NOVELTY - The method comprises supplying an endothermic fuel flow from an endothermic fuel reserve (18) contained in a fuel reservoir (16), bringing the endothermic fuel flow into thermal communication with operating systems (12) that generate waste heat while performing their intended functions during operation of an aircraft, exposing the endothermic fuel flow to a cracking catalyst, and delivering the endothermic fuel flow to a jet engine (20) that is configured to receive and selectively combust some of the endothermic fuel flow.    USE - The method is useful for rejecting waste heat produced by operating systems carried on a military aircraft.    ADVANTAGE - The method is capable of effectively rejecting waste heat produced by operating systems thus maintaining supersonic speeds of the aircraft and avoiding inefficient fuel consumption and the systematic loss of prioritized functionality of the aircraft.    DETAILED DESCRIPTION - The method comprises supplying an endothermic fuel flow from an endothermic fuel reserve (18) contained in a fuel reservoir (16), bringing the endothermic fuel flow into thermal communication with operating systems (12) that generate waste heat while performing their intended functions during operation of an aircraft, exposing the endothermic fuel flow to a cracking catalyst, delivering the endothermic fuel flow to a jet engine (20) that is configured to receive and selectively combust some of the endothermic fuel flow, and delivering some of the endothermic fuel flow not combusted in the jet engine back to the fuel reservoir. The endothermic fuel comprises: an effective amount of branched alkanes (greater than 30 wt.%) so that the endothermic fuel participates in endothermic catalytic cracking of the branched alkanes and achieves a net endothermic effect; less than 0.5 wt.% of olefins; and less than 100 parts per million of sulfur-containing compounds. The cracking catalyst comprises: a superacid operative to catalytically crack the branched alkanes present in the endothermic fuel at a reaction temperature of 20-80 degrees C; and an inert support substrate. The superacid is: present in an amount ample to drive cracking of the branched alkanes and to increase a heat sink capacity of the endothermic fuel flow at the reaction temperature; and present at 200-400 g for every kilogram of the endothermic fuel that flows over the cracking catalyst per minute. The inert support substrate supports and immobilizes the superacid within a flow path of the endothermic fuel flow. The bringing and exposing steps comprise passing the endothermic fuel flow through a fuel segment of a reactive heat exchanger, and circulating a cooling fluid flow through the operating systems. The fuel segment comprises a washcoat within the flow path of the endothermic fuel flow that includes the cracking catalyst. The method further comprises circulating a cooling fluid flow through additional operating systems, where waste heat is accepted and through a cooling fluid segment of the reactive heat exchanger to transfer heat to the endothermic fuel flow in the fuel segment, and circulating the cooling fluid flow back to the operating systems. An INDEPENDENT CLAIM is included for a fuel circulation system for rejecting waste heat produced by one or more operating systems carried on an aircraft.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a fuel circulation system.    Operating systems (12)    Fuel reservoir (16)    Fuel reserve (18)    Jet engine (20)    Cracking catalyst cell. (26)