Analysis and discussion on launching mechanism and tactical electromagnetic railgun technology

Bo-ming Li,Qing-hu Lin

aNational Key Laboratory of Transient Physics,Nanjing University of Science and Technology,P.O.Box,210094,Nanjing,PR China

b China Academy of Ordnance,P.O.Box,100089,Beijing,PR China

Keywords:Railgun Launching mechanism Dynamic measurement Numerical simulation

ABSTRACT This paper begins with a discussion on the significance of multi-physical fields in the research of railgun launching mechanism,followed by an introduction of research work about dynamic measurements and numerical simulations.The application of some measurement methods including the atomic emission spectrum, fiber-optic strain,optical level,pulsed X-ray,and high-speed video in observing the launching process was introduced.The models about the electromechanical dynamic processes and multi-physical fields in a railgun were developed.The mechanisms of the grooving,arc transition,and gouging were analyzed.These work provided a deeper understanding of launching mechanism of electromagnetic railgun.Some issues about the tactical railgun,such as the fiber composite overwrapped barrel,recoil,loading modes and eddy current loss were also discussed.Taking a medium-caliber launcher as an example,the basic process of design and analysis was introduced.

1.Introduction

The solid armature electromagnetic railgun has demonstrated launch velocities in the 2-3km/s range and shows a good potential in military applications,such as air defense and long-range attack.Over the past few decades,extensive research on launching mechanisms of railgun has been carried out[1-3].However,knowledge about the details of launching process is still incomplete,and some theoretical and technical problems remain to be solved before the railgun becomes a practical weapon.

During the several milliseconds when the railgun system launches,components in the launcher and integrated launch package experience severe mechanical and thermal shocks because a powerful electrical energy is instantaneously released in a limited volume.The launching process is combined with some complicated dynamic responses and special physical phenomena,for example the grooving,wear,gouge,arc transition and erosion.Also,the extreme conditions which the railgun components experience has been close to even exceeded their material limits.Therefore,research on the launching mechanisms is of great significance,and technologies of tactical railgun are challenging.

We have been committed to the research of transient physical phenomena in railgun for almost 20 years.Based on the understanding of the generation of electromagnetic field,thermal field and structural field,as well as the interactions among them,we treatedthe launch of railgunas a problem of coupled multi-physical fields.Some models have been developed to describe the multiphysical fields and some methods of measurement have been applied in observing the multi-physical fields.In this paper,we present the main progresses in our measuring and modeling of railgun in this paper.Some issues about tactical electromagnetic railgun technology were also discussed.

2.Research on launching mechanism

2.1.Experiments and measurements

It is generally known that the railgun is a pulsed power system and generates strong electromagnetic interference(EMI).Electronic equipments near the railgun are difficult to work well.In order to avoid electromagnetic interference and get more information about the launching process,we applied some optic measurement methods in the firing tests,such as the atomic emission spectrum, fiber-optic strain,optical level,pulsed X-ray,and highspeed video[4].

2.1.1.Temperature in bore

Boltzmann plot method was used to determine the electron temperature of the plasma generated in the bore of railgun[5].Spectrum signals are converted to electrical signals by a photomultiplier tube(PMT),and recorded by a data acquisition system.The system block diagram of temperature measurement,as well as the measured curves,is shown in Fig.1.We measured the electron temperature before and after the arc transition,and found that some metal vapor generate in the bore despite the fact that the contact voltage is of a lower magnitude when the rails and armature keeps metallic contact.We called this phenomenon as the intermittent transition and thought it is related to the non-ideal sliding electric contact.Moreover,the difference of the electron temperature before and after the arc transition is thousands of Kelvin degrees,which provide another judging method on the arc transition besides the muzzle voltage method.

