SwRI-designed liquid motion experiment to fly aboard space shuttle Atlantis

Data will help solve spinning satellite instability

For immediate release--

May 9, 1997 -- San Antonio, Texas -- A new experiment to be launched May 15 onboard the space shuttle Atlantis will help spacecraft designers better understand how liquid propellants behave in spinning spacecraft. The liquid motion experiment, or LME, is a joint effort by Southwest Research Institute^® (SwRI^®)  and NASA's Lewis Research Center (Cleveland, Ohio) to improve dramatically the performance, reliability, and lifetime of such spacecraft.

According to Dr. Franklin T. Dodge, an Institute engineer in SwRI's Mechanical and Fluids Engineering Division and principal investigator of the LME project, more than 50 percent of all spacecraft spin by design to obtain gyroscopic stiffness during the transfer from low earth orbit, to control propellant location in fuel tanks, or to distribute solar heat loads. "Just like any gyroscope, the spacecraft will wobble (nutate) around its spin axis," explains Dodge. "The attitude control thruster of the spacecraft must be fired when the wobbling amplitude becomes too large to bring the spacecraft back to its desired upright position. If the wobbling amplitude grows too rapidly, the thruster must be fired often, and the liquid fuel is used at a faster rate than planned for in the design; thus, the spacecraft lifetime in orbit is shorter than expected."

Carrying additional fuel might circumscribe the problem, but to do so would add weight that otherwise could be used for potentially valuable payload. Additionally, the energy dissipation caused by the liquid fuel is the primary source of the wobbling amplitude increase, so the problem may be worsened by carrying more fuel. In extreme cases, the energy dissipation caused by the propellant motion in the tanks is so great that the spacecraft goes into a flat spin and cannot be returned to its upright position, a result that translates into a loss of the entire spacecraft, which typically cost $100 million or more, notes Dodge. As an example, the tanks of a satellite had to be redesigned recently after ground tests revealed the energy dissipation rate was less than that predicted using analytical models. "Had the tanks not been modified, this spacecraft would have become unstable."

Because this problem cannot accurately be studied on earth to give spacecraft designers the information they need on fuel motions and the resulting energy dissipation, it was necessary to create a dynamics laboratory in space where liquid motions could be studied in a zero gravity environment. Institute engineers designed scale models of satellite fuel tanks that mount on a spin table. Two motors drive the table -- one provides a steady rotation and the other a nutation with an independent, adjustable frequency.

Four transparent tanks of two different shapes and fill levels will be tested simultaneously to determine the liquid torque exerted on the tanks and the phase angle of the torque relative to the table motion. Three sets of tanks will be used to simulate various liquid viscosities and propellant management devices. A small camera will make visual recordings of the bulk liquid motion and free surface waves. The self-contained experiment will be housed in the experiment lockers of SPACEHAB, a habitable laboratory module onboard the space shuttle. The experiment will be conducted over a three-day period by Atlantis Mission Specialists Carlos Noriega and Dr. Edward Lu, and monitored at the Johnson Space Center in Houston by Institute and NASA Lewis Research Center personnel.

"Our objectives are two-fold," explains Dodge. "We hope to obtain data that will provide guidance for improving analytical models of liquid motion in spacecraft. We also expect to compare these results with energy dissipation rates realized from drop-tower tests already conducted on earth." As a result, manufacturers will be able to design spacecraft with better confidence and less conservatism. Operators should also be able to make more accurate estimates of spacecraft lifetime and how rapidly propellant is expended, thus allowing for more timely replacement of satellites. "The LME is an example of successful collaboration between government and the private sector," says NASA Lewis LME Project Manager Penni Dalton. "LME was designed with input from multiple government and industry sources. The results are anxiously awaited for incorporation into future spacecraft."

The LME is one of the primary experiments to be conducted during the nine-day mission that will feature an astronaut exchange during a five-day docking with the Mir Space Station. Atlantis is scheduled to land at Kennedy Space Center on May 24.

SwRI has more than 40 years of experience in solving liquid propellant problems for the nation's space program and has a long-term, successful collaboration with NASA.

For more than 50 years, the NASA Lewis Research Center has been conducting research and technology programs critical to the nation's aeronautics and space goals. The center is NASA's Center of excellence in aircraft propulsion, space power, communications, propulsion systems, and microgravity science.

For more information about SwRI's Liquid Motion Experiment, contact Deborah J. Deffenbaugh, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-2046, Fax (210) 522-3547.

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