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Fig. 25c Two views of the third vibration mode from the STAGS model. The modal vibration frequency predicted by STAGS is 13.46 Hz. This mode is a combination of shell “breathing” (n = 0 circumferential waves) and axial motion of the propellant tank approximately as a rigid body with significant tension and compression in the supporting struts. The axial tank motion with associated tension in the lower set of struts and compression in the upper set of struts can most easily be appreciated from the image on the left-hand side of the figure. In that image the axial component of the total motion of the tank is upward in the plane of the paper, as can be seen by the deformation of the aft struts (elongation) versus that of the forward struts (shortening). The axial motion of the propellant tank in the GENOPT/BIGBOSOR4 model is shown in Fig. 16a(A). It is associated with the modal vibration frequency 12.069 Hz, which is the lowest frequency determined by the GENOPT/BIGBOSOR4 model.
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