Woohoo, 12 days! I worked hard on that - let me know if you can beat it!
If you care here's how I did it.
Day 1 Money and fuel efficiency power ups.
Day 2 Fuel Tank
Day 3 Engine
After that my basic strategy was to try to buy things in sync:
Buy Wings and Engine together always (keep wings a level or two below and use skillful flying,) and this is top priority whenever you can afford both.
Buy Fuel efficiency, tank, and money together, and do it when you have a good engine and sufficient control. I kept fuel tank a relatively low level because it's so expensive. Efficiency was pretty high.
Aerodynamics and radar are important too, get them especially if it's sort of difficult not to hit stuff.
These are guidelines by the way, be sure to spend most / all money every day.
This works because wings are very expensive but help a ton, and same with engine. So if you buy wings and fuel together, you'll just have a really easy flight. You want to keep your ship difficult to fly so that your money is most efficiently going to your altitude and getting more money. But you also don't want to focus completely on one thing because if one thing is 2 levels ahead, it's like more than triple the price of all of the other things and doesn't deliver quite well enough.
I think on day 9 my ship still wasn't good enough to try to fly sideways to get powerups.
On day 10 I had the red wings and a decent engine and enough fuel to get up to the first $100 bag, but halfway there used the fly sideways trick to completely refuel, and then flew sideways to get about $6000 from the $100 bag before running out of fuel.
Then on day 11 I managed to buy enough stuff to make it to the moon - like 2 engine upgrades, 3 wing upgrades, even more aerodynamics and radar, and a good bit of fuel. I got one hull upgrade and about 4 of the shield, just so I didn't die immediately if I hit a meteor. But I flew way carefully on the last flight (had plenty of wing and radar upgrades for this one, to be safe.) Had to refuel a couple times on the way by flying sideways, but I made it. Also, only get 2 boosters, and I left it at 0 for the first 9 days but if the money works out you could get 1 a day or two earlier than that. But you really need the money for the other stuff as boosters don't go to your bottom line unless you hit something.
Super fun, I love this game. Now I'm going to see what record I can get without flying sideways for powerups. I think I've gotten it in 22 days like that but I can do better. I think the first I played it was like 27 days.
I don't think I would change anything about this game, but maybe the collision mechanics could be a little bit more forgiving... (less spinning completely out of control,) and maybe make armour powerups heal less.