PROPULSION SYSTEM

A propulsion system (300) for an aircraft (10) comprises fuel reservoirs (100 and 800) configured for storing hydrogen (H2-1 and H2-2) and a fuel reservoir (200) configured for storing a liquid hydrocarbon fuel (HC). The propulsion system may be provided with a gas turbine engine (400) comprising a fuel injection unit (500) comprising : a hydrogen pilot burner (510) in fluid communication with hydrogen fuel reservoirs (100 and 800) via flow control valves (512 and 542). There may also be provided a dual fuel main burner (520) in fluid communication with the hydrogen fuel reservoirs (100 and 800) via a flow control valve (522) and in fluid communication with the hydrocarbon fuel reservoir (200) via a third flow control valve (532). A control system (600) may be provided which is operable to control: the flow control valves (512 and 542) to control the flow of hydrogen from the hydrogen fuel reservoirs (100 and 800) to the hydrogen pilot burner (510); the flow control valve (522) to control the flow of hydrogen from the hydrogen fuel reservoirs (100 and 800) to the dual fuel main burner (520); and the flow control valve (532) to control the flow of the hydrocarbon fuel (HC) from the hydrocarbon fuel reservoir (200) to the dual fuel main burner (520).