. The PCBM path during capture is induced by the RMS (1). The RMS interacts with modules ranging in weight from the Cupola (2) and the PMAs (3) to Kibō (4). The mass interacts with lighting to drive temperature differences between the CBM rings. This adds to pressure-induced deflections, especially for Radial Ports (5). The US space program's concept of berthing was developed to mitigate issues of orbital mechanics that were encountered during the evolution of . Although not the first mechanism developed specifically for berthing, the CBM was the first such device designed in the US specifically to assemble structural joints that would hold sea-level pressure. It integrates four
archetypical features: • experience internal pressure in addition to their other primary loads. They are considered life critical when used as the pressure hull of a crewed compartment. In that context, they receive special attention for issues like loads, leak rate, seal redundancy, and verification practices. They also draw close scrutiny to the effects of their failure. • External
flanges are subject to both mechanical loads and loads induced by pressure in their parent . The relative stiffness of the flange determines how the free end will change shape. Distortions must be accommodated when something is being attached at the flange. • transmit forces differently as their posture changes. Their loads are influenced by internal friction, and often require more iterations of analysis and design than structures. In the case of CBM, the load path includes both the module and the RMS, so can be very complicated. • Structural joints that resist high vacuum are engineered to strictly limit gaps across the joint, and the conditions under which they are assembled are carefully managed. For the CBM, these issues are compounded during boltup by seal scrubbing as pre-berth deflections are conformed, and by any dust and debris trapped in the joint. The use of these features on a spacecraft entails special considerations due to the aggressive environment. At the typical ISS altitude, NASA identifies seven factors for that environment: • The composition, properties, and condition of the ambient neutral atmosphere. In particular, atomic oxygen (AO) is highly corrosive to many materials. Elastomers, such as the PCBM's face seal, are particularly sensitive to AO. Low pressure and low absolute humidity also impact the coefficient of friction for many material combinations. Exposure to very low pressures also changes the chemical composition of certain materials over time. • Strongly directional sources and sinks of
radiant energy. The mounting, optical properties, and insulation of exposed spacecraft components are engineered to maintain acceptable temperatures. In some cases, the orbital orientation of an entire spacecraft is dynamically controlled to mitigate these effects. • The
geomagnetic field can interfere with sensitive electrical components (such as those of the ACBM's sensors, switches, and controllers). The effects can include outright failure as the components are carried through the field. •
Ionized gasses that contaminate and charge exposed surfaces, of which the CBM has many. Most spacecraft deal with this issue by careful grounding of the exposed components. • Electromagnetic radiation that can alter the energy state of electrons in powered equipment. The motors, sensors, and control electronics such as those on the ACBM are susceptible to these effects unless shielded. • Meteoroids and orbiting debris, some of which can be both heavy and fast-moving, that can strike the spacecraft. Although the CBM design has been augmented several different ways in this regard, the issue was engineered at the integrated spacecraft level; quantitative requirements are not allocated in either CBM specification. • The balance between gravitational and centrifugal accelerations (often referred to as "zero gravity"), which has substantial implications for verifying the motion of mechanisms on the ground because gravity dominates there. CBM followed typical spacecraft engineering practice, iterating between analysis and test to develop and verify designs for this condition. Several of these features and factors interacted through a long sequence of decisions about the station's orbit, configuration, plans for growth, launch vehicles, and assembly techniques. The berthing operation finds its origin in programs of the 1960s and 1970s as they explored the practicality of physics related to these issues. The CBM concept itself began to emerge with the first studies of the program in the early 1980s, experienced multiple iterations of concept, and completed development shortly before launch of the first flight element as the 1990s drew to a close.
Origins (prior to c. 1984) 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 and 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 plumes hitting the vehicle during . 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 . A standardized Berthing Mechanism was perceived as an external flange on module ports, and a "6-port Multiple Berthing Adapter" roughly corresponded to the eventual Resource Node concept. Deflections induced by internal pressure acting on radially oriented ports of cylindrical modules became recognized as a critical developmental issue. The Task Force's final report also appears to be among the earliest references to "common...berthing mechanisms".
