A combined cycle power generation system for an aircraft includes fuel cell and supercritical CO2 cycles. The fuel cell cycle includes a compressor and turbine disposed on a first shaft, a fuel cell in fluid communication with the compressor and a fuel source, and a combustor in fluid communication with the fuel cell and the turbine. The combustor is configured to combust partially spent fuel from the fuel cell and produce combustion exhaust gas for delivery to the turbine. The supercritical CO2 cycle includes a compressor and turbine disposed on a second shaft, a supercritical CO2 fluid circuit in thermal communication with the combustor and configured to deliver CO2 to the turbine and compressor, and a heat exchanger in thermal communication with the supercritical CO2 fluid circuit and a source of cooling fluid. A mechanical linkage is configured to transfer power from the second shaft to the first shaft.