Analysis of Critical Disturbances for Propellant Management Devices in Future Cryogenic Upper Stages

摘要

In future cryogenic upper stages the ability of multiple starts is required. Consequently, either propulsed flight phases with upward orientated accelerations or, in ballistic phases, microgravity conditions will act on the stage. Disturbing accelerations could position the liquid propellant away from the tank outlet during the orbital flight phases. A necessary amount of liquid fuel has to be accumulated over the outlet before re-ignition. To manage that, the gas free expulsion can be provided with a Propellant Management Device (PMD) in the European cryogenic upper stage A5ME so that no additional firings are necessary to settle the liquid over the outlet. During the microgravity phase the PMD also accumulates the necessary amount of liquid for chill-down of the propellant system. To analyze the behavior of the fluid interfaces during draining drop tower tests were performed. For this purpose the complex geometry of the LOX PMD was simplified to a peripheral inlet divided into segments by vertical baffles. Moreover, a central capillary tube was assembled as the venting tube of the PMD. All inner structures were neglected to allow observation of the menisci. The performed draining tests with different model geometries represent an engine restart preparation phase prior to the last boost. Hence, a low fill level in the tank is assumed. Another assumption is that the liquid propellant in the PMD has no contact to the bulk liquid fuel in the tank. During the ballistic flight phases a critical spin rate which drives the liquid out of the PMD can occur. The critical Bond numbers in dependence of the aspect ratios of the model and its number of segments in which the model is divided into have been computed with Surface Evolver.