© 2016 Sierra Nevada Corporation   368 also c onsidered improvements to mechanically constrain brushes to keep them within assigned rings  and  below insulation barrier s. This modification could either be implemented with a deployable launch lock  configuration or by sn ubbing brushes to limit  stroke once install ed and assembled in the ir assigned  grooves. However, associated  design changes would have required  significant development effort  and  include d an array of additional technical risks. C ost and schedule constraints combined with related  technical risks  ultimately  made the feasibility of fundamental design changes to  fully assembled and  qualified hardware in the middle of final acceptance testing very undesirable.  

Without making physical changes to the SADA, the only other viable options were to either re -evaluate  and modify test boundary conditions to more accurately represent spacecraft  test and launch  environments or tailor  the random vibration test profile (via force limiting, notching, and/or other profile  changes ) to limit input loads to  reasonably margined levels. To address the first  potential solution, the  team further reviewed spacecraft test  and launch data and confirmed that the required component test  levels combined with the rigid SADA/fixture boundary conditions resulted in very different SADA input  loads . Review found SADA component level test loads were significantly higher than any loads measured  at the SADA when  integrated with the payload and mounted to the spacecraft. In addition, data also  revealed that the majority of energy input into the SADA during launch and spacecraft level tests occurred well below the SADA resonant  frequency of 650 Hz. These findings confirmed beliefs that component  tests resulted in SADA loads far above of levels needed to demonstrate compliance with launch and  spacecraft test environments .    It is well known that accurate physical simulation of launch/ interface boundary conditions  at the  component level  is ex tremely difficult and complex  and t he team believed that attempts  to lower SADA  input loads by changing test boundary conditions would be very risky. Any change would significantly  lower  the team’s understanding and certainty related to SADA response during test and t he value of a  known response far outweighed  the risks of uncertainty . Major design changes  to the SADA test  boundary conditions  were not implemented. However, o ne minor change was feasible.    To more accurately match cable bundle loads and related SADA rotor shaft dynamic response during test, a minor change to the SADA test boundary conditions was implemented. This in volved  modifications  to the output cable bundle support and clamping method.  Brackets were manufactured and installed on  the SADA to more accurately represent the as -integrated SADA/spacecraft  configuration.  FEM analysis  confirmed the change would improve margins , and any easily -implemented change for the better with  relatively low impact to program cost and schedule made sense.  This configuration difference can be  seen in Figure 2 where the installation of a cable clamp is shown in the image on the right.    Because major changes to boundary conditions were not feasible, the team changed focus to solutions  that involved vibration test profile modifications . Because early test instrumentation did not include an  accelerometer at the cantilevered end ( i.e., motor end) of the SADA, review of available test data  identified  the SADA primary structural mode related to  brush movement to be an axial mode near 800 Hz.  The protoflight profile ( Figure 3)  coincidentally  included a pl ateau out to 800  Hz. The team believed  excessive loads on the rotor high enough to cause brush movement could be minimized by decreasing  the test profile roll-off point  from 800 Hz to 550 Hz. Re-tests to this modified profile did not solve the  problem and related slip ring short circuit conditions persisted. This indicated that the initial understanding  of the brush movement was incomplete.    It is important to note that at the time of this 550- Hz modified profile testing, development  of a high -fidelity   FEM  model was incomplete.  In fact, it was the recurrence of the short circuit anomaly following the   550-Hz test  that caused an aggressive shift in the investigation and ultimately lead to the discovery of the  650-Hz lateral mode discussed earlier.    Rather than continuing to resolve the problem by marginally chang ing test spectrum profiles , output from  the new high- fidelity FEM model was used to justify the application of more rigorous test techniques  and  manage excessive SADA responses . Vibration test profile- notching and force- limiting were two