Study On The Aeroelasticity And Ballistics Of Flexible Rocket

Large thrust to weight and length to diameter(slenderness)ratios of rocket are needed in order to launch heavier payloads,increase range,and reduce aerodynamic drag.Obviously,the above requirements lead to a highly flexible rocket.A flying flexible rocket is a time-varying coupled system consists of aerodynamic loads,elastic deformation and rigid motion.Research has shown that the effect of the aeroelasticity on the ballistic performance when the slenderness ratio is greater than 15.In this paper,a flexible rocket's trajectory is simulated to analyze the effect of the aeroelasticity on the ballistic performance,the main work is as follows:(1)The rocket is idealized as a non-uniform beam,the rigid and elastic coupled equations of motion are derived using the principle of virtual power.The frequencies and modeshapes of the rocket in the flight are calculated by the transfer matrix method of the multi-body system dynamics with thrust and mass depleting effects accounted for,the expression of the bending rocket's forces and moments is given to establish a complete flexible ballistic model.(2)Computational fluid dynamics(CFD)method was used to calculate a rocket's lift and drag coefficients distribution along the rocket's axis.On the basis of quasi steady theory and linear theory,the rocket's aerodynamic loads are calculated in the flight using the aerodynamic coefficient derivatives distribution obtained in advance.To judge the reasonability of the aerodynamic model,the aerodynamic forces of two bending rockets the deformations of which are based on the first order mode shape were calculated under the assumption of quasi-steady theory and linear theory.The results are compared with CFD method.(3)Rocket's trajectory is simulated to analyze the effect of the aeroelasticity on the ballistic performance under the conditions of different structure parameters and initial disturbance.The results show that the deformation of the rocket causes the center of pressure to move forward.Bending-roll resonance is one of the main causes that lead to shorter firing range..The coupling resonance occurs when rocket's spin frequency is close to the first-order bending vibration frequency,which increases the angle of attack,increases the resistance and shortens the range.For large-aspect-ratio low-speed spinning rockets,the bending-roll resonance problem can be avoided by increasing the stiffness,reducing the rolling torque and designing the engine reasonably.(4)The system's characteristic equation is derived according to rocket's dynamic equations with the thrust considered on the basis of the existing theory.The root locus of the rocket's entire trajectory was calculated in account of the changing of the mass,thrust and aerodynamic loads to analyze the effect of thrust and spinning speed on flexible rocket system's dynamic stability and rsearch the dynamic instability mechanism of the low-speed revolving fin-stabilized rocket of large slenderness ratio.It is found that the critical point of the dynamic stability of the rocket is the intersection of the spin frequency and the first-order bending frequency time trajectory.This shows that the coupling resonance between the first-order bending vibration and the spin motion causes the system dynamic instability.Compared with the rigid rockets with low-speed spin and the flexible rockets without spin,the critical point of dynamic stability of the flexible low-speed spining rocket is more sensitive to the structure parameters and aerodynamic shape.