Galleries are porous element covered closed flow paths extending from the outlet to the propellant pool and are designed to communicate liquid from a pool to the outlet. The porous element prevents gas ingestion while allowing liquid acquisition. The PMD shown to the right is a simplified screen covered channel PMD constructed from rectangular cross section tubes and screen (shown in green).
Thruster firings could deposit the bulk space propellant anywhere in the tank and the galleries create a flow path the outlet. Since porous elements prevent gas penetration while allowing liquid flow, the channel is always filled with liquid providing the flow path between the pool and the outlet. As long as the flow losses and hydrostatic losses do not exceed the porous element bubble point, gas free flow up the channel will occur.
Gallery PMDs are the easiest to understand and are the most capable of PMDs. This makes them very attractive. Unfortunately, they are also heavy, complex, inefficient and less reliable. For missions needing omnidirectional and omnidurational capability at higher accelerations and flow rates, they are the appropriate PMD choice. For example, galleries are used in the Shuttle RCS tanks and the MILTAR communication satellite. In addition, gallery PMD components are found within most traps; which are typically designed to supply the propellant trap volume during omnidirectional and omnidurational demand.