This paper presents a new concept of Hybrid Rocket Motor (HRM) called Double-tube.In this configuration, the gaseous oxidizer is injected using a head end injector and an inner tube with injector holes distributed along the motor longitudinal axis. The inner tube is located inside the combustion chamber, is coaxial to the motor case and goes from the motor head end to the aft end of the fuel grain. In order to test the potential advantages of this design, a series of three-dimensional numerical simulations have been carried out with gaseous oxygen (GOX) as oxidizer and hydroxyl terminated polybutadiene (HTPB) as solid fuel. The simulation model considers realizable k-ε turbulence model combined with eddydissipation combustion model. Furthermore, the solid fuel pyrolysis is computed through customized user-defined functions. This numerical model has been validated for previous conventional HRM designs by comparing the computational results against experimental data. The results obtained for the Double-tube have shown that the regression rate can be increased over 50% with respect to classical hybrid rockets. Indeed, the characteristic velocity, combustion efficiency and species mixing are improved by injecting the oxidizer where it is most needed thanks to an appropriate distribution of the inner tube injector holes. Finally, the Double-tube design allows for a higher control of the oxidizer-to-fuel ratio along the grain because the inner tube provides a customized distribution of oxidizer. NomenclatureA = Arrhenius preexponential constant, mm/s a = preexponential factor of the regression rate law c* = characteristic exhaust velocity, m/s D = port diameter, mm d = diameter of the injector holes, mm E a = activation energy, J/mol e = energy, J/kg G ox = oxidizer mass flux, kg/m 2 s H = enthalpy, J/kg k = kinetic energy of turbulent fluctuations L = grain length, mm ṁ ox = total oxygen mass flow rate, kg/s N = total number of chemical species n = exponent of the oxidizer mass flux O/F = oxidizer-to-fuel ratio p c = chamber pressure, atm or Pa p t = effective pressure, Pa R 2 = coefficient of determination R u = universal gas constant, J/kg-K 1 Master's Degree candidate, School of Astronautics, Beihang University, arnau.pons.lorente@gmail.com, and AIAA Student Member. 2 r = regression rate, mm/s T = temperature, K t = time, s u = velocity, m/s Y m = mass fraction of the mth species α = inner tube injection angle ε = turbulence dissipation rate η = c* efficiency, ratio between the simulated and the theoretical characteristic velocity λ = ratio of inner tube oxidizer mass flow rate to total oxidizer mass flow rate λ' = thermal conductivity, W/m-K μ = viscosity, kg/s-m ρ = density, kg/m 3 Subscripts f = fuel g = gas HE = head end IT = inner tube n = normal direction ref = reference s = fuel surface

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This experimental study investigates the solid fuel regression rate and performance of a hybrid rocket using a double-tube configuration. The paper analyzes the results of a series of static firings of a laboratory-scale hybrid rocket motor with two coaxial cylindrical fuel grains of high-density polyethylene. The gaseous oxygen is injected into the combustion chamber using two different injector types: a double-tube configuration and a conventional axial showerhead injector, which is used as reference. In the double-tube design tested, all the oxidizer is injected through a coaxial inner tube with injector holes distributed along the motor longitudinal axis. Moreover, the inner fuel grain, which is supported by the inner tube, allows the oxidizer jets to enter the combustion chamber at a given injection angle. In this case, the gaseous oxygen is injected counter-flowing generating as a consequence strong recirculation zones that improve significantly the species mixing. The experimental test firings reveal that, for the same oxidizer mass flux rate, the double-tube configuration achieves a regression rate over twice faster than the conventional axial showerhead injector. In addition, the double-tube configuration performed with very stable motor operation and smoother pressure traces than the conventional axial showerhead injector. However, the characteristic flow field of the double-tube configuration provokes a higher unevenness in the fuel consumption. NomenclatureA p = port area, m 2 Subscripts A t = nozzle throat area of the lab-scale motor, m 2 c* = characteristic exhaust velocity, m/s exp = experimental D = port diameter, mm f = fuel G ox = oxidizer mass flux rate, kg/m 2 s if = inner fuel L = grain length, mm of = outer fuel ṁ f = total fuel mass flow rate, kg/s _0 = initial ṁ ox = total oxygen mass flow rate, kg/s _f = final O/F = oxidizer-to-fuel ratio t b = burning time, s p c = combustion chamber pressure, MPa R 2 = coefficient of determination r = regression rate, mm/s α = injection angle, º η = c* efficiency λ = ratio of the inner tube oxidizer mass flow rate to the total oxidizer mass flow rate ΔM = fuel mass burnt during the test, kg ρ = density, kg/m 3 1 Master's Degree, School of Astronautics, Beihang University, arnau.pons.lorente@gmail.com, and AIAA Student Member.

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