115 bronze sleeve between the cup and the housing, but the review board deemed such a drastic change to be  too risky.  There was no available volume for the additional  thickness of a sleeve.  Despite being considered  a risk, t he original coated and lubricated titanium parts remained in the design.         The clamshells were already slated  to be integrated late in the SC  I&T schedule, so the delays associated  with this investigation and corrective action necessitated a c onsiderable amount of re- planning and  schedule modification. Though EM2 clamshells were present in two locations on the flight structure during  SC vibration testing, the new flight units had to be qualified individually, separate from the SC . The final  integration of the clamshells occurred in earl y 2018, weeks before the full SC  TVAC test. This was a hard  deadline; four  deployed clamshells had to be present on the SC  to gather relevant thermal balance data.  Final alignments of the complete ARS system were performed months later, in May 2018.   

Conclusion  and Lessons Learned  

There is an adage that states generally, “All tes ting failures are good failures”, and this anomaly is a perfect   example of the truth in that statement . Though severity of this  issue can be debated, t he marginal nature  of the failure (the pinpuller almost had enough force to overcome the cold- welds) presents a potential  scenario that had this failure not occurred when it did, it could have resulted in a much larger problem  later  in the mission. Had the V3 clamshell deployed as expected, the damage may not have been detec ted until  it was too late. That  damage could have further compounded during SC  vibration tests and launch, resulting  in a worst -case scenario wher e the mechanism didn’t deploy when it needed to in space. In addition to a  partial loss of Level -1 mission requirements  on a flagship NASA mission, the failure investigation would  have been impossible to complete with certainty. A lack of comprehensive inspection data, an d the parts  impossible to recover, only postul ations could be made about a true  final root cause. Ultimately, this  anomaly and subsequent investigation resulted in a more robust , and reliable mechanism that enabled  mission requirements to be met.           

Primary Lessons Learned:  

• Avoid dynamic titanium on titanium interfaces, and do not rely on surface coatings  and lubrication   to fully mitigate galling and cold welding effects.  

• Fully inspect all size and form critical mechanism components .        • Anomalies can beget additional anomalies, which can become red herrings and distractions in an  investigation. This was the case with the  EGSE- damaged pinpuller.  

• Take steps, however conservative, with GSE to protect the hardware.  Both MGSE and EGSE used  during this development exacerbated the anomalous  conditions . 

Acknowledgements  

The fantastically supportive and professional Parker Solar Probe development team at the Johns Hopkins   University Applied Physics Lab.  

References  

1. Parker Solar Probe, parkersolarprobe.jhuapl.edu/index.php#the- mission.   2. “Pinpuller.”  TiNi Aerospace, Inc., 27 Oct. 2017, tiniaerospace.com/products/space- pinpuller/.   3. P. D. M iller and J. W. H olladay . " Friction and Wear Properties of Titanium."  Wear, vol. 2, 1958- 1959,  pp. 133.