Cheats:

 During the game pause the game by pressing [Esc]. At the password
 line, switch a testing aid mode by pressing [C] and then type in one
 of the codes below. If you did it right, you will here "outstanding."

 The Codes:

 13593 Maximum fuel and armor
 14214 Invincibility
 17357 All missions playable
 25245 Maximum weapons
 33124 All missions completed