Very unrewarding and ungratifying game.
I realize we're jaded and definitely prefer games with upgrades and instant gratification, but I would have still liked a little action to assuage my boredom...
How to beat 6000 feet : First, buy and max all. Then fly with a air resistance of 3%(so that you gain altitude, up, not down). Use your rocket till' u are at 200 in "speed". When u sink to 175, use your rockets and get back up to 200, repeat till' you run out of fuel, then let it glide.. PLEASE KEEP THIS COMMENT ALIVE!!!
easy until lvl 7 first, upgrade cash flow while shooting down the little ships with the cannon, then build destroyers and upgrade how much u get for each kill, more destroyers(maybe some planes to take some of the hits...) then use bomber planes to dominate...TO BEAT FLAGSHIP: let it get close enough so u can use your ground cannons/turrets and load up on destroyers, use bombers to kill the flagship...(don't forget to upgrade health regen. some)