Cryogenic bottoming cycle with variable fluid mixture

A gas turbine engine comprising:

• a core engine comprising a main compressor where an inlet airflow is compressed and communicated to a combustor to generate an exhaust gas flow that is expanded through a main turbine section to generate power used to drive the main compressor;
• a bottoming cycle where a working fluid mixture within a closed circuit is heated and expanded through a bottom turbine to generate shaft power, wherein the working fluid mixture comprises at least two fluids having different properties that change a capability of the working fluid mixture at different bottoming cycle operating points;
• a cryogenic fuel system comprising a cryogenic fuel storage tank, a fuel flow path for routing the cryogenic fuel to the combustor of the core engine;
• a first heat exchanger where heat from the core engine is input into the working fluid mixture;
• a second heat exchanger where the working fluid mixture is cooled by a flow of the cryogenic fuel;
• and
• a controller programmed to operate the bottoming cycle to vary a composition of the working fluid mixture in response to an operating condition of the core engine.

A method of operating an aircraft propulsion system comprising:

• generating an exhaust gas flow in a combustor by igniting a mixture of compressed air and a cryogenic fuel;
• communicating thermal energy from the exhaust gas flow into a working fluid mixture within a bottoming cycle where a heated working fluid within a closed circuit is heated and expanded through a bottom turbine to generate shaft power, wherein the working fluid mixture comprises at least two fluids having different properties that change a capability of the working fluid mixture at different bottoming cycle operating points;
• cooling the working fluid exhausted from the bottoming turbine in a fuel/working fluid heat exchanger where a cryogenic fuel accepts heat from the working fluid mixture;
• and
• adjusting a composition of the working fluid mixture by removing a condensed portion of the first fluid and the second fluid from circulation through the closed circuit of the bottoming cycle.