Improved stability analysis of rocket engine nozzles

This master’s thesis was conducted at Volvo Aero Corporation (VAC) in Trollhättan at the department of combustion chambers and nozzles. This department is responsible for the development of the main engine nozzle extension to the European Ariane 5 rocket.

During space flight there are a variety of loads which may cause structural instability in the main engine nozzle extension. To be able to predict the buckling margin accurately, realistic boundary conditions at the interface between the combustion chamber and the nozzle extension are crucial. The aim of the thesis was to design a FE-model of the combustion chamber that would provide more realistic boundary conditions at the interface than the currently used superelement. Comparison between the new and the currently used model was made to investigate whether the current analyses could be considered as conservative or not. The load sequence impact on the buckling margin and an alternative load application method was also studied within this thesis.

Both axial and radial stability are considered in this thesis and the results show that compared to the new combustion chamber model, the superelement leads to non-conservative results. The margin towards axial buckling for the Vulcain 2 NE is lowered by approximately 7 percent when using the combustion chamber model instead of the superelement. A reduction of the radial buckling margin for the V2+ nozzle has also been found but no definite conclusion could be drawn for this model. It has been seen that the modelling of the actuators greatly influence the results for both nozzle models and the combustion chamber model needs more work to produce trustworthy results.

When using an external pressure to apply the nozzle ovalisation instead of a force/moment field, no large differences were found. This means that the force/moment field can be replaced by an external pressure without causing any major effects in the stability analysis. More investigation should be done but the method based on an external pressure may be preferable since no loads are applied to the already plasticized inner wall. This method would also simplify the procedure of the stability analyses since the modal analysis used to determine the size of the force field is no longer necessary.

Finally, the analyses show that there are no significant effects of the evaluated loading sequences. The recommendation is thereby to continue with the currently used load sequence since a more detailed sequence lead to an appreciable increase of the analysis time.

Author: Berggren, John

Source: Lulea University of Technology

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