The interaction of supersonic underexpanded exhaust jets for a four-nozzle clustered rocket configuration is studied using three-dimensional RANS simulations. Such interactions can lead to reversed flow of the nozzle exhaust gases towards the base, and result in high pressure and heat loads on the vehicle. Multi-jet interaction flow fields are characterised by a number of shocks and expansion waves, along with turbulent shear layers enclosing subsonic recirculating zones. The complexity of the flowfield is broken down by identifying various viscous/inviscid interactions, with particular focus on the interaction of turbulence with shockwaves. The shock-unsteadiness k-! turbulence model is used to capture the physics of shock-turbulence interaction, and its effect on the multi-jet flow field.