MECHANICAL AND BURNING PROPERTIES OF HIGHLY LOADED COMPOSITE PROPELLANTS

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 proper ties due to the increased amount of hard solid particles in the soft p olymeric binder matrix, the optimum combination of the particle sizes and volume fractions of the bimodal ammonium perchlorate and the alumi num 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 dete rmined by changing the sizes and the volume fractions of fine and coar se ammonium perchlorate at constant solid loading. The average size (1 0.4 mu m) and concentration of aluminum powder used as metallic fuel w ere 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 t o coarse ratio of ammonium perchlorate. Mechanical properties of the p ropellant samples were measured by using an Instron tester with a cros shead speed of 50 mm/min at 25 degrees C. The effect of oxidizer conte nt and fine-to-coarse ratio of oxidizer on the burning rate of the pro pellant was also investigated by using a strand burner at various pres sures. From experiments in which the size and the fine-to-coarse ratio of ammonium perchlorate were changed at constant solid loading, a min imum value of initial modulus was obtained for each fine-to-coarse rat io, indicating that the solids packing fraction is maximum at this rat io. The tensile strength and the burning rate increase, while the elon gation at maximum stress decreases with increasing fine-to-coarse rati o of ammonium perchlorate. Experiments in which the total solid loadin g was increased at constant fine-to-coarse ratio of ammonium perchlora te show that the modulus, the tensile strength and the burning rate in crease, while the elongation at maximum stress decreases with increasi ng solid loading. Propellants having solid loading of up to 82% exhibi t acceptable mechanical properties and improved burning properties sui table for rocket applications. (C) 1998 John Wiley & Sons, Inc.