The hybrid combustion phenomenon is more complex than the case of solid or liquid combustion. An understanding of hybrid combustion is dependent upon understanding of the interrelationships between the boundary layer zones characteristics and fuel grain geometry during burning time and along fuel grain. The main objectives of the article are to predict hybrid combustion boundary layer geometry and characteristic parameters variation with fuel grain length during combustion. Specially, the relation between flame zone (propagation) position and regression rate change values during combustion. Based on this mathematical model, a computer code was implemented to identify the hybrid combustion boundary layer (BL) parameters. The comparison between Schlieren photograph of Plexiglas-oxygen flat plate burner and computational results show good agreement. The presented code can be considered as a powerful tool for the design, analysis and can be used to evaluate quantitatively the effect of changes in various design parameters of hybrid rocket
motor.