An Ignition-to-Burn out Analysis of Internal Ballistic in Solid Rocket Motors

In the design of solid propellant rocket motors (SRM), the capability of modeling and predicting the behavior of a given motor, in all its operative conditions, is important in order to decrease the experimental, plan and development costs. In this paper, we will present a new numerical model, named SPINBALL (Solid Propellant rocket motor INternal BALListic), that comes out from the updating and development of numerical, mathematical and physical models of the SPIT code (Solid Propellant rocket motor Ignition Transient). This new numerical tool is able to extend the numerical simulation of SRM internal ballistic, from ignition transient to quasi steady state and tail-off phase, allowing to use a unique numerical model, with an unsteady quasi-1D approach, to predict the whole SRM working lifetime. In particular, to achieve this target, it has been needed to consider that, differently from ignition transient, the subsequent operative phases are characterized by the following main driving phenomena: the grain burning surface evolution in time, the throat area ablation- erosion and the liner and thermal protection mass addition into the chamber flowfield and possible vortex shedding phenomena, with the effect of excitement of the motor longitudinal mode. In order to account all these phenomena, a new approach of the non reactive one phase gas mixture gasdynamics eulerian model is considered to describe the variations of the thermophysical properties of gases added into the bore, due to grain combustion processes and liner and thermal protections, as a consequence of ablative and erosion phenomena. Moreover, a grain geometrical processor module, based on the use of off-line pre-evaluated grain geometrical parameters (port area, wet perimeter and burn perimeter) versus web, has been considered in order to evaluate the geometrical configuration variations of the grain propellant into the chamber and, above all, the burning surface variation with time and grain mass addition into the bore. At last, even if more accurate modelings are going to be analyzed, validated and integrated in SPINBALL, simple models of ablation phenomena of the liner and case thermal protections and ablative materials and ablation-erosion phenomena of the nozzle thermal protections have been considered, to evaluate the qualitative effects of these events on the internal ballistic. Preliminary results of the internal ballistic numerical simulation of the third stage of the Vega launcher, Zefiro 9, for its entire combustion time, are presented, with a comparison between the experimental head-end pressure time history and the numerical one.