2.1.2.Vibration of rails

Measurement of dynamic strain is helpful to better understanding the structural response of rails to the electromagnetic force,as well as the effect to the launching process.We designed a rail strain measurement device based on the fiber-optic differential interference method[6].The system block diagram of the rail strain measurement device and a typical dynamic strain curve are shown in Fig.2.Experimental results show that this device is able to operate reliably and stably in the harsh EMI environment during the launching process.Moreover,the device did capture the vibration characteristics of rails under the dynamic loading condition as the projectile was accelerated along the rails.This measurement method provides some further data about the launching process and a basis for study on structural optimization of railgun launchers.

2.1.3.Balloting motion of armature

The in-bore balloting motion of armature has a significant effect on the launching stability.We measured the attitude angle of armature by the optical level method[7].As shown in Fig.3,The measurement device is composed of a laser emission system,laser receiving system and PSD.When the armature moves along the rails,the reflector MO attached on the front surface of the armature continues to deflect,and the reflected light also deflects.The returned laser beam passes through the receiving system and forms an image point on the PSD target surface.The position of the image point can be measured continuously by the PSD.The measurement results shown in Fig.3 confirm the existence of balloting motion of armature and provide a reference for the modeling of in-bore dynamics.

2.1.4.Imaging of armature after launch

We used several high-speed imaging methods to observe the armature that is leaving the muzzle[8].The images can provide the direct information of armature after launch.With the sequential images captured by the orthogonal CCD cameras,the muzzle velocity was calculated.Moreover,using the multiple angle photographs,the shape of armature after launch was reconstructed,and the mass loss of armature was also estimated.That is to say,the 3D volume reconstruction provides a soft recovery technique for armature.The schematic of the 3D volume reconstruction device and a reconstructed armature are shown in Fig.4.The reconstruction method is especially suitable for the study of armature wear and erosion.

At the muzzle,the armature leaving rails is always shielded by the muzzle arc, flame and smoke.Visible light imaging technology does not satisfy the requirements of measurement.Pulsed X-ray imaging,which is of strong penetrability,has a unique advantage in this case.By the X-ray method,not only can we get some image information including the shape and posture of armature,but also get the accurate instantaneous velocity.The principle of the X-ray imaging,as well as the sequential photos of an armature leaving the muzzle,is shown in Fig.5.The X-ray method can provide reliable experimental data for the research of launching mechanism.

2.2.Modeling of launch process

The measurements of dynamic temperature,strain and motion contributed to our knowledge of the launch process and led to a deeper understanding of the special physical phenomena in railguns such as grooving,arc transition and gouging.We proposed a launching mechanism that derives from the multi-physical field effects and the interactions between rails and armature.The occurrence of grooving,wear,arc transition and gouge is related to the dynamic behaviors of multi-physical fields.Two kinds of model were developed to describe the launch process,one was the electromechanical dynamic model and the other was the coupled multi-physical field model.

2.2.1.Electromechanical dynamic model

The electromechanical dynamic model describes the discharge process and the macroscopic movement under the lumped parameter framework.As shown in Fig.6,the launching system consists of a pulsed power source(PPS)and a launcher.The launcher is modeled by a series circuit mainly composed of rail inductance Lr,rail resistance Rr,initial resistance R0and armature resistance Ra.The armature is accelerated by a force,whereis the gradient of rail inductance and i is the circuit current.The ordinary differential equations including the armature motion equation and circuit equations are solved by a numerical integration method.The calculated traces of voltage,current and armature motion is in good agreement with experiments.Furthermore,the contact resistance sub-model and the wear sub-model were added to model the interface behaviors,which resulted in a better approximation to experiments[9].

The armature and rails are subjected to the transient electromagnetic force,and structural vibrations take place due to some disturbances,for example rough surface and electrical current fluctuation.The vibrations affect the sliding electrical contact between rail and armature,and even lead to arc transition.

