Dynamic Characterization of Large Structures and Its Application to Solid Rocket Motor Testing and Launch

Abstract

Launch vehicle stages and sub-assemblies experience severe dynamic loads due to extreme launch vibration environment, thrust oscillations from solid motors, etc. Launch vehicle/spacecraft structural members and ground support structures suffer higher dynamic stress levels when the excitation frequency of vibration environment matches with its natural frequency. In addition to this, pressure oscillations inherent of large solid rocket motors tends to generate thrust oscillations with magnitude up to 1% of mean thrust. True thrust oscillation component is essential in coupled load analysis (CLA) and also in deciding the control plant requirements of launch vehicle. However, during the static test, the measured thrust suffers the structural dynamics’ interactions of the test stand. To understand the behavior of structural members under a dynamic environment, its modal parameters, viz., natural frequency, modal damping and mode shapes need to be estimated. In the present paper, dynamic characterization of large structures is discussed which has applications to launch vehicles and solid motor testing. In the first part, the dynamic characterization of a large solid rocket motor ground static test stand is studied for estimating the actual thrust oscillations generated during the ground test. The force transfer function was obtained between the motor interface to load cells using experimental modal analysis with modal shaker-based excitation. From the force transfer function, actual thrust oscillations generated by the motor are determined. In the subsequent part of the paper, dynamic characterization of the entire launch vehicle (mass of 500 tons), using the transport vibration measurements was discussed. Vibration responses of the launch vehicle at different locations were measured using servo accelerometers and its modal parameters were derived from operational modal analysis. The mode shapes are compared with that of predicted natural frequency, which match quite well. The damping ratios obtained are essential for finite element model updating to estimate the dynamic loads on the launch vehicle.