The Common Mechanism (CBM) connects habitable elements in the US Orbital Segment (USOS) of the International Space Station (ISS). The CBM has two distinct sides that, once mated, form a cylindrical vestibule between modules. The vestibule is about long and 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 . Latches and bolts on the active CBM (ACBM) side pull fittings and floating nuts on the passive CBM (PCBM) 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.

Design overview

All CBM types feature an aluminum ring that is bolted onto the pressure shell during fabrication of the parent . The bolted joint compresses two concentric o-ring seals: one is silicone (for better temperature performance), and the other is fluorocarbon (for better resistance to scrubbing). A mated pair of rings is for life-critical pressure loads, so the rings and seals were engineered to the same standards as the module shells. If the primary seals deteriorate, they can be augmented by secondary seals that were designed and qualified as part of the CBM. The secondary seals can be installed as an .

Most of the vestibule's volume is reserved for crew passage, and a closeout is typically installed around the perimeter of the hatch as a boundary for the passageway. In most locations, volume is reserved for utility connections outboard of the closeout. The set of utilities is specific to each pair of mated modules.

In addition to its structural characteristics, the ACBM performs and reverses the basic functions associated with berthing:

Alignment physically constrains motion between the modules in five of the six degrees of freedom as the distance between them changes. The constraints are imposed by successive sets of structural components. An indication of readiness to operate capture latches is provided to the RMS operator when the incoming module has been correctly placed within reach of the latches. Ready-to-latch indication is provided by four mechanisms: one in each quadrant, associated with each latch. The incoming module is captured by four latches. They draw it through a combined rotation and translation to align the PCBM to the ACBM with a small residual gap. A rigid structural connection is established. Each of 16 powered bolts on the ACBM crosses the residual gap to thread into a nut on the PCBM. The bolts are tightened in a multi-stage process that gradually conforms the two flanges, compresses the CBM/CBM seals, and the CBM/CBM joint.

Two functional types were specified for the ACBM. The Type I ACBM, with a complement of 24 independent mechanisms, can be found either axially or radially oriented on the parent module. It can face any of the six orbital orientations, so can be anywhere within a wide range of temperatures at the start of berthing operations.

The Type II ACBM augments the design of the Type I with components to protect its parent module when nothing is berthed on a . Four of the components are mechanisms that can be deployed to get out of the incoming module's way. Others are removed by the crew after the vestibule is pressurized. The Type II is used where ports would otherwise be exposed for long periods of time, or in directions that experience aggressive pre-berth conditions. The Type II ACBM is found on the radial ports of resource nodes, and can face in any orbital orientation.

The PCBM incorporates fittings and alignment structures corresponding to those on the Type I ACBM. 32 of the fittings are themselves spring-loaded mechanisms, actuated during capture and rigidization by corresponding components of the ACBM. The primary CBM/CBM seal is also part of the PCBM, as are preloaded stand-off/push-off springs to stabilize its relative motion when the CBM/CBM joint is nearly mated.

Two types were specified for the PCBM, differing only in the durability of their seal. The S383 silicon material of the Type I PCBM seal is more forgiving of pre-berth temperature differential between the two modules than the V835 fluorocarbon of the Type II. S383 is also more resistant to atomic oxygen encountered on orbit prior to berthing. The Type II was used to launch small elements in the shuttle payload bay while bolted to an ACBM or to similar flight-support equipment because the V835 material is more resistant to the damaging effects of scrubbing under vibration.

The PCBM is always located on an end of the parent module. It can be attached to a bulkhead or as an end ring on a barrel section of primary structure that is open to vacuum before berthing. PCBMs are attached to modules having a wide range of , so can also experience a wide range of initial temperature conditions. By the nature of the operation, the PCBM always faces in the flight orientation opposite that of the ACBM, so the temperature differentials can be significant.

Operations See the Operations Gallery for more graphics. See the Missions Table for individual berthing events.

Post-launch

ACBMs require EVA to prepare for first use on orbit. Type I ACBMs, usually found on axial ports, typically have a "shower cap" cover that takes two EVA crew members about 45 minutes to remove and stow. Type II ACBMs, found on Node Radial Ports, require release of launch restraints for the Deployable M/D Covers. Release of the spring-loaded covers requires actuation of Capture Latches to close them again afterwards and, therefore, exercises the Ready-to-Latch Indicators. Including inspection, each Radial Port is budgeted about 15 minutes for a single EVA crew member, assisted by IVA crew to operate the ACBM as necessary.