The dynamic response of the trailing arm of a U-shape armature shown in Fig.7 is modeled by the trailing arm mass ma,armature stiffness ka,damping coefficient D,magnetic force Fm,rail stiffness krand rail variationy1.The rail vibration is modeled by the response of a cantilever beam on an elastic foundation under the moving uniform load.The symbols v,L,and x represent the armature velocity,total length of the rail and the length in the force,respectively.Coupled with the lumped parameter model,the sub-models of vibration were established to model the transverse micro displacement of armature arms and rails.

Furthermore,we developed a dynamic model for the in-bore balloting motion of armature[10].This model has three DOF,which are the displacement along the barrel y,the lateral displacement of the mass center x,and the rotation of the armature around the mass centerθ.The armature is propelled by the electromagnetic force T.The interactions between armature and rails are modeled by the contact stiffness.In Fig.8,the symbols a,d,ρ,ε denote half of the armature length,half of the rail distance,the distance from the mass center to the equivalent action point of the electromagnetic force,and the distance between the mass center and the shape center of the armature,respectively.The balloting model is coupled with the lumped parameter model.The calculations show that the balloting motion is affected by the geometry of armature,the stiffness of launcher,and the driving current.Moreover,the stability of balloting motion is closely related to the in bore mechanical damage and the direction accuracy of projectile.The unstable balloting motion is likely to cause a severe armature wear and rail gouging.

2.2.2.Multi-physical field model

To study the launching mechanism under more detailed scale,we developed a coupled multi-physical field model[11].It is composed of an electromagnetic field sub-model,thermal field sub-model and structural field sub-model.Because the Joule heat load and the electromagnetic force load acting on the thermal and structural fields are all derived from the electromagnetic field,the electromagnetic field plays a key role in the multi-physics field problem.With the magnetic vector potential and electric scalar potential as unknown quantities,the electromagnetic field was modeled by magnetic diffusive equations in the Lagrange coordinate system.The Magnetic diffusion equations were solved by a finite-element boundary-element coupling method.The thermal field was modeled by thermal diffusion equations which were established under the assumption of energy balance.The governing equations of the structure field were based on the momentum conservation.The thermal diffusion equations and structural equations were solved by finite element method.A coupled calculation was achieved by the transfer data from the electromagnetic field to the thermal and structural fields.

The multi-physical field model was used to investigate the arc transition mechanism.We conducted numerical simulation for the armature and rails system.Simulation results showed that the multi-physical field model can capture the velocity skin effect which is specific in an electromagnetic railgun.It was found that the sliding electrical contact is affect by the melt-wave,electromagnetic force and thermal stress[12-14].The melt wave develops from the armature tails,as shown in Fig.9,meanwhile the frictional heat is accumulated on the contact surface.Rapid temperature rise leads to the contact surface melting of armature.

The temperature and current concentration at a small area on rail surface leads to a large thermal stress.Calculations show that the contact surface of rails is of a thermal stress about 400 MPa when a driving current of 1 mA is applied,as shown in Fig.10.The high stress easily leads to the grooving and gouging damages.

To investigate the gouge mechanism,we developed an impact dynamic model[15].The governing equations were based on the continuum mechanics theory and consisted of the mass,momentum and energy conservation equations.With the Johnson-Cook failure material model and Mie-Grüneisen equation of state,the impact dynamic model can simulate the large deformation process,for example,the formation of rail gouging shown in Fig.11.We attributed the gouge to the small-angle oblique impact of armature to rail induced by the rail vibration and armature balloting motion.The results show that material flow occurs on the contact surface when the high-speed armature impacts the rail with a small entry angle.The numerical simulation is in line with the experiments.

The multi-physical field model was also used to investigate the in-bore dynamic behavior of an integrated launch package[16].For the system composed of railgun launcher and ILP,a 3D transient simulation of electromagnetic,thermal and structural processes was conducted.The motions of a projectile in three directions are shown in Fig.12.

3.Issues on tactical railgun launcher

The issues on tactical railgun launcher contains many aspects,such as

-Bode life,

-Firing rate,

-Projectile dispersion,-Launcher lightweight.

