The ship combat starts when the attacker ship tries to move into the space taken by the defender ship. The combat is played in rounds. Each round the attacker ship shoots and deals damage to the defender, then the defender ship - even when destroyed completely (DL 1.2 and all later versions) - shoots and deals damage to the attacker. Below is my best understanding of how the combat is calculated, it has not been confirmed by StarDock yet. How the combat is performed if at least one side is a fleet, see fleet combat.