Ejecting a projectile with compressed air under water has a complicated mechanism and includes several parameters. Here, thermodynamic, hydrodynamic and dynamic equations governing the discharging process of the compressed air are derived. Having solved the equation set, effects of effective parameters in the projection system are studied. The result shows that about 90% of behavior of the projection process is affected by opening and closing the compressed air injection valve (Fire Valve). Therefore, it is necessary to relate the changes of fire valve throat area to the time for different depths as a function whose results are consistent with experimental data of a real system.Items such as pressure of launcher tube and projectile speed at the moment of jumping out of the tube, the maximum allowable acceleration exerted on the projectile, the maximum allowable pressure inside launcher tube and the residual pressure inside the cylinder of compressed air At the moment of stopping air injection, are verification parameters based on real data. According to these parameters, the function of valve opening and closing time are estimated in a polynomial and cosine form, respectively. Coefficients of these functions are computed for five depths of 10, 50, 100, 150, 200 meters.Optimization in such systems is a time consuming and troublesome process. Having analyzed the compressed air launcher, it is needed to validate the assumptions by means of CFD simulation in Fluent Software during some phases. Such CFD simulation by the Fluent software is time-consuming. Therefore, the Projection parameters are studied just in depth of 100 meters of water. To do this, the numerical results of axisymmetric simulation accomplished by Fluent Software are compared with the results of system equations solution. Then, a 3D numerical simulation is done in the Fluent software to investigate the 3D launcher model. Finally, some intermediate results of 3D model are presented. Key words: Positive discharge system, Air injection valve, Poppet valve, Underwater Launch control, Fluent Simulation
