cruiser ships are third in line of your fleet

 light fighters

are sent in first to attack the enemy troops at the front of the line: they act as troop fodder so they take more loss in troops than your other unit types.

 

heavy fighters

are sent second and although better at attacking than light fighters they are much slower and they are more for a defense ship there bonus stats give plus in enemy probes,

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 cargo ships which are what the enemy send to you so if your looking for good defense troops against probes and enemy taking all your resource this is the troop you need.

 

cruiser ships

are third in line of your fleet, if the enemy gets through first 2 troop types this is where you need to kill them off: they have the best attacking stats for troop to troop basis; they have a bonus to rockets, and light fighters which is most commonly used troop types, they also fight against solar satellites so if your enemy has any this will kill them off ( if the enemy uses them for a large amount of power and you kill them all off you could send there production rate to 10% if there energy is low enough!)

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 so make sure to stock up on these trust me not only are they strong but also move fast.