A method of cooling an exterior surface of a flight vehicle, the method comprising: advancing the flight vehicle at a flight speed of at least Mach 3, wherein the surface of the flight vehicle is aerodynamically heated; circulating a supercritical working fluid through a fluid loop of a closed loop system that is provided independent of a propulsion system of the flight vehicle, including, in sequence: compressing the supercritical working fluid through a compressor; heating the supercritical working fluid through a heat intake of the fluid loop, wherein the heat intake of the fluid loop is thermally coupled to the surface of the flight vehicle; expanding the supercritical working fluid in a thermal engine, thereby to generate a work output from the thermal engine; cooling the supercritical working fluid, wherein cooling the supercritical working fluid comprises exchanging heat from a low pressure leg of the fluid loop, located downstream of the thermal engine, to a high pressure leg of the fluid loop, located upstream of both the thermal engine and the heat intake of the fluid loop; and recirculating the supercritical working fluid to the compressor; and operably coupling the work output of the thermal engine to the compressor. advancing the flight vehicle at a flight speed of at least Mach 3, wherein the surface of the flight vehicle is aerodynamically heated; circulating a supercritical working fluid through a fluid loop of a closed loop system that is provided independent of a propulsion system of the flight vehicle, including, in sequence: compressing the supercritical working fluid through a compressor; heating the supercritical working fluid through a heat intake of the fluid loop, wherein the heat intake of the fluid loop is thermally coupled to the surface of the flight vehicle; expanding the supercritical working fluid in a thermal engine, thereby to generate a work output from the thermal engine; cooling the supercritical working fluid, wherein cooling the supercritical working fluid comprises exchanging heat from a low pressure leg of the fluid loop, located downstream of the thermal engine, to a high pressure leg of the fluid loop, located upstream of both the thermal engine and the heat intake of the fluid loop; and recirculating the supercritical working fluid to the compressor; and operably coupling the work output of the thermal engine to the compressor. compressing the supercritical working fluid through a compressor; heating the supercritical working fluid through a heat intake of the fluid loop, wherein the heat intake of the fluid loop is thermally coupled to the surface of the flight vehicle; expanding the supercritical working fluid in a thermal engine, thereby to generate a work output from the thermal engine; cooling the supercritical working fluid, wherein cooling the supercritical working fluid comprises exchanging heat from a low pressure leg of the fluid loop, located downstream of the thermal engine, to a high pressure leg of the fluid loop, located upstream of both the thermal engine and the heat intake of the fluid loop; and recirculating the supercritical working fluid to the compressor; and operably coupling the work output of the thermal engine to the compressor.