once you're in the cave, use the pickaxe to break off a piece of one of the pointy things on the ceiling and put it into the little hole in the wall next to the entrance.
This is the second time I've played this game. I beat the game on the 5 foot ramp, i swear it. I could have gone another 500 feet or so. I used all my fuel to go really high and then descended from there. It's a sad ending though...