Focus of this study is the pressurization system of a cryogenic liquid propellant tank for the launcher application. Objective of the pressurization system is to provide the required pressure evolution in the propellant tank for a proper operation of the engine. The motivation of this study is an improved understanding of the complex fluid-dynamic and thermodynamic phenomena during the active-pressurization process of cryogenic propellants in order to optimize the on-board pressurant gas mass. Therefore ground experiments, numerical simulations and analytical considerations were performed for the investigation of the initial active-pressurization phase. For the performed experiments, liquid nitrogen was used as cryogenic model propellant, which was actively pressurized under normal gravity conditions up to different final tank pressures. As pressurant gases, gaseous nitrogen and gaseous helium were used with different inlet temperatures. For the numerical analyses the commercial CFD program Flow-3D, a three-dimensional Navier-Stokes equation solver, was used.