Common Berthing Mechanism (CBM)

When two pieces of the International Space Station need to become one airtight room, something has to hold them together with no leaks and no gaps. On the U.S. side of the station, that job belongs to the Common Berthing Mechanism, or CBM.

Quick facts

• Developer: NASA together with Boeing Defense, Space & Security.
• What it does: joins large pressurized (air-filled) modules in the U.S. part of the ISS into a rigid, sealed connection.
• Hatchway: a roughly 50-inch (127 cm) square opening, big enough to pass full-size experiment racks between modules.
• Mating ring diameter: about 71 inches (1.8 m); the joined “vestibule” between modules is about 16 inches (0.4 m) long.
• Key hardware: 4 motorized capture latches and 16 powered bolts per joint.
• First flight: on the Unity module (Node 1), launched on STS-88 on 4 December 1998.

What it is and how it works

A CBM is built from two matching rings. The Active CBM (ACBM) carries all the powered parts (latches, bolts, indicators) and sits on the module already in orbit. The Passive CBM (PCBM) carries spring fittings, floating nuts, and the primary seal, and rides on the incoming module. Putting all the motors and power on one side keeps the arriving piece simple.

The connection is made by a process called berthing, which is different from docking. In docking, two spacecraft fly together and gently bump under their own control. In berthing, the incoming module just holds its position nearby, and a robotic arm (the station’s Canadarm2, also called the SSRMS, or a visiting vehicle’s own arm) grabs it and moves it into place. Think of docking as two cars carefully parking nose to nose, and berthing as a crane lifting one piece and setting it precisely onto another.

As the passive ring nears the active ring, alignment guides and pins force the correct orientation. At about 4.5 inches (11.4 cm) of separation, four spring-loaded Ready-To-Latch (RTL) indicators trip to tell the arm operator the halves are close enough. The four motorized capture latches then extend and close, pulling the passive ring into rough alignment in seconds. This is called “soft capture.”

With the rings nearly together, 16 powered bolts on the active side reach across the gap, each threading into a floating nut on the passive side. Made of Inconel 718 (a tough nickel alloy) and about 0.625 inch in diameter, these bolts tighten in several staged passes. Each pass draws the two halves flush, compresses redundant rubber-like (elastomer) seals, and builds up structural tension, or preload, of roughly 19,300 lbf (about 85,900 N, or 86 kN) per bolt. The result is a rigid, air-tight joint. Once it is pressurized and the hatch is opened, crew can float between modules and large payload racks can pass through. The whole process is reversible, so cargo ships can later be unbolted, released, and sent to reentry.

Why it matters

The CBM is what made the ISS’s modular, robot-assembled design possible. Because berthing positions a module at almost zero contact speed, it puts far less stress on the structure than an impact docking would. That let engineers join very large, heavy pressurized modules without bulky shock-absorbing hardware, and it allowed a much wider passage than narrow docking ports give. That 50-inch square hatch is essential for moving full-size International Standard Payload Racks (the standard equipment cabinets used for experiments) and bulky cargo.

The trade-off is autonomy. Berthing needs a robotic arm and a crew member to operate it, so it is slower and more hands-on, and it cannot be used for a crew arriving on their own. That is why crewed vehicles like Crew Dragon and Soyuz use docking ports, while cargo ships favor the CBM’s larger hatch.

Where it is used and notable examples

• Unity (Node 1) — the first CBM in orbit, launched on STS-88 in December 1998; the connecting hub of the early ISS with several CBM ports.
• Harmony (Node 2), Tranquility (Node 3), the Destiny lab, Kibo (the Japanese Experiment Module), and the Cupola — all permanently attached to the station through CBM ports.
• Northrop Grumman Cygnus — a cargo spacecraft captured by Canadarm2 and berthed to a CBM port.
• JAXA H-II Transfer Vehicle (HTV / Kounotori) — arrived on its own, then was grappled and berthed, using the wide hatch for large cargo.
• SpaceX Cargo Dragon (CRS Dragon 1) and the Multi-Purpose Logistics Modules — berthed to CBM ports, with the 50-inch passage allowing standard payload racks to be transferred.