The most urgent problem at present is bore life,which is degraded by various damages,for example,the erosion between the rail and armature,rail gouging,rail grooving,and so on.The design of railgun launcher should be considered from material,electrical and mechanical aspects.Unfortunately,the performance of existing materials can not fully meet the requirements of railgun,and the development of new materials is a long process.Because the material and structure damages in railgun are related to the working current,the most feasible way at present is to reduce the working current by ingenious structural design and parameter optimization.The technical approaches include increasing the inductance gradient of rails,decreasing the eddy current losses,improving the initial contact by a suitable way of projectile loading,improving the barrel stiffness and decreasing the weight by fiber composite large tension winding,and so on.

3.1.Fiber composite overwrapped railgun barrel

A 3D progressive damage model based on Hashin's failure criterion and progressive damage theory is established to study the failure mechanism of filament-wound housing for railgun barrel by finite element method[17,18].The model shown in Fig.13 can predict the onset and evolution behaviors of damages in the composite housing of railgun.Also,the prestressing based on the filament winding pattern with iso-hoop stress can be calculated.The model is helpful to analyze the effects of the winding angleθiand winding stress Fion the progressive damage of the filament-wound housing and predict several damage modes including matrix crack and delamination.

3.2.Recoil of railgun launcher

A model was established to study the recoil motion of a railgun launcher[19].The recoil motion was divided into three periods according to the armature position,that are in-bore motion period,residual electric energy release period and inertia recoil period respectively.The motion at each period is decomposed into the free recoil motion and the restrained recoil motion.Based on the interior ballistic calculations and the design parameters of railgun,the recoil motion law of each period can be solved by the motion equation and the force balance equation.The curves of the free recoil velocity(vrf),free recoil displacement(xrf),and recoil brake force(FR)are shown in Fig.14.This model provides a useful reference for design of a recoil brake device.

3.3.Eddy current loss

In order to improve the launching efficiency and reduce the energy loss in the railgun system,a model considering contact resistance,velocity skin effect,temperature-dependent conductivity and friction force was established[20,21].It was found that the eddy current in the vicinity conductors of rails,which is induced by the pulsed current up to several MA,has significant effects on the system efficiency and the electromagnetic compatibility.We investigated the eddy current problem by finite element analysis.The calculated eddy-current losses are shown in Fig.15.The results show that eddy current can be reduced by high permeability and low conductivity materials.

3.4.Loading of solid armature

The initial contact between armature and rail has an important influence on the launch performance.Usually,an interference fit is used,and the armature experiences an engraving process before it is pushed to the designated position.We investigated three engraving modes including the uniform-speed loading,impact loading and combined loading[22].The loading processes were simulated by finite element method.The uniform-speed loading and impact loading show very different stress contours,as shown in Fig.16.By comparing the structural deform,contact pressure,and wear of armature,we found that the combined loading manner can acquire high initial contact pressure with low loading force and small interference size.

3.5.Basic process of railgun launcher design and analysis

A collaborative design and simulation platform was built by commercial software and secondary development programs.The flow chart of railgun launcher design and analysis is shown in Fig.17.Firstly,a prototype of launcher is design according to the requirements,and a series of simulation are conducted to acquire the key parameters and main properties,such as the inductance gradient,resistance gradient,magnetic structure,Lorentz force,ohmic heating and mass properties.By multi- field coupling model,the launching process,temperature rise and structural deformation are calculated.Various performances including the interior ballistics,structural strength and stiffness and thermal behavior are evaluated.Then,the design prototype is revised according to the satisfaction degrees of requirements.After several iterative loops,an optimized design is gradually formed.

3.6.Design of a medium-caliber railgun launcher

A medium-caliber railgun launcher was designed and tested to demonstrate the tactical electromagnetic railgun technologies.The launcher is required to have the following performances:

-Capable of shooting an integrated launch package(1kg)at a velocity of 2.0km/s.