Full-sized elements launched on the NSTS had protective covers over the seal on the PCBM. Two EVA crew members required 40 – 50 minutes each to remove and stow the PCBM's covers, inspecting the seal as they did so, and cleaning it if necessary. Type II PCBMs used as a launch interface were inspected after unbolting, since no covers were installed. For logistics flights, inspection is by camera only.

Berthing

Preparation

The PCBM requires no preparation for berthing beyond what is required post-launch. Preparation of the ACBM for berthing takes about an hour, beginning with selection of supporting utilities (power, data) and sequential activation for each Controller Panel Assembly (CPA). Two CPAs are selected as the Primary and Secondary Master Controllers.

Activation executes Built-in-Test and initializes position counters for the actuators. Each bolt actuator is extended two revolutions, then retracted three to verify operability of both the bolt and the motor. Latches are driven one at a time to the open position which, for Node Radial Ports, deploys M/D Covers. All 20 actuators are set to the operational initial positions (0 revolutions for the bolts, 202° for latches). A remote inspection is conducted to verify that the latches are fully deployed and the mating corridor and surface are clear of obstructions.

Contingencies considered during preparation include cleaning the face of the ACBM ring, and EVA corrective actions involving the M/D Covers as well as the CPA, Capture Latch, and Ready-to-Latch Indicators. Specific resolution procedures are available for the loss of power and communications support to the CBM.

Maneuver The PCBM-equipped module is maneuvered into the capture envelope by a tele-robotically operated Remote Manipulator System (RMS). Two different RMSs have been used to berth modules: the 6-joint Shuttle RMS (SRMS, or "Canadarm") and the 7-joint Space Station RMS (SSRMS, "Canadarm2").

The maneuver operation starts with acquisition of the payload by the RMS End Effector. This step is variously referred to as "capture" or "grappling". During the NSTS era, payloads typically arrived in the Shuttle's Payload Bay. During grapple, the SRMS' joints were "limped", allowing it to conform its posture to the exact location of the payload. The SSRMS typically grapples a free-flying payload that has maneuvered itself to maintain a constant distance and orientation with respect to the ISS. Once grappled, the RMS moves the module by changing its joint angles. The motion of the module must often be choreographed with other moving parts of the ISS such as the Solar Arrays. Visual feedback on the motion of the PCBM has been provided to the RMS operator by at least two dedicated systems. Early berths were guided using a photogrammetric feedback technique called the Space Vision System (SVS), that was quickly determined unsuitable for general use. The SVS was replaced by a task-dedicated Centerline Berthing Camera System (CBCS), first used on STS-98.

The time required to complete the RMS maneuver depends entirely on the trajectory to be followed and on any operational constraints that must be accommodated. The same is true for all contingency planning. Near the end of the maneuver, the operator negotiates a tight corridor as the PCBM begins to mesh with the ACBM. The operation ends when the RMS Operator either sees four Ready-to-Latch indications on the target ACBM, or concludes that only three can be achieved. Because the RTL is a spring-loaded mechanism, the RMS ends up with stored energy and is left in a state that can resist the separating force.

Mate

The two halves of the CBM are nominally joined in three operations: Capture acquires and aligns the in-coming PCBM with respect to the geometry of the ACBM Nut Acquisition threads each Powered Bolt into its respective nut Boltup fully preloads the joint between the two halves

At least two distinct capture protocols have been executed on orbit. Both protocols issue a "first-stage" capture command to an indicated shaft angle between 185° and 187°. First-stage capture ensures that each latch is positioned above its respective fitting, which is operationally verified by evaluating its switch state. The RMS still controls the position and orientation of the element, and the loads exerted by the Capture Latches remain low. Taking about 15 seconds to complete, first-stage capture is restricted to orbital regions where ground controllers can monitor progress in near real time. To control spurious loads when the berthing element is large, the station Attitude Control System may be maintained in free-drift and crew exercise prohibited.

The two protocols differ in how the latches draw the two halves to within reach of the Powered Bolts. During the NSTS era, a single second-stage "capture" command was issued after the SRMS was placed in "test mode". Five stages of capture are executed when using the SSRMS in order to limit the potential for loads building up in its arm booms if off-nominal braking events occur. In either case, capture drives latches to 12° indicated shaft angle in an actuation time of about 108 seconds. In both protocols, the residual energy in the RTLs might cause them to open briefly because the latches are not "hooked" to their fittings until well below the 187° starting position.