Casting Process Impact on Small-Scale Solid Rocket Motor Ballistic Performance

It has been widely reported that the ballistic response of solid rocket motors depends on the process used to manufacture it. Usually, the empirical parameters necessary to predict the performance of a motor (hump effect and scale factor), linked to the manufacturing process, are deduced from the exploitation of previous firing tests. The physical phenomena linked to those empirical parameters are sought to simulate directly the ballistic behavior of a new motor, in the future. One step in these studies has been to manufacture, with the same propellant, several small-scale grains, using three casting processes, and to study the influence of the manufacturing process on the ballistic behavior. Some of those grains have been fired, and others have been cut in several samples for local measurements of burning rate. The main results of experimental analysis are the profile of the radial burning rate on the web coherent with the hump effect, the strong ratio between radial and axial burning rates on each point of the web, and the visualization of propellant stratification by nuclear magnetic resonance (NMR) imaging due to the casting method. The experimental results have then been compared to a numerical simulation performed using a new code developed at SNPE to compute surface burnback with varying burning rate. In this mathematical model, the propellant should not be homogeneous as it is in constant burning rate simulations. The nature and the form of the heterogeneity are linked to the casting method. They may be deduced from a casting simulation performed at SNPE with the MONTREAL® code or from NMR imaging. The simulation results are very close to experimental results in terms of hump effect, scale factor between two casting processes, local burning rate profiles, and local anisotropy of propellant.