Research On System Dynamics And Control Method Of LOX/LCH4 Variable Thrust Rocket Engines Pressurized By The Electric Pumps

As an important even inevitable component referring to numerous space missions,such as space transportation and propulsion,the deep-throttling rocket engine gains more and more attention in the field of aerospace engineering.With the increasing demand for cryogenic,nontoxic,and low-carbon propellants,liquid oxygen/liquid methane propellants become preferable for a bipropellant propulsion system due to their favorable characteristics for long life and reusability.The electric pump system is easy to control and can respond quickly due to its simple configuration.This makes the electric pump system competitive and promising particularly with the development of advanced battery cells.Based on a liquid oxygen/liquid methane variable thrust rocket engine pressurized by the electric pump,the simulation model with regenerative cooling channels has been established and the variations of parameters during the start-up process as well as throttling process are analyzed.To control the engine thrust precisely during the throttling process,the variable thrust control scheme is put forward.The controllers and observers for the mass flow rates of the pumps are designed and validated by the simulation using the established engine model.Varying degrees of failures those may occur in the operating process are artificially injected into the simulation.Meanwhile,the control effects of the feedback control system on the failures are tested.The dynamics analyses of liquid oxygen/liquid methane variable thrust rocket engine pressurized by the electric pump are carried out.Based on the research object in this dissertation,the dynamics models of the electric pumps,valves,thrust chamber,and regenerative cooling channels are formulated and established.The precooling,filling,and start-up processes are simulated using the established engine model.In the simulations of the start-up process,the influences of the valve actuation and the chamber volume before methane injection are discussed and analyzed.The results demonstrate that the valve responses lead to a time delay and additional overshoot on engine performance parameters.Meanwhile,the chamber volume before methane injection cannot be oversized.Otherwise,the mixture ratio will have a significant overshoot at the beginning.Besides,simulations are conducted on several operating conditions and the dynamic characteristics of the engine during the start-up process are analyzed.With the decrease in operating conditions,the methane mass flow rate needs more time to reach its steady-state value.Compared with the methane path,the liquid oxygen mass flow rate can reach its steadystate value quickly no matter what the operating condition is.At low operating conditions,the methane in cooling channels becomes a two-phase flow and the heat transfer is intensified.The controllers for the mass flow rates of the pumps are designed.The thrust control scheme for the rocket engine is put forward.The regular control targets,including the chamber pressure and mixture ratio,are converted to the mass flow rates of liquid oxygen and liquid methane,respectively.The nonlinearity of the electric pump system is taken into consideration by analyzing the gap metric,which determines the control methods.As the nonlinearity of the electric pump is not obvious,it is a good choice to adopt either linear approximation or gain-scheduling controllers.Then,the proportional-integral(PI)controllers and gain-scheduling linear quadratic regulators(LQRs)are designed for the electric pumps in liquid oxygen and liquid methane paths correspondingly according to the dynamics equations and the parameters of the electric pumps.Analyzing the amplitude-and phase-frequency characteristics as well as the pole-zero distribution of the system,the results show that the designed controllers can stabilize the linearized equations in the incremental form at different operating points and improve the dynamic response of the system.This indicates that these two controllers are available for the original system in the whole range of working conditions theoretically.Subsequently,the validation of the controllers in the simulation is conducted in MATLAB/Simulink.With external disturbances,the dynamic responses of the electric pump are compared with and without a PI controller or LQR.The results verify the effectiveness of the controllers.Considering the fact that LQRs have higher requirements in the accuracy of the model and the parameter tuning process is more complex,the PI controller is adopted to conduct further research on thrust control.The observers for the mass flow rates of the pumps are designed.To repress the impact of the measurement noise and sensor failure,the nonlinear observers,i.e.,extended Kalman filter(EKF)and unscented Kalman filter(UKF),of the propellant mass flow rates are designed according to the dynamics equations and the parameters of the electric pumps.The robustnesses of two different nonlinear observers against modeling bias,initial deviation,external disturbance,and initial covariance matrix are compared when estimating the mass flow rates.The results show that the unscented Kalman filter has better estimate accuracy.The simulations of the throttling process are carried out and the results are compared between the systems with and without controllers.To get a proper injection pressure drop,the variation of the injection area is investigated and determined.The target thrust curve contains a main deceleration phase and a continuous throttling phase.The main deceleration phase includes three steady-state processes with constant thrusts while the continuous throttling phase has two continuous linear throttling processes.Therefore,the step response and ramp response during the thrust regulating process are analyzed.Subsequently,the same simulation is conducted on the system with feedback control.The mass flow rate feedback point is determined and the variations of the system parameters with and without controllers are compared.The results demonstrate that with the feedback control system,the engine system can operate normally referring to the target mixture ratio and combustion chamber pressure.In the meantime,the PI controller can reduce the overshoot of the mixture ratio and the temperature of the thrust chamber wall,thus ensuring the service life of the thrust chamber.The simulation on the mission profile shows that the PI controller can guarantee the throttling process and the thrust control errors are within 3%.The simulations of the system with artificially injected faults possibly occurred in the operating process are conducted.The models of pump failure,pipeline blockage,pipeline leakage,and sensor failure are established.Based on the open cycle of the engine model developed in chapter 2,the simulations of the system with artificially injected faults in varying degrees are conducted and the variations of the system parameters are analyzed.In the following,the faults are injected into the engine system with PI controllers.The control effects are investigated and the variations of the electric pumps are studied.When coping with sensor failures,observers are needed.Subsequently,the designed unscented Kalman filters are integrated into the engine system model.The simulation on the mission profile shows that PI control based on the unscented Kalman filter can eliminate the impact of the sensor failures.