The nozzle of the first stage rocket engine of present launchers (Vulcain, SSME or LE-7A) is usually designed in order to achieve full flowing at sea level for the nominal chamber pressure. However the engine will inevitably encounter flow separation during transient startup and shutdown. The separated flow usually exhibits, in some pressure ratio range, a strong unsteadiness and sometimes a loss of symmetry which generates lateral forces. In the present paper, a unsteady computational investigation of transient flows in an axisymmetric over-expanded thrust optimized contour nozzle is presented. A two-dimension experimental investigation has been carried out over an axisymmetric rocket nozzle to validate present results.