© 2016 Sierra Nevada Corporation   366 Table 1.  Example of Various Hardware Test Configurations  

              Note: Use of  the term  ’diving board’ is synonymous with output cable bundle and clamp    The second issue with the historical data involved accelerometer placement and related availability of response data. Standard vibration tests include application of strategically placed ac celerometers on the  exterior SADA surfaces and surrounding test fixtures to recover selected mechanism dynamic responses   during test ; this  placement on the exterior is typical. Other more sensitive components are housed within  an enclosed structure to protect them from contamination and handling prior to launch. These sensitive components are very seldom accessible,  making instrumentation and recovery of dynamic response  difficult . Because this short circuit anomaly involved dynamics of sensitive internal SADA components, the  lack of sensors and response data nearer to the brushes raised concerns regarding FEM correlation accuracy .    To address th ese two challenges , a tailored low -level vibration test ( -6 dB) was conducted on flight  hardware reworked to as -designed configuration ( i.e., verified  slip ring rotor preload and properly  repositioned and aligned brushes ). The test article was instrumented with strategically placed  accelerometers  consistent with the orientation and position defined in protoflight test setup documentation  and found during data review of historical tests  with one exception. One additional accelerometer was  tempor arily installed deep inside the SADA on the slip ring rotor shaft . This additional sensor was critical  to SNC’s understanding of the SADA response within the mechanism nearer to the brushes. T est results   were then used to further refine the FEM and complet e the development of this test -correlated predictive  tool to within 1% of measured values .  

Figure 8 illustrates an example of results from a lateral (z -axis) low-level random vibration test compared  to simulated output of the refined FEM. As shown, the peak level and resonant frequency of t he rotor  shaft for both the test and predictive model align very closely around 650  Hz, which indicates a highly  refined FEM model. All remaining axes were test correlated in a similar manner to complete the effort. Armed with a newly -developed, high- fidelity test correlated FEM, SNC was able to analyze earlier  anomalous tests and confirm low brush force margins when tested to the specified levels. FEM model predictions using baseline protoflight test levels shown in Figure 3 confirmed that the slip ring rotor shaft  modes closely aligned with the SADA primary structural modes. Resonance at this aligned frequency  significantly increased the likelihood of very high peaks (6 sigma and greater) and explained higher -thanexpected brush contact point response loads. Figure 9 illustrates the first mode of the SADA structure at  the far end of the cantilevered assembly (i.e., motor end) aligned with data collected on the interior of the  SADA at the end of the slip ring rotor shaft nearest the brushes that shorted. Analysis results confirm that when tested to the required protoflight vibration levels while constrained in test fixtures as defined in related protoflight test procedures, brushes would exceed preload and gap with as little as 3.8- sigma  events. And as discussed earlier , greater than 6- sigma events were not uncommon. This finding clearly  supports the statement of cause noted below.  

Primary Physical C ause:  Protoflight  testing to specified vibration levels induced loads into SADA and  related slip ring brushes sufficient to overcome brush preload and force brushes to adjacent slip ring circuits. This condition resulted in ring -ring short circuit.