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 . <EOS>