Mechanisms leading to the enhancement of gas generation rates in ETC guns

One of the main drivers behind the electrothermal-chemical (ETC) gun concept has been the use of electrothermal energy to enable enhanced gas generation rates (EGGR) of propellant compositions. Previous work has shown two distinct enhancement phases: a temporary increase during the electrical discharge (EGGRDED) and a more long-lasting effect, post electrical discharge (PEDEGGR). EGGRDED has been shown to be composition independent. However, only certain compositions have gone on to produce levels of PEDEGGR that would be of benefit when applied to gun systems. PEDEGGR is of particular interest as it would be most beneficial for the increase in efficiency of gun systems. Several mechanisms have been identified that may lead to PEDEGGR. These are: radiative energy transfer leading to physical damage and/or sub-surface heating; mechanical ablation and mechanical grain failure; electrical conduction through conductivity-enhanced flame leading to ohmic heating close to the pyrolysis zone and a newly identified effect with electrical conduction through the condensed reaction zone of the grain, called electrothermal pyrolysis (ETP). This paper is concerned with PEDEGGR due to radiation, mechanical ablation and ETP. Measurements of the radiative energy transfer to the surface of propellant during an electrothermal discharge from a capillary type plasma generator are presented. These are compared to modelling work predicting the levels of radiation required to produce the measured levels of PEDEGGR. Further experiments are described that generate (albeit inefficiently) high levels of PEDEGGR through mechanical ablation. The concept of ETP is explained and experimental work described that lead to promising levels of PEDEGGR with the potential for high efficiency and simple, robust plasma generator designs.