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The gas for the flight is taken at the moment of departure, and is not gived back if you cancel and return the fleet prior flying to the destination planet.

In your case, the fact that you have returned some flight is not important - the gas is already used and it won't return to the planet just because you have changed your mind for one or other reason. In this game this fact haven't changed since it's start - the gas for flight is taken at the beginning and won't return if you changed your mind. This is the same when a rocket ship is launched - you need fuel at the start, in order to overcome the gravitation