. Retiring and rebuilding is likely more economical over time than maintaining unnecessary freighters; and it's good for the State too in that the State benefits from the rebuilding of the fleet.

About cargo sizes:
I remember reading a post (I think it was Shark7) where he had reached the conclusion cargo sizes could and should be kept low for efficiency.
I can't recall exactly but I think he said something like 2, 4, 6.
Another poster (turtlefang) said his tests lead him to optimize at 4, 8, 16.
So these are quite different conclusions.

At the moment I'm using something like 4, 8, 12 once I have the tech to optimize speed etc.
I think speed is a vital factor both for the economy and for freighter safety.

But my question is still unanswered - does the AI distinguish in a meaningful way between different cargo sizes?
And if so how?

I get the impression you think variable size may not matter except cosmetically.

I'm not convince that fuel economy is either necessary or desirable. I think the important factor is fuel supply.
So long as there is adequate provision of supplies and stocks the more fuel used the better for the economy.
The more everything used the better for the economy.

Seems to me the AI freighters have two jobs - to maintain planetary stocks and to fulfill specific orders. I presume the State and Private sectors benefit from all resource movements through commerce centres on space ports and mines etc; and the State benefits from the AI building freighters; so keeping the Private sector busy benefits everyone. When the AI begins to reduce its fleet that's a sign there is room in the economy for expansion.

Most of this is speculative of course. I dont have enough experience or knowledge yet to actually know.

Thanks Aeson