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Tinaa
2008-Feb-10, 09:34 PM
I've been watching NASA TV. The astronauts, like scuba divers, have to spend time on pure O2 to get rid of the nitrogen in their system before a space walk. This was news to me. Has this always been the case? Did a astronaut ever suffer from decompression sickness?

grant hutchison
2008-Feb-10, 10:10 PM
IIRC, capsule atmosphere was pure oxygen until the Apollo 1 fire. So prebreathing presumably wasn't required for the Gemini spacewalks.
Don't know about cosmonauts, though.

Grant Hutchison

mugaliens
2008-Feb-11, 07:02 PM
Breathing pure O2 is called "prebreathing." It's required for space walks, as well as non-pressurized activities such as the aerial delivery of HALO (high altitude, low opening) military personnel.

The concept is simply - pre-breath pure O2 long enough (thirty minutes is the standard), and you no longer have any appreciable amount of nitrogen gas dissolved in your bloodstream or body tissues.

This doesn't mean that your blood won't boil below a certain pressure level. It simply prevents nitrogen narcosis, which is what happens when you do have nitrogen in your blood and the pressure surrounding your body drops below a certain level.

As it turns out, the human body can easily survive a 10-second exposure to pure vaccuum, without the blood boiling, eyes popping out, etc.

Not much longer, though...

grant hutchison
2008-Feb-11, 09:34 PM
It simply prevents nitrogen narcosis, which is what happens when you do have nitrogen in your blood and the pressure surrounding your body drops below a certain level.Nitrogen narcosis is actually caused by exposure to high pressures of nitrogen: usually over three bars. It's a hazard for high-pressure workers, rather than in aerospace medicine.
What prebreathing prevents is the bends, which is caused by dissolved nitrogen bubbling out of solution at low ambient pressures.

Grant Hutchison

Tinaa
2008-Feb-11, 10:48 PM
Did Apollo astronauts go through prebreathing before moon walks? Has this always been done or when did it start? If we ever get to Mars, will the explorers have to spend time prebreathing?

GeorgeLeRoyTirebiter
2008-Feb-12, 06:18 AM
This was mentioned in a recent thread (http://www.bautforum.com/space-exploration/70101-orion-atmosphere.html), but even after the fire the Apollo spacecraft continued to use a pure oxygen atmosphere at low pressure (5 psi) while in space. The high pressure cabin atmosphere used while on the ground was changed to 40% nitrogen.

The Apollo (and earlier) astronauts probably did need to prebreathe O2, but only before launch. If the suits used pure O2, the time spent sitting on the pad before launch probably counts as prebreathing. Prebreathing before each spacewalk wasn't needed until the advent of the Shuttle because it's the first US spacecraft to use a mixed-gas environment at or near normal atmospheric pressure.

Noclevername
2008-Feb-12, 08:15 PM
It's also an added protection in case the suit springs a leak. No chance of the Bends from sudden decompression.

idav
2008-Feb-14, 08:50 PM
From reading up on it it seems that nitrogen saturates body tissues more thoroughly than oxygen. Pre-breathing O2 for long enough will sustain a loss of dissolved nitrogen in your blood and body tissues. Evidently, it seems that nitrogen is the primary gas that causes the bends. I didn't know that, I thought it was just gases in general.

Nitrogen Narcosis is a completely different thing.(Correction: the narcosis is different but if you are narced that means you have more N in your body and the bends will be worse, so they aren't completely disassociated.) At great pressures the body will absorb more and more nitrogen from the air we breath. A build up of nitrogen will cause the subject to become intoxicated and loose proper judgment. Not much different than breathing laughing gas I guess. It's called getting narced.

