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Fig. 25a The lowest vibration mode from the STAGS model. The modal vibration frequency predicted by STAGS is 12.19 Hz. This mode is a combination of shell deformation with n = 3 circumferential waves and uniform lateral motion of the propellant tank approximately as a rigid body with significant tension and compression in the supporting struts. The lateral tank motion with associated tension and compression in the struts can most easily be appreciated from the image on the right-hand side of this figure. In that image the uniform lateral component of the total motion of the tank is upward in the plane of the paper. The two components of vibration motion, lateral tank motion and n = 3 shell deformation motion, which are combined in a single mode in this STAGS model, are decoupled in the GENOPT/BIGBOSOR4 model displayed in Figs. 16a and 16b: the lateral/pitching motion depicted in Fig. 16a(C) with a frequency of 12.161 Hz and the n = 3 shell deformation motion depicted in Fig. 16b(F) with a frequency of 13.327 Hz.
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