Abstract: 1. A nuclear engine and nozzle arrangement for a nuclear rocket, said arrangement comprising a cluster of elongated fissile fuel bearing and high temperature capacity modules suitably supported in a pressure vessel, said modules each having a plurality of coolant-propellant channels extending therethrough, a convergent-divergent nozzle structure of fixed cross-sectional dimensions secured to the end portion of each of said modules said modules, a divergent-only unitary skirt member connected directly to the propellant exit end of said modular cluster in series with and diverging from the divergent ends of said convergent-divergent nozzle structures, said modules being formed to conduct a compressible propellant therethrough at sub-sonic velocities, said nozzle structures being formed to develop supersonic velocities of the propellant and said divergent-only skirt being formed to develop further the supersonic velocities of said propellant.