Modeling and control of sounding rocket airframe vibration

Vibrations in the cylindrical structure of sounding rocket airframes during flight are stimulated by combustion in the rocket motor, and by buffet excitation from airflow around the vehicle. These vibrations can cause a number of undesirable affects. Low frequency bending can change the relative position of the center-of-gravity and center-of-pressure of the airframe and adversely impact its aerodynamic stability. Vibrations can become confused with changes in vehicle attitude and cause guidance problems. Finally, vibration can interfere with the operation and life expectancy of on-board components. Every rocket payload is assessed for its robustness to vibratory excitation. Tests are conducted that mimic the vibration environments of flight. The structural and functional integrity of on-board systems is monitored. Additionally, simple beam models of the airframe are used to predict bending in the airframe and its affect on flight stability. What does not exist is an efficient means for predicting the longitudinal and circumferential vibratory response of an airframe, and with which one can design stiffening structures into a payload to achieve a desired vibratory response. In this research, an axisymmetric shell finite element model was developed that can predict the bending vibrations of a payload, as well as the higher order circumferential modes of vibration that also exist. This model was validated using data from the literature, and from a series of vibration experiments. Principal among these experiments was a sub-scale replica sounding rocket. Baseline experimental results were strong and showed good correlation with the predicted results. The validated model was then used to optimize the size and placement of a set of stiffening rings to achieve a prescribed shift in the frequency response of the replica rocket. This optimal stiffening ring configuration was experimentally tested and its effectiveness assessed. Finally, the application of this modeling approach and optimization process is assessed, and the potential effectiveness of the stiffening ring concept for full-scale sounding rockets is demonstrated.