Advanced Development/Phase B (c. 1985 – c. 1988) In parallel with the on-going system-level configuration studies, NASA anticipated that concept development projects for advanced docking and berthing mechanisms "...to substantially reduce docking loads (velocities less than 0.1 ft/sec) and provide payload berthing capabilities...will be initiated beginning in Fiscal Year 1984." The Berthing Mechanism Advanced Development program actually started in 1985, leading to full-scale testing in the Six-Degree-of-Freedom test facility at Marshall Spaceflight Center (MSFC). In that effort, "common" appears to have meant that a single family of mechanism designs accomplished both berthing and docking (inheriting the divergent requirements for both) and that any member of the family could join with any other member. "Active" and "passive" referred to whether mechanisms were provided for attenuation of residual kinetic energy after docking. Motor-deployed capture latches of two different designs (fast- and slow-acting, having short- and long-reach, respectively) were mounted on the outboard radius. Outward-oriented guide petals were also located on the outboard radius, giving the mechanism an overall diameter of about 85 inches. Structural latching was accomplished by a "bolt/nut structural latch" of 0.500 inch nominal diameter. Designed for a tensile load of , both the bolt and nut were fabricated from A286 steel, coated with a tungsten disulfide dry film lubrication as specified by DOD-L-85645. Bolt/nut locations alternated in orientation around the perimeter of the 63-inch diameter pressure wall and the faces of both rings included seals, so that the mechanism was effectively androgynous at the assembly level. The bolts were designed for manual actuation, using sealed drive penetrations through the bulkhead. An option for motorized torquing was identified, but not designed. The bolt could be tightened from either the head side, or the nut side. Neither the torque nor the uncertainty in are reported in the available documentation. One of the study's four variants incorporated an aluminum bellows, allowing a loop of modules to be closed. Tension loads caused by internal pressure were carried across the bellows by a continuous cable loop threaded through 47 pulleys arrayed around the outside of the bellows. Not all of the issues with the bellows design appear to have been fully resolved by the end of the developmental test series. Although the dimensions accommodated internal utility connections and a 50-inch square hatchway, the mechanism envelope had limited compatibility with the eventual recessed Radial Port locations on USOS Resource Nodes. The apparent incompatibility with Radial Port locations might be explained by the as-yet unstable configuration of the Nodes, being shown as spherical 10-ports modules in some configurations, but cylindrical 3-port modules in others. Many other features of the baseline station configuration of the time also appear quite different from the eventual ISS.
Space Station Freedom (c. 1989 – c. 1992) As 1990 approached, the size of the CBM had been stabilized by a specific engineering approach to the design of modules. Indirectly constrained by the circular cross-section of the NSTS Payload Bay, the internal volume of the module was divided into eleven regions. A center aisle running the length of the module is surrounded by four banks of equipment. The equipment banks meet along four lines running nearly the full length of the pressure shell. Immediately outboard of those points, wedge-shaped utility volumes run parallel to the aisle. The utility runs allow them to be tapped from many stations along their length. Other equipment, some of which facilitated utility connection between modules after they are mated on orbit, is more efficiently packaged in the endcone volumes than in the cylindrical portion of the module. Penetrations for these utility runs to connect between modules received significant attention in the layout of the vestibule and, therefore, of the CBM. Each bank of equipment was divided into "racks" of standard size that could be installed on orbit in order to repair, upgrade or extend the station's capability. Racks holding related equipment could be integrated and tested on the ground before launch. This approach to integration facilitated a higher level of verification than would have been available using replacement of smaller components, providing for "...easy reconfiguration of the modules over their life span of 30 years." It also permitted the architecture to accommodate the subsequent change in orbital inclination by moving some of the heavy racks off the initial launch of the module. The distinctive size and shape of both the common hatch and CBM enabled this concept of module integration because they permitted movement of the large racks into, and out of, the modules while on orbit. Other system-level decisions in this time frame also affected the eventual design of the CBM. The idea of a "common" mechanism for both docking and berthing appears to have been discarded, and major mechanisms specific to each of those distinct operations were identified. The concept of a "common" module pressure shell with a range of Radial Port configurations, still being studied by NASA at least as late as 1991, was discarded in favor of dedicated "Resource Nodes" having four Radial Ports near one end of a cylindrical pressure shell. Closure of the "module pattern" was deferred from the initial system-level design by 1992, eliminating the bellows-based variant of the PCBM. in 1992. By the early 1990s, a more detailed picture of the CBM began to emerge. The initial release of the PCBM development specification was in October 1991, followed by that of the CBM/PE ICD in February, 1992 and the ACBM development specification in January, 1993. Several elements of the Advanced Development concept were retained with little change. The bolt/nut structural latch and 4-bar capture latches remained, although the bolt diameter had increased to . Both the bolts and the capture latches were motorized with manual backup being available, although the individual mechanisms were still driven by way of sealed couplings that passed through the bulkhead. The term "active" had evolved to mean the co-location of all powered devices on the side of the interface already present on orbit when the mating operation took place. Other features had been changed more significantly since the Advanced Development concept. "Androgyny" had been discarded: all 16 bolts were collected on the same side of the CBM/CBM interface, and the nut side was no longer described as being drivable. An 8-channel multiplexing motor controller could be remotely switched between latches, with two controllers required for each module having an ACBM. Differential pressure sensors had been included to monitor potential leak locations. Until it was cancelled, the Passive Flexible CBM still had an aluminum bellows, but the cable/pulley concept had been replaced by a set of 16 powered struts, driven by the multiplexing motor controller. The CBM/CBM seal design was a "face" design, on one side of the interface only. Alignment guides were deployable, and their orientation was reversed to face inward. The four capture latches had acquired friction clutches, allowing them to be back-driven. New features emerged in this time frame. A debris cover had been added to the ACBM concept. It was a full-diameter unit of a single piece, removed and replaced with the RMS. Attachment of the rings to their bulkheads had been defined as a 64-bolt pattern, but no differentiation of the bolt pattern is mentioned in any of the sources. A shear tie had been added to the design to carry loads parallel to the CBM/CBM interface plane.
