Radiation-Augmented Combustion of Homogeneous Solids

The thermal effects of incident radiation on the burning characteristics of homogeneous solids are considered and the practical implications of these effects in solid rocket motors are assessed. The applicability of the equivalence principle (equivalence between radiation and initial temperature) is investigated experimentally and analytically. Experimental steady burning rates as a function of initial temperature and mean radiant flux are presented for a fine ammonium perchlorate (A P) composite propellant which indicate that the equivalence principle is accurate to within experimental error. The equivalence principle is also assessed analytically by considering the worst case conditions of condensed phase controlled burning. For deflagration and pyrolysis of solids controlled by condensed phase reactions it is shown that a modification of Ibiricu and Williams’ high activation energy asymptotic burning rate expression allows consideration of the effect of incident radiation on steady burning rate over a wide range of propellant opacities. Numerical simulations are used to verify this modification. Thermal radiation from combustion gases in typical non-metalized solid rocket motors is examined using band model calculations. These calculations show that even pure molecular gas radiation (no particles) can reach near blackbody levels for realistic motor conditions. The effects of thermal radiation on burning rate and temperature sensitivity are also examined using the equivalence principle. It is shown that differences in burning rate and temperature sensitivity between motors and propellant strands which in the past have primarily been attributed to erosive burning, could in some instances be due to thermal gas radiation. The implications of gas radiation on combustion stability are also discussed. It is shown that radiation may have either a stabilizing or destabilizing effect, according to the initial temperature dependence of the temperature sensitivity σ_p(T₀), although in most cases it is a stabilizing influence.