Abstract
Grain design takes the central place in a solid rocket motor design activity. Ballistic quality of a designed grain can be evaluated by two vital indexes known as neutrality and sliver content. Sliver content results in tail-off of the thrust-time curve. These two measures of merit are an important part of acceptable grain design. This paper is restricted to the study of convex port star grain geometry and describes parametric evaluation to assess the effects of seven independent and defining geometric variables of the star and other ballistic factors including density of propellant and characteristic exhaust velocity on the burn pattern and performance profile for qualitative analysis of sliver fraction (tail-off)and neutrality. The purpose of the study is to expand the design domain by evaluating entire convex Star family under both neutral and least sliver content conditions.The computer program associated to it is essentially the ballistic design analysis of the convex star grain configuration.Results showed that neutrality and sliver fraction are dependent oncertain parameters. It has been observed that for good neutrality, higher angular fraction and star angle close to neutrality must be maintained. Sliver fraction depends upon the star geometry and can be reduced by decreasing angular fraction leading to reduced tail-off. Thus neutrality and reduced tail-off cannot be achieved simultaneously and trade-off has to be made. However, higher value of characteristic exhaust velocity (C*) will reduce tail-off.Key Words: Grain design, tail-off, ballistic design, neutrality1 Department of Mechanical Engineering, University of Engineering & Technology, Peshawar.INTRODUCTIONThe propulsion device used to propel space launch vehicles, rockets etc producing thrust by ejecting stored matter is termed as Solid Rocket Motor (SRM). The specific shape/configuration of the propellant casted inside the SRM is termed as grain or propellant grain. Since the desired characteristics (e.g. thrust, pressure) vary with time during any mission, design of SRMsis a complicated process. The geometric shape of grain configuration and properties related to propellant being used influences the performance parameters of the SRM. In fact, the only way in which the characteristics can be controlled is by managing the rate of combustion of the propellant set accordingto specific grain configuration’s geometry and its chemical formulation. The grain geometry is depen-dent upon various independent design variables which define that specific geometry. During operation, as the grain regresses, exposure of new burning surface areas dictates the burn pattern that in turn defines the thrust time profile. Propellant properties including characteristic exhaust velocity (C*) and density have also significant impact on the thrust time profile.Different grain configurations are currently being used according to the operation requirements including End Burner, Rod and Tube, Multi-fin, Double anchor, Tubular, Star, Wagon Wheeletc1. The shape of these grains may be two dimensional (cross sectional area does Figure 1. Definition of the star grain configuration1