When I was 12 and getting SCUBA certified I went on a dive at Lake Rawlings, near Petersburg Virginia. It turns out that instead of opening my first stage fully and then a 1/4 turn back, I closed it fully and turned it a 1/4 turn back open. At atmospheric pressure the apparatus still seemed to be working fine. However, once I got below about 40 feet my breaths started getting cut short. Because the valve wasn't opened enough there wasn't enough back pressure to aid my breathing and force air into my lungs at ambient pressure. This startled me, I signaled my father and made my ascent. This was my first introduction to the necessity of staying coolheaded. I ascended to fast because I was in a panic and shot from 40 feet to surface in a matter of seconds. (FYI air gains buoyancy as it decompresses so I ascended at an increasing rate and didn't have the experience to regulate my BC's inflation) When I got to the top I most certainly could feel the bends. Definitely the oddest feeling I've ever experienced. Luckily I was with an entire Rescue Squad of folks so if the worst did happen I was in about as good company as I could of had.

joema
2008-Feb-14, 10:17 PM
...The Apollo (and earlier) astronauts probably did need to prebreathe O2, but only before launch. If the suits used pure O2, the time spent sitting on the pad before launch probably counts as prebreathing....
Correct on Apollo, however I think Mercury & Gemini used a pure O2 cabin environment at about 5 psi, on the pad and during missions. This eliminated any need to pre-breath O2.

I don't think a pure O2 cabin at 5 psi has much greater fire risk than a 80/20 N2/O2 mix at 14.7 psi. The O2 partial pressure isn't that much higher than sea level.

The Apollo 1 fire happened during a ground test when the cabin was at about 16.7 psi (!!) of pure O2. Yes, that's a fire risk.

The command module required net positive internal pressure for structural reasons. With a single-gas system, the only way to achieve that in a 14.7 psi ambient situation is over-pressure the cabin with pure O2.

If you think about it, if the interior was at 5 psi and exterior at 14.7 psi, there would be over 100 tons of differential pressure on the capsule.

I think Mercury initially planned for a 80/20 N2/O2 mix at 14.7 psi on the ground, changing to 5 psi pure O2 in orbit. However I recollect an engineer was killed in a ground test (or nearly so) because the N2/O2 ratio got out of whack -- N2 was too high. That's when they switched to a pure O2 ground environment, which I assume Gemini also used.

GeorgeLeRoyTirebiter
2008-Feb-17, 08:53 AM
Correct on Apollo, however I think Mercury & Gemini used a pure O2 cabin environment at about 5 psi, on the pad and during missions. This eliminated any need to pre-breath O2.

I did some research on the Mercury and Gemini environmental control systems, so I'll have to disagree with you here. On the pad, they used O2 at or slightly above ambient pressure (the capsule would be pressurized to 3 psig for a leak check after boarding and the cabin purge, but I can't find whether or not this pressure was maintained for the entire prelaunch phase). During launch, a pressure relief valve would open to maintain 5.5~6.0 psig. This would be allowed to decay in orbit to about 5.1 psia.

I'm not sure why these conditions wouldn't require prebreathing. USAF crews on high altitude flights (above 50,000 ft) must prebreathe for an hour, and they experience conditions almost identical to those of the Gemini astronauts: a cockpit pressure of slightly more than 5 psia, pure O2 suit supply, and in the event of a decompression, a 3.5 psi suit pressure.

joema
2008-Feb-17, 01:11 PM
I did some research on the Mercury and Gemini environmental control systems, so I'll have to disagree with you here....I'm not sure why these conditions wouldn't require prebreathing...
You're correct, Mercury/Gemini/Apollo required pre-breathing pure O2 before launch. This was because the in-flight cabin environment was roughly 5 psi pure O2, so nitrogen had to be washed out of the bloodstream before launch.

I shouldn't have said Mercury/Gemini used pure O2 at 5 psi on the pad -- I don't know. It was pure O2, however. For structural reasons it would make sense to be roughly 14.7 psia, then bleed down to about 5 psi during the ascent.

Pre-breathing before EVA generally wasn't required on those missions, unlike shuttle and ISS. However some Gemini flights having an early EVA required a pre-breath period, as all the N2 hadn't washed out by then, considering the lower space suit pressure.

mugaliens
2008-Feb-18, 01:23 PM
Nitrogen narcosis is actually caused by exposure to high pressures of nitrogen: usually over three bars. It's a hazard for high-pressure workers, rather than in aerospace medicine.
What prebreathing prevents is the bends, which is caused by dissolved nitrogen bubbling out of solution at low ambient pressures.

