As a natural result of the electrified integration and electrical energy revolution, the electromagnetic launch (EML) technology will be inevitably used for future launch missions. This paper has summarized research focuses in this field according to the published papers on EML in China during the past decade and mainly introduced the current research and progress in several key technologies, including launcher, integrated launch package, high-power high-density supply, and overall control/maintenance. Some common pressing problems are pointed out to provide direction for the application of EML technology in engineering and many other fields, such as launcher life, miniaturization of power supply, and improvement of energy availability.

Research Progress of Electromagnetic Launch Technology

An inductive coil gun is a solenoid surrounding a cylindrical metallic armature (the projectile), both electromagnetically coupled. One or more capacitors discharge a stored voltage quickly through the coil, configuring a kind of RLC circuit, with the pulse-shaped current generating a magnetic field inside the coil that accelerates the projectile. Once the projectile crosses an equilibrium point inside the coil, the existing magnetic field pulls the armature back, therefore reducing the final muzzle velocity and decreasing the efficiency—defined as the ratio between the available capacitor electrostatic versus the slug kinetic energies. This paper describes a system that monitors the armature position inside the coil, with a laser beam, and automatically cuts the current when the projectile reaches the equilibrium point. The system, named current switch circuit, enables higher velocities using the same set of coilgun and projectiles, providing a higher efficiency.

Coilgun Velocity Optimization With Current Switch Circuit

An improved large-caliber synchronous induction coilgun was tested to demonstrate the launch of 120-mm projectiles. This induction coilgun is made of 15 identical coils. The coils are stacked end-to-end forming a barrel, and each coil is energized by the same pulsed power topology in sequence to generate a magnetic force to accelerate the projectile. Tracking of the projectile location during launching provided precise feedback to control. A computer simulation based on the viewpoint of FEA software is developed to predict the performance of the launcher system. 5-kilogram copper projectiles were accelerated from rest to 220 m/s through the 2.5-m launcher assembly with a projectile kinetic energy to capacitor stored energy efficiency of 14.5 %. The 15-stage induction coilgun provides a significant improvement in the armature capture effect, system efficient and coil structure. Further experiments studies are being continued.

Design and testing of a 15-stage synchronous induction coilgun

The structure and trigger control strategy have become the most important factors that restrict the performance of the multistage synchronous induction coilgun (MSSICG). However, it is still a difficult task to design MSSICG under overload constraint due to coupling between the multiple parameters. In this paper, the maximization of the emission efficiency and acceleration stationarity is treated as a multiobjective optimization problem. By analyzing the relationship between the number of turns and the other structural parameters of the launch, the multiobjective optimization model of MSSICG is established by the current filament method which was verified by the experimental data and finite-element method. And then the second generation nondominated sorting genetic algorithm (NSGA-II) and multiobjective particle swarm optimization (MOPSO) were employed to optimize the model in order to maximize the energy transfer efficiency while achieving the smooth acceleration of the armature. With the formulated optimization model, a five-stage synchronous induction coilgun is optimized as a special case. A decision-making procedure based on the fuzzy membership function is used for obtaining best compromise solution from the set of Pareto-solutions obtained through NSGA-II and MOPSO. In addition, the optimization performance of the proposed multiobjective optimization model and the single-objective optimization model of the MSSICG was compared. The result of optimization shows that the proposed multiobjective optimization model of MSSICG can effectively improve the performance of the coilgun compared with the single-objective optimization model which takes of the launch velocity and overload acceleration as the combination objective function or only the launch velocity.

Multiobjective Optimization of Multistage Synchronous Induction Coilgun Based on NSGA-II

A coilgun is an electromagnetic launcher based on electromagnetic induction. The projectile with a heavy load can be accelerated to a high speed. The currents that flow through the driving coils and the projectile would produce joule losses. All of the properties are drastically determined by the geometry and power of a coilgun. This paper aims to optimize the geometry and power parameters of the coilgun. The muzzle velocity is selected to be the objective when the temperature rise limitations are taken into consideration. To analyze the performance, an equivalent circuit model including the calculation of temperature rises is employed. The performance of a three-stage coilgun is calculated based on this model and then this coilgun is optimized with the utility of adaptive genetic algorithm taking the influence of temperature rise into consideration. The result of optimization shows that the muzzle velocity is improved and the temperature rises are conformed to the design requirements. The optimized coilgun is simulated by the finite-element method to verify the performance.

