Various tests showed a significant side load peak for low nozzle pressure ratios during engine start-up and shut down phase. DLR Lampoldshausen carried out tests to examine the flow field in a truncated ideal contour nozzle for low NPR. For NPR20 a slight concave shaped Mach disk was found. Its curvature is limited to the centre and its height trend correlates with measured side loads. A concave shaped Mach disk being responsible for re-attached flows at low NPR could be excluded. The experiments were accompanied by numerical simulations of the flow field on various pressure ratios with regards on the shock pattern. The predicted Mach disk shape compares well with the experiments
