The recent extension of the nation's space activities into the ambit of human space exploration demands for safer and more reliable cryogenic ullage management techniques. The objective is to ensure that ullage pressure meets the NPSP requirements during the engine start transients and steady state operation as well as meets the tank structural requirements. If the ullage pressure drops below a threshold limit, the pump may cavitate and the tank can also buckle, both of which will lead to mission failure. In this regard, a transient thermodynamic model of liquid hydrogen (LH2) tank was developed using finite difference based SINDA/FLUINT solver for determination of ullage conditions at various phases of flight from pre pressurization to cryogenic engine ignition. Simulations were conducted to study fluid transients due to ullage pre-pressurization in the ground, tank dilation during ascent phase of the rocket, slosh excitations induced due to external vibratory loads. The results were compared with ground and flight measurements of the ullage pressure. Subsequently the studies were extended for a larger ullage volume. Results showed that in a larger ullage the fluid response due to flight disturbances is slower and the pressure is well within safe limits. A study was also conducted to determine the effect of helium concentration on the ullage response during the transient events.