The optical system has been developed in order to investigate a mobile rocket combustion chamber through schlieren technique. The design process has been presented in a consecutive perspective, despite of the non-sequentiality that characterizes any decision-making route, in order to facilitate the reading. A brief introduction to the combustion chamber test bench, as well as to the rising interest of space industry and agencies in green propellants, is promptly followed by theoretical background about schlieren technique and its application in an optical diagnostic perspective. It is also presented a computational method for numerical evaluation, implemented in a Matlab routine for image-processing.