inside of the stator, minimizing the impact on the overall assembly height. Additionally, the chosen grating  has a resolution of 7,200 counts per revolution, which on its own leads to an angular position knowledge of 0.05 deg (3 arcmin). Coupled with the 4x interpolation of the encoder head, the system has a total of 28,800 counts per revolution, providing angular position knowledge down to 0.0125 deg (0.75 arcmin). The Aeroflex *CAY-1398 miniature slip ring capsule was selected to provide the electrical interfaces  between the bus and payload.6With a diameter of 0.95 cm (0.375 in) and an overall length of around  2.5 cm (1 in), the compact size of this unit allowed it to be mounted within the shaft of this motor assembly. Additionally, its twelve electrical lines exceeded the minimum of eight set by the design  requirements.  The final COTS component is the bearing. Given the assembly volume constraint and the height already  occupied by the stator, cover plate and encoder head, it was determined that only a single bearing could  be accommodated in the assembly. This led to the selection of an NHBB *deep groove, thin-section radial  ball bearing,7as it did not need to be mounted in a duplex configuration like angular contact bearings. Simulations of the spacecraft attitude control system were used to determine the maximum static  imbalance of the rotating payload that the system could tolerate before control authority no longer met mission-required levels. From this, the team was able to determine that the CG of the payload had to be within 1.3 mm (0.051 in) of the axis of rotation. 1The selected bearing is manufactured to Class 5  tolerances as specified by the Annular Bearing Engineering Committee (ABEC 5), which allows for amaximum radial runout of 0.005 mm (0.0002 in), well below the static imbalance requirement. 8To prevent  a loss of contact between the inner race of the bearing and the shaft that would be caused by differential thermal expansion between the steel and aluminum, a stainless steel ring was added to the shaft to serve  as the bearing mounting surface. Its diameter was sized to provide a light interference fit of 0.0025 mm  (0.0001 in), as recommended by the manufacturer. Furthermore, the height of the mounting ring was  sized 0.13 mm (0.005 in) shorter than the bearing height, allowing the inner race of the bearing to be compressed by tightening the ring used to axially retain the bearing on the shaft. The deformations to the inner race caused by the interference fit and the compression serve to preload the bearing and mitigate some of the inherent radial play. Design Units The development cycle of the scanner assembly leveraged the use of an iterative design process, using  stages of prototypes and engineering models such that any lessons learned and design changes in the current iteration could be integrated into the following iteration. Each prototype served as a risk reduction  measure, proving different aspects of the design (and therefore increasing TRL) with each iteration.  3-D Print Unit Once the basic design of the scanner assembly was established, a 3D printer was used to fabricate  mockups of each component (shown in Figure 5). This was done early in the design process in order toidentify any issues that may have been overlooked during the initial design. The component mockups were used to visualize the physical envelope of each part in the assembly, check the fit between components (especially those that were custom designed to interface with and between COTS  components), visualize the volume of the overall assembly, and most importantly, verify that the unit could be assembled.  *New Hampshire Ball Bearings, Inc., Chatsworth, CA 527