Multibody Dynamic Approach Of Modeling Liquid Rocket For POGO Stability Analysis

POGO vibration, resulted from the closed-loop instability between propulsionsystem and structure modes, is highly concerned in manned space flight. In analysis,only few important structure modes should be artificially selected, this complex themodeling work for checking stability of powerful bundled liquid rocket since a lot ofdensity structure modes has to be considered carefully. Hence, it is very necessary todevelop another easier and efficient approach for stability analysis. Besides, an idealrocket should be pogo robust, which means pogo free for a certain kind of payload andwide range of working conditions. While, flight data show that pogo stability behaviorsmission-specific and is highly sensitive to the dynamical working conditions. Therefore,it will be meaningful to qualitatively investigate the effect of physical factors onstability, and figure out the important parameters domain for achieving robust pogostability. This thesis is aimed at studying these two major problems.Firstly, the effects of physical parameters on pogo stability are qualitativelyinvestigated through analytically approach. In the analysis, coupling between rocketstructure and a single-propellant system is transformed to a number of two coupledoscillators, which describe the involved structure mode and the propulsion mode,respectively. Analytical results on the pogo stability are obtained in terms of3dimensionless parameters. The effects of these three dimensionless parameters and alldimensional physical parameters on pogo stability are extensively discussed. Resultsshow that regulating the frequency of propulsion with accumulator until it is lower thanthat of structure is an efficient way to achieve robust pogo stability.Secondly, a finite element approach of analyzing one dimensional fluid-structureinteraction problem is developed for modeling the suction line with arbitrary spatialconfigurations subjecting to large deformation. The pipe structure is divided through thenewly proposed Euler-Bernoulli beam element based on quaternion. Quaternionsinstead of Euler angles are adopted as nodal variables to avoid the traditional singularityproblem when describing the attitude of cross section. To meet the Euler-Bernoullibeam requirement, a requital interpolation method is also specially developed toguarantee the perpendicularity of cross section and tangent. The liquid in pipe isformulated through finite volume method whose control volume keeps on coinciding with the corresponding beam element during deformation. The governing equation ofcompressible and uncompressible liquid is established, and the reaction force tostructure is formulated. Numerical results show this approach is suitable for modelingfluid-structure interaction in one dimensional pipe.Finally, a multibody dynamic approach of modeling liquid rocket is proposed forpogo stability analysis. All of components in propulsion system are successfullyimplemented into multibody dynamic program after fixing the converging problem metat solving motion equations of liquid and structure simultaneously. POGO stability of apropulsion system with both axisymmetric and non-axisymmetric configurations isnumerically addressed. Results show that, on the one hand, except longitudinalvibration modes, instability might happen at the bending modes or local models evenwith axisymmetric propulsion configuration, and this kind of instability could bemitigated by accumulator. On the other hand, non-axisymmetric configuration offeedline might introduce another kind of instability, which could be eliminated throughdesign of feedline instead of accumulator.