Sensitivity of solid rocket motors to electrostatic discharge : History and future

The characterization of the sensitivity to electrostatic discharge of solid propellants and the adaptation of the corresponding procedures in the production, handling, and operation of motors and systems using them have always been an essential part of the safety analysis of rocket systems. However, incidents or accidents have occurred during the past decades, which showed that the appreciation of the risks and the knowledge of the mechanisms of ignition were not sufficient. These incidents always involved hydroxyl-terminated polybutadiene based aluminized composite propellants. After an analysis of the incidents, a new test was designed in France at the beginning of the 1980s in order to characterize propellants as sensitive or not sensitive, and. a criteria based on a percolation model was developed to classify propellant formulations. This test has never been contradicted by further experimental evidence. But other incidents or accidents happened later which showed that the model is not sufficient for some practical situations. and that the evaluation of the level of sensitivity of a sensitive propellant remains today a difficult problem. After a history of the main events in the field and of the efforts conducted for characterization, testing, and modeling of the sensitivity to electrostatic discharge during the last 25 years, recent findings on the effect of some factors are presented, and some directions for the future are described.