Alternative Methods of Dynami Model Verification for Ascent

Of particular concern to most spacecraft programs is the verification of the finite element model used to predict launch loads. The verification process requires a test-to-model correlation of the high loads producing modes. Presently, this usually requires building/using a test fixture to simulate the spacecraft-booster interface fixity conditions. This method, usually referred to as fixed-base modal testing, introduces significant uncertainties to the test results in the form of test stand flexibility, friction at the interface, etc. To avoid this myriad of problems and possibly reduce the overall structural test program duration significantly, alternative test programs have been developed. These test programs couple a standard modal test of a freely suspended structure with modified modal tests designed to highlight the character of the payload-booster interface without the need of simultaneously constraining all the interface degrees of freedom (DOF's). The initial free-free modal test is used to verify the high load producing modes of the structure which are due to internal components of the structure reacting with the rest of the structure, while the two alternative methods to be discussed in this paper, a mass loaded interface freely suspended modal test and the interface verification test, are used to verify the the character of the high load producing modes in which the structure significantly reacts against the booster interface.

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