Experimental resonant frequencies and mode shapes from vibration tests with a 1/5-scale model of the Titan III-A launch vehicle are presented, Measurements of longitudinally excited vibration modes of the model structure supported in a suspension system offering negligible restraint to longitudinal motion for five different propellant loadings are shown. The data indicate the quantitative and qualitative effect on the overall model vibration response due to different amounts of simulated liquid propellant in the tanks. The experimental data show the response of the model to be complex due primarily to structural coupling of modes. Two mathematical models are used to calculate the model vibration response; one represents a simple lumped-mass analysis and the other a more refined interpretation which considers axisymmetric shell, fluid, and mass components of the structure. Longitudinal vibration mode shapes and frequencies computed from the component model show better correlation with measured basic longitudinal structural. modes than those obtained using the lumped-mass representation. / M.S.