Electrothermal Chemical Guns

 


There is much interest in the design and implementation of electrothermal chemical (ETC) guns. ETC guns use a capillary discharge plasma to ignite the solid propellant providing explosive force to a projectile. This ignition method leads to an enhancement of the ignition and combustion of the energetic propellant. Among the enhancements are reduced ignition delay time, highly reproducible ignition time, enhanced combustion of the solid propellant, and increased muzzle exit velocity for the projectile.

 

The Nonequilibrium Gas and Plasma Dynamics Laboratory at the University of Michigan's Department of Aerospace Engineering is involved in computer modeling of the plasma-propellant interaction at the heart of ETC enhancements, under contract from the United States Army. A detailed understanding of the physics of the plasma-propellant interaction is considered one of the key elements to the future success of practical ETC gun implementation.


Model of a Capillary Sustained Plasma

A capillary model for an ETC that includes self-consistent consideration of the ablation phenomena is developed. Figure 1 shows some characteristic regions in the interface between the discharge plasma and the dielectric wall such as an electrical sheath near the dielectric, the Knudsen and hydrodynamic layers, and a quasi-neutral plasma. Different kinetic and hydrodynamic phenomena determine the main features of the plasma flow including Joule heating, radiative and convective heat transfer to the dielectric, and electrothermal acceleration of the plasma up to the sound speed at the cavity exit. The central region is the quasi-neutral plasma that occupies almost the entire capillary since typically the transition region scale length is much smaller than the capillary radius. The plasma region is separated from the dielectric surface by the vapor layers (Knudsen layer and hydrodynamic layer). The plasma is heated due to electric current flowing through the capillary. The energy transfer from the plasma column to the capillary wall consists of the heat transfer by particle fluxes and radiation heat transfer. Energy is absorbed by the capillary walls and dissipated by thermal conductivity and material evaporation.

The dependence of the ablated mass on the peak discharge current is shown in Fig. 2. The ablated mass has a linear dependence on the discharge peak current. For comparison, experimental data are also shown in Fig. 2. It can be seen that both simulations and experiment suggest that the ablated mass increases as the capillary inner diameter decreases. This effect is explained by increased current density in the capillary, which affects the ablation rate through Joule heating.

ablation_Cap_fig1 ablation_Cap_fig2 ablation_Cap_fig3


Plasma-Propellant Interaction Modeling

An analytical ablation model based on kinetic theory was developed and used for modeling propellant ablation during the plasma interaction. This model was coupled to experimental data for the ablated mass of two different solid propellants being considered for ETC application after exposure to a discharge plasma. This allowed the effective heat flux from the plasma into the propellant to be determined as a function of the plasma density. This revealed that a semi-transparent propellant, JA2, experienced a greater heat flux than did an opaque propellant, XM39. This means that radiation is a significant heat source in the plasma-propellant interaction.

Recently, a hydrodynamic analysis was used to determine how the convective heat flux to the propellant bed would be affected by the formation of a plasma sheath, which should naturally arise at any plasma-wall interface. This study indicates that XM39 has a higher convective heat flux than does JA2. This means that radiation has an even smaller affect on an opaque propellant like XM39 than previously believed.

Current and future work will involve including chemical effects such as film growth and recombination reactions at the propellant surface during the plasma discharge to the ablation model to see what effects they have on the ablation rate and heat flux.

ablation_PPI_fig1 ablation_PPI_fig2

 


This work is supported by the Army Research Office.