An improvement in the performance of solid rocket motors was achieved by increasing the oxidizer content of HTPB-based solid propellants. To minimize the adverse changes in the mechanical and rheological properties due to the increased amount of hard solid particles in the soft polymeric binder matrix, the optimum combination of the particle sizes and volume fractions of the bimodal ammonium perchlorate and the aluminum powder in the solid load was obtained from the results of testing a series of propellant samples prepared by using ammonium perchlorate in four different average particle sizes, 9.22, 31.4, 171, and 323 mu m. The maximum packing density of solids in the binder matrix was determined by changing the sizes and the volume fractions of fine and coarse ammonium perchlorate at constant solid loading. The average size (10.4 mu m) and concentration of aluminum powder used as metallic fuel were maintained constant for ballistic requirements. Optimum sizes and fine-to-coarse ratio of ammonium perchlorate particles were determined to be at mean diameters of 31.4 and 323 mu m and fine-to-coarse ratio of 35/65. Solid content of the propellant was then increased from 75 to 85.6% by volume by using the predetermined optimum sizes and fine to coarse ratio of ammonium perchlorate. Mechanical properties of the propellant samples were measured by using an Instron tester with a crosshead speed of 50 mm/min at 25 degrees C. The effect of oxidizer content and fine-to-coarse ratio of oxidizer on the burning rate of the propellant was also investigated by using a strand burner at various pressures. From experiments in which the size and the fine-to-coarse ratio of ammonium perchlorate were changed at constant solid loading, a minimum value of initial modulus was obtained for each fine-to-coarse ratio, indicating that the solids packing fraction is maximum at this ratio. The tensile strength and the burning rate increase, while the elongation at maximum stress decreases with increasing fine-to-coarse ratio of ammonium perchlorate. Experiments in which the total solid loading was increased at constant fine-to-coarse ratio of ammonium perchlorate show that the modulus, the tensile strength and the burning rate increase, while the elongation at maximum stress decreases with increasing solid loading. Propellants having solid loading of up to 82% exhibit acceptable mechanical properties and improved burning properties suitable for rocket applications. (C) 1998 John Wiley & Sons, Inc.