© 2016 Sierra Nevada Corporation   372 Complex mechanical systems require a significant amount of scrutiny to ensure all fundamental  frequencies are characterized and understood. This is especially true for systems that contain compliant  structure housed in a very rigid primary structure . In the case of this SADA, the design guideline to  evaluate dynamic loads against 3- sigma events for certain components seemed appropriate during the  design and development phase. However, the investigation that followed the slip ring short circuit  anomaly  clear ly identified a need to look beyond this 3- sigma guideline and apply a much higher  standard, closer to 6 sigma . This need to increase load margins must be balanced with associated  increased costs as well as other technical trades that may drive design complexity and technical risk.    During any design evaluation phase to consider whether -or-not increased load margins are required, test  boundary conditions deserve a very critical review. Test boundary conditions can significantly impact technical trades and d esign decisions. Often, as seen in this case, component boundary conditions during  protoflight or acceptance vibration testing rarely represent the as -integrated final flight configuration with  the spacecraft. Understanding these differences and working to tailor test requirements prior to test to  match component input levels (often at test specific resonant frequencies) can save countless investigation hours and wasted costs.  

Familiarization  with all forms of test and hardware evaluation technology  is also beneficial . During this  investigation, the team utilized complex force- limiting vibration test methods, high- fidelity test correlated  FEMs, and advanced technology 3D radiographic imaging to resolve a very difficult technical challenge. However, the selection of applicable  technology (new or old) is a far more important consideration than  the use of new high- tech methods and equipment in general. For example, this investigation warranted  the use of 3D imaging because the problem confirmation and re- test validation would have been  significantly more difficult without it. This may not be true for future programs when reviewing new  designs   where alternatives to this technology, such as design modification to include view ports, may be more  appropriate.   

As with any complex problem, it is important to exhaustively consider alternatives, utilize technology  appropriately, and support highly collative design experiences.  In this case, SNC successfully did this to  link high brush loads encountered during pr otoflight vibration testing to high SADA resonant peaks  atypical of flight -like conditions. Findings from extensive data review, analysis, and design assessments  highlight the need to thoroughly understand the impact of test boundary conditions and related   mechanism resonant modes when designing complex mechanisms . Brush preload, beam shape and  cross section, rotor support structure, and fundamental mechanism frequencies all interplay to present  significant design and test challenges.     Despite the challenges faced during component level testing, SNC remains convinced that the SADA  design is well suited to meet all mission requirements. To date, seven of these SADAs are successfully  performing all required on-orbit operations  and have accumulated more than 35 years of total operating  time.