The design optimization of air-breathing propulsion engine concepts has been accomplished by softcoupling the NASA Engine Performance Program (NEPP) analyser with the NASA Lewis multidisciplinary optimization tool COMETBOARDS. Engine problems, with their associated design variables and constraints, were cast as non-linear optimization problems with thrust as the merit function. Because of the large number of mission points in the ยฏight envelope, the diversity of constraint types, and the overall distortion of the design space, the most reliable optimization algorithm available in COMETBOARDS, when used by itself, could not produce satisfactory, feasible, optimum solutions. However, COMET-BOARDS' unique featuresยฑwhich include a cascade strategy, variable and constraint formulations, and scaling devised especially for dicult multidisciplinary applicationsยฑsuccessfully optimized the performance of subsonic and supersonic engine concepts. Even when started from dierent design points, the combined COMETBOARDS and NEPP results converged to the same global optimum solution. This reliable and robust design tool eliminates manual intervention in the design of air-breathing propulsion engines and eases the cycle analysis procedures. It is also much easier to use than other codes, which is an added beneยฎt. This paper describes COMETBOARDS and its cascade strategy and illustrates the capabilities of the combined design tool through the optimization of a high-bypass-turbofan wave-rotortopped subsonic engine and a mixed-ยฏow-turbofan supersonic engine.