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Effect of Similarity Distortion on the Scaled Model of a Pressure Vessel- A Case Study of Propellant Tank

This paper investigates the effect of similarity distortions on the structural similitude performed on the full scale prototype propellant tank in order to develop its scaled model. Propellant tanks act as pressure vessels and find their use in various aerostructures ranging from launch vehicles to missiles. They require extensive and costly testing to qualify which can be reduced by performing tests on the scale down model and using its results to accurately predict the prototype performance. Structural similitude using dimensional analysis is the widely used method of developing scaled models of pressure vessels in order to reduce the cost of manufacturing and testing. One of the problems encountered in dimensional analysis is the lack of complete similarity due to variations in the material properties, geometrical parameters and other factors between the prototype and the scaled model. These variations, called similarity distortions, result in partial similarity model that can affect the overall testing results. In this paper, the similarity distortions due to the change in material, wall thickness and head ratio of the propellant tank are studied. The objective to analyze these distortions is to achieve reliable test results and at the same time identify the structural parameters whose complete similarity cannot be violated. For comparison and overall analysis, a linear pattern of scale factors for different levels of distortions is presented for the development of scaled models. Results show that partial similarity in the scale factor of head ratio does not give satisfactory results. However, partial similarity in the material properties and wall thickness of the scaled pressure vessel can be overcome and the method to achieve this is presented.