page_title: Common Berthing Mechanism
section_title: Origins (prior to c. 1984)
hierarchical_section_title: Common Berthing Mechanism / Developmental history / Origins (prior to c. 1984)
caption_attribution_description: English: Screen shot from pdf.
context_page_description: The Common Berthing Mechanism connects habitable elements in the US Orbital Segment of the International Space Station. The CBM has two distinct sides that, once mated, form a cylindrical vestibule between modules. The vestibule is about 16 inches long and 6 feet across. At least one end of the vestibule is often limited in diameter by a smaller bulkhead penetration.
  The elements are maneuvered to the berthing-ready position by a Remote Manipulator System. Latches and bolts on the Active CBM side pull fittings and floating nuts on the Passive CBM side to align and join the two.
  After the vestibule is pressurized, crew members clear a passage between modules by removing some CBM components. Utility connectors are installed between facing bulkheads, with a closeout panel to cover them. The resulting tunnel can be used as a loading bay, admitting large payloads from visiting cargo spacecraft that would not fit through a typical personnel passageway.
context_section_description: The CBM is just one branch in the long evolution of the United States' ability to assemble large spacecraft. At least as early as the late 1950s, the capability had been recognized as “...necessary for building space stations and assembling vehicles in low Earth orbit...”.  By the end of the Apollo program, standardized rendezvous and docking practices to support it had been proven in practice.  The basic challenges of propellant management were well understood, as were control stability and contamination issues resulting from the chase vehicle’s propulsive RCS plumes hitting the target vehicle vehicle during proximity operations.
  The advent of the Space Shuttle Program mitigated some issues with docking, but introduced new ones.  Significant differences between the masses of chase and target vehicles provided for less equal sharing of momentum after contact, and the larger mass of the Shuttle required significantly more braking propellant than was needed during Apollo.  Simple coaxial alignment between chase and target inertial properties during terminal approach operations was not possible with the asymmetric Orbiter, which was designed for aerodynamic lift during return from orbit.  Impingement of large Shuttle RCS plumes on relatively small target vehicles also disturbed control over target orientation during proximity operations.  These issues forced changes in braking strategy on the Shuttle program.  Not all strategies were easily implemented in all orbital directions, which threatened the ability to assemble in some of those directions.  The use of a long tele-robotic device (the RMS) reduced that threat by moving the point of first touch away from the chase vehicle.
  By 1972, requirements analysis for the Shuttle Program estimated that almost 40% of mission objectives would involve assembly by placing a payload into the Orbiter's Payload Bay.  It was envisioned at that time that many of the retrieved spacecraft would not be designed for such operations, further raising the importance of solving (or eliminating) issues with docking.  The berthing operation was developed to do so: a requirement to gently grasp a nearby spacecraft with near-zero contact velocity was allocated to the Shuttle's planned RMS.  Using the RMS to assemble objects on orbit was regarded as a driving requirement for accuracy in both position and orientation of the emerging system.
  Although not foreseen at the time of RMS development, this period saw the emergence of requirement topics that would become important to the CBM: the accuracy and precision of RMS control, limitations on its ability to force things into alignment, and the magnitude of structural loads peaking in the booms and joints during capture.  These proved to be crucial to the design, qualification, and operation of the mechanism's development.
  The SRMS did not accomplish its first retrieval and payload bay berth until STS-7 in June, 1983.  The date of first operation was two months after submission of final reports by the eight contractors of NASA's Space Station Needs, Attributes, and Architectural Options Study.  Even though no flight results were available when the final study reports were written, at least three of them identified “berthing” as the primary means of assembling a Space Station from pressurized modules delivered in the Shuttle's payload bay.  Of the concepts described and illustrated, none strongly resemble the eventual design of the CBM, and little discussion of the technical details is readily available.
  In early 1984, the Space Station Task Force described a Berthing Mechanism that would attenuate the loads incurred when two modules were maneuvered into contact with each other, followed by latching.  Contact conditions were identified as important, but were not quantified at that time.  The same is true for the diameter of the internal passageway.  Internal connection of utilities between the modules was explicitly required, as was “androgyny”.  A standardized Berthing Mechanism was perceived as an external flange on module ports, and a “6-port Multiple B
caption: The Space Station Task Force identified berthing as a primary assembly technique.[6]
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