OPTIMIZATION OF AIR-BREATHING PROPULSION ENGINE CONCEPT

The design optimization of air-breathing propulsion engine concepts ha s been accomplished by soft-coupling the NASA Engine Performance Progr am (NEPP) analyser with the NASA Lewis multidisciplinary optimization tool COMETBOARDS. Engine problems, with their associated design Variab les and constraints, were cast as non-linear optimization problems wit h thrust as the merit function. Because of the large number of mission points in the flight envelope, the diversity of constraint types, and the overall distortion of the design space, the most reliable optimiz ation algorithm available in COMETBOARDS, when used by itself, could n ot produce satisfactory, feasible, optimum solutions. However, COMETBO ARDS' unique features-which include a cascade strategy, variable and c onstraint formulations, and scaling devised especially for difficult m ultidisciplinary applications-successfully optimized the performance o f subsonic and supersonic engine concepts. Even when started from diff erent design points, the combined COMETBOARDS and NEPP results converg ed to the same global optimum solution. This reliable and robust desig n tool eliminates manual intervention in the design of air-breathing p ropulsion engines and eases the cycle analysis procedures. It is also much easier to use than other codes, which is an added benefit. This p aper describes COMETBOARDS and its cascade strategy and illustrates th e capabilities of the combined design tool through the optimization of a high-bypass-turbofan wave-rotor-topped subsonic engine and a mixed- flow-turbofan supersonic engine. (C) 1997 John Wiley & Sons, Ltd.