The combustion behavior of composite fuel-rich propellants based on hydroxy-terminated polybutadiene pre-polymer (HTPB)/ammonium perchlorate oxidizer (AP) studied experimentally at 0.1-1 MPa. Results show that high content and small particle size of AP, high pressure promotes sustained combustion and increases burning rate, combustion efficiency and decreases ignition temperature. The copper chromite (CC) as a burning rate accelerator, moderately, increases the burning rate over the whole pressure regime. Moreover, it creates an acceptable combustion efficiency of about 96%. 6% CC decreases ignition temperature by about 16%. It is indicated that the Vieille burning rate law for this propellant family may be extended to extremely low pressures and combustion efficiency marked from "highly poor" to "poor" for the formulations without copper chromite.