Transition to ISS (1993 – c. 1996) By December 1990, Space Station Freedom's cost estimate had risen from the 1984 estimate of $8 billion to reach $38 billion. Although the estimate was reduced to $30 billion by March of the following year, calls to restructure or cancel the program were prominent in Congress. In March 1993, NASA Administrator Dan S. Goldin communicated that President Clinton wanted "...the current Space Station redesigned as part of a program that is more efficient and effective...[to]...significantly reduce development, operations, and utilization costs while achieving many of the current goals...". The redesign team submitted their final report in June 1993, describing three distinct space station concepts. Each concept was assessed at orbital inclinations of 28.5 and 51.6 degrees to expose any issues of support from the US and Russian launch complexes, respectively. None of the three configurations precisely matches the design of the ISS as it exists today, although some of them bore strong resemblance to the eventual configuration. The CBM was the only explicitly identified structural/mechanical subsystem included in all options at all inclinations. An increased exploitation of vestibule volume for utility connections was recommended for all options in order to decrease EVA time. Removal of automated controllers, motors, and latch mechanisms was conceptually identified as an option for one of them. The specific conceptual designs that emerged from the Task Force were soon overcome by events. By late 1994, the US, Russia, and International Partners agreed in principle to merge their national efforts into a single "international
[sic] Space Station" project. The cooperation led to hybridized assembly operations such as installation of the docking module atop the Orbiter Docking System on
STS-74. This blurred common distinctions between berthing and docking, being positioned by the RMS but actuated by Orbiter thruster firings. Both CBM specifications were completely re-written in 1995 (PCBM) and 1996 (ACBM) as part of the transition process. This period also saw the splitting of the ICD into dedicated Part 1 (interface requirements) and Part 2 (physical and functional definition) at Revision D (June 1996). By the time a final framework for the international effort was contractually established in December 1996, the first CBM simulators had already been delivered to NASA.
Qualification (c. 1994 – 1998) Having been specified independently, compliance for most requirements of the ACBM and PCBM was verified separately. In addition to assembly-level activities for the ACBM and PCBM, compliance data were generated for subassemblies such as the Capture Latch, Powered Bolt, Powered Bolt Nut, and Ready to Latch Indicator. For example, the Powered Bolt and Nut functionality was qualified by component-level tests that included ambient functional, random vibration, thermal vacuum, and, for the bolt, thermal cycle. Load tests at the yield and ultimate static conditions were conducted at the component level, as were dynamic conditions. The success criteria for these tests were generally based on the torque required to establish and relieve preload, on electrical continuity, and on the accuracy of the bolt's load cell. In contrast, at least 11 specified verification activities required conjoint verification of mating and/or demating the two sides. Of those, five called for validated by and/or that required a specific combination of circumstances and interfaces. For example, the specifications directed capture to be qualified "...by analysis under dynamic loads imposed by the SRMS and SSRMS...validated by assembly-level test that includes variation of performance resulting from temperature and pressure on the ACBM and PCBM and on their interfacing structures." Boltup analyses of the ACBM/PCBM interface, and subsequent leakage, required similar validation by element- and assembly-level tests that included the distorting effects of pressure and temperature. End-to-end demonstrations were also required at the assembly level to verify "...mechanical functionality...without interruption from accomplishment of ready-to-latch indication and capture." Imposing the combined effects of capture dynamics and distortions required iterations of analysis and validating test for each aspect. The dedicated test setup was developed in three parallel threads: :*
Contact dynamics analysis of early CBM versions had begun by 1992, and was incorporated into MSFC's RMS Model for use in Boeing's CBM model development tests. The model was based on the "method of soft constraints", assessing "...intersection or penetration between the corresponding surfaces and calculating mutually perpendicular forces proportional to the depth of penetration". Preliminary model validation testing for these "rebound" forces and subsequent accelerations was conducted in MSFC's Contact Dynamics Laboratory from 1992 through at least 1997. The loads were locally linearized and imposed on the back end of a PCBM test article in the conjoint tests and demonstrations by a counter-balanced "Resistive Load System" suspended from the top of MSFC's V20 Vacuum Chamber. :*
