1D heterogeneous dual-phase model of a cryogenic liquid Hydrogen (LH2) propellant tank pressurization system was developed. Simulations were carried out using SINDA/FLUINT version 6.3, a finite difference, lumped parameter tool, to determine the quantity of gaseous Helium (GHe) for pressurization of the fuel tank in the cryogenic upper stage of a rocket. As there was no feasibility of establishing a lab-level twin of the total system, the model was validated on a sub-system level with data from other flight measurements. The design strategy was to identify the worst-case scenario so that the proposed active pressurization system would cater to most adversities in flight. The non-dimensional GHe mass and time required for pressurization were determined to be in the range of m* = (0.058 to 0.145) and Δτ = 0.18 × 10–6 to 0.38 × 10–6 respectively. Post-flight simulations indicated that m* = 0.1 of GHe was required to pressurize the tank by ΔP = 0.174 in Δτ = 0.32 × 10–6