A solid heterogeneous, high performance rocket propellant, operable at high pressure with a burn rate relatively insensitive to changes in pressure and temperature, is disclosed. The propellant includes a binder formed from the reaction of a hydroxy terminated polybutadiene with a diisocyanate, ammonium perchlorate as an oxidizer, aluminum as a fuel, and iron oxide as a burn rate modifier. The ammonium perchlorate is a multimodal mixture of large paryticles with a weight mean diameter of about 70 to about 110 m and small particles having a weight mean diameter of about 7.5 to about 15 m. The propellant may also include bonding agents, curing catalysts, a plasticizer, antioxidant/peroxide scavengers, and pot life extenders.