I knew there was something I forgot to do! I wanted to make it so that if you ran out of fuel, you had to wait for the bar to fill a bit first before you could thrust upwards again. As it is, it essentially flickers between using fuel and refilling fuel every other frame, so your thrust power will be less, but it's likely almost unnoticeable to the player.
It's a shame cause it would just take a couple lines of code to fix!