Large eddy simulation methodology coupled with volume of fluid(VOF)method was used to study the spray atomization process of N2H4/MMH liquid-liquid swirl coaxial injectors.The spray characteristics such as spray angle,breakup length,thickness of liquid sheets and droplet size were investigated quantitatively.The bipropellant jets mixing and liquid jet instability was analyzed.Compared with LES and linear instability theory analysis for the single swirling coaxial jet,a shorter breakup length,lager spray angle and wider spatial distributions of droplets position was produced in two swirling coaxial liquid jets.A strong pressure,velocity and turbulent fluctuations was calculated near the contact position of the two coaxial jets.The results indicated that additional perturbations were generated due to the strong radial and axial shear effects between bipropellant jets.Due to different flow velocity and propellant density,interaction between the two coaxial jets enhanced Kelvin–Helmholtz instability and Rayleigh-Taylor instability,and then accelerated the reduction in the liquid breakup length,breakup time,ligaments and fragments length scales.The interaction processes also contributed to achieve liquid-phase mixing of bipropellant and lead to a narrowing of the droplet size spectrum instead of a broadening.