The objective of this research is to develop a prediction code for the Air Force Research Laboratory Propulsion Directorate that can accurately determine the gross thrust coefficient for a user defined non-axisymmetric two-dimensional converging diverging nozzle. The code includes the effects of friction, angularity, and expansion losses on nozzle efficiency. To demonstrate the prediction method, the generated computational results were compared to experimental data, as well as computational results from other existing nozzle performance codes, for a number of different nozzle geometries. The nozzle internal performance prediction code showed excellent agreement with experimental data in predicting the gross thrust performance for all nozzle geometries considered. It was shown, however, that when the experimental data showed evidence of flow separation, a flow phenomenon this code is unable to predict, the code results underpredicted the experimental by up to 10%.