Actually tanks arent perfectly armored (the typical tradeoff of design) and they still have weak points (like their top/rear where the armor is significantly lighter - usually there are some radiator openings )

Exactly. I was thinking of making something like two numbers to represent armour rating "armour 80mm-50mm" and during combat there is an armour roll (which part of the armour was hit, in this case rand(80,50)). And/or rules like "if the weapon has twice bigger caliber than needed it deals +50% damage".

Another way to look at it is to have a chance of a critical hit, and if so, let it bypass some amount of armor, with a ship with a defect/flaw could have a higher chance of receiving a critical hit, while a ship with a better armor design / less weakpoints could have a perk that lowers the critical hit chance. Certain classes of weapons could have increased critical hit chances as well, and crew skills could also modify it.

How to make combat (hit/damage/etc calculations)?

It depends what you want your battlesystem to support in terms of game mechanic.

Based on the above post, I would theorize that you'd like to create a relationship between big and small ships and that you are ok with hard counters.

It seems to me that the above post suggests that "fighters" are nearly useless unless fielded en masse against capital ships whereas capital ships can one shot pretty much any fighter but don't have sufficient weapons to kill them fast enough.

The problem I perceive with that approach is that, while these mechanics should increase the depth of your combat mechanics, they result in a zero sum game: it takes longer for fighters to kill a capital ship and a capital ship dies slower but can't decimate fighters all that well.

The end result is that a specific amount of fighters (or more) is required to down a capital ship, whereas any smaller number will be killed by the capital ship. The battles would be unnecessarily long and uneventful.

Do you have middle-sized ships as well?

If so, you could build a rock paper scissor effect:

1 Capital ships can annihilate 100+ fighters

10+ Frigates ships can annihilate 1 Capital Ship

10+ Fighters can annihilate 1 Frigate Ship

Means to achieve the above:

- Capital ships need to survive better against fighters: add "static armor reduction" (capital ships reduce all damage taken by 1). If fighters' weapons generally do 3 dmg, then they do -33% against capital ships (these numbers can scale at will).

- Insure Frigates marginalize the above effect: slower firing rate, but high dmg (a 20 dmg missile -1dmg is still 95% dmg output).

The above ruleset would indicate that fielding a certain amount of Frigates would naturally result in lethal damage on a Capital Ship before the Capital ship has fired 10 shots. This can be balanced by calculating the amount of shots per tick (assuming Frigates fire every 2 ticks and capital ship every tick).

Tick 1: 10 Frigates fire (1 Frigate killed by Capital ship)

Tick 2: No Frigates fire (1 Frigate killed by Capital ship)

Tick 3: 8 Frigates fire (1 Frigate killed by Capital ship)

Tick 4: No Fridates fire (1 Frigate killed by Capital ship)

...

10+8+6+4+2 = 30 Frigates will fire in this exchange. As a result, 30 missile fires should be enough to down a Capital Ship (that's how you known how to balance their respective life).

You also need to bear in mind that the Frigate's life can't exceed the same curve for the Fighters:

Tick 1: 10 Fighters fire

Tick 2: 9 Fighters fire

...

10+9+8+7+6+5+4+3+2+1 = 55

Thus the Frigates must not be able to survive 55 hits (*3 dmg = Life <165)

And there you have it, should support a simple 3 ship type system, and you can tweak individual ship class' life and weapon systems so long as they roughly fit this range.

Of course, this requires testing in a combat where all 3 ship classes are present in various numbers to see what works best, so long as priority targeting works. (Capital ships shooting Frigates, then Capital Ships, Frigates shooting Capital Ships, then Frigates, and Fighters shooting Frigates, then Fighters)

Even the mighty can fall to the sting of a bee.

Even the mighty can fall to the sting of a bee.

I fully agree, in fact, I tend to make my "own" systems in such a way that fighters are actually best against larger crafts when massed, but the OP seemed to suggest a system that worked the other way around and I didn't want to argue the specifics here, just provide a context that supports the core precepts.

but the OP seemed to suggest a system that worked the other way around and I didn't want to argue the specifics here, just provide a context that supports the core precepts.

Yes I should have provided more context -- my response was a simple rebuttal to the idea of larger ships being 'effectively" indestructible to smaller ships as being one of limited value.

First I would like to solve the typical situation like 3x battleships (big) and 8x destroyers (medium size). Without worring about fighters, special cases, etc yet. How those, relatively big, ships fire at each other and how the armour and caliber work for them?

First I would like to solve the typical situation like 3x battleships (big) and 8x destroyers (medium size). Without worring about fighters, special cases, etc yet. How those, relatively big, ships fire at each other and how the armour and caliber work for them?

As per the above, the Battleships would win out of utter attrition. Medium-sized ships' purpose is to deal a lot of damage with reduced costs (low life count) and it takes 10 destroyers to assault 1 battleship. So you'd likely end with one moderately damaged battleship and 2 unscathed ones? (as 3 battleships are mincing down destroyers much faster, but Destroyers still get to "alpha-strike" the first battleship with high dmg)

You'd need to lay down actual numbers to make sense out of it though, but I predict the above based on my test numbers.

It really depends on how you want things to play out. How do you want armor to work? How do you want different tech levels to work? An example is the first armored ships used during the American Civil War. They basically were invincible against the old wooden ship designs.

So in the example above, do you want armor to be ablative, a la MechWarrior, like shields in Star Fleet Battles, which is ablative but regenerating (self healing materials aren't that fanciful), or more like an all or nothing when it comes to penetration. Similar to WWII tank and naval warfare, where if one can't penetrate the armor, you're basically stuck hoping for critical hits and/or damaging external components.

And armament comes into play too, how are those battleships kitted out? Are they anti-capital ships? Are the the destroyers kitted out as anti-capital ships? Or are they supposed to be escort ships, anti-fighter ships? One way to look at it is the recent World of Warships, which is kind of rock paper scissory. They have Battleships, Cruisers and Destroyers. Battleships are slow and with heavy guns, best at fighting other BBs and killing Cruisers. Destroyers tend to be nimble enough to dodge BB fire and take them down with Torpedoes. While Cruisers are the midline that have fast firing guns that can take down destroyers, but are reduced to firing HE at BBs for reduced HP damage and in the hopes of setting fires and damaging things like the propeller.

(And course you have some Cruisers with torpedoes, some destroyers that are more gun heavy and better at fighting other destroyers, and Carriers with bombers that drop torpedoes, and some ships that have better AA than others, but the basics are as above)