You get Ace, Double Ace, and Triple Ace levels based on the number of kills you have made. After 70 kills, you are an Ace. 150 kills mean you are a Double Ace, and 275 promotes you to Triple Ace. This is a number, independent from the ship class you have killed. So if you kill 275 Containers, you are a triple ace, but if you kill 274 Juggernaughts, you are only a double ace. Difficulty level has great importance in promotions. If you play on Very Easy, you get one fifth score for killing a ship, on Easy, you get one forth, Medium is 1/3, Hard is its half, while you get full score on Insane skill level. Achieving promotions in FS2 is extremely hard, and you must play with it a lot, in order to achieve the highest rank(Admiral)