The invention concerns an aircraft propulsion system having an engine core within which fuel is combusted to produce an exhaust containing water produced from the combustion of fuel, at least one propulsive fan for generating a mass flow of air which mixes with the exhaust of the engine core, and one or more sensor arranged to sense a condition indicative of vapor trail formation by the exhaust flow from the engine; and a controller arranged to control the ratio of the mass flow of water in the exhaust to the mass flow of air propulsed by the propulsive fan such that the ratio is reduced upon sensing of said condition by the one or more sensor.