378 Early HST RWA -Induced Jitter Tenets /Insights Driven by First Principles and  Simplified  Analyses  

Early HST studies , founded in approaches used on classified satellites , were  solely  devoted to  predicting   disturbance effects  on an LOS  central pointing  vector  along with minimizing other known imag e-distorting  effects  (e.g., tight thermal control, PCS -specific sources ) along with large separation of primary structural  mode frequencies  from the maximum active  control bandwidth frequency  (target of  5-to-10x). To support  this, Lockheed’s proposal efforts  built a representative full-scale Structural Dynamics T est Vehicle, a  medium -fidelity prototype/ demonstrator with flight -like structural components’ design, analysis, materials ,  fabrication, and test approaches . It was also  sufficiently versatile to support ongoing development efforts.  This Structural Dynamics Test Vehicle formed a core structural building block  subsequently detailed and   outfitted (circa 1987) as  the Smithsonian’s National Air and Space Museum HST e xhibit article10. Although  RWA  IV forces  are small ( conveniently measured in milli- lb/milli- newtons ) input s to a mammoth 11500- kg  (~25000- lb) spacecraft, HSTs milli -arcsecond- level LOS resolution/stability jitter requirements ( ~100X   below prior satellite limits ) posed a substantial  analytical and testing challenge. RWA’s required finebalance limits  (see below ) relate directly to primary  (1.0 harmonic)  IV amplitudes  at rotation speed(s).   Engineering tasks focused on 4 areas perceived as the most -difficult for achieving stringent jitter levels :  1) High “Scissors M ode” (Fig. 6)  frequency , where in the inner Optical Telescope Assembly  and outer  Support Systems Module  (SSM)  structures  pseudo- kinematical ly pivot in opp osite  directions (pitch & yaw  axes), is highly dominant jitter contributor;  driven as high as practicable (target >15 Hz).   2) RWA rotor size to be  relatively large and kept at low operating speeds  (nominally  <5 Hz) to minimize   primary u nbalance radial forces (believed  highly dominant) to avoid stimulating Scissors M ode jitter.  3) RWA  radial force driven to  minimum attainable l evels for state- of-the-practice- unbalance/I V-setup   detection levels  (<milli- newton)  by applying challenging  unbalance/tolerance minimizing  approaches.   4) RWA bearings were specified to be of proven tightest  tolerances , acquired in same- production lots,  and then checked by fine balancing for acceptable primary balance disturbances. In principle, it was thought  that higher -frequency vibrations were insignificant (and not specified) compared to the paradigm on  runout/unbalance threat s: more extended efforts not considered needed nor practicable.  

Figure 6.  LOS Reflective Optical Path Diagram &  Scissors Mode Dynamic FEM Plot   (Pitch Direction)9,11 

Lessons Learned:  Historical Background and Perspectives  on Unforeseen HST  Jitter Problem s 

Early HST efforts ( rudimentary computing technology ) predicted structural  dynamic disturbances  by use of   laborious  hand calculations and relatively simplistic  calculator/ computer analysis . Finite Element Model  (FEM) static stress analysis was barely  more advanced and transitioning simultaneously alongside classic  hand calcula tions/checks;  as an example,  stress analysis result  uncertainties still necessitated classic  fullscale extensively -instrumented P roof Load tests.  Maturing during HST Preliminary/Critical Design Rev iew  efforts (circa 1978) were sufficiently capable, more readily available, and reasonably reliable Structural  Dynamic modeling -analyses tools to begin to compute end-to-end disturbance/ jitter characteristics with  improved detai ls including, for the first time,  more refined secondary effects. Around 1982,  emergent ray - trace model ing tools ( e.g., employing Zerneke polynomials) provided actual magnification -derived LOS  values for individual optical elements moving/ distorting  relative  to each other and a central ized reference