Study On The Working Property Of A Solid Propellant Microthruster

With the development of astronautical technology and the upping requirement of space missions,micro satellites are paid greater importance.As an important actuator for attitude and orbit control of a micro satellite,the micro propulsion system is becoming the key field of research facilities worldwide.However,problems exist in most recent developed micro propulsion systems,such as complicated structure,low reliable fuel and ultra-high voltage.Solid propellant microthruster is a kind of solid rocket motor,which has simple structure,stable fuel and high reliability and solid propellant microthrusters can be integrated in the form of array to avoid the disadvantages of low specific impulse and one-shot use.The solid propellant microthruster is gaining popularity due to above advantages.The working process inside the solid propellant microthruster is complicated.The flow,combustion and heat transfer processes are affected by microscale effects.Large viscous loss and heat loss through the wall exist in the gas,and the flame scale is in the same order as that of the microthruster.These phenomena differ much from solid rocket motors and should be investigated intensively.Due to the difficulty of directly measuring the internal flow field of the solid propellant microthruster,numerical simulation is the major method applied in our research.The research content include following aspects:(1)From the viewpoint of flow field analysis,a proper geometric model of the solid propellant microthruster is chosen and an unsteady integrated simulation model for solid propellant microthrusters loaded with end-burning grain is hence established.The model takes into consideration the microscale flow effects,fluid-solid coupled heat transfer process and heat feedback of gas to solid propellant's burning surface in the case of burning surface regression,where the process of burning surface regression is realized by using dynamic layering method.On the basis of above model,varies wall materials are taken into consideration respectively.The wall slip effect to the performance of the solid propellant microthruster,the variations of heat loss and viscous loss of gas in the case of burning surface regression with time and their effects on the performance of the microthruster are investigated.(2)Due to the restrict of ignition resistor's scale and existence of local heat hot spot,the ignition of varies positions on the burning surface is not parallel.The local remeshing model and fluid-solid coupled heat transfer model are implemented to establish a local ignition model of the solid propellant microthruster loaded with end-burning grain and the model is realized by using re-development function of the simulation tool.The unsteady simulation is performed for the local ignition process of the solid propellant microthruster by applying the established model.The heating effect of gas to unburned propellant surface,variation of propellant's burning surface and internal flow field of the microthruster with time is analyzed.The calculated variation of thrust with time is compared with that of the local ignition model proposed by Jwongkwang Lee and measured thrust-time curve,which verified our model's effectiveness in predicting thrust-time curve.(3)The conventional surface mass adding model is no longer suitable for simulation of solid propellant microthruster's working process taking into consideration the fact that the scale of propellant's gas phase flame is close to that of microthruster and a more detailed combustion model for solid propellant should be implemented.Existing one-dimensional condensed-gas phase combustion models of solid propellant haven't been extended to two-dimensional cases and existing two-dimensional combustion models of solid propellant don't include multi-step condensed phase reactions.So,a two-dimensional condensed-gas phase combustion model of solid propellant is established by using re-development tool of simulation software to extend a one-dimensional condensed-gas phase combustion models of a double base solid propellant to two-dimensional cases.The model is then applied to analyze distribution of condensed phase temperature,condensed phase species,gas phase temperature and gas phase species.The effect of fluid viscosity on flame structure,temperature of propellant's burning surface and regression rate at varies positions of propellant's burning surface is also analyzed.The calculated burning rate at the symmetry is compared with experimental values and the model is then implemented in a solid propellant microthruster with a burning surface to throat ratio of 100 to verify its validity.(4)Due to restricted burning surface to throat ratio of end-burning grain and high difficulty for inner hole grain's fabrication,exploratory investigation on solid propellant microthruster's working process and performance characteristic is performed.A prototype of 10×10 scale solid propellant microthruster array is fabricated by using high-precision mechanical fabrication method.The ignition powders and propellant pellets are prepared by grinding and pellets with proper diameter are selected by implementing vibration method.The thrust-time curve of a solid propellant microthruster loaded with porous propellant is derived by solving zero-dimensional inner ballistic equations.The thrust measurement system is established to directly measure the thrust-time curve of the microthruster,of which the curve is compared with calculated thrust-time curve.According to comparison of calculated results and experimental results,the extinction due to rapid pressure decay might be the main reason of too short thrust duration measured in experiment.(5)Considering the restriction of zero-dimensional inner ballistic model in analysis of the solid propellant microthruster's ignition process and the internal fluid field,an one-dimensional model is established to depict the working process of the microthruster loaded with porous propellant based on Euler equations.The model includes mass equation,momentum equation,energy equation,species transport equation and heat transfer equation in pellets.The model also takes into consideration drag between phases,heat transfer from gas to pellet surface and solid phase reaction inside the ignition powders.TVD scheme is selected to solve the flow equations and the scheme is implemented by programming using MATLAB.The one-dimensional inner ballistic model's validity is verified by comparing calculated thrust-time curve with result of zero-dimensional inner ballistic model and measured result.The distribution of variables such as pressure,temperature,gas velocity and their variation with time are analyzed based on the model.The ignition process of ignition powders and propellant is also analyzed.