An ablative system we used to RPG system we ran back in the day worked something like this:

A new off-the-line ship should have its stock resistance. If you get hit with a beam weapon (10 atk) and you have defenses (5 def) then your defenses should protect you (5 damage gets through), but they should also "wear down" over time down to a certain minimum.

So let's say we have two 10/5/100 ships duking it out (damage/armor/HP) where 10 percent of all damage that exceeds defense gets taken off it until it's repaired over time where you get a minimum of 10% of the original defenses.

Round 1:

Ship 1 does 8 damage to Ship 2 (5 is blocked, 3 damage gets through, .3 armor removed)
Ship 2 does 5 damage to Ship 1 (5 is blocked, 0 gets through, 0 armor removed)

Round 2:

Ship 1 does 6 damage to Ship 2 (4.7 is blocked, 1.3 damage gets through, .1 (.13 rounded down) armor removed)
Ship 2 does 10 damage to Ship 1 (5 is blocked, 5 damage gets through, .5 armor removed)

Hopefully at this point the system is clear. Though defenses should never drop below .5 here (10% of original).

Armor would then replenish like HP over time. This would also be a simple way of simulated crippled ships. For even more fun 10% of damage could also be removed from weapons simulated damage to the ship.