This paper analyzes the results obtained from a series of static firings of a lab-scaled hybrid rocket in which gaseous oxygen was supplied into axial-symmetric polyethylene cylindrical grains through two different injector configurations: an axial conical subsonic nozzle and a radial injector. The axial injector is interesting because of its relatively easy design, the higher regression rates, and the noticeably stable motor operation. To exploit its qualities, not only the assessment of the regression rate but rather the entire behavior of the motor is required. For the investigated set of operating conditions, the instantaneous regression rates exhibit a time dependence caused by the impinging jet zone dynamics, while the average regression rates are higher and less mass flux dependent than those achieved with the radial injection motor and expected from the classical turbulent-boundary-layer diffusionlimited theory. A comparison to the data from the radial injector was further drawn in terms of combustion efficiency and fuel regression uniformity. Concerning combustion stability, some observations are made. The radial injector, at the same mass flux and pressure, produces lower regression rates, high pressure oscillations, and worse combustion efficiency, but more uniform fuel consumption. Nomenclatureof ultrasounds in reference conditions c * = characteristic exhaust velocity c * 0 = theoretical characteristic exhaust velocity calculated at O/F c * = theoretical characteristic exhaust velocity calculated at OF D = space average port diameter D 0 = port initial diameter D 2 = average port final diameter D 2x = local port final diameter d = diameter E a = activation energy F = motor thrust G = mass flux H v = effective heat of vaporization I m = mixing index L = grain length L c = prechamber length L * = chamber characteristic length l = exponent of the Reynolds number in the mixing index definition m = exponent of the geometrical ratio in the mixing index definitioṅ m f = fuel mass flow ratė m ox = oxidizer mass flow rate = isentropic one-dimensional mass flow rate through the nozzle N = number of data points in the port diameter profiles n = exponent of mass flux OF = spatially averaged oxidizer-to-fuel ratio OF = mean value of spatially averaged oxidizer-to-fuel ratios O/F = average oxidizer to average fuel mass ratio p a = ambient pressure p e = nozzle-exit pressure R = gas constant Re = Reynolds numbeṙ r = space average regression ratė r x = local regression rate s = fuel grain thickness T w = fuel surface temperature t = time V e = isentropic one-dimensional velocity at nozzle exit x = axial abscissa z = nondimensional axial abscissa (x/L) α = nozzle divergence cone half-angle M f = measured solid fuel mass loss M f = computed solid fuel mass loss t = time step t o = ultrasounds time of flight η = c * efficiency, ratio between the actual and theoretical c * th η 0 = c * efficiency calculated at O/F λ = momentum-thrust reduction coefficient μ = gas viscosity ρ f = solid fuel density σ = standard deviation...

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