-Has a bore life over 100 rounds.-Compact in size.

The barrel configuration is shown in Fig.18.Its main technical features are:

(1)The in-bore magnetic field is enhanced by double-turn rails,and the double-turn section accounts for 2/3 of the total length of the barrel.

(2)The main-rail has a convex shape to provide lower current densities for armature.

(3)Ceramic insulators are used to withstand the preloading and resist the arc erosion.

(4)Both the carbon fiber winding and the bolt preloading are used to balance the magnetic force and diminish the bore growth during the railgun firing.

The flux lines shown in Fig.18,as well as the inductance gradients shown in Fig.19,demonstrate the effect of the magnetic field enhancement by double-turn rails.

With the inductance and resistance gradients given by the electromagnetic model,the interior ballistic process was calculated.The histories of discharge current i,projectile velocityυ,muzzle voltage Um,and breech voltage Ubare shown in Fig.20.

Besides the curves of the launcher,the 0D interior ballistics model also gave the discharge curves of each module of the pulsed power source.The curves of current output and capacitor voltage are shown in Fig.21.With this information,the launch efficiency and safety of the launching system can be evaluated.

The Lorentz force given by the electromagnetic FEM model was transferred to the mechanical FEM model,and the structural responses were calculated.The structural deformation and equivalent stress of different barrel segments when the current reaches its peak are shown in Fig.22.With these results,the structural strength and stiffness can be evaluated.

For the composite structure of the barrel composed of various materials,the connection between components is of significant effect on the launch performance.The contact and separation of components are considered in the structural simulation.A contour of contact pressure is shown in Fig.23.The contact state during launch,which is associated with the barrel configuration,preloading and launch parameters,is also one of factors in evaluating the barrel configuration.Good configuration and appropriate preloading ensure that no component separation occurs under any working conditions.

Taking the ohmic heating as the source term of heat conduction equation,we calculated the thermal process.Fig.24 shows the final temperature rise of a single shooting.Due to the skin effect of current diffusion,local temperature at the rail corner rises to about 200oC.By smooth treatment of the corners,the local temperature rise can be greatly reduced.

In order to reduce the size and weight of the railgun launcher,a compact breech design was proposed.The coaxial cables are symmetrically arranged on the left and right sides of the barrel.Vectors of Lorentz force on the cables and breech components are shown in Fig.25.

3.7.Experiments of the medium-caliber railgun launcher

The medium-caliber railgun launcher has been manufactured and tested.In a series of firing experiments,it showed a good performance,and the key technical parameters satisfied the expected requirements.The medium-caliber railgun launcher was successfully used to fire an integrated launch package.Histories of the discharge current,breech voltage and muzzle voltage are shown in Fig.26.

The high-speed digital camera was used to capture the motion of the projectile,sabot and armature.The image sequences of integrated launch package leaving the muzzle is shown in Fig.27.

4.Conclusions

This paper presents our main progresses in the research of launching mechanism and tactical electromagnetic railgun technology,including:

(1)Observation of the dynamic firing process in a strong EMI environment was achieved by optical methods.

(2)The electromechanical dynamic model provided a simple,quick,convenient and practical method to simulate the firing process.

(3)The multi-physical field model was established to describe the detailed firing process.

(4)A collaborative design and simulation platform was built and successfully used in the technology demonstration of a middle-caliber tactical railgun launcher.

The experimental and theoretical research on the launching process and physical phenomena of railguns provided a deeper understanding of the launching mechanism.The models and measurement techniques are helpful to engineering design and performance evaluation of a tactical electromagnetic railgun system.The emphasis in future work should be on the development of more advanced measurement methods to further explore the launching process of railgun,and the technical emphasis of tactical railgun launcher is on the issues related to the light weight,long life,high fire cycle,and high accuracy.

Acknowledgements

The authors would like to acknowledge the excellent technical support of their associates in National Key Laboratory of Transient Physics at Nanjing University of Science and Technology,China.