Afterburning of solid rocket exhaust

Abstract

AN EQUILIBRIUM CALCULATION of the solid fueled rocket exhaust of the proposed NASA Space Shuttle has been carried out and parameterized in terms of oxidant/fuel ratio. Results comparing the equilibration times with experimentally determined dilution times show that Hz and CO are completely burned to H20 and CO2, whereas HC1, although thermodynamical ly usable, is not oxidized to C12. NO formation is less certain, but an upper limit of HCI/NO > 5 can be determined. Studies to date on exhaust product reactions by Susko and Kaufman[1] and Nybakken and Harris [2] either include an assumption of intense afterburning of H2 and CO, or use as an arbitrary parameter zero or complete afterburning [3]. In this paper a simplified model is used to estimate the extent of afterburning. Data are parameterized in terms of oxidant/fuel ratio (derived from the work of Rhein[4]). Effluent f rom the proposed Space Shuttle is fuel (F) and oxidant (O) is air. The lifetime (z) of several key reactions in the afterburning equilibria are compared to the time rate of change of species concentration. When reaction lifetimes (~-) become long compared to cloud dilution times then the equilibria will clearly be frozen. An inventory of exhaust products HCI, H20, CO, CO2, N2 and N204 at -r = 13 sec after ignition (solid particles are excluded) results in 1932 moles of products assumed evenly distributed throughout a cloud volume of 1.25 x 106 m 3 (Hart [5]). This gives an oxidant-to-fuel ratio of 26.9 at ~ = 13 sec, which increases logarithmically through O/F = 1100 at 40 sec. Applying cloud rise and growth factors f rom Nybakken and Harris[2] one finds an O/F ratio of 7000 at ~= 150 sec. This curve enables one to set an approximate dilution time. Equilibrium analysis was carried out by the minimization of Gibbs free energy with mass balance constraints. The thermodynamic state of the gas mixture was specified by a given pressure and enthalpy. Our model makes use of two main assumptions: (a) that stagnation enthalpy is constant throughout the mixing region