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."

Codes:

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