491 Microswitch Testing Defects   Microswitch es are used throughout the SpaceX docking system as position indicators. In addition to the  soft capture petal switches, which are used to declare capture, and the ready to hook switches, which  indicate when the IDA hard capture plane has been contacted, s witches are used within  the attenuator  arms and the hard capture hooks to indicate the mechanism state.  

Durin g system and sub- scale testing, microswitch failures were observed. Investigation revealed fracture  of internal components to the microswitch caused during shock events. These failures lead to the selection  of a different series of microswitches with fully potted internals that were more robust to shock events. This  was an important lesson learned in assessing the robustness of commercial of the shelf components to  shock induced failures. Additionally, it was found that the harness stiffness was such that was subject to  fatigue failure modes, even from the most careful of handling. A less stiff harness material was used to  prevent this failure mode.  This was an important lesson in selecting harness potting materials that are  robust to nominal handling.  

Figure 11: Fracture of Ceramic Contact Pin in Shock  (left) and undamaged switch  (center) ; harness  damage (right)  

Inspection  Lessons Learned  

In addition to thoroughly testing  hardware, the SpaceX docking system has gone through intensive  inspection campaigns . Visual inspections and tear downs are a critical component of qualification  campaign. SpaceX has also greatly increased its use of Computerized Tomography (CT) inspections. This  allows hardware internals to be inspected without a tear down that inherently changes the condition. CT  inspection of hardware has proven to be an immense resource for both failure investigations  as well as  identifying potential failure modes . Two examples of SpaceX’s use of CT scanning are outlined. It is  important to note that CT scans do provide radiation dosage to active components. A dosimeter scanned  with a development unit can estimate this dosage to determine impact on the unit’s usable life.  

Retaining Ring Vendor Verification   After qualification testing, the eddy current damper  was CT scanned, though it performed nominally  during  the campaign. The scan revealed a significant defect : a reta ining ring had become dislodged from i ts  groove. If settled in a slightly different position, t his retaining ring could become stuck in the gearbox,  causing the damper to jam. This led to additional development testing, where it was  discovered that only  improperly seated retaining rings were at risk of becoming dislodged; properly seated retaining rings  survived qualification environments  as indicated by pre- and-post environment CT scanning. Through this  discovery, SpaceX worked with the vendor to implement  robust  quality control practices on retaining ring  install. CT scan videos are stored for future evaluation, which proves  useful if a different failure mode was  discovered: videos can be reviewed without requiring hardware to be removed and re- scanned. This was  an example where CT scanning enabled discovery of  a potential problem without experiencing a dramatic