A. Gocmez et al., MECHANICAL AND BURNING PROPERTIES OF HIGHLY LOADED COMPOSITE PROPELLANTS, Journal of applied polymer science, 67(8), 1998, pp. 1457-1464
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.