The launch vehicles using the liquid fuel, which can control the thrust, are used for large space vehicles. However, the mass of the launch vehicle decreases due to fuel consumption during launch sequence lift off this affects the dynamic loads and vibration characteristics of the launch vehicle. In order to examine the dynamic characteristics, the experimental study for the actual launch vehicle is essential. However, the modal test of the full-scale launch vehicle model is considerably restricted by time, space, and cost. Therefore, an alternative test method using scaled model or simplified cylindrical tank is required.
However, there are few disadvantages when free vibration characteristics are investigated by the modal test. The modal testing time is excessively increased due to trial and error when finding a desired mode shape, and the accuracy of the result is reduced because the every natural frequency must be distinguished depending on the mode shape. In addition, it is very complicated and difficult to investigate characteristics of the fluid-structure coupled problems since the effects of the fluid and the structure of the launch vehicle are non-linear coupled by the oxidant of the launch vehicle and the liquid fuel of the fuel tank.
In this paper, a modal test method with finite element analysis (FEA) is suggested to overcome these drawbacks. Modal tests and finite element analyses are conducted by considering fluid-structure coupled effect. In addition, the free vibration characteristics and tendency of the cylindrical tank in terms of the water level are also investigated.
Acrylic cylindrical tank simulating oxidant tanks of the liquid launch vehicle is used for the research model. In the finite element analysis, the free vibration characteristics of the cylindrical tank are verified by using the normal modes analysis to compare with the vibration characteristics of the cylindrical tank verified by the modal test. In addition, the modal tests are conducted based on the determined accelerometer and impact position, which are determined by frequency response analysis. In order to investigate the free vibration characteristics in terms of water level of the cylindrical tank, modal tests and finite element analyses are conducted for the cylindrical tank filled with 0%, 25%, 50%, and 75% of the liquid (water) corresponding height of the cylindrical tank. Liquid (water) is implemented by using the virtual mass method of MSC.NASTRAN for the finite element analysis. The frequency response curves calculated from the modal analysis through the modal test, and the result were compared with each other. In addition, the free vibration characteristics determined through the modal test and the finite element analysis are also compared.
From the comparison results, it is verified that the natural frequency and mode shape investigated through the modal test are very similar to those obtained from the finite element analysis. Also, it is confirmed that the frequency response curves obtained from the modal test and the finite element analysis are similar. Therefore, the validity of the modal test method with the finite element analysis is confirmed. In addition, the results of free vibration characteristics according to water level tell that the natural frequency of the cylindrical tank decreases as the water level increases. Finally, when a launch vehicle uses liquid fuel, natural frequency increase as the fuel consumption increase and some vibrational mode such bell mode can also appear during the launch sequence.