132 was a significant risk reduction effort; therefore, the flight bits were installed in the FM1 BC just prior to  delivery to ATLO .   

While the FM1 ACA was in ATLO testing, the FM2 ACA was being assembled . The FM1 and FM2 ACA  units were intended to be identical, but this did not occur due to findings from the FM1 ACA thermal vacuum  test. A modification to the linear stage mechanism on the FM2 SHA was implemented to improve robustness   over temperature and life. Because of this improvement, the FM2 SHA was designated the flight unit, and  ultimately the decision was made to upgrade the FM2 ACA to flight  as well . This decision was not made  lightly and would mean that the FM2 ACA would not participate with the flight Rover at environments until  actual launch and surface operations at Mars . This was deemed acceptable for the following reasons:  1)  ACA units are interc hangeable in the Rover at the three bipod interfaces, with no mass impact or structural  feature differences between the FM1 and FM2 units, 2) the FM2 ACA was exposed to a complete  environmental test program at the ACA -level, which included 3- axis random vi bration and thermal vacuum  functional testing which incorporated full end -to-end sequences (with the exception of bit exchange and  sample acquisition operations) , and 3) after the FM2 ACA is installed in the flight Rover in ATLO, a ground  test was conducte d that include d an end- to-end functional test within the ACA using a non- flight sample  tube, hermetic seal and seal dispenser hardware, which exercised/verified force and position sensing using  flight avionics .  

Because the FM1 BC had been verified  in the Rover thermal vacuum test with the flight bits installed,  and  is interchangeable between ACA units, the FM1 BC was designated the flight unit . Removal of the B C from  the ACA is required as part of Rover integration/de- integration, so an interface break is  necessary  regardless . As a result, the eventual flight ACA will be a mix of FM1 and FM2 ACA hardware. Maintaining  a consistent test flow and history was critic al for understanding mechanism life as well as for  documentation/configuration management purposes.  

Due to PP and CC constraints on SIH and SHH items, additional STSA and DVT assemblies were required  to support ground testing and not compromise cleanliness of the flight hardware. In addition to an EM and  QM STSA and DVT assembly, an ATLO flight model was developed to support Rover level activities in  ATLO, including environmental testing, while a sterile flight model was established for flight with extensive  PP and CC oversight to ensure stringent cleanliness constraints are being adhered to. To verify functionality  of the sterile flight model hardware prior to flight, a test program was  conducted that exercised sample tube  interac tions with sheaths and bits, as well as seal dispensing at both ambient and Mars temperature and  pressure conditions . The sterile flight model assemblies were also exposed to a 3-axis random vibration   test for launch dynamics verification. Because of the s tringent cleanliness requirements that had to be  maintained for the hardware, clean tents and special handling requirements  were implemented. At the  completion of the test campaign, the hardware went through a final cleaning operation followed by bakeout. The sterile flight model hardware is integrated into the Rover as late as possible prior to launch, directly  from the bake- out, and requires a clean air purge within  the ACA volume up to the moment of launch to  maintain cleanliness . Functional testing onc e integrated in the Rover is prohibited  to avoid compromising  cleanliness . As a result of this limitation, the QM STSA and DVT assembly will be exposed to the flight  bake- out as well followed by testing at Mars pressure and temperature in QMDT to verify functionality and  performance by similarity.  

Challenges  

The ACA has had to overcome a number of significant design and test challenges, including volume  constraints, tight tolerances , hardware cleanliness and operational  issues . As a result, quite a bit of planning  and re- planning was necessary to ensure requirements could be met, and the right tests were conducted.  Being flexible is key when working in this mode. Having two ACA flight models, FM1 and FM2, and the  ability to swap hardware between t he two, allowed for  some flexibility . As previously noted, the final flight  ACA configuration is  a combination of FM1 and FM2 ACA stations.