Dynamic responses at critical locations of a spacecraft due to excitations expected during the ascent phase of a launch vehicle mission are usually estimated through a Coupled Loads Analysis (CLA) using the structural dynamic finite element model of the launch vehicle coupled with that of the spacecraft. Generally, the full physical structural dynamic model of a spacecraft has lakhs of degrees-of-freedom (DOFs). Coupling such a model with a similar model for the launch vehicle results in exorbitantly high computational costs for CLA. Hence, dynamic analysis of such large and complex structural assemblies usually employ sub-structure coupling or Component Mode Synthesis (CMS) methods. The most widely used CMS method for dynamic analyses is the Craig-Bampton (CB) method. Conventionally, a full launch vehicle CLA involves one level of CB-reduction wherein a reduced-order dynamic model of the spacecraft is first generated using the fixed-interface CB-method. This reduced-order model is coupled with the launch vehicle model through the interface DOFs and CLA is performed using this coupled dynamic model.
For test missions of ISRO’s manned space program, a simulated Crew Module (CM) is interfaced to the launch vehicle in place of the spacecraft. For CLA of this launch vehicle, a CB-reduced model of the CM is required. The CM comprises several sub-systems, including the Crew Seat Assembly (CSA), and dynamic responses at critical locations of these sub-systems need to be obtained from CLA. Structural dynamic model of the CSA is complex and involves lakhs of DOFs. Interfacing this detailed finite element model of CSA directly with the full model of CM for CB-reduction was found infeasible considering the prohibitively high computational time and resources required. An alternate approach is utilized to overcome the problem wherein the reduced-order dynamic model of CM is generated with two levels of nested sub-structuring. The CSA is represented as a CB-reduced model within the full physical structural dynamic model of CM, which is further CB-reduced and coupled with the launch-vehicle model for CLA. This paper presents the approach adopted to extract internal point dynamic responses on CSA from a dynamic analysis using the reduced-order dynamic model of CM with two-level CB-reduction. To validate the proposed approach, structural dynamic model of a skeletal structure of CM is generated encompassing the full model of CSA. A reduced-order dynamic model of this skeletal structure is also generated with two-level CB-reduction. A typical transient force excitation is considered and dynamic responses at internal points of CSA are estimated from the reduced-order dynamic model using the proposed approach. These responses are compared with corresponding responses obtained from a similar analysis with the full physical model of the skeletal structure and validity of the approach is established. The presented approach is generic and can be conveniently extended to extract responses from any dynamic analysis with models having even more than two levels of nested sub-structuring. Using this approach, the computational time and resource requirements for dynamic analysis studies are minimized. Dynamic analysis capabilities of MSC Nastran software are used for this study.