Modeling Liquid Motions in Spinning
Spacecraft Tanks, 18-9946
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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.
Schematic shows gyroscope-like motion that causes liquid
motion in the tanks of spinning spacecraft.
Liquid resonant frequency predictions are compared to the
Energy dissipation rates are shown for a liquid in
Fluid and Machinery
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