Identification of Cyclically Symmetric Resonance in Experimental Data for Engine Failure Analysis

A small crack was noticed during inspection in mid-2002 in the liquid hydrogen feed system flowliner of the Space Shuttle Main Engine. To investigate the hypothesis that the cracking was caused by resonance, a comprehensive structural and fluid dynamics analysis and test program was initiated. Evaluating the structural aspects of the problem required the development of an innovative multi-step methodology discussed in this paper. Due to orthogonality, resonance occurs only when the integer wave number of the mode equals the integer wave number of the excitation, as well as when the excitation frequency is close to the natural frequency of the mode, so fluid physics personnel initially characterized the amplitude, frequency, and shape of the excitation. Secondly, a Matlab ® Fourier series decomposition algorithm was written to break down structural modes derived from modal test and strain-gage derived operational response shapes obtained from hot-fire testing into their constituent wave numbers. A routine was then written that could be used for animation of the hot-fire strain gage data at specific resonant response frequencies. Finally, a comparison was made between the Fourier decompositions, operational response animation, modal analysis and modal test results, and excitation characteristics to pinpoint which modes were being excited. This information provided the proof of the resonance condition and also assisted in identifying helpful modifications to the excitation and structure.