The fou rth and final ste p in rover stand-up is the deployme nt of the su spension bo gie membe r. A  partne rship was e stablished with the wheel asse mbly  subsystem s to use the aft bogie wheels to pull the  telescopi ng bogies out an d latch them into their dep loyed positio n (se e Step 4). By utilizing the wheel   drives, the bo gies can b e deployed without the need to add two ad ditional si ngle use a ctuators. The only   changes to the rover/la nder system to e nable thi s capability were a small rede sign to the aft bogie whee l  restraint an d the addition o f a cleated pl atform to en sure the aft wh eel had th e required tra ction to pull the  aft bogie out of the forwa rd bogie. 

Step 4: Bogie Deploy 

Material Selection   Titanium wa s used excl usively for the stru ctural comp onents of the suspe nsion. This material was  selected fo r several re asons. Th e hig h stre ngth-to-weig ht ratio made it attra ctive for a m ission whe re  volume and mass wa s at a premium.  In addition, the rover wa s mounted  to a graphite/ep oxy comp osite   lande r basepetal. Titaniu m’s low CT E matched t he lander’s better than other p otential suspension  material s. Fi nally, the a bility to weld  titanium a llowed the suspen sion stru ctural  comp onents to b e  optimize d for stren gth and flexibility. Eight of the ten suspension tube mem bers we re welde d. The de sire  to increa se the Ti-6AL-4V  from the anneale d to a solution treate d and aged (STA) state was resisted.  While th e ST A pro cess would incre ase the strength of the titanium from 90 0 MPa (13 0 ksi) t o 1100 MPa  (160 ksi), the  weld seam s would rem ain in the anne aled co ndition, creating a n obvious a nd unacce ptable  weak li nk that  could only be mitigated if the STA pr ocess was performe d after welded. T he possibility that  the weld me mbers would distort signi ficantly durin g the STA pro cess due to th eir thin walle d constructio n  was deem ed too risky to accept. Therefore, all the su spension tub e membe rs were kept in their ann ealed  condition. 

Structu ral Me mber F abrication  The de sire to cre ate a su spen sion that efficiently ab sorbs e nergy leads to st ructural m embers th at are  thin wall ed box beam s. A b ox beam design is a m ass efficient ge ometry for compone nts subjected to  both   bendi ng and torsio nal loads. The  beams are also tapered wherever po ssible t o incre ase m ass saving s.   Based  on these desired design fe atures, the  fabri cation meth od selected to create the me mbers wa s  electron beam wel ding. T he use of we lding in  the space industry is usually reserved only for propulsion  tanks and tu bing due to fears of poor workm anship and difficulty of inspectio n. Propulsi on subsystem s  can be pressure te sted to verify weld  integrity. Sp acecraft structure, on th e oth er hand, does not typically   lend itself to such pro of loadin g meth ods. Becau se of these p otential com plexities, other altern ative  avenue s of manufa cturing were inve stigated, the  most promi sing of whi ch wa s invest ment ca sting .  However, the  small nu mber of pa rts m ade this option prohibitive ly expense per piece, so weldi ng was  ultimately cho sen as the fa brication met hod. 

The ba sic constru ction of each weldme nt is the joi ning of two C-cha nnels.  A less expe nsive option o f  weldi ng a pl ate on an o pen box pla ces the weld seam  in th e corner of th e box beam  was initially  investigate d. This l ocation  is un desirab le due to  the fact that the  corner of th e bea m is th e location of   maximum stress in torsion al load cases (se e Figu re 5), and the refore ne cessitates the more compli cated  C-cha nnel approach. T he typical wall thi ckness of 1 mm incre ases to 1.5 mm along th e weld seam fo r  extra stre ngth. This de sign process al so en ables additional feat ures to be machi ned int o the weld ed  membe rs. The aft bogi e member has a recess machined i nto it for the bogie latch p awl to reside. Th e  “knees” in the aft rocker and center wheel struts are thickened up to accommo date the localized increase  in stress. 

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