Most traps are simply screen covered containers which retain any gas ingested for the remainder of the mission. Another type of trap can vent the gas back into the bulk space during periods of high acceleration. These are called start baskets because they are most often used to start large settling engines.

Like a conventional trap, the start basket retains propellant during adverse accelerations (like that shown on this page). This propellant can be used for demand between burns and/or to accommodate the main engine ignition transient.

Once propellant is settled over the outlet, the gas in the trap can be vented if a vent tube is incorporated into the trap and the settling acceleration sufficiently high to overcome the vent tube porous element bubble point. Once the gas is vented, the start basket is ready to repeat the sequence of demand followed by main engine ignition.

The very first PMD was a start basket used for the Agena upper stage in the 1960s. The start basket eliminated the need for solid ullage rockets, which had proven not as reliable as desired.

A screen entranced trap keeps gas from entering the trap by using the bubble point of the screen. If the pressure difference across the screen does not exceed the bubble point, gas will not enter - even as liquid enters the trap from a sponge or from a propellant pool (as shown in the figure at left).

The propellant illustrated in the figure will enter trap gas free only if the pressure difference across the screen does not exceed the bubble point. In the most basic terms, the bubble point must exceed the sum of the the viscous losses through the screen and the hydrostatics. The simple force balance can be expressed as:

Details

Traps are designed to allow gas penetration when the trap propellant is required. The entering gas is "trapped" either for the life of the mission or until it can be vented with a ventable trap (or start basket). Because the trap is sized by the volume required and not by the flow path needed, the flow losses and, more importantly, the flow transients are generally not significant. With low internal velocities, the basic bubble point equation above is sufficient to design a trap. The more interesting physics occurs within the trap where a gallery arm or vanes are required to access the propellant.

The main functionality of a trap is to provide a fixed volume of propellant. Traps are most often sized with a safety factor of two on volume to ensure proper operation and some off design capacity.

The entrance windows in traps are most often screen because screen wicks and can maintain a bubble point even with gas on both sides of the screen. Since gas will reside inside the trap and will contact the porous element on the outside, screen is preferred. However, it is not required if special measures are taken to ensure that the perforated sheet is always wetted. This can be done with fins inside the trap which always hold liquid against the perforated sheet.

For more information on the design, use, and physics of traps please see

AIAA 95-2531 Propellant Management Device Conceptual Design and Analysis: Traps & Troughs