In recent years, plug nozzles have gained a renewed interest in space business. In principle, this nozzle concept offers a continuous altitude adaptation up to its design pressure ratio. But, the flow adaptation achieved with this advanced rocket nozzle concept induces shocks and expansion waves resulting in exit profiles far away from idealized one-dimensional flow profiles, and inducing additional losses which may lower the theoretically achievable performance under idealized conditions by several percents. These flow phenomena observed in experiments and numerical simulations of different altitude adaptive plug nozzles are highlighted and discussed in this paper. A numerical approach for plug nozzle flowfield simulations is presented, which is based on the method of characteristics. First results obtained with this method are encouraging. Predicted flow topology is well comparable with Schlieren images and wall pressure measurements of recent plug nozzle experiments performed in Europe. Nomenclat ure Symbols CF thrust coefficient h height k coefficient 1 length m