In order to start the main engine of an upper stage it is necessary to position the liquid propellant stable above the tank outlet. Accordingly, supplying the thrusters with gas- and vapor-free propellant is a further fundamental criterion. This can be accomplished by a small pre-acceleration prior to firing the engine. An alternative can be the use of Propellant Management Devices (PMD) in order to provide gas- and vapor-free propellant. Therefor a propellant refillable reservoir (PRR) will supply the essential amount of liquid propellant until the liquid outside the PRR has settled. In consequence of reorientation the will be refilled through capillary vanes in the tank.
PMDs have been applied numerous times in case of storable propellants like MMH and hydrazine to date. In the current work the application of a PMD in a restartable cryogenic upper stage shall be obtained. In between of the driven flight phases the stage will be under conditions of compensated gravity. Within these ballistic phases handling of the cryogenic propellants is complex in case of additional thermal boundary conditions of the system. Additionally, disturbing accelerations may remove the liquid propellant from the outlet. Before starting the engine the liquid propellant has to be settled above the exhaust. Vaporization of the liquid and bulk boiling has to be taken into account. In this regard vaporization can be caused by incident heating through the walls of the tank, heat input from the engine to the feed lines and equipment, and by pressure fluctuations in the tank due to venting or condensation of the pressurization gas.