Board Thread:General Discussion/@comment-4576340-20140623154410/@comment-1913529-20140702172159

If they don't want to change the fuel system, then at least change some of the challenges so it is possible to complete them in a single day without spending money, such as the Sortie 10x challenge - you begin with 3 supply, use it up and you wait 12 hours to fully resupply back to 3. If you don't have any stock left over, you can't complete it. Even if you get the additional 3 stock fuel from challenges, you're still at least 1 fuel short. They should make it 10x within 24 hours.