When you run low on fuel, missiles, or armor, press Start on Controller 1 to pause the game. On Controller 2, press A, B, A, B, Select, A and B.
Super A10
Choose the A-10 and begin flying. As soon as the "Good Luck" message disappears from the screen, shoot your 30mm cannon once, then hold X. Continue holding it until "Super A-10" appears in the bottom left corner of the screen.