Study On The Electrostatic Discharge Hazard To HTPB Composite Solid Propellant

With the development and application of high-energy solid propellant and solid rocket motor, more attention is paid to the sensitivity and safer processes. Electrostatic discharge hazard is a kind of usual hazards to the solid propellant which can induce combustion or explosion secretly. The processes of electrostatic discharge hazard on the solid propellant consist of the following steps: electrostatic accumulation, electrostatic discharge, dielectric breakdown, and initiation of the propellant after the breakdown. Among those steps, electrostatic discharge is the key step. The research of electrostatic discharge hazards can be divided into many fields, such as, assessment methodology of electrostatic discharge hazard of solid propellant, the relationship between the ingredients of propellant and its sensitivity to electrostatic discharge, characteristics of electrostatic discharge, the influence factors on electrostatic discharge process, simulation and numerical analysis, and so on. Obviously, these results are of great significance for the safer production, handling and operation of propellant.In this paper, focused on the two important steps, dielectric breakdown and initiation after the breakdown, the electrostatic discharge hazards of the HTPB propellant were investigated. Equipment and procedure for accurate measurement of the sensitivity of solid propellant to electrostatic discharge were established. The effects of the ingredients and mechanical property of HTPB propellant, as well as the defects in it on basic electrical properties and sensitivity of electrostatic discharge were systematically studied. The key influence factors on the characteristics of dielectric breakdown and initiation after the breakdown were determined. The mechanism of electrostatic discharge hazard of HTPB propellant was deduced reasonably. An electrostatic discharge hazard model of solid propellant, which is based on forming and development of the hotspot, was established. The effect of propellant ingredients and performance on electrostatic discharge hazard was numerically analyzed.The dielectric breakdown characteristics are mainly affected by the physical properties of propellant, such as electrical and mechanical properties. The dielectric strength of HTPB propellant increases with increasing volume resistivity or decreasing dielectric coefficient. As more energy is required for dielectric breakdown and crack growth, the greater the tensile stress of propellant is, the higher the dielectric strength. The dielectric strength of HTPB is high because of its high volume resistivity and low dielectric coefficient. The dielectric strength decreases by adding some substances with low volume resistivity and high dielectric coefficient, such as Al and AP. Formulation of the propellant is the intrinsic factor which determines the electrical and mechanical properties of the propellant. The content of Al powder is the most effective parameter on the dielectric strength of the HTPB propellant. The current methodology for assessment of electrostatic discharge hazard has some disadvantages. For example, excess high sensitivity of electrostatic discharge due to lose insight of the energy consumed in the circuit and dielectric breakdown of air, and illusive non-initiation due to excess short electrical discharge time. These disadvantages were overcome by means of direct voltage measurement on the sample, contacting the sample with the electrodes closely, and introducing a resistance with certain value in the circuit. Therefore, a novel methodology to accurately assess the sensitivity of solid propellant to electrostatic discharge was established. The sensitivity of propellant to electrostatic discharge increases with higher contents of Al powder and catocene, lower value of RT, and thinner sample.The initiation characteristic after dielectric breakdown is greatly related to the reaction characteristics of the propellant, which are mainly determined by the contents of AP and binder. The initiation energy after dielectric breakdown of HTPB binder is higher because the decomposition of HTPB binder is an endothermic reaction. The initiation energy after dielectric breakdown of HTPB with oxidizer is low because the decomposition of AP and the reactions between decomposition products of AP and HTPB are exothermic. The Al powder can not react in the electrostatic discharge process. The initiation energy after dielectric breakdown of HTPB propellant with catocence is also low, because catocence can accelerate the decomposition of AP.The process of electrostatic discharge hazard of the HTPB solid propellant is as the following steps. Firstly, the gas in the defects of propellant is dielectric breakdowned by high voltage caused by the electrostatic accumulation. Secondly, the dielectric breakdown is occurred in the whole sample and the random cracks through the sample are formed. Thirdly, the propellant in and around electric channel is heated by thermal effect of the breakdown current. Finally, the propellant is ignited by the electrical discharge energy.An electrostatic discharge hazard model of solid propellant, based on forming and development of hotspot, was established. The processes of crack and hotspot development in dielectric breakdown were emulated numerically using the model. The effects of some factors, like decomposition characteristics, mechanic properties and physical dimension on the electrostatic discharge hazards of the propellant were analyzed numerically. The results show that lower energy required for the crack growth and higher energy released from the propellant reactions lead to easier ignition electrostatic discharge, which means higher sensitivity of propellant to electrostatic discharge. Thicker the propellant is, lower the electrostatic sensitivity, but the critical discharge energy of unit thickness decreases with the increasing thickness of the propellant.