Reduced gravity undergraduate student flight experiment - Liquid transfer in reduced gravity

Undergraduate students in Mechanical and Aerospace Engineering at The University of Alabama in Huntsville designed, fabricated, and tested a reduced gravity flight experiment which flew on NASA's KC-135A aircraft during the Spring of 1997. This special course included scientific scholarship, hands-on test operations, interpretation of the results and educational and public outreach activities. Specifically, the student project involved transfer of liquid from one container to another in reduced gravity. The behavior and transfer of liquids in reduced gravity and variable acceleration environments is a topic of importance to material science, fluid physics, combustion science, life sciences, and spacecraft design. The test data package design addresses a number of issues raised by previous research concerning fluid physics and liquid transfer. Among these issues include the effect of microgravity on liquid transfer, the role of surface tension devices (baffles) in effective liquid transfer, the gas ingestion phenomena, the distribution of liquid in reduced gravity and varying acceleration environments. Bubble formation is observed to occur after six to ten seconds after entering into the reduced gravity portion of the parabola. The results show that liquid transfer is more efficient when using baffles or surface tension devices inside the tanks. INTRODUCTION AND BACKGROUND During space operations in reduced gravity the transfer of a liquid from one tank to another is required. One element of propellant management that will be necessary during future space operations will be the transfer of liquid from a tanker vehicle to a receiver vehicle. Examples of such include: 1) transfer of propellants from the shuttle to the space station, 2) transfer of propellants to tanks of vehicles designed for lunar or planetary missions, 3) transfer of vital fluids to the space station, and 4) restart of propulsion systems in space. Also, applications occur in high speed aircraft under high "g" and variable "g" loading where liquid transfer issues are encountered. The fluid management challenges encountered with the transfer process include: 1) Gas free outflow from the supply tank, 2) Spill free inflow into the receiver tank, 3) Control of the liquid-vapor interface during acceleration changes, and 4) Positioning of the fluid at the drain port. In microgravity liquid behaves dramatically different than on the surface of the earth in 1-g. In environments where gravity is reduced, the body force * Assistant Professor, Senior Member. Copyright © 1998 The American Institute of Aeronautics and Astronautics Inc. All rights reserved. of gravity no longer dominates. Instead, the effects of surface tension, adhesion, and other applied forces such as rotationality and random accelerations determine the distribution of liquid. On the surface of the earth liquid inside a container will fill the bottom of a container, minimizing liquid/gas interface surface energy. One also observes a meniscus develop as the liquid "climbsup" the side of a container due to surface tension. In microgravity liquid inside a container in equilibrium will no longer be at the bottom of the vessel. Rather, the liquid will be distributed inside the vessel such that the liquid/gas interfacial surface energy is minimized. This results in a bubble of constant curvature being formed within container. Much work has been done concerning the transfer of liquid between a supply and receiver tank. Initially, several investigations were completed in the NASALewis drop tower facility which demonstrated various aspects of liquid transfer phenomena. The first comprehensive demonstration of liquid transfer was performed by Apollo 14 astronauts during transearth orbit. ' The objectives of this experiment were to demonstrate the effectiveness of two surface tension baffle designs during fluid transfer. The unbaffled tanks were found to be unsuitable for transfer of liquid in weightlessness. The baffled designs; however, were shown to be effective in transferring liquid in weightlessness. Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. Liquid motion in microgravity was also investigated on Sky lab 3. This research was designed to investigate liquid/gas interface behavior including wave propagation and surface tension effects. The results of this study provided insight into low-g liquid motion, wave propagation, and manipulation. More recently several shuttle experiments concerning liquid transfer have also been conducted. On the STS-51-G shuttle mission a hemispherical tank filled with fluid was subjected to acceleration inputs and the resulting sloshing forces were measured.3 The objectives of this STS Get Away Special (GAS) canister included 1) determining the low-gravity, dynamic behavior of a partially filled tank using experimental methods, 2) correlating the experimentally obtained data to mathematical models, and 3) evaluating the outflow of the liquid in the test tank through a tube and solenoid valve and into an evacuated chamber. The study was initiated to examine the coupling between the dynamic response of the fluid in the tanks and the tank design. Another shuttle experiment was conducted to evaluate the effects of liquid sloshing. 4 If propellant tank liquid sloshing should occur in a spin stabilized satellite the energy of the vehicle would be dissipated and wobbling motion about the spacecraft's axis could occur. The objective of this experiment was to measure slosh forces and observe slosh motion in a model satellite propellant tank. On shuttle mission STS-017 a Get Away Special (GAS) 5 experiment was conducted to (1) investigate the use of advanced surface-tension, propellantacquisition concepts in the design of propellant tanks, and (2) demonstrate the bubble-free delivery of fuel from a partially filled propellant tank to the spacecraft engines. More specifically, the experiment was designed to demonstrate an off-load capability for a fuel-tank propellant acquisition system, a capability that did not exist with current fuel-tank surface tension devices such as the Shuttle RCS propellant acquisition system. Hassan et al. 6 designed a tank pressure control experiment which flew on board the space shuttle (STS-52). On the Orbiter each test generally started with a heating phase to increase the tank pressure and to develop temperature stratification in the liquid followed by a fluid mixing phase for the tank pressure reduction and fluid temperature equilibration. Analysis of data from the flight experiments and their comparison with the results obtained in drop tower experiments suggest that as the Bond number approaches zero the flow pattern produced by an axial jet and the mixing time can be predicted by the Weber number. The equilibrium configurations of free liquid/gas surfaces 7 in reduced gravity environments are of particular importance for liquid transfer phenomena. These conditions generally yield surfaces of constant curvature meeting the container wall at a particular contact angle. The time required to reach and stabilize about this configuration is not completely understood for sudden changes from elevated to reduced gravity. An objective of this program was to provide students with the opportunity to propose, design, build, test, and fly a reduced gravity flight experiment. The scientific objective of the experiment was designed to address a number of the issues raised by previous research in fluid physics and liquid transfer. Among these issues include the effect of microgravity on liquid transfer, the role of surface tension devices (baffles) in effective liquid transfer, the gas ingestion phenomena, the distribution of liquid in reduced gravity and under changing acceleration environments. Also, an objective of the proposed project was to quantify the time scale required to stabilize a liquid/gas interface within a partially filled container. EQUIPMENT DESCRIPTION