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The question ISS Cupola window coatings includes this diagram of the cupola windows. cupola window cross section

According to the diagram the space between the Primary pressure pane and the Redundant pressure pane is "vented to space". I cannot see how this makes sense, damage to the Primary pane would create a path from cabin to space which seems undesirable.

How does it actually work?

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2 Answers 2

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Woody has given one reason why the space between the two pressure panes is vented to space. But what happens if the primary pane fails?

The space between the panes is vented through a flex hose that is accessible inside the station.

From The International Space Station: Operating an Outpost in the New Frontier:

A vacuum is drawn on the inter-pane space to prevent condensation from forming between the two panes of window glass. That is the purpose of the flex hose that Mike Foale is inspecting in Figure 19.

Diagram showing the flex hose

So far the only window leak has been in this hose, not the panes. But in any case:

Once the leak point was identified, the hose was disconnected (which stopped the leak to space) until a replacement hose could be flown to the space station.

The flex hose covers and their associated label can be seen in this photo:

Cropped piece of a photo of ISS Cupola. Shows labels such as "Vacuum Line Cover 1".

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    $\begingroup$ I think the key point is the flex hose and ports act as a flow restriction so that if the inside pane failed, the leak rate would be manageable. Further, the volume at vacuum is limited so the energy imparted to a broken pane is small. Airplane passenger windows use something similar with a tiny hole in the inner pane. $\endgroup$
    – user71659
    Commented Oct 22 at 19:10
  • $\begingroup$ ... and if the inside pane fails, even the small leak can be eliminated by removing the flex hose and plugging up the ports, which can easily be done inside the station. It seems a failed inner pane could even be completely removed without affecting the pressure integrity of the outer pane if necessary, allowing clear observation while waiting for a replacement pane, and easy replacement of the inner pane without requiring space suits. $\endgroup$
    – user9557
    Commented Oct 23 at 12:48
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    $\begingroup$ @user9557 Yeah, though according to this answer, "Internal Pressure Cover is installed in the event of pane failure or pane seal leak." Which makes sense for safety. $\endgroup$
    – jpa
    Commented Oct 23 at 16:16
  • $\begingroup$ I gave the tick because jpa mentions "the flex hose accessible inside the station". But both answers are excellent, thanks @woody. $\endgroup$
    – NL_Derek
    Commented Oct 23 at 20:23
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I think “primary” is being used in the sense of “of chief importance” since it separates the cabin from vacuum.

Violation from impact is more likely to occur to the “Redundant Pressure Pane” than the “Primary Pressure Pane”.

If the space between the two pressure panes was pressurized, violation of the outer (“Redundant") pane would result in vacuum shock-loading of the inner (“Primary”) pane with 5 tons of pressure. In theory, this shock loading increases the risk of simultaneous catastrophic failure of both pressure panes.

The vacuum between the pressure panes in the ISS differs from the Russian window design. In the Russian design, the space between the panes is pressurized and the outer pane is designated the “Primary Pane”. What are the windows on the Russian segement of the ISS made of?

Early in the NASA assessment of the SM windows there were concerns that failure of the primary pane might cause failure of the secondary pane as well due to near instantaneous change in differential pressure since the volume between the two panes is pressurized. However, tests conducted by NASA using SM window hardware have demonstrated that the loss of the primary pane does not result in the loss of secondary pane.

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    $\begingroup$ My reading of this question (or at least my own question) was: if the primary pane breaks for some reason, but the redundant does not, does ISS maintain pressure? Or is the "venting to space" slow enough that some manual process can be enacted to seal up that venting process? Or will they just have to live with a slow leak until they can fix the primary pane? $\endgroup$
    – Seth Robertson
    Commented Oct 22 at 14:20
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    $\begingroup$ I can imagine that since the panes are presumably pretty strong, that it's considered a relatively low risk that a primary pane would be broken from the inside accidentally. Whereas high speed impact by space debris or meteoroid, or perhaps an out of control visiting spacecraft, would have a much greater chance of breaking a window. $\endgroup$
    – Steve Pemberton
    Commented Oct 22 at 19:57
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    $\begingroup$ I'm now curious if the outer pane were damaged, is there some process by which it could be replaced without depressurizing the station? (Or just the cupola module if it can be internally isolated) And would that be a potential difference between pressurizing or not between the panes? $\endgroup$
    – Darrel Hoffman
    Commented Oct 23 at 16:11
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    $\begingroup$ @DarrelHoffman ... on-orbit window replacement: space.stackexchange.com/questions/31988/… $\endgroup$
    – Woody
    Commented Oct 23 at 16:22

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