This thesis deals with the flow field at a two-dimensional SERN-Nozzle. Main focus lies on the flow around the nozzle flap and the occurring separation of the boundary-layer of the outer flow. The objective is to get a better understanding of the interacting flow fields and to improve the nozzle performance by an optimized flap geometry. In particular this means a higher axial thrust coefficient and a smaller rotation of the thrust vector in the transonic range. To analyse the flow field two-dimensional numerical calculations and wind tunnel experiments in the hypersonic wind tunnel facility H2K of the DLR in Cologne are utilized. The location of the boundary-layer separation is measured depending on different Reynolds numbers and different nozzle pressure ratios, and a criterion to predict the separation location depending on the static pressure is derived. Further on, the influence of the Reynolds number on a three-dimensional corner flow upstream of the flap is shown. Different nozzle configurations are tested and the results are applied to the flap of the hypersonic configuration ELAC of the collaborative research center 253. Numerical simulations confirm the obtained improvements concerning the axial thrust coefficient and the rotating thrust vector in the Mach number range between Ma = 1.23 and Ma = 7. Further studies deal with the changes of the flow field when the nozzle flow is heated to temperatures up to 1000 K. Focal point is the influence of temperature effects such as the change of viscosity or the change of the ratio of specific heats. With increasing temperature (and constant nozzle pressure ratio) the position of the nozzle shocks vary and the nozzle plume widens. Further on, the static pressure at the expansion ramp is increased. This pressure increase exceeds the pressure increase expected by the decreasing ratio of specific heats, and thus the axial thrust coefficient is increased, too. Key words: hypersonic, SERN-nozzle, ELAC, nozzle flap, temperature effects