Best Components
Mission Specific


Mission Optimized
High Reliability
Low Residuals
Low Recurring Cost


Design Cost


All Kinds




Most PMDs are constructed from components of different types. This results because one component may meet one mission objective and another component a second objective. For example, a repeatable spin recovery might require a sponge and unlimited lateral thruster firings may require vanes. Thus a sponge and vane PMD would be the optimal solution for these two mission objectives.

By combining different components, the PMD can be optimized for the mission requirements as well as address cost and weight concerns. In addition, risk can be averted and reliability increased (e.g. why have screen where you do not need it).

The PMD pictured to the right makes use of every type of PMD component. It contains 1) a trap to retain gas ingested during contingency operations, 2) a gallery (liner) within the trap to access the trap's propellant, 3) vanes within the trap to bring liquid to the trap liner, 4) a trough in the trap entrance to prevent the trap from spilling during horizontal handling, 5) a sponge to supply propellant during repeated orbital operations, 6) another gallery under the sponge to access the sponge's propellant, and 7) vanes in the tank bulk space to refill the sponge. By combining these components, the PMD is small (except the thin vanes), lightweight, cost effective, and most importantly highly reliable.

Some combinations are required by one of the components. For example a sponge often needs vanes for zero g refilling or galleries may need a trap to trap the gas ingested during launch. The combinations here show these relationships.