The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 % by weight), aluminum (fuel, 16 %), iron oxide (a catalyst, 0.4 %), a polymer (such as PBAN or HTPB - a binder that holds the mixture together, 12.04 %), and an epoxy curing agent (1.96 %). This mixture develops a specific impulse of 242 seconds at sea level / 268 seconds vacuum.
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