435 It was a nother  lesson learned about the contribution of torsion spring preload to the flexure ’s pitch and roll  angles . The flexure to chassis bracket contact from flexure roll was found after DDR with the chassis bracket  already fabricated. Therefore, it  was a considerable challenge to accommodate the additional roll limiters   at this stage of hardware development . When having larger springs attach to hardware with degrees of  freedom, the influence on the hardware’s orientation from spring preload should be accounted for.  

Magnetic Hardstop   Similar to how the kickoff spring added additional fail- safe margin and a fault case to the deployment, the  magnetic latching hardstop had similar additional benefits for the final deployed position . Due to the larger  flexure  deployable, there was close clearance concern to the surrounding hardware  after deployment ,  namely the aft bridle tower and its surrounding harnessing on the chassis top deck.  A final deployment  angle of 40° was chosen to split the difference between the minimum rotation needed by the f lexure to clear  the range of motion of the d ifferential and aft bridle tower.  

From initial testing, there was concern that with the initial hardstop design induced a substantial amount of  flexure rebound due to its stiff nature stemming from a C -channel profil e design.  After initial contact with  the hardstop, the f lexure would then require three re- contacts with the hardstop to come to a complete rest.   Around the DDR, the hardstop went through two major redesigns. First, the structural shape was made to  be a st andard aluminum sheet metal profile. The final design was the same sheet profile with a bore feature  added to house the magnet and its casing (Fig. 9).  This diving -board like design added significant  compliancy such that the hardstop would dampen out the i nitial contact of the f lexure and reduced the  amount of flexure rebound after initial contact . However, during developmental tests, this elastically  compliant hardstop rotated nearly 8° over the planned 40° rotational travel for the final deployed position.   This exceedance approached the close clearance to the aft bridle tower but did not violate the clearance  margin.  It was deemed acceptable after being closely monitored at flight level functional testing.  

Figure 9. Development of the hardstop: (a) initial  stiffer C -channel profile, (b) DDR sheet metal design, (c)  thinner profile with a feature to house the magnet in the final magnetic hardstop.  

As a second function, the hardstop is to restrain the deployed flexure once it’s deployed such that the rover  differential is completely free to rotate.  This hardstop design utilized the flexure’s Maraging 300 steel  magnetic properties to make a magnetic latch. The magnetic force of this latch would be added to the  residual capability of the t orsion springs. The magnet to be used is of the same material as other magnets  on Mars 2020, Grade N42 Neodymium. There was concern about the likelihood of magnet fracture due to  its brittle nature directly from impact of the f lexure to hardstop contact . To address this, a  protective  stainless -steel  housing was incorporated.  For securement, a ring magnet design was chosen to allow a  fastener to run through the magnet and housing and the magnet was to be potted inside the assembled  housing (Fig. 10) .  

Several lessons were learned in the prototyping  of the magnet  housing. The magnet housing is two  separate pieces that when mated together there is a void that would fit the ring magnet. To have the