How is that functionally different than my model? They fire all their missiles every round, and only use the exact number of missiles against a target that it takes to destroy it. If something with ATK 10 is battling 2 ships with DEF 5, it shoots 5 missiles at the first target, destroying it, then 5 missiles at the next one. There is no concept of a target receiving more "attack power" than they have defense.

Anyways, you are looking for a physical model of the combat behavior, but there's no real reason for it. Just like in any game like this, you abstract certain concepts to make it simpler.