Aircraft vapour trail control system

An aircraft propulsion system comprising:

• an engine having an engine core configured to combust a quantity of fuel to produce an exhaust flow containing water, the water being a product of combustion of the fuel;
• at least one propulsive fan configured to generate a mass flow of air when rotating, the at least one propulsive fan being coaxially located with the engine core, and the mass flow of the air generated by the at least one propulsive fan mixing with the exhaust flow;
• one or more sensors configured to detect an occurrence of conditions indicative of contrail formation by the exhaust flow from the engine, the conditions indicative of contrail formation including conditions indicating that contrail formation is happening and ambient conditions predictive of contrail formation;
• and
• a controller in communication with the one or more sensors and configured to:
• receive a respective signal from each of the one or more sensors;
• monitor the respective signal from each of the one or more sensors to determine whether the occurrence of conditions indicative of contrail formation has been detected;
• and
• increase a ratio of the mass flow of air generated by the at least one propulsive fan to a mass of water in the exhaust flow when the occurrence of the conditions has been detected,
• wherein the at least one propulsive fan is located within a duct having an annular flow opening downstream of the duct, and
• the flow opening comprises a portion of a bypass duct of the engine to allow the mass flow of air to bypass the engine core and flow towards the exhaust flow.