Reusable rocket stages is a trend in launch vehicles that have been observed for some time now. Even reusable upper stages have been seen in the Space Shuttle, Kistler K-1 and now the SpaceX Starship. This thesis is on the development of a genetic optimisation algorithm to determine the economic feasibility of upper stage reusability. Genetic optimisation has been chosen due to the mixed-integer nonlinear nature of the Reuse Index proposed by United Launch Alliance. The optimisation algorithm uses a modified version of ParSim v3 made by the Parachute Research Group (PRG) of Delft Aerospace rocket Engineering (DARE). The thesis gives an overview of the various dynamic, mass and cost models used in the tool together with the verification and validation of these models. Based upon the conclusions of the various runs performed recommendations are given on if and when upper stages can be reused.