Temperature predictions were based on standard thermal analysis modeling techniques. The model was validated by stand-alone thermal balance testing of both assemblies at AEDC's 12V Thermal Vacuum/Solar Simulation Chamber in 1995/96. These ensured use of the correct interface conductances, internal re-radiation, and internal thermal capacitances. Validation was supported by select contact conductance testing, reducing the number of variables to be resolved in thermal balance. Temperatures were imposed during assembly-level qualification testing by a combination of strip heaters, cryogenic shrouds, and direct LN2 Injection. :*
Pressure-induced deflections of pressurized elements were estimated by finite element modeling of their primary pressure shells, which led to validating pressure tests in mid-1996. For CBM assembly-level testing, the Active Pressure Vessel (APV) emulated boundary conditions on a flight-like radial port berthing plate. Emulation used 32 external structural doublers ranging in thickness from , 32 internal struts and 16 pneumatic actuators to tailor stiffness, constrain deflections, and apply local radial loads, respectively. The simpler Passive Pressure Vessel emulated an axial port. Manufacturing of the APV overlapped with discovery of negative margins in the design of Node 1 radial berthing plates. Redesign of the plate could not be accommodated in the APV's manufacturing schedule. It was compensated for by the relative rotation of nut acquisition commands during test. Setup for the assembly level test began with chamber modifications in August 1996, with the two pressure vessels being delivered for characterization testing in December. Integrated checkout of the assembled setup in the V20 chamber began with baseline testing of developmental CBM hardware in August 1997, and was completed in November of that year. Formal testing ran in three phases from February to September 1998: :
Phase A executed 62 boltup cycles under a range of atmospheric and temperature conditions to evaluate leak rates and Powered Bolt/Nut life cycle. :
Phase B ran 35 partial cycles (capture and nut acquisition) under an expanded range of temperature conditions. :
Phase C conducted five round-trip demonstrations under "challenge" conditions: extreme temperature differentials combined with PCBM positions more distant than those previously executed in hardware. No leak test was ever failed in this test. The contact dynamics model correlated to the test results with high statistical confidence and was shown to have no discernable sensitivity to deflections. Wear-out signatures for the Powered Bolt were identified and validated, and several integration issues were identified and resolved through minor re-designs. Significant issues with test-specific off-loading of gravitational effects were encountered, ultimately leading to changes in flight procedures. Nominal and contingency procedures were investigated and, in some cases, extensively revised prior to flight operations. Tests were subsequently conducted in the facility to qualify the IVA seals, and to support resolution of mission operations issues about bolt reach, contact corridors for alignment, RTL clearance, M/D Cover clearance, and RTL activation. The facility also provided real-time support for the first three flight uses of the CBM to assemble the ISS on orbit.
Field modifications (c. 2000 – present) • The decision to install Node 3 on the port-facing CBM of Node 1, instead of the originally planned Nadir-facing orientation, resulted in "...a unique circumstance: an exposed axial port berthing mechanism. Because this had never been planned for, a new design was developed...similar to the forward facing radial port...to provide a deployable shield to cover the exposed areas." The unique covers were installed during EVA #4 of
Expedition 50. • In late 2017 and early 2018, modifications were made to the attachment of CPAs to the hatch beams on two Nadir-facing ports. These modification allowed for rotation of CPAs "...into the vestibule rather than requiring that the crew remove them completely after a vehicle arrives. This will save both crew time and stowage space during a berthed mission. The CPAs must be installed for proper CBM operation during berthing activities, but they obstruct the pathway into the vehicle once the hatch is opened, so they need to be moved out of the corridor prior to cargo operations." ==Galleries==