Heat exchanger with active buffer layer

A hypersonic aircraft, comprising:

• a hypersonic propulsion engine comprising a hot bleed air source, a combustion section and a ducting assembly comprising an outer case;
• a fuel delivery system for providing a flow of fuel to the combustion section of the hypersonic propulsion engine, the fuel delivery system comprising a fuel tank;
• a heat exchanger disposed outside the outer case and in fluid communication with the fuel tank and with bleed air from the hypersonic propulsion engine, the heat exchanger comprising:
• an outer wall;
• a first chamber defined within the outer wall, the first chamber at least partially defined by a first wall disposed within the outer wall, the first wall defining a first outer surface, wherein the first chamber includes a first inlet fluidly coupled to the fuel delivery system and a first outlet fluidly coupled to the combustion section;
• a second chamber defined within the outer wall, the second chamber at least partially defined by a second wall disposed within the outer wall, the second wall defining a second outer surface, wherein the second chamber includes a second inlet fluidly coupled to the hot bleed air source and a second outlet;
• a third wall extending within the outer wall, the third wall having a first side and a second side, wherein the third wall separates the first wall of the first chamber from the second wall of the second chamber;
• a pump for providing a buffer fluid;
• a first buffer fluid flowpath defined between the outer surface of the first wall and the first side of the third wall, wherein the first buffer fluid flowpath includes a buffer fluid outlet fluidly coupled to the pump;
• a second buffer fluid flowpath defined between the outer surface of the second wall and the second side of the third wall, wherein the third wall fluidly isolates the first buffer fluid flowpath from the second buffer fluid flowpath, and wherein the second buffer fluid flowpath includes a buffer fluid inlet fluidly coupled to the pump;
• and
• a plurality of cross-flow tubes disposed within the outer wall and extending through the first chamber, the third wall and the second chamber, wherein the plurality of cross-flow tubes fluidly couple the first buffer fluid flowpath to the second buffer fluid flowpath.