Compatibility and simulation of the heating up of the propellant charge body of a high precision machine gun

Abstract

The new precision machine gun in development at company Diehl uses a propellant charge without any case. It is based on consolidated NC double base ball powder. For ballistic reasons a defined distance between propellant charge body (PB) and the wall of the combustion chamber must be adjusted and for this a new feature was developed in cooperation with Fraunhofer ICT. The distance is achieved with foam stripes based on polyurethane energized by HMX. To avoid a degradation of the ballistic properties during use time, compatibility investigations using heat flow microcalorimetry have been performed to select suitable foam formulations. For this special assessment the criteria have been developed. A further design feature is a thermal insulation between the combustion chamber wall and the PB. This is also achieved by the energetic PUR foam. To design and predict this thermal insulation, FE calculations have been made using the thermal property and decomposition characteristics of the energetic foam and the PB, which have been parameterized by iso-conversional analysis. The calculations have been compared with real measurements of the times to autoignition at different burning chamber temperatures.