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Modeling Liquid Motions in Spinning Spacecraft Tanks, 18-9946

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Principal Investigator
Franklin T. Dodge

Inclusive Dates: 04/01/96 - 10/30/98

Background - Many spacecraft spin to increase their pointing stability, to equalize solar heat, or to settle the liquids in their tanks. The spinning and unavoidable wobbling and precession cause the liquids in the spacecraft tanks to oscillate. The dynamic forces created by these liquid motions form a feedback loop with the wobbling and precession, which can destabilize the spacecraft. The degree of feedback is critically dependent on whether the liquid oscillations are excited into resonance and the rate at which kinetic energy is dissipated by viscous stresses in the liquid. In earlier research, analytical models were developed to determine the resonant frequencies of liquid motions in spinning, precessing tanks. In the present project, these models were generalized to incorporate viscous effects so that energy-dissipation rates could be predicted. The predictions of resonant frequencies and energy-dissipation rates were compared to SwRI's Liquid Motion Experiment (LME), which was funded by NASA and flew on Space Shuttle Flight STS-84 in May 1987.

Approach - The analytical approach to quantifying energy dissipation rates in spinning, precessing, spacecraft tanks involves several steps, including: 1) modifying the models of liquid motion developed in earlier research to include an unsteady viscous boundary layer; 2) from these unsteady viscous boundary layer models, developing models of the total energy dissipated by the liquid motion, for specified tank precession amplitudes and frequencies; and 3) developing numerical methods to compute quantitative results from the models.

Accomplishments - The unsteady boundary layer models were developed and put in form suitable for computation. Predictions of liquid resonant frequencies and energy dissipation rates were computed and compared to LME results. The predicted resonant frequencies and the energy dissipation rates were generally in good agreement with the space flight experiments. Efforts are continuing to develop an LME re-flight to acquire data on other tank shapes and to resolve the remaining discrepancies between the analyses and the test results.

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Schematic shows gyroscope-like motion that causes liquid motion in the tanks of spinning spacecraft.

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Liquid resonant frequency predictions are compared to the LME results.

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Energy dissipation rates are shown for a liquid in cylindrical tanks.

Fluid and Machinery Dynamics Program
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