AIRCRAFT PROPULSION SYSTEM

An aircraft propulsion system (400) comprises a gas turbine engine (440) arranged to provide propulsive thrust and a fuel cell system (452) having an air input port (455), the aircraft propulsion system being configured such that bleed air from a compressor (412) of the gas turbine engine is provided to the air input port during operation of the aircraft propulsion system. Gaseous output comprising water vapour produced by the fuel cell system at an output port (457) thereof is provided to a heat-exchanger (413) disposed between consecutive compressor stages (411, 412) of the gas turbine engine to provide cooling of compressed air between said stages, thus improving the efficiency of the compressor section of the engine.