Combustion behavior of AP/HTPB/Al composite propellant containing hydroborate iron compound

We report a first attempt to investigate the reaction mechanisms of hydroborate iron compound (HIC), a promising candidate for combustion catalyst, in AP/HTPB/Al composite solid propellants using ammonium perchlorate (AP) as the oxidizer and aluminum as the metal fuel. The effects of HIC on the combustion properties of propellants were experimentally examined to determine burning rates, combustion flame structures and flame temperature distribution. The combustion residues were collected and examined; the mechanical sensitivity of propellant samples were tested. It was found that the addition of HIC particles can increase the burning rates with only minor changes of the pressure exponent. When 5.0 wt% of Al is replaced by HIC, the pressure exponent increases from 0.31 to 0.35 over the pressure range 1-12 MPa while the burning rate increases from 33 mm s(-1) to 40 mm s (-1) at 7 MPa. The combustion flame images of burning propellants surface loaded with HIC at various pressures present multi-flame structures, and the flame brightness increases with increasing the pressure. Many sparks are observed near the burning surface during the propellant combustion, due to burning aluminum particles in gas phase. With 5.0 wt% HIC included, the propellant produces very different condensed combustion products (CCPs) with a variety of morphologies, and the most typical ones are porous shape, large alveolate and loose structures. The relative combustion residue mass decreases with increasing pressure (1-5 MPa) and HIC mass fraction in the formulation (0-5%); While HIC powder is insensitive to friction and impact stimuli (0% at height is higher than 125 cm), the tested propellant formulations containing different mass fraction of HIC particles result sensitive to impact and friction a bit more than the reference formulation without HIC particles. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.