38 J. C. Akers and Joel Sills Fig. 5.3 MSFC space shuttle mated ground vibration test facility (Test Stand 4550) with shuttle orbiter enterprise being lowered into place (l) and being integrated to the external tank (r) This paper discusses the ML Only modal pretest analysis, which in addition to standard pretest analysis checks verifying adequacy of the exciter and accelerometer layout also included a force response analysis to generate “test like” acceleration time histories from which modal parameters were extracted. All the three test configurations were looked at: ML on the VAB Support Posts, ML on CT-2, and ML on the VAB Support Posts and CT-2. For the force response analysis to generate “test like” acceleration time histories, the planned instrumentation, shakers, excitation types, expected range of modal damping values, estimated ambient and sensor noise levels were used. These “test like” acceleration time histories were processed and had modes extracted using the same data processing software available to the test team. This “as-run end-to-end” simulation aspect was done to ensure a high likelihood of successfully identifying the primary target modes. 5.2 Verifying Validity of Results A series of intermediate checks were built into the pretest analysis to support verifying the validity of the results. Model checks were performed on all FEM’s, which included computing mass properties, free-free normal modes, fixed-base normal modes, 1 g static loading in three orthogonal directions, unit displacement in three orthogonal directions, and static loadings applied in all three orthogonal directions at all excitation locations. All intermediate checks were successfully completed. Static loadings applied in all three orthogonal directions at all excitation locations were used to compute their local compliances. The shaker suitability study used force to acceleration (A/F) frequency response functions (FRF) synthesized from the FEM mode shapes and modal frequencies assuming 1% modal damping. These drive point A/F FRF were double integrated in the frequency-domain to obtain the corresponding force to displacement (D/F) FRF, which were compared to the statically derived drive point compliances to verify their validity. Finally, the force response analysis generate both clean and noisy “test like” acceleration response time histories. These “test like” acceleration response time histories were then processed into A/F FRF and compared to those from the shaker suitability study to verify the validity of the force response analysis. Figure 5.6 shows this comparison for one of the shaker drive point locations. The sensor and ambient background
RkJQdWJsaXNoZXIy MTMzNzEzMQ==