Structural Design And Mechanical Analysis On C/C Grids For Ion Thruster

The ion electric propulsion technology is a kind of electric propulsion methods with high specific impulse,high efficiency and longevity.The thrust is generated by leading out the ions,which are ionized in the high-voltage field,through the ion optical system.The ion optics system,also known as the grids assembly,consists of a screen grid and an accelerating grid,which is one of the key components of ion thruster.It determines not only the performance,but also the reliability and longevity of ion thruster.This thesis is focused on the design and mechanical evaluation of the grids assembly.C/C composites have the characteristics of small thermal expansion coefficient,strong resistance to ion sputtering,low density and high strength.It is an ideal material for grids of ion thruster.In this paper,C/C composite grids for a 10-cm diameter Xenon ion thruster were designed and fabricated.New fabrication techniques were devised to improve their structural strength.The basic idea is to minimize the broken number of carbon fibers in making grid holes.Resin-impregnated carbon fiber bundles were woven among an array of pins based on the grid hole position.The screen or accelerator grids were then solidificated through carbonization by an isothermal chemical vapor infiltration(CVI)process.Since the grids of the ion thruster are thin-walled and porous parts,the mechanical behavior of the developed of C/C composite grids should accommodate the mechanical environment of ion propulsion.The finite element models of the accelerator and screen grids have been established,respectively.By introducing the periodic boundary condition,a homogeneous model is established to analyze the stress distribution and the mechanical properties of the representative volume element for each grid.Then the effect of mechanical properties of the composite components on the global lateral stiffness of the grids are discussed and compared with the Molybdenum grids.The results show that the global lateral stiffness of the C/C grids is a function of the tensile modulus of the carbon fiber and the modulus of matrix.The stiffness of C/C composite grids meets the requirement of the thruster in the extreme environment in space.The spacecraft experiences harsh vibration and impact during launch.As the weak parts of thruster,grids assembly has to withstand greater shock and vibration.In this paper,the impact of the ion electric propeller is simulated using a pendulum impact response spectrum tester.Two types of carbon fibers,including T700,and M50 J,were selected as the constituents of the grids assembly,and the modal,frequency response,shock response(x,y,z axis)were analyzed and compared with the molybdenum grids.The results showed that the strength of the two types of materials was not enough to withstand the impact of instantaneous explosive generated by the rocket engine.Some special measures to reduce vibration must be considered to ensure intactness of the grids assembly in launch period.