On the exergetic effectiveness of combined-cycle engines for high speed propulsion

Air-breathing engines are utilized in the hypersonic regime through thermal integration of the fuel into the propulsive cycle, which improves the efficiency by recovering thermal energy from the freestream and the aeroshell. The classical efficiency figures based on First Principle analyses are inaccurate performance indicators of the resulting combined cycle. Instead, this paper deduced the engine thermal and airframe transfer effectivenesses based on thermodynamic availability, related to the overall losses of the thermally integrated vehicle for a given mission. The engine overall effectiveness, derived from the propulsive efficiency and the engine thermal effectiveness, was found to be a generalization of the Bréguet equation. The developed methodology was demonstrated in a combined cycle engine operating at flight speeds from Mach 2.5 to 5. In particular, the propulsive efficiency, thermal effectiveness, total loss and subcomponent losses were evaluated using the common framework of thermodynamic availability.