Stability Of Large-loop System Coupled Between Attitude Control And Pogo For Liquid Rockets

As the scale of the liquid rockets getting larger, the distributing characteristic of the structural modal becomes intensive and spatialized. Thus, the coupling effect not only exists between the propulsion system and the longitudinal modal of structural system, but also exists between the propulsion system and the lateral, torsional modals. It is known that elastic vibration plays a non-negligible role on stability of the attitude control system. The coupling effect between the propulsion system and structural system may also result in the coupling between the attitude, structural vibration and the propulsion system, which further affects the stability of the attitude control system. Thus, it is necessary to take the effect of Pogo loop into account in the design of attitude control system. The main works of this dissertation are as follows:The mechanism of Pogo is analyzed. The limitation of the critical damping ratio method is pointed out firstly and the critical coupling strength method is used to evaluate the stability of Pogo. The propulsion system is simplified as a monopropellant model and the necessary and sufficient conditions of Pogo stability is derived according to Hurwitz stability criterion. By using the critical coupling strength method, the effects of the parameters such as the damping ratio, on the stability of Pogo are investigated. It is found that the effects of the damping ratio (structural or propulsion system damping ratio) on the Pogo stability are non-linear. Based on the critical damping ratio method and the critical coupling strength method, the effects of the physical parameters, such as equalivent stiffness, combustion damping on the Pogo stability of a type of Chinese CZ liquid rocket is analyzed to further reveal the limitation of the critical damping ratio method.An improved modeling method for Pogo analysis and simulation is proposed. Based on the description method of "independent weight-displacement", the eight physical elements of Rubin’s method are redivided into nine independent elements. Then the improved Rubin’s model is established with the perturbational pressures of the inlet and outlet points of the elements considered as the continuity condition. This model is non-singular and can be directly used for time-domain simulation and frequency-domain analysis. Compared with Rubin’s model, the dimension of improved Rubin’s model is almost half of Rubin’s, which significantly improves the numerical efficiency and stability. According to the modeling process of improved Rubin’s model, an automatic assembling method for the coefficient matrixes in the improved Rubin’s model is presented. To describe the assembling method concisely, the dynamic equations of the nine types of independent elements are described in a standard manner. The mapping relationship between the local and global numbers of elements and nodes is obtained by numbering all of the elements and nodes. By integrating the element stiffness matrixes to obtain the whole stiffness matrix that used in the finite element method (FEM), the coefficient matrixes of the improved Rubin’s model are automatically assembled from the coefficient matrixes of all of the elements. The automatic assembling method described in this study can be used to repeat Pogo modelling and analysis with high efficiency and veracity.With the improved Rubin’s model, the Pogo stability of XXI type of Chinese CZ rocket is analyzed by frequency-domain analysis and time-domain simulation. Further more, the model for Pogo stability analysis considering longitudinal, lateral and torsional modal in structural system is developed based on the improved Rubin’s modeling method. The Pogo stability of XX2 type of Chinese CZ liquid rocket is studied both in frequency-domain and time-domain by investigating the effects of pump gain and accumulator energy value to the system stability. The result shows that the variations of the propulsion system paramaters not only results in the instability of longitudinal structural modal, but also the instability of lateral structural modal, which means that the coupling exists between the longitudinal, lateral and torsional modal of structural system and propulsion system.The coupling model of attitude control system, structure system and propulsion system is derived to investigate the effects of the propulsion system on the stability of attitude control system. Based on mechanism that the effects of attitude control system on the attitude motion and structural vibration as well as the interaction of propulsion system and structural vibration, the coupling model of attitude control-structure-propulsion system is derived. The large-loop coupling model can be used to investigate the coupling stability of the large-loop coupling system. Besides, the coupling model can be directly used for frequency-domain analysis and time-domain simulation for its non-singularity characteristic. Based on this model, the large loop stability of XX2 type of Chinese CZ liquid rocket is analyzed.According to the modeling and analysis theory, the "Propulsion-Structure-Control Coupling Dynamic Analysis" software is developed in Matlab. This software can be used by both GUI and command to the modeling and analyzing of propulsion system, Pogo system and large-loop system.