5 Space Launch System Mobile Launcher Modal Pretest Analysis 43 Fig. 5.11 Autocorrelation function of ML Shaker Forces From each MSFC provided ML shaker force time history, five uncorrelated ML shaker force time histories were generated for use in the modal pretest Multi-Input Multi-Output (MIMO) force response analysis. Since the data processing frame length used to process the force response analysis time histories into PSD, FRF, and coherence significantly exceeded 5 seconds, a “Slinky Approach” was used to generate five uncorrelated shaker force time histories. ML shaker #1 force time history was created by taking the original recorded lateral shaker force time history and appending it to itself as many times as necessary until the assembled time duration equaled or exceeded the desired time duration. ML shaker #2 force time history was generated by lopping off the first data processing frame length of ML Shaker #1 time history and appending it to its end. This process repeated for the remaining ML shaker force time histories. Because the data processing frame length was significantly longer than 5 seconds, lopping off only first 5 seconds would have end up with the five ML random shaker force time histories being correlated and having very high coherences. This is due to each ML shaker force time history essentially being time delayed versions of themselves [37]. 5.7 Drop Hammer Forces The MSFC test group designed and fabricated a Drop Hammer to supplement the ML shakers excitation of the ML Deck to help identify higher frequency ML Deck primary target modes. The Drop Hammer can shape its impulsive force, both in terms of peak force and frequency bandwidth, by adjusting its drop height and shock absorber. MSFC test personnel recorded an impulse train from four drops of the Drop Hammer in their laboratory. These four impulse Drop Hammer force time history, after removing transients due to resetting of the Drop Hammer between drops, were converted into a 16 impulse Drop Hammer force time history having the same sampling frequency as the modal pretest analysis force response analysis. Validity of the 16 impulse Drop hammer force time histories was verified by comparison of its PSD’s to that of the MSFC supplied four impulse Drop Hammer force time history. 5.8 ML Shaker and Drop Hammer Suitability Study This study was performed to determine if the locations and orientations of the ML shakers and Drop Hammer were adequate to excite all target modes. FRF were synthesized from the FEM modes assuming 1% modal damping, since this was believed to be the lowest modal damping level present during the ML Only modal tests. A spectral resolution equal to 1/tenth the half power bandwidth of the lowest frequency target mode was selected to ensure all resonance peaks in the FRF were well captured. Normal Mode Indicator Functions (NMIF) and Multivariate Mode Indicator Functions (MMIF) were computed on these synthesized FRF to determine if the ML shakers and Drop Hammer were adequate to excite all of the target modes. NMIF and MMIF values below 0.25 indicated potentially well excited target modes, and between 0.25 and 0.4 indicated
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