Vanes are structures located along the tank walls or within tanks that liquid clings to and are designed to communicate liquid from a pool anywhere in the tank to the outlet. The PMD shown to the right is a typical center posted vane PMD made simply from three pieces of sheet metal.
Thruster firings could deposit the bulk space propellant anywhere in the tank and the vanes create a flow path the outlet. All modern propellants are wetting and will climb into every crevice of the PMD. By creating corners (such as the corner formed by the side vanes and the tank wall or the corners formed by the center post itself), a flow path is established from the pool to the outlet. As propellant is consumed, more propellant will flow “uphill” to replace it.
Used for low acceleration thruster firings, a vane PMD provides propellant continuously for any burn duration and direction. This unlimited duration and direction capability coupled with the vane PMDs light weight and low cost, make the vane PMD the most desirable option for any mission. Unfortunately vanes are capable of lifting propellant only under very low g conditions - such as that created during 0.2 lbf hydrazine thruster firings. This make vanes viable as stand alone PMDs only in hydrazine systems. In bipropellant systems, vanes cannot lift propellant against the typically higher accelerations. Vanes are used in bipropellant systems for zero g resupply of control devices.