Comparing Free-Free and Shaker Table Model Correlation Methods Using Jim Beam

Abstract

Finite element model correlation as part of a spacecraft program has always been a challenge. For any NASA spacecraft, a Coupled Loads Analysis (CLA) is used to predict the coupled system response of the spacecraft and launch vehicle. The accuracy of the CLA is highly dependent on the precision of the frequencies and mode shapes extracted from the spacecraft model. NASA standards require the spacecraft model used in the final Verification Loads Cycle to be correlated by a modal test. Due to budgetary and time constraints, most programs opt to correlate the spacecraft dynamic model during the environmental qualification test, conducted on a large shaker table.

For any model correlation effort, the key has always been finding a proper definition of the boundary conditions. This paper is a correlation case study to investigate the difference in responses of a simple structure using a free-free boundary, a “fixed” boundary on the shaker table, and a base-drive vibration test, all using identical instrumentation. The NAVCON Jim Beam test structure, featured in the IMAC XXVII round robin modal test of 2009, was selected as a simple, well recognized and well characterized structure to conduct this investigation.

First, a free-free impact modal test of the Jim Beam was done as an experimental control. Second, the Jim Beam was mounted to a large 22,000 lbf shaker, and an impact modal test in this pseudo-fixed configuration was conducted. Lastly, a vibration test of the Jim Beam was conducted on the shaker table. The free-free impact test, the “fixed” impact test, and the base-drive test were used to assess the effect of the shaker modes, evaluate the validity of fixed-base modeling assumptions, and compare final model correlation results between these boundary conditions.