Which one of the large ships? I don't believe a Terror Ship could keep on flying if all 12 avalanches connect, even with minimum damage of 50 per missile you would still reach the half health breakpoint, so unless you have to do more than half health to shoot down, that's very unlucky. On the other hand, a Supply Ship can keep on flying with 12 hits, it could go down with 11 if they all hit with full damage but it could also take up to 22 avalanches to reach the half health point.