Segment-to -Segment Attachmen t System (SSAS ) 

The SSAS is an unpressurize d mechanism used for structurally conne cting the inboard truss elem ents  (truss P1 to S 0, S1 to S 0, P3 to P1 and S3 to S1 ). The SSAS is a f ully automated mec hanical s ystem that   aligns , captures , and then bolt s together t wo truss elements . The SSAS includes  a centrally located  captu re latch asse mbly, three  alignment guid es and fou r discrete po wered bolts to  structurally atta ch  each inbo ard truss elem ent to the station as it is delive red by the orbiter.   

SSAS Des ign Features  and Operation   Since the SSAS is  an unpress urized mec hanism, it is not required to provide a pres sure seal between  element s, all owing a le ss compli cated desig n soluti on than the CBM. A cent rally located captu re latch  enga ges a capture b ar and provid es the force req uired to bri ng the interface into conta ct and ensu re  alignment for the bolting sys tem (Figure 6). As  with t he CBM, the ac tive elements  of the SSAS, inc luding  the rea dy-to-l atch sensors, capture latch assem bly, and remotely  actuate d bolting system are o n the  active ISS side of the interface a nd interact with  passive compo nents on the ele ment being b erthed u sing  the Station’s robotic a rm (Fi gure 7 ). 

Using input from a  digit al imagi ng  system a nd ma king visual   observation s of the interface, the  SSRMS operator moves t he eleme nt  being atta ched into the  berthin g  corrid or. The  SSRMS require s the   berthin g m echanism n ot induce  constrained motion while th e  SSRMS op erato r is m oving the   payload (Thi s was the mai n driver for  designing  the shap e/size of th e  captu re env elope  an d align ment  guide s). At about 7.6 cm (3 in)  separation, rea dy-to-latch (RTL)  mech anisms provide “positive” status  feedba ck to the SSRMS o perato r. At  RTL the inte rface is within  sufficient   alignme nt and reach for t he Captu re  Latch to ta ke over the remainde r of  the berth.   

The Ca pture Latch i s drive n by either  of two ind epen dent m otor d rive  mech anisms for redu ndancy and  has, additionally, a mechanical  EVAoperated contin gency override   mech anism for the even t of total  electri cal f ailure (both motors  inope rable). As the latch cla ws clo se  around the capture b ar, they pull the  element s into  alignment a nd prel oad  the inte rface. The  latch p reload  provide s the force to suffici ently seat   the cup/con es, which align the  powere d bolts with thei r respective nut s. Once captu re is complete , the four po wered  bolts are drive n into  the nuts.  The  bolts are en gaged a nd tightened on e at a time to prov ide the re quired bolt preloa d. The two   Bolting Syste m controlle rs and th e two integrate d motor controlle rs on the Lat ch Assembly are linked to  the ISS internal comp uter n etwork an d cont rol the bolt s and captu re latch re spectively.  MOTORIZED BOLT ASSYS (MBA) (4 PLACES) SHEAR CONE (4 PLACES)BBC STRUCTURAL SCARRING FOR EVA CONTINGENT BOLT (2 PER MBA) CAPTURE LATCH EVA OVERRIDE INPUT (7/16" HEX) ALIGNMENT PINS (3 PLACES)CAPTURE LATCH STRUCTURAL SCARRING FOR EVA CONTINGENT BOLT W/ NUTPLATE  (2 PER MBA)FIXED CUP NUT ASSY MT RAILSCAPTURE BARALIGNMENT GUIDE (3 PLACES)FLEXURE COMPLIANT NUT ASSY (3 PLACES) NADIRREADY TO LATCH SENSOR (3 PLACES) READY TO LATCH STRIKER (3 PLACES) Figure 6.  SSAS Compon ents 

Figure 7.  Truss Segmen t S1 being p ositioned  for mate to S0 (on-orbit p hoto ) Capture Bar SSRMS S1 Truss 

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