An aircraft propulsion system, including a turbojet and a heat exchanger system including a main heat exchanger, a hot air supply pipe and regulating valve, a high pressure pipe bleeding high pressure stage hot air through a first valve, an intermediate pressure pipe bleeding intermediate pressure stage hot air through a second valve, a pipe transferring hot air to an air management system, a main supply pipe supplying fan duct cold air including a main regulating valve, an evacuation pipe expelling air to the outside, a sub heat exchanger, wherein the supply pipe from the regulating valve goes through the sub heat exchanger, a sub supply pipe supplying cold air from the main supply pipe, a sub evacuation pipe expelling air to the fan duct, a temperature sensor measuring hot air temperature exiting the main heat exchanger, and a controller controlling the valves according to the measured temperature.