Steady-state burning of a propellant containing, by mass, 82% of a mixture of ammonium perchlorate and sodium nitrate, 14% HTPB, and 4% Al was investigated in a constant pressure bomb up to 7 MPa of nitrogen. Two different regions of burning rate versus pressure dependence were found: r_b = 0.326 p~(0.79) mm/s, for 1.1 < p < 5.0 MPa, and r_b = 3.33 p~(0.50) mm/s for p > 5.0 MPa. At pressures less than 1.1 MPa, steady state burning of the propellant could not be observed. Independent investigation of the similar composite propellant containing only AP as oxidizer gave the dependence: r_b = 0.450 p~(0.48) mm/s in the pressure interval 0.1 to 7 MPa. The main result in this respect consists in that the burning rate versus pressure relations are similar for both formulations under high pressure, p >5.0 MPa, (with the exception that burning rate of the SN propellant is slightly less) and quite different in the lower pressure interval. The basic assumption discussed in this paper consists in that for propellant burning in the lower pressure region the very reactive chlorine oxides are eliminated by a reaction with sodium nitrate, and burning rate is determined by the NO_x oxidation processes similar to those defining burning rate of double-base propellants. In the high pressure region the independent burning of the HTPB and small particles of AP and Al interrelation becomes effective, and the process of heat generation is implemented basically in the zone where ClO_x oxidation reactions prevail.
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