Pressurant effects on cryogenic liquid acquisition devices

On-orbit storage and resupply faculties for cryogenic liquids will likely require surface-tension liquid acquisi- tion devices (LADs) in the storage tanks to provide reliable extraction and transfer of vapor-free liquid. The investigation reported in this paper comprised experiments with a screened-channel LAD in liquid hydrogen to measure the performance of the channel as a function of the type and temperature of the pressurant gas used to expel liquid from the test tank through the LAD. With cold pressurant (near the saturation temperature of hydrogen), vapor penetration of the LAD screen occurred at near the predicted pressure difference across the screen for both helium and hydrogen pressurant, although helium provided better results. With warm helium pressurant, there was no observable degradation in screen performance. With warm hydrogen there was a substantial degradation, and if the LAD outflow was interrupted, allowing the liquid in the LAD to stagnate, vapor penetrated the screen at very low differential pressures. ANAGING liquid propellants and other fluids in mi- crogravity will be an essential element of the on-orbit facilities infrastructure needed to support future operations in space. Space transfer vehicles, Space Station Freedom, space- based defense systems, and serviceable satellites, for example, will require on-orbit servicing to replenish liquid propellants, fuels, coolants, commercial stocks, or life support fluids. Much of the technology required for storing and transferring significant amounts of these fluids in microgravity remains under development and is yet to be demonstrated. This is especially true regarding cryogenic liquids: For liquid hydro- gen, oxygen, and nitrogen—which are important for in-space propulsion and life support systems—data describing the ther- mal and dynamic phenomena involved in storage and transfer in space are quite limited. One important aspect of in-space fluid transfer is acquisi- tion of vapor-free liquid from a storage tank. In the absence of gravity or acceleration forces to settle the liquid contents of a tank over the outlet, special devices must be used to transfer the liquid out of the tank. Bellows, bladders, and capillary (surface-tension) devices have been used effectively with non- cryogenic liquids. With cryogens, surface-tension devices (e.g., screen-covered channels) appear to be the best approach because heat conduction may form unwanted vapor bubbles within bellows and bladders. Further development followed by an in-space demonstration is necessary, however, for cryo- genic applications. Liquid hydrogen in particular imposes po- tentially troublesome design constraints because of its low surface tension and ease of vaporization. A liquid acquisition device (LAD) employing fine-mesh screen would usually be designed to serve either as a "total communication" LAD for withdrawing liquid from one tank for transfer into another or as a "start basket" in a propellant tank in an upper stage booster for providing vapor-free pro- pellant during engine startup until the thrust settles all of the propellant. In either application, wherever vapor contacts the wetted screen, the surface tension at the liquid-vapor interface within the small pores in the screen inhibits the passage of