Thrust

To unlock the bonus game, make it to the reverse gravity planets on the normal setting, which ZSB will be unlocked on the main menu afterwards.

On the Hard+ setting, there is a way to get extra fuel on the seventh level of every planet. First, get past the two sets of big doors. Shoot the second button again, then fly down and head right, going past the area with all the bunkers, factory and pod. The door past that area will be open; fly through the tunnel for several additional fuel tanks.

Reach the reverse gravity planets. Once the game ends, a third option on the main menu will appear, which is to press 3 to play ZSB, the hidden mini-game.