Sponges are open control devices consisting of metal panels in close proximity; generally located over the tank outlet or a trap. The PMD shown to the right is a typical radial PMD constructed by extending thin sheet metal sponge panels from a central support tube. The panels form tapered gaps in which propellant clings. The taper forces any bubbles in the sponge outboard and also ensures that as the sponge propellant is consumed, the propellant surface moves from outboard to inboard. A porous element located near the center tube would be required to access the sponge propellant.
Large sponges are used for CG control and/or for gas bubble control. For example, the Orbital Express program used a sponge to control the gas bubble and allow gas access (for transfer) at high fill fractions and the SDO program used a large sponge to retain all orbital use propellant to minimize slosh disturbances. Small sponges are used for repeated maneuvers such as stationkeeping maneuvers on a geosynchronous communication satellite or to supply a settling main engine during the ignition transient.
Sponges are refilled either during periods of zero g coast or by a settling acceleration. Like vanes, sponges are open devices which do not rely on porous elements and which will refill/reset after any acceleration environment. This makes them highly reliable.