The aim of this thesis is to give a deeper understanding of Power Spectral Density analysis and how it can be used to predict the impact of fluctuating loads induced directly to a rocket nozzle. The initial study mainly deals with understanding how the response may vary with changed geometrical setup of the reinforcement structure. That is why a great deal of the time has been spent on parameterize the model to make it easy to automatically loop over many different geometrical and material properties of the nozzle extension. The work was implemented on a simplified FE-model of the next generation sandwich nozzle extension that was under development at GKN Aerospace Engine Systems. The thesis will only address the impact of buffeting loads during atmospheric ascent as it is the most critical stage of the flight. The occurrence of buffeting loads during atmospheric ascent is due to varying pressure fields as it passes through the atmosphere. This load depends primarily on the shape of the vehicle, but the severity can change rapidly depending on the dynamic pressure, angle of attack and Mach number. To analyze the behavior of a random load it is important to understand which critical structure modes are present in the specified frequency range. Excitation of these modes can lead to structural failure of the nozzle due to excessive displacement. Evalaution of the critical mode frequencies indicated a linear relationship, up to 60 mm stiffener height, between the eigenfrequency of the critical modes and the stiffener height, where both 3-wave and 4-wave increase modes more rapidly for higher stiffeners than for the ovalisation mode. The pendulum and torsion mode will have nearly constant frequencies. Both the torsion and pendulum mode are caused by motion at the nozzle throat and are unaffected by the increased stiffness of the lower part of the nozzle. Connecting these linear relations to the energy distribution of the PSD spectrum showed an increase of available energy for the ovalisation mode and a decrease for 3-wave and 4-wave modes for higher stiffeners.A further study of how the reinforcement structure affects the eigenfrequencies and the magnitude of the critical modes states that the stiffener height has the largest contribution. The increased stiffener height gives an exponentially decreased radial displacement of the nozzle wall. The stiffener thickness has a minor effect on the mode frequencies and contributes only to an increased structural stiffness of the reinforcement structure. This states that it is better to use higher stiffeners than an increased number of lower stiffeners. At the same time the allowed stiffener height is limited by an increased stiffness of the wall at each stiffener due to increased risk for axial buckling between stiffeners.An important notice is that the PSD spectrum for the Ariane 5 rocket was not available during the thesis. Instead a PSD spectrum from a slightly larger nozzle extension was used. How this PSD spectrum relates to the Ariane 5 PSD spectrum needs to be further investigated to verify the magnitude of these critical modes.Finally, the study has shown that PSD analysis is a time efficient and competent tool that could be used as a first approach during the design phase. A problem that was stated early on was the lack of support to handle nonlinear behaviors such as plasticity in ANSYS. This meant that large temperature changes occurring during flight could not be accounted for in this PSD analysis. However LS-DYNA has recently implemented a non-linear solver for the aerospace industry, based on Boeing software N-FEARA, a NIKE3D-based finite element tool for structural analysis of vibro-acoustic loads. If the LS-DYNA non-linear solver can account for large thermal and structural loads, the PSD analysis might be an alternative to perform time history analysis. It shall be emphasized that this is a reduced report; company sensitive results and developed methods are not included.