I have read about the common berthing mechanism on ISS and how the attachment process works and how the pressure seal is established, and I think I have that part figured out. There is a passive CBM and an active CBM which mate together and provide a pressure tight seal against space around them.

Passive CBMActive CBM

I have also seen plenty of diagrams of the hatches, so I know how they look and what parts they have:

CBM Hatch

However, what I haven't been able to figoure out from the documents is exactly how the hatches work with regards to modules in general. I have a lot of questions that I'm just going to list here - but I think a better answer might be to just explain in very concrete terms how the CBMs work.

  • Is there a hatch on both the passive and the active CBM? If so, does that mean there are two hatches to go through when going from one module to another?
  • Or are the hatches somehow just halves of a single hatch that is somehow combined during the docking?
  • Can (both of?) the hatches be opened from both sides?
  • Can the manual pressure equalization valve (MPEV) be operated from both sides? Are there two valves (one per hatch) or only one?
  • If there are two hatches, do both of them open towards the inside of the module they are in?
  • Can a module be moved from one place to another on the ISS while it is pressurized?
  • What happens to the air that is in between the passive and active CBM when they are detached? Is there a way to use a depress pump on it or is it simply lost to space? Is the air vented to space somehow before the detaching operation or does it just whoosh out when the seal is opened?
  • Do all the modules using CBM have exactly the same kind of hatch? Does the cupola have a hatch at all? Is the pressurized mating adapter also using the same hatch on the CBM side?
  • Harmony module is said to have six CBMs, with only the one connecting to Destiny being passive and the rest active. Yet in schematics and pictures, the CBM towards PMA-2 (forward) looks different (no covers). Is there any difference there, or is it a 100% standard CBM with active side on the Harmony side and passive on the PMA-2?
  • Oxygen, power, water, etc. are routed through connectors in the vestibule, but how does it concretely work? Do the routings require that the hatches stay open? What is needed to route them? Is it just a wire/pipe between the passive and active CBMs, or is there something to connect on the module side of the hatch as well? Are the connectors automatically sealed if there is nothing connecting to them (for example, in oxygen lines)?

2 Answers 2


There are several kinds of hatches on the ISS to permit many different kinds of vessels and modules from different sources to connect to the station. CBM is one of those, and as you note is an asymmetric gendered connection system with active and passive nodes. The ISS is equipped with both active and passive CBM nodes. The full range of connectors available and their locations can be illustrated by this diagram:


Source: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf

The CBMs connect in pairs, one passive and one active, each of which has a hatch. Both hatches can be opened from either side, but are normally opened from the active port side. The volume beyond each hatch can be (and is) pressurised and remains pressurised. When a volume berths it is pressurised and when it unberths it remains pressurised.

There is a small space between the two hatches which is called is vestibule which becomes exposed to the vacuum of space. The pressure within this vestibule must be managed as part of the berthing and unberthing procedures. The temperature gradient between the two modules must also be equalised as part of the berthing procedures as unequal thermal expansion can affect the firm and safe connection of the two module mechanisms.

The hatches of each CBM open inwards and upwards at 90° to the connection and have a latch to hold them in the open position. You also asked about the cupola. Unusually this does not have its own hatch, but it uses a passive CBM. This is because it is so short and there is no room for an opening hatch. Only the main node can close it off with a hatch. This is shown by the two illustrations of the Cupola before installation:

Copola shown no hatch

Source: Wikipedia

ISS Cupola diagram

Source: BBC News

Much of the illustration of the CBM hatches was shown when when the MPM (Multipurpose Module) Leonardo was moved on the ISS. It was originally positioned on the Unity Module; the hatches were closed and it was relocated (using the Canadarm) to the Tranquility module whilst still pressurised. There are videos of the hatches opening and closing as well as its original installation:


(1) Cook, Aksamentov, Hoffman & Bruner (2011), ISS Interface Mechanisms and their Heritage
(2) Newby, Mark A (2000), International Space Station Structures & Mechanisms
(3) McLaughlin & Warr (2001), The Common Berthing Mechanism (CBM) for International Space Station
(4) Harding, Pete (2013), NASA planning ISS module relocations to support future crew vehicles


I can comment that there are two hatches. Each side has its own and they are independently opened.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.