An aircraft propulsion system, including a turbojet and a heat exchanger system which includes a main heat exchanger, a hot air supply pipe, a transfer pipe transferring hot air to an air management system, a main supply pipe supplying cold air from the fan duct, an evacuation pipe expelling air to the outside, a sub heat exchanger with a high pressure pipe going therethrough, a sub supply pipe supplying cold air and including a sub regulating valve, a sub evacuation pipe expelling air, a temperature sensor, and a controller controlling the system according to the temperature measured The sub regulating valve comprises a door articulated between closed and open positions, a return spring constraining the door in the open position, and a realizing system, controlled by the controller, mobile between a blocking position and a realizing position in which the door is released to move to the open position.