134 While ensuring the ACA functions  properly has been a challenge on Earth, this is only exacerbated by  having to operate in a Mars environment where extreme temperature differences as well as exposure to,  and operations within, a dusty environment are commonplace. With an operating range of -110°C to +50°C,  this can result in significant  CTE effects  and/or  gradients , especially due to heat -to-use conditions at cold,   that need to be accounted for to ensure proper fits and functionality . Testing at Mars temperatures was  conducted to verify hardware functionality and performance, which identified a number of issues . One  example is significant drag increases at cold temperatures due to Teflon spring- energized seals  used in  dynamic applications , which are implemented in a number of locations within the BC. Ultimately, the springs  were removed from the seal, leaving the Teflon jacket , which still exhibited too much drag on its own.  Therefore, the jacket was cut to allow for expansion and contraction significant ly reduc ing drag, while  not  detrimentally affecting the performance of the seal . These changes were successfully validated by test .  Using spring- energized seals in a dynamic application over a large temperature range, and particularly cold,  must be evaluated closely for potential signs of fit or drag issues due to CTE.  

Because of cold temperature exposure, all of the wet lubricated mechanisms require heating for operation,  with a minimum allowable heat-to-use temperature of -70°C used for ground testing. Braycote 600 and  601EF are used in various ACA mechanisms, which were found to contribute to high drag conditions  impacting performance  at cold temperatures  (starting below approximately -40°C) . The SHA and EE both  contain high life linear motion mechanisms where cold temperature testing revealed operational issues  resulting in warmer heat-to-use temperatures being prescribed. These changes are implemented to  preserve mechanism life and/ or to support mechanism functionality with appropriate force/torque margins  for operations on Mars .  

During ACA testing in thermal vacuum at Mars pressure and temperature, a number of sample tubes  failed  to insert properly into bits and sheaths at - 110°C . These failures did not occur  during Earth ambient testing  using the exact same hardware prior to thermal vacuum. An investigation identified two key findings. The  first was related to the tight clearances between the tube and sheath/bit, which were required for proper  ball lock mechanism functionality . However, system errors due to factors such as tolerances, deflections,  controller errors, etc. exceeded the clearances of the mating hardware. In addition, the geometry of the  hardware included non- axisymmet ric features  which offered catch opportunities due to the tight clearances   and resulting machining. As a result, the compliant mechanism on the EE struggled to compensate.  Performing lateral offsets  prior to sample tube insertions favorably aligned tube features with the bits and  sheaths  resulting in successful operations . While the operational fix was successful, being mindful of what  a compliant mechanism is being asked to do within a very constrained design might lead one to different  implementation strat egies.  

The second key finding was related to a critical  hardware configuration that was incorrectly modified for  testing . The EE contains a cable bundle that exits the force- torque sensor and was strain relieved with  specific tie -downs such that the cabling would not affect EE operations , including at cold temperatures   where stiffness could increase by about a factor of two. For the thermal vacuum test, thermocouple (TC)  wires were routed along the EE cable bundle and tied down in a few locations . The TC wires were added  just before the thermal vacuum test and therefore not part of any ambient testing prior to environments . As  a res ult, the EE RCCM ended up being stiffer than the intended design point  at cold temperature, which  contributed to tube insertion failures . The test was actually stopped to allow for correction of this issue once  identified . The cable bundle tie downs with t he TCs were removed and the test continued successfully .  When instrumenting hardware for a test, be very careful that the introduction of test components  (i.e., TCs  and their respective cabling)  will not impede expected functionality of the hardware under test .  

Friction values were found to be much higher than expected during testing with hardware that had been  exposed to both stringent precision cleaning and the high temperature bake- out at 350°C in air for 1 hour  (for combustion cleaning) . This was determined during FM2 ACA testing using  non-flight sample tubes and  gloves that had been processed flight -like (cleaning and bake- out) and failures in the ball lock mechanisms  were observed. This was the first opportunity to test hardware processed in a flight -like manner . The final  flight hardware cleaning and bake- out processes were developed concurrently with hardware development .