I noticed that I have limited amount of fuel only when I had 10 percent left. I was just digging planets until nothing is left from them and forgot that I actually have a mission :D.
Really liked the mechanic and whole upgrade system. The only problem for me was that these resource colors sometimes really hard to distinguish. I thought that I need granite and went mining for stone and so on.
Great submission, I will go and try to finish your game now instead of just drilling planets for fun :D