392 Table 14.  HST On -Orbit Acceleration Results; RWA Primary Harmonic (4) Inputs -to-HST Locations (3 )20  HST Prin. Direction  Forward    Equip.   RSU Near 

 Distance from HST C.G.  194 inches  57.2 inches  81.3 inches   RWA Speed = 3 Hz (180 RPM)  - - -      V1 RMS 0.08 µg  0.18 µg  0.10 µg       V2 RMS 0.33 µg  0.30 µg  0.11 µg       V3 RMS 0.05 µg  0.49 µg  0.08 µg   RWA Speed = 10 Hz (600 RPM)  - - -      V1 RMS 4.45 µg  8.17 µg  7.76 µg       V2RMS 8.62 µg  13.23 µg  5.63 µg       V3RMS 5.44 µg  3.41 µg  7.24 µg  

Systems Engineering (SE) Introduction/Overview f or Basic RWA  Sizing and Affordability for Jitter  

Looking ahead to future  uses and affordability issues,  spacecraft SE RWA choices for jitter -performanceversus -cost prompts many considerations. Because 1) costs are critical overall pro gram drivers, 2),  ACS/GN&C subsystems are a notable contributor to satellite costs,  3) RWAs are an essential stabilized  spacecraft need, 4) RWAs are, on a mass -sizing basis, one of the more costly single components , 5)  supplier alternatives  are a relatively small set of options to push for  competitively low er pricing,  and 6) cost - performance trades are a basic system engineering tool to determine component selection, there is a need  for quantitative cost evaluations . To a first order, RWAs are chosen (allowing for adequate margins in  power, mass, angular momentum, etc.) as s mall as possible via two key values:  1) spacecraft size ( i.e.,  maximum spin -axis-control inertia), and 2) maximum required slew rate. However, considering stringent  jitter limits, RWA tradeoffs become more complex. Smaller RWAs otherwise capable of the basic station - keeping/pointing mission commonly have several inherent deleterious impacts:  1) smaller RWA designs have smaller parts ( i.e., bearing components and reduced dimensions) that impact efforts to keep  tolerances low (without other compensating eff orts) and raise harmonic ratios’ frequencies ( i.e., see prior  304H -vs-101H), 2) smaller RWA mass lowers local reaction mass for mitigating prime IV disturbance  energies originating at bearing imperfections, 3) higher spin speeds for a target angular moment um value  forces IV amplitudes ( i.e., non-spin- axis harmonics) to increase significantly ( i.e., by speed squared) that  also drives more responsive transmissibilities, and 4) smaller rotor fine- balance adjustments are often more  challenging. A seemingly  straight forward more- affordable choice for minimal -size RWAs, with upfront  acqui sition cost savings, can actually  translate to higher  final cost to attain and successfully verify IV limits.  Figure 12 shows NASA -mission subsystem percentage costs and simil ar relationships for other large  satellites 21,22. Note that, very generally, atypical elements of NASA’s widely -varied missions (planetary  landings/fly -bys, manned space, solar monitoring, etc.) can drive more complex orbital mechanics/GN&C  rather than RWA  costs compared to GEO satellite platforms driven by station- keeping commonalities , e.g.,  normal transfer -to-orbit plus pointing stability . Although a complex military satellite program can drive costs  from roughly three t o eight times higher than basic co mmercial/NASA missions due to demanding  requirements (reliability, threat hardening, etc .), proportions shown in Figure 12 are roughly scalable and  percentages are representative. Note that ACS is non- trivial: typically of the order of 5- 8% of mission  costs21,22.