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Author
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Topic: Apollo 1: Cabin and spacesuit pressures
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robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 02-21-2016 10:31 AM
  
I believe that the Apollo 1 crew were on an independent suit loop of 3 to 4 psi while the general cabin pressure was around 16.7 psi. That means there was a higher pressure bearing down on the crew. Did this have any effect similar to when divers go deep into water? In other words, would the crew have been able to feel the higher pressure pressing down on them? Would this have caused their suits to press against them?Also, Ed White removed his glove just before the fire. This meant that he went from a 3 to 4 psi environment to a 16.7 psi environment. Would this sudden change have any effect on the human body? |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 02-21-2016 12:04 PM
The suits were not hard shelled so couldn't support an "underpressure." In the situation as described, there would be separate suit loop airflow, but for all intents and purposes the suit static pressure would be the same as cabin pressure. If the cabin depressurized at altitude or in space the suits would remain pressurized or "over pressure." |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 02-21-2016 03:54 PM
So for Apollo 1 the cabin pressure and the suit loop pressure were both 16 psi of pure oxygen. What happened when they switched to a mixed nitrogen oxygen combination for launch on the flights after Apollo 1? |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 02-21-2016 05:15 PM
My understanding is that on the pad after hatch closure ground support equipment (GSE) was used to pressurize the CM cabin at sea level pressure with a 60/40% oxygen/nitrogen mix via the CM purge port. This is much leaner in nitrogen than normal ambient atmosphere and reduced fire hazard whilst expediting nitrogen purge. The pressure suit circuit (PSC) was 100% oxygen input at slightly over cabin pressure ensuring that the PSC remained free from contamination.During ascent this was maintained until ambient pressure fell to 6 psi below cabin at around 14,000 feet and 50 seconds into the flight. The cabin and PSC pressures then followed ambient down to maintain their respective differential pressures. This also had the effect of gradually purging ("washing out") cabin nitrogen as it was replaced solely with oxygen. |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 02-21-2016 11:12 PM
Thanks David. That's a great explanation and clears up my initial questions. For Apollo 1 it was 100% oxygen at the same pressure in both the suit loops and cabin. If someone removed a glove in those conditions, it would have no effect. A similar action could not be done in later tests because of the different environments that existed in the suits and the spacecraft. |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 02-22-2016 01:28 AM
Well for a test I guess a glove could have been removed with care. For an actual launch, although the mission rules stated "continue mission" for loss of suit integrity, I doubt that would apply if a glove was removed. To my mind you'd have to assume that the PSC was contaminated with nitrogen thereby negating the pre-breath - so at the very least a launch delay. |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 02-23-2016 09:17 AM
Just read a passage from John Young's book, "Forever Young." In it he describes a test that the AS-205 backup crew were conducting on the same day as the fire. He wrote, ...there we were — Tom, Gene, and myself — sitting inside the command module wearing space suits pressured to the full O2 pressure of 3.75 psi. With the air at 14.7 psi outside our suits but inside our cabin, the virtual pressure working on us inside our suits was equivalent to about 18 psi. That seems to imply that there was a different pressure in the suit loop and the cabin during testing. On another note, wouldn't breathing 100% oxygen at high pressure for several hours be harmful? And also, were the suits rated for an internal pressure of over 16 psi? |
NukeGuy
Member Posts: 113
From: Irvine, CA USA
Registered: May 2014
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posted 02-23-2016 11:14 AM
...there we were — Tom, Gene, and myself — sitting inside the command module wearing space suits pressured to the full O2 pressure of 3.75 psi. With the air at 14.7 psi outside our suits but inside our cabin, the virtual pressure working on us inside our suits was equivalent to about 18 psi. I don't believe this is correct. The Apollo-Saturn 204 Review Board noted that there were periods of increased O2 flow in the suit loops. This was attributed to crew movement which would increase suit leakage rates. The suits could only leak if they were at or above cabin pressure.Thumbing through John Young's book I noted other questionable statements. It seemed he was taking credit for the proposal to send Apollo 8 around the moon. I haven't seen anything that supports this assertion. |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 02-23-2016 01:33 PM
