To enable cheats, start a game, when you're playing press F8, then quit the
game (only to the main menu - not to DOS!). Then start a new game with
the same pilot you used in the last game. In the shop, press 5 on the numeric
keypad to receive 5000 credits. This can be repeated. In the game, press
F8 to toggle invincibility. 

Submitted by Tippy (tippya@coca.net)