An aircraft hydrogen propulsion system comprising: a fan; an electric motor (206; 508) operably connected to the fan by a drive shaft and configured to convert electrical power to rotational energy to drive rotation of the fan through the drive shaft; and an aircraft power generation system (208; 500) operably coupled to the motor and configured to supply power to the motor, wherein the aircraft power generation system comprises a fuel cell configured to generate at least 1 MW of electrical power using hydrogen fuel; characterised by: a closed-loop supercritical CO 2 bottoming cycle comprising a turbine, a compressor, a recuperating heat exchanger, a waste-heat heat exchanger, and a pressure-balanced heat exchanger, wherein the closed-loop supercritical CO 2 bottoming cycle is configured to generate work through the use of a working fluid which flows along a working fluid flow path through the compressor, the recuperating heat exchanger, the waste-heat heat exchanger, the turbine, the recuperating heat exchanger, the pressure-balanced heat exchanger, and back to the compressor; wherein the hydrogen fuel for the fuel cell is passed through the pressure-balanced heat exchanger of the closed-loop supercritical CO 2 bottoming cycle prior to being supplied to the fuel cell. a fan; an electric motor (206; 508) operably connected to the fan by a drive shaft and configured to convert electrical power to rotational energy to drive rotation of the fan through the drive shaft; and an aircraft power generation system (208; 500) operably coupled to the motor and configured to supply power to the motor, wherein the aircraft power generation system comprises a fuel cell configured to generate at least 1 MW of electrical power using hydrogen fuel; characterised by: a closed-loop supercritical CO 2 bottoming cycle comprising a turbine, a compressor, a recuperating heat exchanger, a waste-heat heat exchanger, and a pressure-balanced heat exchanger, wherein the closed-loop supercritical CO 2 bottoming cycle is configured to generate work through the use of a working fluid which flows along a working fluid flow path through the compressor, the recuperating heat exchanger, the waste-heat heat exchanger, the turbine, the recuperating heat exchanger, the pressure-balanced heat exchanger, and back to the compressor; wherein the hydrogen fuel for the fuel cell is passed through the pressure-balanced heat exchanger of the closed-loop supercritical CO 2 bottoming cycle prior to being supplied to the fuel cell. a closed-loop supercritical CO 2 bottoming cycle comprising a turbine, a compressor, a recuperating heat exchanger, a waste-heat heat exchanger, and a pressure-balanced heat exchanger, wherein the closed-loop supercritical CO 2 bottoming cycle is configured to generate work through the use of a working fluid which flows along a working fluid flow path through the compressor, the recuperating heat exchanger, the waste-heat heat exchanger, the turbine, the recuperating heat exchanger, the pressure-balanced heat exchanger, and back to the compressor; wherein the hydrogen fuel for the fuel cell is passed through the pressure-balanced heat exchanger of the closed-loop supercritical CO 2 bottoming cycle prior to being supplied to the fuel cell.