Each berthi ng mechanism design wa s also drive n by unique  requirements fo r that interfa ce. The CBM3  must provide a pressure seal and a passageway for crew and equipment  between modules.  The SSAS4  is fully automated to minimiz e EVA op erations  for t he inboard truss  element berthing, while the RTAS is  fully manuall y operate d due to difficult y in getting power and dat a acro ss th e rotary joint s betwe en the  inboa rd and outboa rd tru ss elem ents. The CAS, u sed for temp orary in stallati on of payloa ds and cargo  carrie rs, h as no req uirement for a bolted st ructural atta chment, but inst ead relies on the ca pture  mech anism to carry structural loa ds. 

Common Be rthing Mec hanism (CBM) 

The CBM is the ISS-devel oped mechanism u sed to assemble  the i nhabite d volu me of the S pace Statio n.  All pre ssurized mod ules, with the ex ceptio n of th e Ru ssian m odule s, use this me chanism desi gn to  provide  a common atta chment interfa ce. The CBM (Figure 2 ) is a fully automat ed me chanical sy stem tha t  align s, captu res, and the n bolts togeth er the two elem ents. The CBM also p rovides the atmo spheri c seal   feature s for establi shing the pre ssurized passa geway between th e module s (shirt sleeve e nvironm ent).  The d evelop ment of thi s mechani sm was cove red in a  paper presented  at the 26th  Aero space  Mechani sms Symposium, “Space Station Freedom Comm on Be rthing Mechanism ”, by Eri c Illi of The  Boeing Comp any5. 

CBM De sign Features a nd Operatio n  The CBM i s a comple x system of  mech anisms that work together to  accompli sh berthing  of pressuri zed ISS  module s (Figure 3). A  major d esign  driver for this mechanism is the  requi rement to provid e an  unob structed  passa geway throug h the CBM betwee n  the module s. This requi rement pre cludes  the use of a central ca pture me chanism  to pull the  interfa ce togethe r. Thi s  requi rement drove all of the align ment,  captu re and prelo ad me chani sms to t he  perim eter of the CBM system. At each   berthe d interf ace th ere is an Active CB M  half and a Passive CBM h alf.  The Activ e  CBM ha lf is located on the ISS s ide of  the interfa ce and e ncompa sses all of th e  static and motor actuated mechani sms,  telemetry a nd control system. The  incoming eleme nt to b e berthed contain s the Pa ssive  CBM half, with  all of th e rece iving me chanisms for the Active Half. T he Pa ssive CBM also contains the  atmo spheric seal  that mates to  the Active h alf seal surface. The CBM system is designed such that any Passi ve CBM half   can berth to any Ac tive CBM half. 

Figure 2.  CBM ber thing of MPLM on   Node 1 (on-o rbit photo)  

As the elem ent bein g berthed approaches the ISS, The A ctive and Pa ssive halves begi n to interact.   Coarse align ment guid es begin to ali gn and center t he two h alves as they are  moved clo ser togethe r by  the Shuttle or Space Stati on Rem ote Manipul ator System (SRMS/SSRMS). At about 11.4 cm (4.5 in )  separation, Ready to L atch me chanisms are tripped, sending  a signal to th e RMS ope rator that the two   halves are close enough for the  CBM t o take ove r the remai ning bert hing operation s. Capt ure latches,  with el ectro-mech anical actuators, on the Active  half extend and pull the  Passi ve half into  the Active half.  Coarse alig nment guid es contin ue the aligning until hand off to the fine alignm ent com ponents. A set of                                                    3 S683-2 9902 “Active Com mon Berthin g Mecha nism Prim e Item Developme nt Specifi cation ”, 01 Oct 19 98  4 SP-M-598 D “Config uratio n Item Specification for the Segment to Segment Attachment Syste m”, 05 De c  2001  5 Proce edings of the 26th Aerospa ce Me chani sms Sym posium, May 13-1 5, 1992, page 2 81  303