1. Many previously published regression rate correlations were developed on the basis of somewhat crude assumptions regarding the effects of chamber pressure, oxidizer composition, radiative heat transfer, duct-velocity profile, chemical kinetics of the combustion processes, and composition of the fuelpyrolysis products. Green [1] reviewed early theoretical analyses (up to 1963) of hybrid motor combustion processes and compared the assumptions made in the different models. Recently, a brief chronicle survey of hybrid combustion studies was provided by Helmy [2]. Kuo [3] has presented a comprehensive review of previous analyses, existing models, current programs, and major challenges in hybrid propulsion. Classical analyses of hybrid combustion have used turbulent boundary-layer assumptions to determine the convective heat flux to the fuel surface and deduce a regression rate correlation [1,3-6]. The well-known basic model, developed by Marxman and co-workers [4-6], is based upon convective heat transfer controlled combustion in a turbulent boundary layer over a flat plate. Many attempts have been subsequently made to modify •Marxman's model by including the effects of radiative heat transfer, fuel thermal degradation processes, gasphase chemical kinetics, and density variations in the boundary layer [2,7-12]. However, each of these modified models usually considers only one or two of the above effects.
2. Recently, a windowed, slab-geometry hybrid motor has been designed and used at the High Pressure Combustion Laboratory (HPCL) of the Pennsylvania State University to provide instantaneous local regression rate measurements under realistic operating conditions for correlation development and computational model validation [14-16]. The motor used GOX as the oxidizer and a family of HTPB-based solid fuels. Both ultrasonic pulse-echo and real-time X-ray radiography techniques were used to determine the local instantaneous regression rate. In a parallel pyrolysis of several HTPB-based solid fuels under realistic high heating rates with identification and quantification of the pyrolysis products. These data have been utilized in the regression rate correlation development of the present work.
3. The high-pressure, widowed, classical hybrid motor has been described in detail in previous papers [14-16]. The motor utilizes two opposing fuel slabs each 584 mm. long and 76.2 mm wide. Normally, a uniform initial port height of 8.9 mm to 12.7 mm separated the two fuel slabs. As discussed in Ref. [16], the motor had operating conditions within the range of interest for practical applications: G0up to 530 kg/m2-s, p upto 95 atm, L/Dhof about 40, and variable O/F ratio. A real-time, X-ray radiography system was used to deduce the local, instantaneous solid-fuel regression rates. Chiaverini, et al. [15] discussed the procedure to deduce the instantaneous solid-fuel regression rates, including the effects of mechanical compression/decompression of the fuel slabs caused by pressure transients. An array of fine-wire temperature profiles and pyrolyzing surface temperatures at various axial locations. Several pressure transducers provided the motor pressure history along the motor port. Pure HTPB, 80% HTPB/20% Al, and 80% HTPB/20% Alex were tested in the motor. Gaseous oxygen was employed as the oxidizer for all tests.
4. where e is the turbulence dissipation rate, and should not be confused with the emissivity. In Eq. (6) and (7), the molecular diffusivity and kinematic viscosity were calculated using the Boundary Layer Integral Matrix Procedure (BLIMP) mixture equations described by Kuo [25]. For the global reaction of 1,3 butadiene gas (the largest pyrolysis product of HTPB) and gaseous