317 kg. The estimated MGA brought the total to 13.7 kg. The estimated MGA was validated when the  assembl y’s mass ended up at approximately 13.4 kg. This final mass even included a deviation after CDR  that added more mass to the interface plate between the SCS and the RSA to increase the stiffness of the  interface.   A limit on the use of high melting point materials limited the mass of titanium that could be used in the  design. For thermal reasons, a titanium bracket was used as an installation bracket for the microwave  feedhorn. The mass of this bracket was only around 150 g. Other titanium parts throughout the design   added roughly another 150 g for a total titanium mass of 300 g. In hindsight, there was room in the mas s  allocation to add titanium mass to other areas such as bearing shafts and housing s. However,  adding more  cantilevered mass to the design reduced the natural frequency of the system. Also, preliminary test results ,  described later, provided confidence in the design choices  made through CDR .  

At PDR, the q ualification temperature range defined was  relatively benign; ranging from - 45°C to +55°C. Prior to CDR,  this range was extended with the upper limit moving to +70°C.  Nevertheless , the thermal environment was not considered a  huge obstacle for the mechanism.    Alignment  of the mirror over the temperature range ultimately  played into tolerancing and fits on the SCS bearing assembly.  The temperature range over which the positioning requirement  applied was reduced from qualification to Flight Acceptance  (FA) temperatures. This  change was a mitigation approach to  reduce the preload stresses in the bearing by reducing the  magnitude of interference fits across the temperature range.   Meeting the first mode requirement was by far the most challenging part of this design effort. Margin of 20  Hz above the first mode frequency  was design ed into the system  to ensure the SCS could meet the 100  Hz first mode requirement  once integrated into the larger system. The mass of mechanisms  and mirrors   made 12 0+ Hz a difficult target . This is because the cantilevered nature of this system drove the center of  gravity away from the interface ( Figure 3). This led to a constant battle between function, mass, and  stiffness. Additional design challenges came from a relatively high random vibration requirement  at the SCS  interface which started at  16.8 grms , but later reduced to 14.1 grms shortly before CDR .  Risk Reduction Effort   To proactively reduce risk , Brassboard versions of the thin section bearing assemblies were designed,  procured, assembled, and tested. The  primary objective of this effort was  to reduce uncertainty in the drag  torque of the bearing assembly  across the operating temperature range.  This was important because the  control approach for this mission was to perform open loop control over the rotation of the Secondary Mirror  using a fixed voltage input into the motor , and s cience was sensitive to the rotational velocity of the  Secondary Mirror . Because of this, a  require ment on  the velocity  was written to specify an operational  band  that ranged from 1.5 deg/s to 2.9 deg/s.  Predictions on velocity prior to PDR showed little margin  compared   to the velocity requirements over the operating temperature range of the system ( Figure 5). With  so little  margin against velocity , predictions on friction were important to validate.   The Brassboard assembly consisted of the output bearing assembly, input bearing assembly, torque  transducer, motor and structure  (Figure 4). Config uration of the output bearing assembly  (designed to  support the mass of the Secondary Mirror)  was a back to back duplex pair of angular contact bearings with  a spring preloaded  outrigger  radial bearing to help support moment loads. A back to back duplex pair of  angular contact bearings were also selected for the input bearing assembly. In this case, these bearings  were separated by match ground spacers . Requirements on the input bearing assembly were much more  benign because this assembly did not have to support the mass or position requirements of the cantilevered   Figure 3. Center of Gravity of SCS