Propulsion system of an aircraft, said propulsion system including a turbojet including an intermediate pressure stage and a high-pressure stage, a fan duct, and a heat exchanger system, which includes: - a main heat exchanger including a main hot supply connection, a main hot transfer connection pneumatically connected to the main hot supply connection through the main heat exchanger, a main cold supply connection and a main cold evacuation connection pneumatically connected to the main cold supply connection through the main heat exchanger, - a supply pipe which is connected to the main hot supply connection, and which supplies the heat exchanger with hot air, and which includes a regulating valve, - a high pressure pipe which bleeds hot air from the high-pressure stage through a first valve, - an intermediate pressure pipe which bleeds hot air from the intermediate pressure stage through a second valve, wherein the high-pressure pipe and the intermediate pressure pipe are connected to the inlet of the regulating valve, - a transfer pipe which is connected to the main hot transfer connection, and which is adapted to transfer the hot air that has passed through the main heat exchanger to an air management system of the aircraft, - a main supply pipe which is connected to the main cold supply connection, which supplies the main heat exchanger with cold air from the fan duct, and which includes a main regulating valve, - an evacuation pipe which is connected to the main cold evacuation connection and is adapted to expel the air to the outside, - a sub heat exchanger including a sub hot supply connection, a sub hot transfer connection pneumatically connected to the sub supply connection through the sub heat exchanger, a sub cold supply connection and a sub cold evacuation connection pneumatically connected to the sub cold supply connection through the sub heat exchanger, wherein the high pressure pipe issued from the first valve goes through the sub heat exchanger between the sub hot supply connection and the sub hot transfer connection, - a sub supply pipe which is connected to the sub cold supply connection, which supplies the sub heat exchanger with cold air from the fan duct and which includes a sub regulating valve, - a sub evacuation pipe which is connected to the sub cold evacuation connection and expels the air to the fan duct, - a temperature sensor, which measures the temperature of the hot air exiting the main heat exchanger through the transfer pipe, and - a control unit which controls the main regulating valve and the sub regulating valve according to the temperature measured by the temperature sensor and the temperature desired for the hot air exiting the main heat exchanger through the transfer pipe. - a main heat exchanger including a main hot supply connection, a main hot transfer connection pneumatically connected to the main hot supply connection through the main heat exchanger, a main cold supply connection and a main cold evacuation connection pneumatically connected to the main cold supply connection through the main heat exchanger, - a supply pipe which is connected to the main hot supply connection, and which supplies the heat exchanger with hot air, and which includes a regulating valve, - a high pressure pipe which bleeds hot air from the high-pressure stage through a first valve, - an intermediate pressure pipe which bleeds hot air from the intermediate pressure stage through a second valve, wherein the high-pressure pipe and the intermediate pressure pipe are connected to the inlet of the regulating valve, - a transfer pipe which is connected to the main hot transfer connection, and which is adapted to transfer the hot air that has passed through the main heat exchanger to an air management system of the aircraft, - a main supply pipe which is connected to the main cold supply connection, which supplies the main heat exchanger with cold air from the fan duct, and which includes a main regulating valve, - an evacuation pipe which is connected to the main cold evacuation connection and is adapted to expel the air to the outside, - a sub heat exchanger including a sub hot supply connection, a sub hot transfer connection pneumatically connected to the sub supply connection through the sub heat exchanger, a sub cold supply connection and a sub cold evacuation connection pneumatically connected to the sub cold supply connection through the sub heat exchanger, wherein the high pressure pipe issued from the first valve goes through the sub heat exchanger between the sub hot supply connection and the sub hot transfer connection, - a sub supply pipe which is connected to the sub cold supply connection, which supplies the sub heat exchanger with cold air from the fan duct and which includes a sub regulating valve, - a sub evacuation pipe which is connected to the sub cold evacuation connection and expels the air to the fan duct, - a temperature sensor, which measures the temperature of the hot air exiting the main heat exchanger through the transfer pipe, and - a control unit which controls the main regulating valve and the sub regulating valve according to the temperature measured by the temperature sensor and the temperature desired for the hot air exiting the main heat exchanger through the transfer pipe.