A propulsion system for an aircraft comprises a gas turbine engine; a plurality of fuel tanks arranged to contain different fuels to be used to power the gas turbine engine, wherein the fuels have different calorific values; and a fuel manager. The fuel manager is arranged to store information on the fuel contained in each fuel tank and to control fuel input to the gas turbine engine in operation by selection of a specific fuel or fuel combination from one or more of the plurality of fuel tanks based on thrust demand of the gas turbine engine such that a fuel with a lower calorific value is supplied to the gas turbine engine at lower thrust demand.