Screen vs. Perf Sheet

Both screen and perforated sheet have characteristic bubble points & flow losses and may be used in gallery arms successfully. However, screen has two properties that make it the preferred choice for most gallery arms.

First, screen wicks - which means it is self sealing when gas penetration occurs (such as during launch, depletion or off design operations). In fact, I argue and believe that screen ought to be used wherever gas may reside on both sides of a porous element - which includes all full tank gallery arms.

Second, screen has a much larger effective thickness because a) the liquid to metal ratio in the screen is almost always higher than perforated sheet and b) screen is more flexible than perforated sheet. An increase in effective thickness increases transient capability and will allow for smaller arms compared to perforated sheet. Perforated sheet galleries area always larger than screen gallery arms with identical performance.

Perforated sheet does have a place on gallery arms - inside traps. The inside of a properly designed trap will not see gas during launch and gas will never reside on both sides of the arm porous element - thus wicking is not required. In addition, within a trap, the arm length is short and the transient reduced. Perf sheet may not require an overly large gallery arm.

A screen covered channel or gallery keeps gas from reaching the outlet 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 gallery at the propellant pool.

The propellant illustrated in the figure to the left will flow up the gallery against the hydrostatics only if gas cannot enter the arm near the outlet. In the most basic terms, the bubble point must exceed the sum of the dynamics, the viscous losses (through the screen and along the arm), and the hydrostatics. Ignoring unsteady effects, the simple force balance can be expressed as:

Details

The force balance equation would seem to indicate that a finer screen (with a higher bubble point) would be preferable. However, fine screen has higher flow losses and a much smaller effective thickness which as we shall see is very important for transient performance.

The basic force balance above does not begin to capture the intricacies of gallery arm use because unsteady effects are ignored. These transient effects often dictate loads on the screen bubble point many times larger than the steady state loads.

The unsteady governing equations are:

Continuity (equivalent thickness vs. time):

Conservation of Momentum (acceleration and pressure):

Equation of State (pressure vs. equivalent thickness)

The severity of the transient depends upon the arm size (which dictates the liquid velocity change) and the screen compliance (equivalent thickness at bubble point). The smaller the arm, the larger the transient. The less liquid the screen can supply per unit area, the more severe the transient.

The reason transients are so severe with gallery arms is that the liquid within the arm must be accelerated to ensure gas free delivery. These changes in flow velocity in the arms must occur quickly - before gas penetrates the relatively thin screen thickness. Without accounting for the transient loads, one is virtually guaranteed a small puff of gas at every transient event. This gas may accumulate in a trap or downstream filter and be ingested in one fell swoop by the thrusters.

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

AIAA 97-2811 Propellant Management Device Conceptual Design and Analysis: Galleries