Test verification of the Cassini spacecraft dynamic model

The Cassini spacecraft mission to Saturn will be launched in October 6, 1997 on a Titan IV/Centaur launch vehicle. Cassini is the largest interplanetary spacecraft ever developed. The purpose of the mission is to release a Probe through the atmosphere of Titan, while the spacecraft remains in orbit around Saturn. Before launch approval can be obtained, a test verified finite element model of the Cassini spacecraft must be completed and approved by NASA. The correctness of this model is critical to the final verification coupled loads analysis and margin of safety assessments. Modal surveys were performed in August 1995 and January 1996 on a Cassini test article to provide experimental data for the verification of the Cassini test analytical model (TAM). In addition to the modal surveys, static test and component sine swept test results were also used to corroborate analytical model results. Since the Cassini spacecraft is a complex system with numerous components taking part in global modes, a systematic approach was developed to complete the model verification task. The model verification strategy included the selection of modal survey test modes, correlation goals, and the model updating approach based on hardware knowledge, engineering judgment, and analytical methods to produce the final test correlated finite element model. The analytical approach used sensitivity to select optimum incremental improvements to the model parameters, solving an eigenvalue problem to compute the modal properties of the updated model. This method made it possible to vary many model parameters in unison to obtain good agreement between test and analytical modes, especially for complicated global modes. Through the implementation of these various tools, the Cassini spacecraft model was successfully correlated to test verification goals.