Two-dimensional axisymmetric interior ballistics simulations in the projectile launch system utilizing long slotted tubular solid propellant are carried out using solid/gas two-phase fluid dynamics code of Eulerian-Lagrangian approaches. The movement of slotted tubular solid propellant is simulated one-dimensionally by Lagrangian approach. The simulation results are compared with experimental data for validation. The histories of the breech pressure and projectile velocity under various conditions are in good agreement with the experimental data. We examine the effects of the projectile and propellant mass conditions on the energy release rate of solid propellant and the projectile kinetic energy at the muzzle. A series of the simulations by changing the mass of projectile and propellant clarifies that the launch system requires the heavier projectile for ensuring the sufficient time of the projectile staying in the launch tube to convert efficiently the propellant chemical energy to the projectile kinetic energy.