In Learn to Fly to fuel is used by the three tiers of rockets.
The higher the tier (price) of rocket, the more boost each one has and so the less fuel is used. Rocket fuel can be upgraded to make it last longer
Like Learn to Fly, the the more expensive boosts that you buy, the higher the thrust and so the less fuel is used as well as being able to upgrade the amount of rocket fuel each boost holds.
Unlike the previous two games, Learn to Fly 3 is the first game to show the fuel in the individual boost/stages description. 100 fuel is equal to about 2.2 seconds, 2500 fuel is equal to 55 seconds etc. As far as I know the Solar Turbine has the highest amount of fuel.