Twenty-four hours in and I do have a playable game. The game is currently untitled, but the object is to stay alive by adding fuel to the generator so that the shield spheres can destroy the alien ships. If you run out of fuel, or if you hit the shield spheres or enemies, the game is over.
The rest of this evening is to tweak, add some nice effects, maybe revisit some of the 3D models. Tomorrow comes sound, music, title screen, and more tweaking of game play.