Abstract: Vehicle system characteristics will be affected by the variations of propulsion system. It is necessary to rearrange vehicle system in order to fulfill space mission in optimal system state. In this paper the linearized mass models, similar to that in C [1], has been used to derive vehicle system mass equation for expendable, multi-stage vehicle with series or parallel propulsion system. The system mass equation has been utilized to analyze the relationship between take-off mass and design parameters for a vehicle with parallel propulsion system.