"The propellant mixture in each SRB motor consists of an (Click link for more info and facts about ammonium perchlorate) ammonium perchlorate ( (A substance that oxidizes another substance) oxidizer, 69.6 percent by weight), (A silvery ductile metallic element found primarily in bauxite) aluminum ( (A substance that can be consumed to produce energy) fuel, 16 percent), (Click link for more info and facts about iron oxide) iron oxide (a ((chemistry) a substance that initiates or accelerates a chemical reaction without itself being affected) catalyst, 0.4 percent), a (A naturally occurring or synthetic compound consisting of large molecules made up of a linked series of repeated simple monomers) polymer (a binder that holds the mixture together, 12.04 percent), and an (A thermosetting resin; used chiefly in strong adhesives and coatings and laminates) epoxy curing agent (1.96 percent).
"The propellant is an 11-point ((astronomy) a celestial body of hot gases that radiates energy derived from thermonuclear reactions in the interior) star-shaped perforation in the forward motor segment and a double-truncated- (Cone-shaped mass of ovule- or spore-bearing scales or bracts) cone perforation in each of the aft segments and aft closure.
"This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.
http://www.absoluteastronomy.com/encyclopedia/s/sp/space_shuttle_solid_rocket_booster.htm
"Ammonium perchlorate is an inorganic salt used in solid rocket fuel, in
munitions and in the pyrotechnics industry. Perchlorate manufacturers
estimate that approximately 90% of perchlorate is used for solid rocket
fuel. Ammonium perchlorate dissolves as easily as table salt, and the
resulting anion is stable and can persist for decades in the
environment, and moves easily through both groundwater and surface
water. Perchlorate has been detected in 110 public water supply wells
in California from at least 14 distinct sources, in the Colorado River
from sources in Nevada, and in surface or groundwater in Utah, Texas,
New York, Maryland, and Arkansas."
http://www.cpeo.org/lists/military/1998/msg00301.html