An Apollo pressure suit, at root is just a rubber bag with hoses in and out, scrubbers and a pressure regulating valve, so I'm afraid basic physics says that's incorrect.However, if Young meant to say that the suit had a 3.75 psi differential pressure with the cabin (which could only mean greater than cabin, see above), then "18 psi" in the suits would be about right. "Forever Young" is rife with errors, and whilst I'm glad it was published, people need to be very careful attempting to use it as a reference. |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 03-04-2016 10:50 AM
Isn't breathing pure oxygen at high pressure harmful over a prolonged period of time? |
Space Cadet Carl
Member Posts: 305
From: Lake Orion, MI
Registered: Feb 2006
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posted 03-04-2016 12:37 PM
I'm not a doctor, but I was wondering the same thing. Breathing 100% pure oxygen at 16psi sounds like trouble. |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 03-14-2016 04:41 PM
As no-one medically qualified is going to touch this, here's a layman's view.Breathing 100% O2 at sea level pressure is no problem for short periods, some aviators even believe it cures hangovers. Pressures in excess of around 8psi for periods in excess of 24 hours of continuous exposure (time varies tremendously with individual and pressure) results in respiratory damage due to oxygen toxicity and possible eventual death. Repeated exposure for shorter periods can also do damage. There are also other minor irritating effects of breathing 100% O2 at lower pressures, such as "oxygen ear", that military aviators are familiar with. |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 03-25-2016 11:13 AM
quote:
Originally posted by robsouth:
I believe that the crew were on an independent suit loop of 3 to 4 psi while the general cabin pressure was around 16.7 psi.
Just to clarify, could you have misinterpreted the situation? I suspect that under certain test conditions the suits had 3.75 psi ΔP with respect to the cabin. This would then account for both the board's leakage comments and Young's remark in his book.
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oly
Member Posts: 1465
From: Perth, Western Australia
Registered: Apr 2015
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posted 03-25-2016 04:01 PM
Some understanding of what pressure measurements are being considered needs to be made. PSI (pounds per square inch) pressure of standard atmospheric conditions at sea level is 14.7 psi. This is absolute pressure meaning that it is 14.7 psi greater than zero pressure (space considered zero for this topic) so if a spacecraft cabin is inflated to 16.7 psi(a) then in effect it has 2 psi extra air pressure than the standard atmosphere outside. Consider this as 2 psi of stress the spacecraft is feeling.If the spacesuit inside the spacecraft only had a total of 3.7 psi air pressure compared to space then there would be 13 psi air pressure crushing the spacesuit. However, if the spacesuit has 3.7 psi extra (differential) air pressure as compared to standard atmosphere (18.4 psia) then the space suit would be inflated inside the spacecraft. As the spacecraft launches and rises in the atmosphere the cabin pressure can either be maintained at 16.7psi meaning as it climbs the differential between cabin internal pressure and outside air pressure changes until in space there is 16.7 psi pressure on the inside and zero pressure on the outside. Now we have 16.7 psi of stress on the spacecraft (compared to the 2psi pre launch). The spacesuit would still have 3.7 psi pressure inside. However the spacecraft needs to be made as light as possible meaning thin materials. 16.7 psi is a lot of stress to put the spacecraft under so it is safer to allow the spacecraft to hold less air pressure as it climbs into space (Alan Shepard cabin pressure holding at 5.5) An aircraft carrying passengers at cruise altitudes generally maintains a cabin pressure of 10-11 psia pressure. The cabin has air inside it instead of pure oxygen. This pressure allows the crew and passengers to function normally without any significant risk. This cabin pressure is about 5.5 psi greater than the pressure outside at 25000 feet altitude to give the passengers an equivalent cabin pressure of 8000 feet. So U.S. spacecraft were designed to operate with lower cabin pressures. The Apollo Command Module cabin pressure in space was maintained