AI War:Unable To Repair
Why Can't I Repair Some Ships (Such As Forts/Warheads)?
A: Most ships can be repaired by engineers, but a few, such as Fortresses and Warheads, can not. These ships all have "Repair" listed as an immunity. This applies to both human and AI players, of course. From a story sense, these ships are simply too large and complex for engineers to perform field repair. In terms of gameplay, this lets both players and AIs do multiple raids against these larger structures, thus wearing down what would otherwise sometimes be an insurmountable blockade. Generally any unit that is immune to repair will also have a regeneration speed. That is, the unit will repair itself, given enough time. However, this time is usually greater than that of an engineer assisted repair (fortresses take about 3 to 5 hours to repair to full from 0, mines take 30 minutes).
Be aware that if such a unit has a non-zero speed and non-infinite engine health, it can be engine damaged into unmovability permanently. The self-repair (regen) will not repair engines, even after the unit is at full health.