Abstract: Despite that many sophisticated prediction tools are made to explicate the phenomena of internal ballistic for dual thrust rocket motor due to geometry change, none of them discussed uncertainties due to geometric, ballistic and regression simultaneously. Mathematical models are developed on the basic governing theories to estimate the pressure time history for two tubular grains with two different diameters along the grain. A Computer module was made to facilitate this study with consideration to uncertainties as they have a noticed effect on the results. The need for an optimization tool was necessary to reduce the error between theoretical and experimental results, genetic algorithm (MATLAB tool-box) was used as optimization tool. A set of static firing test are made for validation and to determine the operating characteristics of the motors experimentally. It was apparent in this study that some of these uncertainties are applicable in large scale motors only and the others are applicable for both small, and large scale motors.