Rapid-Fire is the ability of a ship to shoot multiple units in one round of combat.
During combat, if Rapid-Fire didn’t exist, each unit (defense or ship) would only have the ability to target and shoot at one enemy unit in each round (at random). However, if the shooting unit has Rapid-Fire against the targeted unit, there is a probability of it to shoot again at another target at random. This cycle of “extra” shots is cumulative, which means that a unit can effectively shoot 1000 times in just one round.
If r is the value of the Rapid-Fire of shooting ship to the target ships, on average, the shooting ship will fire r times.
Consider the following example:
Cruisers have Rapid-Fire of 10 against Rocket Launchers. Let’s consider the battle of 1 cruiser against 2 rocket launchers and 1 light laser. Each defensive structure will shoot once against the cruiser, as it is the only attacker. When it’s the cruiser’s turn, it chooses a target randomly from the three available targets. Let’s suppose it chooses one of the rocket launchers. The cruiser shoots as normal but then, since it has Rapid-Fire against Rocket Launchers, it gets a dice roll to decide if it has another chance to shoot. Let’s suppose the roll is successful, and it gets an extra shot - then it randomly chooses another target. Let’s suppose this time it chooses the light laser. It’ll shoot once, but since cruisers do not have Rapid-Fire against Light Lasers, it doesn’t get a Rapid-Fire “roll of the dice”, and the round ends here. Had it chosen the rocket launcher again, the Cruiser would have been granted the chance to “roll the dice” again for an extra shot, again choosing the target at random.
It is important to note that during combat, a ship has a certain probability of exploding after each shot it receives (this probability is decreased with each upgrade of the ship’s Armor Technology). This is the advantage of Rapid-Fire; even a heavily armored ship is more likely to be destroyed in a single round of combat by a ship or defense that has Rapid-Fire capability against it. This is a key reason for building a balanced defense and fleet.