With the increasing performance demands placed on aircraft, the nozzle is becoming a much more versatile component. This versatility is increasing the afterbody complexity and the difficulty in afterbody drag prediction. This thesis documents a computationally inexpensive method for estimating the afterbody drag of aircraft during the conceptual design stage. The design and creation of a new hybrid approach to afterbody drag prediction is discussed. This hybrid approach uses the Integral Mean Slope Truncated Method in place of an equivalent body of revolution pressure drag. Corrections or drag deltas are then applied to the Integral Mean Slope Truncated afterbody drag estimate. These drag deltas account for various geometric effects and three-dimensional effects. The resulting prediction method is applicable to both axisymmetric and 2-D nozzles. The hybrid approach has been incorporated into ACSYNT Version 2.0.0 (AirCraft SYNThesis), a conceptual aircraft design tool developed at NASA Ames Research Center and Virginia Polytechnic Institute and State University. Comparison of the predictions of the new hybrid approach with those of a more complex and much more computationally expensive method show generally good agreement.