Radiative heating of solid propellant by an electrically-induced plasma source

A model of heating of a sohd propellant due to net radiation loss from a hydrocarbon, ablation-stabilized plasma is proposed to investigate the potentral role of radiation in explaining experimental data from studies of plasma-propellant interaction. In the model, the radiant energy is transmittedl to the solid propellant, the combustion chamber gases, and the chamber walls. The one-dimensional thermal profile wrthin the propellant 1s resolved within the two-dimensional, multi-phase, cornbusting flow simulation. The enhancement in the burning rate of the solid propellant as a result of the temperature mcrease is computed. The model is applied to a closed chamber ETC firing and indicates that absorption of radiant energy m the outer shell of the propellant IS sufficient to increase the temperature of the propellant to a level which would have a significant impact on the resulting propellant combustion. Under the conditrons examined, radiation IS an important mechamsm in ignruon. A hydrocarbon, ablatron-srabrlized plasma has been used in electrothermal-chemical (ETC) guns, currently under active mvestrgauon by the U.S. Army, to rgrute the solid propellant in the combustron chamber and as an additional source of energy In the early portion Approved for public release: cllsrrlbullon IS unllmltcd of the interror ballistic cycle.’ The plasmas utilized have temperatures on the order of l-2 eV which places them in the radiant regime.2‘4 When introduced into a bed of propellant (up to 10 kilograms) in a large-caliber gun (up to 120mm bore diameter), ETC plasmas have been shown to ignite the propellant and to modify the combustion behavior of the solid propellant. For example, JA2 propellant initialized at cold temperatures in gun firing tests and exposed to a controlled ETC plasma will generate gas at its hot temperature condition.’ Experiments have also shown that the ignition of solid propellants is much more rapid with a plasma compared to conventional pyrotechnic igniters used in guns, and that the ignition is more repeatable in terms of ignition delay time.’ At least three hypotheses5”’ have been proposed to explam the change in gas generation rate observed in sub-scale and large-scale experiments: (1) irradiation of the propellant leading to in-depth heating; (2) modification of the chemical kinetics of propellant combustion due to the presence of ions; and (3) mechamcal grain fracture leading to an increase in exposed surface area of the propellant. Limited expenmental data have been collected on propellant irradiation in ETC guns.6-8.‘0.‘4.‘5 However, since plasmas used in ETC guns enter the combustion chamber with temperatures on the order of 20,000 K a large portion of the plasma energy is expected to be radiant energy.3 Measured bummg rates of some solid propellants increase in the presence of plasma, and JA2 propellant has been shown to change its physical appearance when exposed to radiation from an ETC plasma.‘-’ Experimental data collected over many years with conventional pyrotechnic ignition has documented and quantified the change in the burning rates of solid