By another question I was reminded how hard it is to build airtight equipment such as vacuum chambers from multiple pieces. Docking or berthing space ships / space station modules is a rather similar task, only with a reversed pressure gradient. I am wondering how the seals work.

When screwing two pieces of a vacuum chamber together, you can pick between a few materials in terms of seals depending on the target quality of the vacuum. For an ultra-high vacuum, copper is rather common. But copper rings can only be used once (copper gasket vs. knife-edge flange), so they have to be exchanged each time you open up some particular flange. For a high / medium vacuum, you can work with Viton / cheap rubber. Viton and rubber can be used multiple times, but even this has limits.

Space Shuttle - APAS-95 This is an APAS-95 docking mechanism, mounted on a space shuttle. You can clearly see the two brown rings.

ISS/Kibo - Passive CBM This is a passive interface of a Common Berthing Mechanism as found on Kibo at the ISS. There are basically three rings, made of some brown material.

There are a few interesting aspects about those rings in space, which make me curious. Apparently, they can be re-used rather often. Remember, how often for instance modules at the space station MIR were re-arranged and therefore un-docked and docked again. Besides, this stuff stays in space (and vacuum) for very long times while it does not seem to degrade significantly (by out-gassing etc).

Which materials are used and have been used in this context? What are their limits? How many docking / un-docking cycles can they handle? How air-tight are those seals in terms of numbers (e.g. loss of air in kg per day)?

  • 3
    $\begingroup$ Consider the Russian segment docking systems as well. How many Progress, ATV, and Soyuz dockings have there been now? Probably highest usage is the one at the end of Zarya, where ATV docks. $\endgroup$
    – geoffc
    Jul 31, 2013 at 23:10
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    $\begingroup$ @geoffc Hmm actually, the Soyuz, ATV and Progress bring their own seals with them - being the active part of the docking system. The docking port in the back of Zarya is the passive part without its own seal. $\endgroup$
    – s-m-e
    Jul 31, 2013 at 23:23
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    $\begingroup$ That is actually a pretty elegant approach then. The part that wears out, is the part you bring along and discard each flight. Clever. $\endgroup$
    – geoffc
    Aug 1, 2013 at 0:34

1 Answer 1


The material used for the seal is silicone rubber.

Example materials considered for NDS iLIDS are:

  • Parker S0383-70 (as part of Gask-O-Seal product)
  • Esterline ELA-SA-401

Silicone rubber is the only class of space flight-qualified elastomeric seal material that functions across the expected temperature range. NASA Glenn has tested three silicone elastomers for such seal applications: two provided by Parker (S0899-50 and S0383-70) and one from Esterline (ELA-SA-401). The effects of atomic oxygen (AO), UV and electron particle radiation, and vacuum on the properties of these three elastomers were examined. Critical seal properties such as leakage, adhesion, and compression set were measured before and after simulated space exposures. The S0899-50 silicone was determined to be inadequate for extended space seal applications due to high adhesion and intolerance to UV, but both S0383-70 and ELA-SA-401 seals were adequate.

Source: Space Environment Effects on Silicone Seal Materials (PDF)

The metal for the interface is aluminum. NDS IDD specifies MIL-DTL-5541 Type 1, Class 3 chemical conversion coating (containing hexavalent chromium Cr (VI)) over the metal.

iLIDS is spec'ed to withstand 70 docking/undocking cycles in passive mode (20 on the ground and 50 in space), and 24 cycles in active mode (on a spacecraft) (20 on the ground, 4 in space).

Leak limit is specified as follows:

  • iLIDS-to-iLIDS: maximum of 0.0011 kg dry air/day leakage at vestibule pressurization of 101 kPa and an external vacuum pressure when mated.
  • iLIDS-to-vehicle: maximum of 0.0004 kg dry air/day leakage at vestibule pressurization of 101 kPa and an external vacuum pressure when mated.

The leakage rate is determined by two active factors:

  • Atomic oxygen in LEO
  • Ultraviolet radiation from the Sun

Source: International Low Impact Docking System (iLIDS) Technical Requirements Specification (PDF)

  • 1
    $\begingroup$ Thanks for the answer, this is fairly interesting. LIDS is basically a future system. What about the stuff, which is in space right now (or was in the not too distant past)? $\endgroup$
    – s-m-e
    Aug 17, 2013 at 16:20
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    $\begingroup$ Fluor-siloxane rubbers with Cadmium oxide (CdO) fillings. $\endgroup$ Aug 17, 2013 at 17:55
  • $\begingroup$ spectacular answer! $\endgroup$
    – Fattie
    Jul 6, 2017 at 23:41

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