Modal test/analysis correlation for the Centaur G prime launch vehicle

A modal test was performed on the Centaur G Prime launch vehicle for the purpose of verifying the loads analysis model. This paper describes the procedure by which the modal parameters obtained in this test were cor- related with the corresponding analytical predictions. Based on this correlation, the stiffness model of the Shut- tle trunnion system has been modified. The evolution of the model updating and the final results are described. OR aerospace payload structural systems in which the responses and loads dictate the design and thus the size and weight of the structure, the accuracy of the analytical model is of major importance because of the stringent weight constraints. A test-verified analytical model is usually required for the final verification loads analysis. The modal test from which the natural frequencies, mode shapes, modal damping, and other dynamic characteristics are determined experimen- tally is used to verify the analytical model. The comparison of various modal characteristics, such as the natural frequencies and mode shapes obtained from the modal test and its cor- responding analytical predictions, define the accuracy of the analytical model. The modal test should be designed in such a way that these dynamic characteristics can be readily measured from the test. In addition to predicting the eigen- values and eigenvectors, the analysis can yield other valuable information that can be used as criteria for the verification of the model. The Centaur G Prime launch vehicle was the upper stage for the upcoming Galileo and Ulysses interplanetary spacecraft and would have been launched by the Space Shuttle prior to the Challenger accident. The modal test of the Centaur G Prime cargo element was conducted in July-August 1985. Figure 1 shows the schematics of the Centaur G Prime in its modal test configuration. Although several different methods were used in the modal test, the results of the high-level sine dwell method were chosen for the correlation and model updating.