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Analytical investigation of noise suppression effect of water injection to exhaust plume from rocket motors was carried out. The results showed acoustic absorption by water droplets, acoustic scatter by water droplets, absorption through air, and water curtain effect increase as frequency becomes high. It was also confirmed that acoustic absorption by water droplets has the most significant effects among the four effects. Furthermore, steady 2D-axisymmetric Reynolds-Averaged Navier-Stokes (RANS) simulations of a supersonic free jet were carried out in order to evaluate reduction of jet energy due to water injection. The reduction of the acoustic source strength along the jet axis was evaluated considering the difference of r 2 k 7/2 value between with and without water injection. The far field sound power level (SPL) was analyzed using an empirical prediction method, NASA SP-8072 [1] and compared to sub-scale motor test data. The strength of the acoustic source power along the jet axis was set based on the reduction rate of the r 2 k 7/2 value due to water injection and the propagation to the far filed points was analyzed based on the NASA SP-8072 [1] . The results showed that the water injection effect can be reasonably evaluated by using both the analytical prediction methodology and the evaluation methodology of reduction of the jet energy based on the change of r 2 k 7/2 . Nomenclature a = speed of sound in the gas a P = speed of sound in the liquid phase T 0 = mean temperature in the gas T = perturbation of temperature in the gas T P = temperature in the liquid phase P = perturbation of pressure in the gas P 0 = mean pressure in the gas Pv = partial pressure of the vapor Pv e = equilibrium saturation pressure r = mean density of the gas r P0 = mean density of the liquid r P = perturbation of density of the liquid r V0 = mean density of the vapor u = perturbation of velocity in the gas phase u P = perturbation of velocity in the liquid phase k V = mass rate of vapor to the gas k P = mass rate of liquid to the gas C p = specific heat of gas at constant pressure 1 Researcher, JAXA s Engineering Digital Innovation (JEDI) center, AIAA Senior Member. 092407 2 r p = water droplet radius C v = specific heat of gas at constant volume l = thermal conductivity of gas m = kinematic viscosity of the gas h l = latent heat of vaporization of the liquid D p = diameter of water droplet q = angle from jet axis r= distance from the center of nozzle exit D e = diameter of nozzle exit k = turbulent kinetic energy (TKE) X = distance along jet axis from nozzle exit Q p = mass flow rate of exhaust jet Q w = mass flow rate of injected water

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