Research On Dynamic Characteristics And Active Control Of Variable Mass Launch Vehicle

Along with the development of aerospace technology,the demand of launch vehicle is diversified.As a typical time-varying system,variable mass of the launch vehicle caused by the fuel combustion has great influence on the flight of the launch vehicle.At the same time,with the increasing of the launch vehicle size,flexibility of the launch vehicle leads to the problem of rigid-flexible coupling dynamics.In order to meet the requirements of complex tasks,a accurate dynamic model considering the variable-mass effect is very important to the design and control of the launch vehicle.As is well known,fuel mass is vary large in the proportion of the total mass of the launch vehicle.From the economic point of view and orbit design of the launching task,it is necessary to make a further study on the optimal control of fuel in addition to considering the variable-mass effect,thus this can meet the basic requirement of vehicle launch and can achieve the purpose of saving fuel as well.In addition,frequency parameters of the launch vehicle in the flight process of the vehicle are important to the design and control of the launch vehicle.Usually,frequency parameters are iedntified on the ground by modal vibration experiments for the vehicle.But frequency parameter identification in flight can not only eliminate the inaccuracies of the ground experimental,but also can identify the real-time modal parameters.To sum up,variable mass and flexibility of the launch vehicle have great effect on dynamic characteristics of the vehicle.The optimal control of fuel and the modal parameter identification in flight are the difficult problems in the field of spacecraft dynamcs and control.Therefore,it has important theoretical significance and engineering application value to carry out the research of dynamic characteristics and active control of variable-mass launch vehicle.This dissertation is funded by the Key Project [grant number 11132001] and the General Project [grant number 11272202 and 11472171] of Natural Science Foundation of China,and the Key Project of Shanghai Scientific Research [14ZZ021],and the Natural Science Foundation of Shanghai [grant number 14ZR1421000],which presents theoretical and simulation studies of dynamic modelling,fuel optimal control and modal parameter identification in flight of launch vehicle.The main research and achievements are as follows:(1)A study on dynamic model of a flexible launch vehicle considering variable mass effect is carried out.Firstly,dynamic model of a variable mass particle is established based on the Kane equation,and the dynamic model of the flexible body is established by the method of modal truncation.The translational equation,rotational equation and vibrational equation of the launch vehicle are obtained.The launch vehicle model is simplified as the nozzle-beam model,and the dynamic equations of the nozzle and beam are derived respectively.The dynamic equation of the whole system can be obtained by combining the dynamic equations of the nozzle and the beam.Finally,numerical simulations of a single-stage launch vehicle and a two-stage launch vehicle are carried out to verify the validity of the presented dynamic model.Simulation results show that the model can effectively describe the effects of variable mass and flexibility of the launch vehicle on the flight characteristics of the vehicle.(2)The problem of fuel optimal control of launch vehicle is studied.Firstly,fuel optimal control problem of the launch vehicle is equivalent to a time optimal control problem under the assumption that the fuel consumption rate remains constant.The optimal performance index is presented.The initial and terminal conditions of the optimal problem are determined by the initial state of the vehicle flight and the terminal condition of the scheduled orbit.The two point boundary value problem is established by using the Hamiltonian equation.Finally,control law of the swing angle of the vehicle nozzle and the related flight trojectory of the single-stage and two stage rockets are obtained by numerical simulations.The data of swing angle of the vehicle nozzle is applied to the dynamic model of the flexible launch vehicle given in Chapter 2,and a numerical comparison is done for the flight trojectories with and without fuel optimizztion.The difference can verify the validity of fuel optimal control problem using the two point boundary value method.(3)A study on the frequency parameteridentification of the launch vehicle in flight is carried out.First of all,response output data of the vehicle in the flight process can be obtained by solving the dynamic model considering variable mass and flexibility given in Chapter 2,the input of the vehicle thrust data is known,so the pulse response data of the system can be obtained by the OKID method.Then bring the pulse response data into the ERA method we can get the time-varying frequency parameters of the vehicle.Finally the identified time-varying parameters are compared with those obtained by the theory model given in Chapter 2.Simulation results show that the ERA method can achieve almost the same results with those by the theory model,which proves the validity of the ERA identification method.