BKNO3 pyrotechnic igniters for SRM: analysis of performance dispersion from development to production

Abstract. In Solid Rocket Motor, thrust is obtained from the controlled combustion of the propellant grain at high pressure and temperature. This reaction is predictably and repeatably triggered using a system called igniter, which is commonly made of an initiation system and an energy release system. These two components are physically inserted into a series of housings mounted on fixed or temporary (ejected during firing) supports inside the motor. The energy release system represents the core of the igniter and, in case of pyrotechnic igniter, is made of a pyrotechnic charge encapsulated inside a perforated basket. The most common pyrotechnic charge is made in form of pellets composed of BKNO3. In the development phase, different configurations are investigated to select the most appropriate to fit the requirement on ignition time of the motor. In this phase different architectures are compared involving: different ignition chain (pyrotechnic igniter devoted to igniting a pyrogen igniter and then the SRM; pyrotechnic igniter devoted to ignite directly the motor), different basket geometry (cup like vs tube-like), pellets charge mass. Once defined the igniter design and demonstrated its capability in fulfilling the motor ignition requirement, the pyrotechnic igniter undergoes to production and performance dispersion is mostly ascribed to production dispersion related to pellets production lot dispersion, despite the requirement to comply with the same regulation (MIL-P-46994B). In the final paper, for the development phase, the performance dispersion is analyzed with the aim to discern the main parameters needed to define the performance envelope to be used in the design process. This study allows to optimize the safety margins in the design phase assuring the needed performance at motor level, reducing item inert mass. On the other hand, for the production phase, pellets acceptance characteristic dispersion within the same production lot (intra-lot) and between lots (inter-lot) are analyzed to retrieve a correlation between pyrotechnic pressure curve and the corresponding acceptance properties. Great impact has been found from crush strength, heat of reaction and burning rate. This analysis allows to identify the characteristic to be put under strict control in lot acceptance phase to limit performance dispersion.