at 4.8 psi (ref Apollo 12 lunar surface journal) this was a pure oxygen environment allowing the crew to function normally, the lunar module cabin pressure was operated at 3.5 psi when separated from the command module. So the suit pressure inside the lunar module 3.5 psi cabin pressure was still 3.7 psi greater than cabin pressure. When the lunar module was depressurised on the lunar surface the total air pressure acting on the suit would still be 3.7 psi. As the suit is in effect a football bladder, if the pressure is increased it becomes harder to bend and manipulate. The suit was also closed by two zippers so the greater the suit pressure the greater the risk of failure. So on the lunar surface, the astronauts environment would be equivalent to an altitude higher than Mt Everest. |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 03-25-2016 05:53 PM
Thanks, but I know all that. Rob's initial question contains an error stating that in the block I CM the suit was at 3-4 psi whilst the cabin was at 16.7 psi. We both agree that's not physically possible. I'm familiar with block II systems, but am a bit hazy on block I differences. At first I thought the situation was that the suit had sufficient air to pressurize to 3-4 psi if the cabin fell to zero. Now I think perhaps the situation was a suit leak check with the suit pressurized to 3.75 psi above cabin. Happy with basic physics, but not sure what was the actual condition in the block I CM. Any thoughts? |
NukeGuy
Member Posts: 113
From: Irvine, CA USA
Registered: May 2014
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posted 03-26-2016 08:30 AM
It would seem that the suit pressures would have to be reduced during ascent along with the cabin in order to avoid overpressurizing the suit. Was this handled by the Environmentsl Control System or did the suits have a check valve that did this passively?The Block IIs had N2 in the cabin atmosphere so it would be important to maintain a suit pressure higher than the cabin pressure to avoid introducing N2 into the suit. |
NukeGuy
Member Posts: 113
From: Irvine, CA USA
Registered: May 2014
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posted 03-26-2016 08:31 AM
Just read David C.'s comments far above in this lengthening thread. |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 03-26-2016 07:52 PM
The interior of the spacecraft was pumped up to 16.7 psi of pure oxygen. Wouldn't it be harmful to breathe pure oxygen at that level of pressure for over 5 hours? |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 03-26-2016 10:13 PM
Probably not Rob, please read my comments above regarding oxygen toxicity. Also, I'm sceptical that the cabin was at 16.7 psi for five hours. Do you have a specific reference for this? |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 03-27-2016 10:12 AM
Didn't they make the internal pressure 16.7 psi of pure oxygen at the beginning of the test and maintain that level right up to the fire? |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 03-27-2016 10:33 AM
No. The simulated count was held with the hatch open to investigate an unusual odour. It was sealed for a duration of no more than 3 hours 45 minutes prior to the mishap. After sealing it was pressurized and purged of N2 as only O2 flowed into the cabin. Not sure how long a full purge was calculated to take, but it certainly wasn't instantaneous. |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 03-27-2016 03:52 PM
The crew entered the spacecraft just after 1pm and the test was still going after 6.30pm. That means the test was due to run longer than 5 hours in pure O2 at 16.7 psi. |
David C
Member Posts: 1429
From: Lausanne
Registered: Apr 2012
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posted 03-27-2016 06:16 PM
quote:
Originally posted by robsouth:
That means the test was due to run longer than 5 hours in pure O2 at 16.7 psi.
No it doesn't. The crewed portion of the test was running far behind schedule, and the mishap occured during a hold at T-10 minutes when the cabin had been sealed for no more than 3 hours 45 minutes. This doesn't change the fact that 100% O2 at 16.7psia for 5 hours is not a medical concern. |
robsouth
Member Posts: 769
From: West Midlands, UK
Registered: Jun 2005
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posted 03-31-2016 04:12 PM
Can anyone say for certain how long the part of test at 100% oxygen at 16.7 psi was going to be? |
gleopold
Member Posts: 33
From: Reston, VA, USA
Registered: Jun 2010
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posted 03-04-2024 09:57 PM
Frank Borman and Bill Anders both listed Block I (S/C 012) atmospheric pressure as 20 psia on the pad. Are they including suit pressure with the 16.7 psi cabin pressure on the ground? |
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