Solid rocket motors power sounding rockets and their thrust profile are determined mostly by their internal geometry. This study investigates the effect of the changes in internal geometry by changing the aspect ratio, truncation and elongation of grains in multiple BATES grain solid rocket motors. BATES grains are propellant segments in a thick-walled cylinder shape. Longitudinal effects, like erosive burning, augment the burn rate over the motor length changing the thrust profile. As part of the thesis, a model was made accounting for longitudinal variation both in the grain regression and gas flow. This model was validated through data from T-Minus Engineering measuring the pressure at the forward closure. With a sensitivity study, the effect of aspect ratio and length was researched. This resulted in a recommendation for geometry changes of the DART booster of T-Minus Engineering B.V. . Furthermore, additional validation data was acquired of static pressure along the length of the motor, instead of single-point measurements, to provide better validation of one-dimensional models. This was done using a non-invasive and inexpensive method using strain gauges on the chamber.