Screen Channel Liquid Acquisition Devices for Cryogenic Propellants

Abstract This paper describes an on-going project to study the application screen channel liquid acquisition devices to cryogenic propellant systems. The literature of screen liquid acquisition devices is reviewed for prior cryogenic experience. Test programs and apparatus are presented to study these devices. Preliminary results are shown demonstrating bubble points for 200x1400 wires per inch and 325x2300 wires per inch Dutch twill screens. The 200x1400 screen has a bubble point of 15.8 inches of water in isopropyl alcohol and 6.6 inches of water in liquid nitrogen. The 325x2300 screen has a bubble point of 24.5 inches of water in isopropyl alcohol, 10.7 inches of water in liquid nitrogen and 1.83 inches of water in liquid hydrogen. These values are found to be in good agreement with the results reported in the literature. Background Under the influence of earth’s gravity, buoyancy normally dominates the separation of liquid and vapor inside a container, that is, the liquid (heavier fluid) settles to the bottom while the vapor (lighter fluid) rises to the top. In the reduced acceleration or gravity environment of space surface tension, rather than buoyancy, can become dominant in determining the relative positions of liquid and vapor propellants. In low gravity, as the liquid-vapor interface shape assumes the minimum surface and potential energy position, the liquid tends to wet or migrate along the walls or interior structures within the tank. In a cylindrical tank the liquid-vapor interface assumes the shape of a half-sphere in zero gravity; whereas, in the case of a spherical container, liquid-vapor interface becomes spherical, that is, the liquid actually encapsulates the vapor. The propulsion systems of most vehicles require single-phase propellant delivery since two-phase flow in the propulsion system leads to cavitation and engine damage. During the high acceleration engine thrust periods, single-phase expulsion is accomplished simply by withdrawing liquid from the bottom of the tank and utilizing an anti-vortex baffle over the tank outlet. However, in low gravity where fluid is not centered over the tank outlet, withdrawing single-phase fluid becomes a challenge. On current upper stages such as the Centaur, small storable propellant thrusters are used to create acceleration and position the fluid over the tank outlet for second or third main engine firings. The Space Transportation System (STS) or Space Shuttle auxiliary propulsion system utilizes a bipropellant system (N2O4 and MMH) for orbital maneuvering (Orbital Maneuvering System or OMS) and for attitude control (Reaction Control System or RCS). Capillary liquid acquisition devices (LADs) within the Shuttle OMS and RCS tanks have proven quite successful in assuring delivery of single-phase propellant to the engine. Considerable experience exists with LADs for storable propellants and a variety of shapes, sizes, and combinations can be used, depending on the mission application. One type of LAD, referred to as a vane, is a lightweight structure with high surface area. Multiple vanes are located in a central region of the tank and are configured such that the vapor is positioned, by capillary action, in preferable positions. Liquid is wicked down the vane and into a capillary trap that supplies liquid for engine restart. Another, more prevalent type LAD is a screen channel device (used on the Shuttle for auxiliary propulsion). Screen channel devices closely follow the contour of the propellant tank wall (typically within 0.25 inches) and can be of either a rectangular or triangular cross-section. Usually, four channel legs (one per tank quadrant) are used and manifolded together over the tank outlet or feed-line entrance. The channels are positioned such that one or more channels are always in contact with the liquid along the tank wall, independent of the liquid-vapor positions. This property of always being in contact with the liquid is called “total communication.” The channel side that faces the tank wall has multiple openings that are covered with tightly woven screen. As pressurized outflow or expulsion begins in reduced gravity, surface tension forces within the screen weave tend to block the outflow of vapor and allow the passage of liquid as propellant. As discussed later, wicking action assures that the screens surfaces remain wetted and that the vapor blocking effect is sustained provided other conditions are satisfied. Through previous and current applications, analysis and design techniques for storable propellant screen channel LADs have been well established (see references 15 and 16). The resistance to vapor passage is dependent on the surface tension retention ability of the screen. The