380 effects,  e.g., more optical -mode complex cross- couplings.  A highly -iterative effort , involving  stress, design,   and manufactur ing engineers, eventually found a viable design approach meeting  strict mass limit s.  

As a related aspect, significantly stiffer base/mount (in local DOFs)  and improved knowledge of  RWA highfrequency output s initiated  a quick -response RWA vibration isolation feasibility assessment . Those results  led directly  to devel oping a unique fluid- damped passive vibration isolation system  (e.g., Fig. 4) , applying a  pseudo- kinematic interface approach while retaining HST Orbital Replaceable Unit capabilities  via  astronauts using standard tools8,9. Findings and techniques were later applied to large aeronautical and  aerospace systems12,13. Insights  also directly  caused investigation of I V high-frequency harmonics, how  IV  is modeled/bounded, sources for  IV variation  amount  evidence, and data regarding IV measurement s of  ground- vs-orbit uncertainties/variations14. Concerns also  emerged about  the potential for a dense field of   IV harmonics  (see Fig. 5 rightmost resonance z one) to converge via a non-obvious combination of  input  forces -moments to create  a “perfect resonance storm” , collectively stimulating a reactive high- frequency  complex mode involving several  smaller optical components. Data on r ealistic  damping values  were  also  investigated due to emerging evidence that classic high-amplitude damping  approaches  (i.e., elastomerics)   tend to substantially  diminish  at low force levels ; prior analyses  used damping derived from  high- amplitude   forces ( ~3%; i.e., launch- level vibrations) . Fortunately, due to efforts attacking 4 jitter priorities  (above),  IV  test setup detection  capabilities  could effectively collect  sufficiently sensitive amplitude- spectral data with  minimal modifications  thereby  avoiding yet another added  development -improvement effort. The author  had sole responsibility for dy namic modeling, model validity , substantiating damping data,  RWA isolation  system requirement development, and jitter  result  accuracy verification duri ng this time . Regularly , separate  high-detail  dynamic models were  rapidly created for  a key  subassembly  or unit to then integrate a simplified  version  into the full HST model  that closely replicated  local mode frequencies/ shapes. Efforts  included  successful  conduct of a thorough model audit /review with NASA Subject Matter Experts . Due to such   attention/ refinements, subsequent HST  modal tests concluded that analysis results  compared very well  with the pre- test structural dynamic model2. For a period,  circa 1985 , this Level 1 RWA -IV-based LOS jitter  requirem ent threat w as the foremost program  technical  concern, receiving  considerable attention and  visibility  at quarterly reviews.  Although not a heroic save of HST performance, its importance is notable . 

RWA IV Disturbance Source Considerat ions Relative  to Classic Wheel Bearing Imperfections    HST RWAs are deemed state -of-the-art (for their size)  due to several unique as pects:  1) strict  screening of  matched duplex bearing sets via preliminary runout measurem ents and added data checks  acquired  in preacceptance runs on  a fine -balanced EDU  rotor (e.g., <50% yield for flight -qualified ABEC 9 production lots) ,  2) unusually high  number of  fine-balance  iterations , 3) attention to spectral signature/trends  in fine -balance  iterations , and 4) lubricant  (derived Andok C ) tailoring  for robust stability in HST operating conditions.  The  RWA vendor was highly motivated by a potential to advance existing knowledge of low -noise wheels.  IV  arise s from 3 primary imperfect root causes :  1) rotor -bearing assembly  mass  unbalance/asymmetries, 2)  minute bearing/race/retainer imperfections, and/or 3) driv e configurati on/physical forces. These can be   separately differenti ated,  but, as combined, create the total disturbance output spectral  signal.  In theory ,  using first-principle fundamental physic s, precise definition  of bearing parameters/geometry, tolerance  build- up, and mis alignments would allow near -perfect knowledge of all detected IV ratios and provide  some  indication of  relative intensities. However, in practice,  spurious microscopic -level effects ( e.g.,  lubricant/tribological interactions, surface conditions/finish, temperatures, minute ball bearing preload fluctuations, etc.) manifest as  deviations in internal contact -surface placements and amplitude excursions  which exponentially grow with rotation speed,  but are not possible to consistently predict or practicably   ascertain in use. If  a candidate geometric -based imperfection(s) is below a readily  meas urable threshold  (or is excessively difficult/ costly to inv estigate  or measure further ), a deeper qua ntitative root is not tractable;   e.g., ball be aring/ cage localized out-of-roundness  or duplex -pair-to-duplex -pair eccentricities . 

Primary Static -Dynamic  Unbalance on Rigidly Mounted Rotating Masses  as Applied to RWAs    Rotor balance and related  bearing stability/dynamics  is well studied  and analyzed 15,16 and summarized here  for high- precision RWAs . Figure 7 shows key parameters  and balance quality  grades . Once specifying a