AbstractSolid composite propellants contain 80–90% of a crystalline oxidizer like ammonium perchlorate and powdery metallic fuel like aluminum with 10 to 15% organic binders like HTPB or CTPB, to bind the solids together and maintain the shape under severe stress and strain environment. Also, the propellant must not crack or become brittle at subzero temperatures. Formulating and processing of the highly filled composite propellants are difficult tasks and need a thorough understanding of rheology, even apart from a knowledge of propellant chemistry, particulate technology, manufacturing methods, and safe handling of explosives and hazardous materials. The flow behavior or rheology of the propellant slurry determines the ingredients and some of the abnormalities of the motor during firing. The propellant viscosity and mechanical properties are determined by the binder system, and the unloading viscosity of the propellant slurry is dependent on the initial viscosity of the binder system, solid loading, particle size, and its distribution, shape, temperature, and pressure. In the present report an attempt is made to study the dependency of viscosity of the HTPB binder system on temperature, plasticizer level (composition), and torque (angular velocity of spindle). The viscosity measurements were made using a Brookfield viscometer model DV III at different plasticizer levels (10–50%), temperatures (30–65°C), and torques (50–100%). The data indicate that the viscosity of HTPB, DOA, and their mixture decreases with increasing temperature and is constant with torque. The Arrhenius equation gives the energy for viscous flow to be ≅35 kcal/mol for HTPB. The variation of viscosity with temperature of HTPB/DOA and their mixture follows a mathematical model expressed as where T is the temperature and a1, a2, a3, a4, and a5 are the constants. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1002–1007, 2002