An experimental and analytical investigation of the installed thrust and drag of various isolated nozzle and twin-nozzle/aftbody configurations indicated that empirical correlations provide the best means of predicting aft- end performance, especially for the early stages of the aircraft design. Both subsonic and transonic isolated nozzle drag data were correlated using IMS (integral mean slope) as the geometric parameter. A correlation of twin-nozzle/aftbody drag data at subsonic and transonic speeds was developed by combining Spreiter's transonic similarity parameters with the IMS of the equivalent body of revolution. A correlation of inviscid MOC pressure drag, achieved through use of IMS combined with similarity parameters obtained from linearized supersonic flow theory, provided an accurate and rapid means of estimating drag for arbitrary axisymmetric boattail contours at supersonic speeds. Twin-nozzle/aftbody drag data at supersonic speeds was correlated with the equivalent body drag obtained from the axisymmetric MOC correlation. Improved thrust and drag performance was obtained by modifying the aft-end design of five selected aircraft configurations. The rationale for these modifications was derived from design guidelines and criteria developed during the program. Improvements in mission radius for a fixed takeoff gross weight aircraft were obtained, in general, by utilizing convergent-divergent nozzles, a horizontal wedge interfairing with the trailing edge terminating at the exit plane of the nozzles, a single vertical stabilizer, and a narrow lateral nozzle spacing.