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I Was the recent ISS leak imminently dangerous?

  1. Sep 6, 2018 #1
    The ISS suffered a leak apparently caused by a 1/8 inch hole left during manufacture and never repaired. On Quora, the claim has been made that such a hole with one atm of pressure difference would only cause 324 cu ft of air to rush out per hour. I don't think this is close to correct, but I am lacking data re pressure differences, nozzle size, and air flow. Does anyone have a good table of references to help out?
     
  2. jcsd
  3. Sep 6, 2018 #2
    The relevant formula is conductance of a tube. I am familiar with it from John H Moore’s brilliant and essential “Building Scientific Apparatus”, but it appears many places. There are two formulae for two different conditions. Here the viscous flow condition applies and the conductance of air through an orifice D cm in diameter and L cm long is

    12.1 D^3 / L liters/sec

    Lets say the wall is 2 mm thick and you mentioned the hole was the hole was 3 mm in diameter giving a conductance of ~2 l/s

    The flow rate is the the conductance times the pressure differential. Here that is 1 atmosphere, so the hole leaks something like 2 l-atm / s or 7200 l - atm / hour. 1 cu ft is 28 l so that is 260 cu ft / hour. Given the precision of my estimates, I think that is close enough to make 320 plausible as the correct number.
     
  4. Sep 6, 2018 #3

    fresh_42

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    Has anyone references whether they really run the station on 1 atm?
     
  5. Sep 6, 2018 #4
    Ah, yes. Forgot about that. A quick Google search indicates they do actually pressurize to 1 atmosphere, but, that didn’t have to be the case. Actually, why do they? Important for long duration?
     
  6. Sep 6, 2018 #5

    fresh_42

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    I meant it as an interesting question, rather than a criticism. I guess they also lose a bit of pressure while outside missions or during docking maneuvers. As pressure might cause material fatigue it would be interesting to know what medical needs require - or what pressure the MIR had. E.g. on the ASTP mission 1975 there was a difference (Wikipedia):
    Found on some Wiki page:
    "There in the space station, where people are staying, there is an atmosphere that is adapted in terms of pressure and composition of the earth (21% oxygen and 78% nitrogen at 1014 hPa)."

    I've looked at a 37Mb pdf from Nasa where they explain how alien gases are found, how the recycling system works, that the space suits have 100% oxygen and many, many facts, but nothing about the cabin air conditions.
     
    Last edited: Sep 6, 2018
  7. Sep 7, 2018 #6

    256bits

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    https://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eclss/cabinpress.html
    Space shuttle,
    Kind of makes sense, as instrumentation and materials could all be tested at ground level under atmospheric conditions, rather than having one more design criteria of functionality under oxygen rich and reduced pressure.
     
  8. Sep 7, 2018 #7
    The All Oxy Atmosphere, aside from EVA in suits, was discontinued after the Apollo 1 fire on the ground, with Chaffee, White and Grissom. Since then it has been mixed gas due to the extreme fire hazard of the electronics and avionics packages.
     
  9. Sep 7, 2018 #8
    But not always 1 atm
     
  10. Sep 7, 2018 #9

    fresh_42

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    This doesn't quite match up with

    "Stafford and Slayton meanwhile had entered the docking module and closed behind them the hatch (no. 2) leading to the CSM. They raised the pressure from 255 to 490 millimeters by adding nitrogen to the previously 78 percent oxygen atmosphere. In Soyuz, the crew had reduced the cabin pressure to 500 millimeters before the docking. The pressure in the tunnel between the docking module hatch (no. 3) and the Soyuz hatch (no. 4) had been raised from zero to equal that of the docking module. Leonov and Kubasov were the first to open the hatch leading to the international greeting. During the transfer that was to follow, the pressure in the DM and Soyuz would be the same - 510 millimeters."
    https://www.hq.nasa.gov/office/pao/History/SP-4209/ch11-4.htm
     
  11. Sep 7, 2018 #10

    Nugatory

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    A bit of a digression, but the Apollo 1 fire has already been mentioned, and it is relevant to the tradeoffs between pressure and composition of atmosphere......

    Apollo 1 was designed to operate in space with about 5 psi internal pressure, about what you'd find at the top of the highest mountain peaks. This lowered the required structural strength and hence the weight of the spacecraft, but required an oxygen atmosphere just as mountaineers require oxygen at altitude. (The oxygen atmosphere also simplified preparations for space walks, as the space suits operated on pure oxygen).

    However, on the ground the capsule is subject to 14.7 psi external air pressure and cannot be operated with negative internal pressure, so the Apollo 1 test was done at an internal pressure of a bit over 16 psi. 16 psi oxygen is an extraordinarily high fire risk - any random spark can turn anything that will oxidize in air when heated into pyrotechnics, and that's what happened to Apollo 1.

    There were several other oxygen atmosphere accidents at about that time and together they led to a consensus that the engineering advantages of pure oxygen atmospheres justified the fire risk only when low pressure and weight was absolutely paramount. And even then much effort goes into eliminating flammable materials around oxygen under pressure.
     
    Last edited: Sep 7, 2018
  12. Sep 7, 2018 #11

    LURCH

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    Brngs up an obvious question; how did they regulate pressure during the launch? If the capsule had to be at 1 atmosphere on the launch pad, but needed to be 1/3 atmosphere by the time it got to orbit, it sounds as if they would have needed to leave open a vent until they reached a certain altitude, then close it.

    For that matter, how was the leak in the OP dealt with?
     
  13. Sep 8, 2018 #12

    Tom.G

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  14. Sep 8, 2018 #13

    anorlunda

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    I wonder. How significant the internal pressure could be to the ISS structural rigidity calculations?
     
  15. Sep 8, 2018 #14

    Nugatory

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  16. Sep 11, 2018 #15

    bob012345

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    Very likely not too much. However other structures such as the Bigelow inflatable space habitat structures, it's everything.

    https://en.m.wikipedia.org/wiki/Bigelow_Aerospace
     
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