Geometry and Power Optimization of Coilgun Based on Adaptive Genetic Algorithms

To demonstrate the multistage induction launch capability and test the accuracy of the simulation result, a small-scale, 4-stage synchronous induction coilgun is designed, fabricated, and tested based on the simulation result. The four coils are stacked end-to-end, forming a barrel, and each coil is energized by the identical pulsed power supply in sequence to create a traveling magnetic wave that accelerates a projectile. The inductance of four coils is tailored by changing the number of turns in order to decrease the pulsed current risetime. The tracking of the projectile location during a launching provides a precise feedback to control when the coils are triggered to create the wave. Two types of the projectiles are tested. A 0.65-kg sleeve projectile is accelerated from rest to 125 m/s with a projectile kinetic energy to capacitor-stored energy efficiency of 6%. A 1-kilogram sleeve projectile is accelerated from rest to 92 m/s with the efficiency of 5%. The solenoid projectile is broken at the bending point when it induces high current. The launcher provides data for model verification and the engineering basis for proceeding with larger multistage system. Further research is to improve the system efficiency and maximize the magnetic coupling between the driving coil and the projectile.

Experimental Results From a 4-Stage Synchronous Induction Coilgun

For the multistage induction coilgun, many researchers pay great attention to improving the system efficiency. The directions of magnetic field generated by the driving coils are consistent for conventional coilgun, because it is convenient to analyze acceleration characteristic and stress status for the armature in the barrel. In this paper, the launching performance of multistage induction coilgun is investigated in terms of different directions of magnetic field in order to improve the system efficiency. A field-circuit coupled model for a 6-stage coilgun is built. The simulation for the consistent direction of magnetic field is calculated. For the same parameter setup, the directions of magnetic field generated by the last three stages current are changed. The simulation results show that the armature endures a large braking force when going through the last three stages, and the velocity drop is obvious. Meanwhile, the currents in the last three stages are very large. The distribution of the current density and the magnetic field during the transition process of magnetic field are analyzed. The reason for velocity drop when changing the direction of magnetic field is discussed. A method that the trigger position is adjusted for the armature is proposed. The simulation results show that the armature can be accelerated to a higher speed on condition of inconsistent direction of magnetic field with adjusting the trigger position. It is feasible to improve the system launching performance through changing the direction of magnetic field. A 15-stage coilgun is built to validate the method. The experimental results show that the velocity and system efficiency are increased dramatically through changing the direction of magnetic field. It provides valuable reference for multistage induction design.

Investigation of Magnetic Field Arrangement on Launching Performance of Multistage Synchronous Induction Coilgun

Driving coils in a multistage induction coilgun are stacked end to end forming a barrel. During launching, the pulsed current flows through the driving coil, and the electromagnetic force propels the projectile ahead. The driving coil endures not only the radial expansionary force because of the magnetic flux compression, but also the axial force due to the compressive force and counterforce of the projectile. In the process of coil design, both the mechanical strength and the electrical insulation must be considered. Due to the nonlinearity of insulation materials, the theoretical calculation cannot reflect the true state for the driving coil. The conventional static-state test method is analyzed and verified through experiments. The experimental results show that this method is ineffective for the multistage launching, because it neglects the axial counterforce and does not reflect the true state of launching. An improved method for evaluating the coil strength is proposed based on the analysis of the basic principle. The projectile is accelerated by the former stages, and the testing coil accelerates the projectile with an initial speed. Equipment for testing single coil is designed and installed in the muzzle of the existing multistage coilgun. The experimental results show that this method can test the coil's mechanical strength and electrical insulation more effectively than the conventional method. The dynamic-state method can be used to approximately evaluate the performance of different types of the coil. The structure of the driving coil can be improved through testing and evaluating, which provides experimental reference for the multistage induction coilgun design.

Design and Evaluation of the Driving Coil on Induction Coilgun

A large muzzle kinetic energy railgun is developed to demonstrate the supersonic launch of 200 mm projectiles. A trade is studied that evaluates over 30 different railgun cases resulted in the selection of a simple railgun as the choice. The goal of the railgun launches a 200 mm diameter projectile with a mass of greater 20 kg to a velocity of 2500 m/s, and the muzzle kinetic energy is more than 62 MJ. The analysis and modeling of the electromagnetic launch results in an adequate design. The final railgun is designed with an 8 m barrel and characterized by the following parameters: conducting rails with an inductance gradient about 0.4 $\mu{\rm H}/{\rm m}$ , a peak current of about 7 MA, an effective current of no ${ , the projectile muzzle velocity of 2540 m/s, the muzzle kinetic energy of more than 63 MJ, and the system efficiency of 31.8%. A more detailed study is presented in this paper.

Bin Cao

Papers

Design and testing of a 15-stage synchronous induction coilgun

Experimental Results From a 4-Stage Synchronous Induction Coilgun

Investigation of Magnetic Field Arrangement on Launching Performance of Multistage Synchronous Induction Coilgun

Design and Evaluation of the Driving Coil on Induction Coilgun

Design and Simulation of a Large Muzzle Kinetic Energy Railgun