Influence of Boundary Conditions on Dynamic Characterization of Launch Vehicle Structures with Closely Spaced Modes

Dynamic characterization tests play a critical role in launch vehicle applications, as they provide the frequencies and mode shapes required for refining Finite Element Models (FEM) and ensuring structural integrity. While such tests are often routine when mode shapes in orthogonal planes are well separated, practical challenges arise when modes are closely spaced. In these cases, careful test planning and execution become essential to obtain reliable results. A key factor influencing test outcomes is the boundary condition of the test article. Although free-free suspension, achieved through very low-frequency support, is theoretically ideal, it is often impractical. As a result, most dynamic characterization tests are performed with a base-fixed condition, where the properties of the supporting structure can influence the measured response. For structures with asymmetry limited to a single axis, mode shapes are typically expected to align along that axis; however, deviations may occur when boundary effects are significant. This paper presents a case study of a structure in which the first bending modes in orthogonal directions were separated by only 0.1 Hz. The hardware, with a mass of less than 4 tons, was mounted on a 110-ton rectangular platform without strong ground fixity. Although initially expected to have negligible influence, the base altered the bending mode orientations by up to 30 degrees, even though frequencies and mode shapes remained largely unchanged. Through carefully designed experiments, these closely spaced modes were extracted, and the base influence was identified. A repeat test on a rigid, concrete-embedded platform confirmed the true orientations. The results underscore the importance of accounting for boundary condition effects in dynamic characterization tests and provide practical guidance for test engineers to mitigate uncertainties in future launch vehicle applications.