A hydrogen fuel system for aircraft comprising: a gas turbine engine; a fuel feed conduit defined at least in part by, in fluid series: a liquid hydrogen tank fluidly connected to a combustor of the gas turbine engine; a liquid hydrogen pump to drive fuel to the combustor of the gas turbine engine; an evaporator; and an electric heat source (144, 244) in thermal communication with the evaporator to add heat into a flow of hydrogen passing through the evaporator; and an electric energy source associated with the electric heat source to power the electric heat source; wherein the fuel feed conduit is further defined by: a gaseous hydrogen pump downstream of the evaporator drive a flow of gaseous hydrogen from the evaporator to the combustor of the gas turbine engine; and a gaseous hydrogen heater downstream of the gaseous hydrogen pump and upstream of the gas turbine engine, wherein the gaseous hydrogen heater is associated with an engine heat source internal to the engine to add heat to the flow of gaseous hydrogen passing through the gaseous hydrogen heater; and wherein the system is characterized in that: the engine heat source comprises compressor bleed air from a compressor section of the gas turbine engine, and in that: a bleed air conduit is defined at least in part by the compressor section of the gas turbine engine and the gaseous hydrogen heater to add heat to the flow of gaseous hydrogen passing through the gaseous hydrogen heater and cool the compressor bleed air passing through the gaseous hydrogen heater; and further in that: an environmental control system return conduit defined at least in part by the compressor section of the gas turbine engine and an environmental control system is configured to flow the cooled compressor bleed air to the environmental control system. a gas turbine engine; a fuel feed conduit defined at least in part by, in fluid series: a liquid hydrogen tank fluidly connected to a combustor of the gas turbine engine; a liquid hydrogen pump to drive fuel to the combustor of the gas turbine engine; an evaporator; and an electric heat source (144, 244) in thermal communication with the evaporator to add heat into a flow of hydrogen passing through the evaporator; and a liquid hydrogen tank fluidly connected to a combustor of the gas turbine engine; a liquid hydrogen pump to drive fuel to the combustor of the gas turbine engine; an evaporator; and an electric heat source (144, 244) in thermal communication with the evaporator to add heat into a flow of hydrogen passing through the evaporator; and an electric energy source associated with the electric heat source to power the electric heat source; wherein the fuel feed conduit is further defined by: a gaseous hydrogen pump downstream of the evaporator drive a flow of gaseous hydrogen from the evaporator to the combustor of the gas turbine engine; and a gaseous hydrogen heater downstream of the gaseous hydrogen pump and upstream of the gas turbine engine, wherein the gaseous hydrogen heater is associated with an engine heat source internal to the engine to add heat to the flow of gaseous hydrogen passing through the gaseous hydrogen heater; and wherein the system is characterized in that: the engine heat source comprises compressor bleed air from a compressor section of the gas turbine engine, and in that: a bleed air conduit is defined at least in part by the compressor section of the gas turbine engine and the gaseous hydrogen heater to add heat to the flow of gaseous hydrogen passing through the gaseous hydrogen heater and cool the compressor bleed air passing through the gaseous hydrogen heater; and further in that: an environmental control system return conduit defined at least in part by the compressor section of the gas turbine engine and an environmental control system is configured to flow the cooled compressor bleed air to the environmental control system. a gaseous hydrogen pump downstream of the evaporator drive a flow of gaseous hydrogen from the evaporator to the combustor of the gas turbine engine; and a gaseous hydrogen heater downstream of the gaseous hydrogen pump and upstream of the gas turbine engine, wherein the gaseous hydrogen heater is associated with an engine heat source internal to the engine to add heat to the flow of gaseous hydrogen passing through the gaseous hydrogen heater; and wherein the system is characterized in that: the engine heat source comprises compressor bleed air from a compressor section of the gas turbine engine, and in that: a bleed air conduit is defined at least in part by the compressor section of the gas turbine engine and the gaseous hydrogen heater to add heat to the flow of gaseous hydrogen passing through the gaseous hydrogen heater and cool the compressor bleed air passing through the gaseous hydrogen heater; and further in that: an environmental control system return conduit defined at least in part by the compressor section of the gas turbine engine and an environmental control system is configured to flow the cooled compressor bleed air to the environmental control system.