Southwest Research Institute (SwRI) News

SwRI^® liquid motion experiment will help solve spinning satellite instability

San Antonio — November 20, 1995 — A new experiment scheduled to fly aboard the space shuttle in late 1996 will help spacecraft designers better understand how liquid propellants behave in spinning spacecraft. The liquid motion experiment is a joint effort by Southwest Research Institute® (SwRI) and the National Aeronautics and Space Administration (NASA) to improve dramatically the performance, reliability, and lifetime of such spacecraft.

The multi-year, multi-million dollar NASA program was explained at SwRI’s 48th Annual Trustees Meeting by Dr. Franklin T. Dodge, principal investigator of the program and an Institute engineer in SwRI’s Mechanical and Fluids Engineering Division. According to Dodge, 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 (precess) around its spin axis,” explains Dodge. “The attitude control thrusters 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 thrusters 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 planned.”

Carrying additional fuel might circumscribe the problem, but to do so would add weight that otherwise could be used for potentially valuable payload. And, unfortunately, 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.

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 precession with an independent, adjustable frequency.

Four tanks of two different shapes and fill levels will be tested simultaneously to determine the liquid moment exerted on the tanks and the phase angle of the moment relative to the table motion. Three sets of tanks will be used to vary liquid viscosity 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 mid-deck locker region of the space shuttle. The experiment will be conducted by the shuttle astronauts and monitored at the Johnson Space Center in Houston by Institute and NASA 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 that of 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.

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.

Related press release: SwRI-designed liquid motion experiment to fly aboard space shuttle Atlantis

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