Until recently, spinning spacecraft nutation growth rates have been difficult to predict accurately. Analysis was used in the past, but the predictive models used were typically conservative because of unknowns in the behavior of fluids sloshing in the propellant tanks. The conservative results from these models may not have the fidelity required by some spacecraft projects to predict the stability of their spacecraft both during upper-stage flight and post-upper-stage separation. Obviously, if a spinning spacecraft were to become critically unstable during third-stage flight or after spacecraft separation, the mission could be lost.

Several experimental testing methods have been used by various organizations to predict nutation growth rates. Many of these methods require large resources and facilities. Others, such as drop-tower testing, have utilized small-scale models and high spin rates, but scaling factors must be used and these types of tests are typically more applicable for propellant tanks with either rigid internal propellant management devices or no internal propellant management devices at all. Propellant tanks with flexible, bladder-type propellant management devices have been difficult to scale.

NASA/KSC has developed a predictive computer simulation to analyze nutation growth rates of spinning spacecraft. The simulation can use either a pendulum or rotor mechanical analogy to model propellant tank fluid motion and energy dissipation. Several pendulum or rotor parameters must be determined before the simulation can successfully predict nutation growth rates. To obtain the highest prediction fidelity possible, propellant behavior test data are required to populate the model parameters.

To provide accurate test data for the simulation, NASA/KSC and the Southwest Research Institute (SwRI) have developed a new spinning slosh test facility to obtain the propellant force and torque measurements necessary to derive the model parameters. This facility is called the Spinning Slosh Test Rig (SSTR) (figure 1). The facility is owned by NASA/KSC and operated by SwRI. The first use of the SSTR was in support of the Genesis mission, which was launched on a Delta II on August 8, 2001.

After the Genesis tests were completed, the SSTR dynamometer was redesigned to improve the fidelity of the data obtained, incorporate the lessons learned from the Genesis tests, and increase flexibility to accommodate various tank sizes. NASA/KSC utilized the redesigned dynamometer to obtain data in support of the CONTOUR spacecraft project launched on a Delta II on July 3, 2002. Figure 2 shows the CONTOUR test tank mounted to the SSTR.

With the improved force sensors of the redesigned SSTR dynamometer, SwRI was able to detect a previously unsuspected fluid resonance in the CONTOUR propellant tanks. NASA/KSC and SwRI revised the simulation to incorporate additional mechanical analogies to account for the resonance. The improved force sensors and revision of the NASA/KSC simulation greatly improved the accuracy of the simulation results for the CONTOUR mission. Figures 3.a and 3.b illustrate a sample of the data output from the CONTOUR tests. The fluid resonance in the Y-torque can be seen clearly in figure 3.b.

For both the Genesis and CONTOUR missions, the data obtained from the SSTR testing and subsequent simulation results provided much higher confidence in the nutation growth rate values. This higher confidence allowed the respective spacecraft projects to determine that no modifications to their spacecraft or upper stage were required to mitigate the risk of a nutation growth rate problem.

Future use of the SSTR includes testing a soon-to-be-fabricated generic propellant tank with various configurations of an internal, nonrigid propellant management device. The goal of the generic tests is to build a database populated by nutation growth rate values for various bladder configurations, tank placements relative to spacecraft centerline, and multiple fluid levels. Additional spacecraft projects have expressed interest in utilizing the NASA/KSC nutation growth rate analysis capability to obtain accurate nutation growth rate information.

Key accomplishments:

• 1999: Concept study.
• 2000: SSTR fabrication, assembly, and test; Genesis propellant tank tests.
• 2001: Dynamometer redesign; CONTOUR propellant tank tests.

Key milestones:

• 2002: Generic test tank tests.
• 2003: Spacecraft propellant tank tests.

Contact: S.W. Clarke (Steven.Clarke-1@ksc.nasa.gov), VB-A4, (321) 476-3615
Participating Organizations: VB-A3 (J.E. Suderman), Analex (D. Griffin), Southwest Research Institute (Dr. F. Dodge and S. Green), and Boeing (A. Findley and K.J. Betty)