A series of static engine rings, thermal pyrolysis experiments, and gas chromatograph/mass spectrometer tests were conducted to investigate the solid-fuel regression rate and heat-transfer behavior in a lab-scale hybrid rocket motor burning hydroxyl terminated polybutadiene/gaseous oxygen. A real-time, X-ray radiography system was used to determine the local, instantaneousregression rates. A data analysis program was developed to help correlate the experimental data. The semi-empirical regression-rate correlation showed that, in addition to convection, radiation from soot and variable uid properties across the boundary layer had signi cant effects on regressionrate behavior. The radiant heat ux from soot was relatively more signi cant under low mass ux and low oxidizerto-fuel ratio conditions. Radiation from CO 2 , H 2 O, and CO was quite small compared to convection and soot radiation. The nondimensional regression-rate correlation agreed with the experimental data to within § 3%. Stanton-and Nusselt-number correlations were also developed and found to depend on both ow regime and radiant heat ux. The regression-rate correlations predicted independent data from both a lab-scale tube burner and the 11-in. (28 cm) JIRAD motor to very reasonable accuracy. NomenclatureA = Arrhenius preexponential constant, mm/s B = blowing parameter, ½ f r=.Gc f =2/ or Eq. (20) B mod = modi ed blowing parameter, B exp.q 00 r =q 00 c / c = fuel speci c heat, J/kg-K c f = skin-friction coef cient c p = gas isobaric speci c heat, J/kg-K D = mass diffusivity, m 2 /s Da = Damköhler number D h = hydraulic diameter, cm E a = activation energy, kcal/mol F = function de ned in Eq. (22) G = local, bulk mass ux, kg/m 2 -s h = enthalpy, J/kg; port height, mm L = length of fuel slab, 58.4 cm Nu = Nusselt number Pr = Prandtl number p = pressure, atm or MPa q 00 = heat ux, W/m 2 R = function de ned in Eq. (34) Re = Reynolds number R u = universal gas constant, kcal/kg-K r = regression rate, mm/s St = Stanton number T = temperature, K t = time, s u = streamwise velocity component, m/s w = fuel web thickness, mm x = axial location, cm Y i = species mass fraction ® = absorptivity = heat of formation, J/kg 1H r = heat of reaction per unit mass reactants, J/kg 1H v = heat of vaporization, J/kg " = emissivity, or turbulence dissipation rate, m 2 /s 3 µ = temperature ratio, T ;avg =T s · = absorption coef cient ¹ = viscosity, Ns/m 2 ½ = density, g/cm 3 ¾ = Stefan-Boltzmann constant Subscripts avg = average c = convective eff = effective = ame g = gas phase o = oxidizer, or reference r = radiant s = surface or soot t = turbulent

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An experimental investigation of the thermal pyrolysis behavior of several hybrid-rocket solid fuels under rapid heating conditions was conducted to determine pyrolysis laws and to identify and quantify the products of fuel pyrolysis. The study focused on four fuel formulations: pure hydroxyl-terminated polybutadienes (HTPB), 80% HTPB/20% Alex, 80% HTPB/20% Al, and the Joint Industrial Research and Development fuel formulation. A rapid conductive-heating technique was developed and employed to determine Arrhenius-type pyrolysis laws. All four fuels displayed two sets of Arrhenius parameters, depending on the range of surface temperature. For pure HTPB, E a = 4:91 kcal/mol and A = 11.04 mm/s above 722 K, while E a = 13:35 kcal/mol and A = 3965 mm/s below 722 K. These results agree well with those obtained previously using a lab-scale hybrid motor operating under realistic conditions. The gas chromatograph/mass spectrometer tests of the nonmetalized fuels, using a ash-heating oven, indicated that the relative concentrations of the pyrolyzed species depended strongly on temperature. For pure HTPB seven major products were identi ed, with 1,3-butadiene representing the dominant product at all temperatures tested, up to 1073 K. The measured mole fractions of the pyrolysis products and deduced pyrolysis laws of the fuels studied can be utilized in a comprehensive model simulation for combustion performance predictions.

Selected fuel, oxidizer and simulant gels were prepared and rheologically characterized using a rotational rheometer. For fuel gelation both organic and inorganic gellants were utilized, whereas oxidizers and simulants were gelled with addition of silica and polysaccharides, respectively. The generalized Herschel‐Bulkley constitutive model was found to most adequately represent the gels studied. Hydrazine‐based fuels, gelled with polysaccharides, were characterized as shear‐thinning pseudoplastic fluids with low shear yield stress, whereas inhibited red‐fuming nitric acid (IRFNA) and hydrogen peroxide oxidizers, gelled with silica, were characterized as yield thixotropic fluids with significant shear yield stress. Creep tests were conducted on two rheological types of gels with different gellant content and the results were fitted by Burgers‐Kelvin viscoelastic constitutive model. The effect of temperature on the rheological properties of gel propellant simulants was also investigated. A general rheological classification of gel propellants and simulants is proposed.