Processing effects on ballistic response of composite solid propellant grains

Abstract

A synthcsis of works carried out by SNPE and university partners to investigatc processing effects on ballistic response of composite propellant grains is prcsented. It has been organizcd with the following general guidelines : An experimcntal work has been done to investigate the effects of casting processes and correlate burning rate profiles measured inside various grain shapes using subscales (cylindrical and finocyl). A particular attention has been paid to observe propellant flow during casting, analyze striations formation mechanism and determine their influence on burning rate. A constant cKort has bcen supported to identify and remove from observed burning rate anomalies, causes other than processing effects giving (Grain dcformation, erosive burning, pressure drops,...). Improved ballistic analysis and prediction methods have been dcveloped and validatcd, whcrc each part of the motor has its own burning rate profile. This allows, in some cascs, a bctter feedback of experience on predictions than traditional methods using a global Burning Rate Anomaly Factor (BRAF). Propellant flow simulation codes have been used and adapted to produce a graphical reprcscntaton of striations. First results are vcry promising when compared to striations observcd inside snbscale grains. This represcntation of striations can bc used by a numerical model, still undcr developmcnt in cooperation with University of BORDEAUX, which simulates a 2D burning front moving in a given striatcd medium containing altcrnativcly standard propellant laycrs and thin bindcr rich layers. A validation program of thc combincd approach (propcllant flow simulation and combustion in a striatcd medium) is in progress. Comparisons betwccn measured and calculated burning rates, using somc basic configurations of striations, dcmonstratc the consistency of the striation hypothesis and thc ability of thc model to simulatc thc Burning Ratc Anomaly Factor observcd. The final aim of these different works is to build a complete model to predict a BRAF related to a given motor and casting process. Introduction Technical Background Manufacturing processes generate various stresses into uncured composite propellants : Flow through pipes, slit plates and blowholes (in the case of injection), along mandrels and walls, induces mainly boundary layer type shear stresses. Fall of free paste streams tends to create high vclocity gradients and shearing inside propellant mass in the vicinity of dropping zones. Spreading of propellant accumulated around dropping zones leads to bonding of moving frcc surfaces. Plunging after casting can induce again shear stresses and a complete reorganization of propellant structures. I t is now widely admitted that these different strcsses indncc separation of the solid particles and/or changes in propellant composition4 which in turn leads to burning rate variations as a function of location in the grain and relative angle of incidence between flame front and shcarcd lines or surfaces?. In the case of simple geometries (i.e. cylindrical central perforatcd grains) this effcct has bcen quite well characterized. For grains cast mandrel in place it is always rclated to a low-highlow burning rate variation through web. This variation has bccn differently, and sometimes poctically, named : Hump cffcct, Rainbow effect, Burning Rate Anomaly Factor, ... Somc semicmpirical modcls have been proposed and are still efficient. Burning rate measurements on samples (strand burncr) havc confirmed the burnin rate influencc of casting tools geometry (ports, slit plate ) and of thcir location compared to thc motor case and mandrel havc been observed and qualitatively analysedl. For more complcx geomctries (Slotted, Finocyl) burning rate variations as a hnction of web fraction havc been also observed3. Thc corresponding BRAF (Burning Rate Anomaly Factor) features generally a different shape comparcd profile derived from full grain firing analysis 3 5 3 . The