Prediction on Pogo Phenomenon of a Space Launch Vehicle using the Virtual Mass and Modified Finite Element Method

Abstract

In space launch vehicle, there exist many instabilities that can arise from the coupling between the structural and the feedline system. Therefore, it is required seriously to predict various instabilities in the development stage of the space launch vehicle. Pogo phenomenon is one of the coupling problems. Its phenomenon is an instability that the longitudinal modes of the launch vehicle structure resonate with the pressure and flow modes of the feedline in the propulsion system. And such resonance will re-excite the fuselage structure. For the analysis of the pogo phenomenon, accurate analysis of structural modes reflecting liquid propellant and feedline pressure mode in propulsion system will be needed. The purpose of this thesis is to conduct structural analysis of a three-dimensional tank with fluid and to predict the feedline pressure modes in a launch vehicle. In addition, by applying a pogo suppressor in a feedline system, its prevention is implemented. The present pogo analysis is attempted on NASA space shuttle LOX system. Three-dimensional tank structural modal analysis will be conducted using the virtual mass method. To verify the accuracy of this method, a 1/8-scale space shuttle external tank is estimated. Especially, the LOX tank in the external tank is analyzed intensively. The natural frequencies and mode shapes are found to be in good agreement with the existing document results. In the feedline system, the present analysis will be adopted to predict the interpump line pressure modes causing instability of the space shuttle. Modified finite element method is verified by NASTRAN analysis and applied to the interpump line of the space shuttle LOX system. The comparison of numerical results against the reference about space shuttle are found to be satisfactory. In addition, the pogo suppressor will be considered to the interpump line in the space shuttle. As a result, resonance area with the fuselage structure is prevented by varying the natural frequencies of the interpump line. In this way, the possibility of pogo suppression is confirmed.