434 Initial Motion Kickoff Spring   To adhere to f light mechanism design principle s, a kickoff spring to aid in the initial deployment of the flexure   was required to be added after DDR. This leaf spring,  also made out of corrosion resistant steel ( CRES) ,  was placed within the c levis gap of the CDPR’s bracket connected to the aft of the differential , the Horizontal  Swing Arm (HSA)  bracket. With its preload, t he spring was designed to exert  a force on the flexure lug to  minimally displace the flexure  approximately 9.5 mm, or the diameter of the p in-puller pin. Prototype testing   yielded two find ings: first that the leaf spring did not need conditioning, b ut that the end interfacing with the  flexure was digging into the paint  of the flexure and causing the paint to shed. Tw o actions were taken from  this lesson learned:  (1) incorporate a slight curvature on the kickoff spring end contacting the flexure for a  smooth point of contact, and (2)  mask the potential contact area of the kickoff spring and flexure lug side  to avoid any flexure paint scraping.  Lastly, a coating of dry -film lubricate was applied to minimize friction of  the contact surface on the kickoff spring. See Fig. 8 for the kickoff spring configuration with the flexure in  the stowed position.  

The addition of this spring had several benefits for designing a flight mechanism. Not only did  it add extra  deployment force for the mechanism, but also  fail-safe capability in a fault case to  ensure the mechanism  functions  as intended  if the larger and more capable upper torsion spring functionally or structurally fails .  This more robust fault case configuration significantly boosted confidence that the CDPR will work as  required for the critical separation prior to touchdown on the Martian surface.  All these benefits are an  important lesson learned of why it is flight design principle to have an initial motion spring for a deployable.  

Figure 8. Views of initial kickoff spring: (a) view of kickoff spring attached to the HSA bracket, (b) view  inside the bracket clevis where the spring interfaces with the bare flexure lug.  

Travel Limiters   With the new Mars 2020 design, t he larger height and length of the flexure  as well as the rotational degrees  of freedom in the spherical bearings at two ends of the flexure introduced  a few close clearance concerns   in the mechanism.  At nominal (when flexure is engaged or not yet released), the 10.3 mm vertical clearance  between the chassis top deck and flexure was found insufficient in the event if the flexure pitches downward  when it exits out of the HSA bracket during deployment.  During testing without the torsional springs, there  was a roll  rotational degree of freedom in  the flexure, which allowed flexure contact with the chassis bracket  due to residual rotations from the spherical bearings at two ends of the flexure.   

To address these concerns of interference, features on the chassis bracket were implemented to adapt four  (4) vertical travel limiters at the clevis region of the chassis bracket . Two of them are opposite of and  vertically aligned with each other with one above and below the flexure lug that control the vertical up- anddown pitch.  The remaining two are located at two sides of the center lower limiter and are designed to  control the roll of the flexure.  These t ravel limiters are fabricated out of Nitronic 60 for material compliancy  and high surface wear capability and are designed in pin- like shapes with spherical tips, which were also  coated with dry -film lubricant.  A back fin extrusion on the flexure end lug common to the chassis bracket  was added as contact regi on for the travel limiters.  See Fig . 7 for configuration of its design.