Grant Hutchison

Errr... Sorry. Wrong answer.

Military aircrew members conducting HALO (high altitude, low-opening) missions MUST pre-breath pure O2 for thirty minutes before take-off to prevent nitrogen narcosis at HALO "green light" (delivery) altitudes beyong 18,000 feet.

Same goes for the jumpers, who're also on pre-breathing equipment at least thirty minutes before their jump.

This is due to the nitrogen buildup within the human bloodstream at sea-level pressures, NOT "three bars" as you claim.

It is a very REAL concern for aerospace medicine. Please do not doubt this! Every high-altitude (above 10,000 feet) jumper and aviator knows this, and some of us have seen the consequences when this reality has been ignored.

joema
2008-Feb-18, 01:33 PM
Errr... Sorry. Wrong answer...
Grant's answer is correct.

You have apparently confused nitrogen narcosis with decompression sickness (the bends), which are two different things.

Nitrogen narcosis: http://en.wikipedia.org/wiki/Nitrogen_narcosis
Decompression sickness (the bends): http://en.wikipedia.org/wiki/Decompression_sickness

grant hutchison
2008-Feb-18, 02:39 PM
Grant's answer is correct.

You have apparently confused nitrogen narcosis with decompression sickness (the bends), which are two different things.Yes, the name "narcosis" provides the clue: the nitrogen, at higher-than-normal partial pressures, acts in the manner of an inhaled anaesthetic agent. In extreme situations, it can produce unconsciousness. But usually it "just" produces disinhibition and failures of judgement, which are of course very dangerous underwater.

Grant Hutchison

mugaliens
2008-Feb-20, 01:21 AM
Nitrogen narcosis is actually caused by exposure to high pressures of nitrogen: usually over three bars. It's a hazard for high-pressure workers, rather than in aerospace medicine.
What prebreathing prevents is the bends, which is caused by dissolved nitrogen bubbling out of solution at low ambient pressures.

Grant Hutchison

You're absolutely correct, Grant, and I stand corrected! I inadvertantly mixed terms.

Must have been the narcosis...

:whistle:

KLIK
2008-Feb-20, 01:50 PM
Slight hijack, but I was always fascinated by stories of JBS "an inordinate fondness for beetles" Haldane, who did a lot of experiments with his son on the effects of decompression, with themselves as guinea pigs. Also poison gas etc. An amazing constitution.

http://www.dadamo.com/wiki/wiki.pl/J.B.S_Haldane

grant hutchison
2008-Feb-20, 04:43 PM
Martin Goodman has recently written a fine biography of JS Haldane (father of JBS) describing their various gas-breathing experiments: it's called Suffer and Survive. I'd recommend it for the stories, despite the fact Goodman doesn't write very grammatically.

Grant Hutchison

weatherc
2008-Feb-20, 05:21 PM
I haven't seen this mentioned anywhere in this thread (unless I missed it), but aren't the spacesuits only pressurized to a percentage of pressure at sea level? If they didn't, then they would have a lot of trouble bending their arms when doing an EVA (the space suits stiffen up like balloons when filled to 1 atmosphere). Isn't that the reason for the O2 prebreathing?

grant hutchison
2008-Feb-21, 01:01 AM
I haven't seen this mentioned anywhere in this thread (unless I missed it), but aren't the spacesuits only pressurized to a percentage of pressure at sea level? If they didn't, then they would have a lot of trouble bending their arms when doing an EVA (the space suits stiffen up like balloons when filled to 1 atmosphere). Isn't that the reason for the O2 prebreathing?That's right.
The physiology goes like this:
The quantity of nitrogen dissolved in your tissues is in equilibrium with the partial pressure of nitrogen you breathe. If I give you less nitrogen to breathe, you'll expire some dissolved nitrogen until a new equilibrium is reached. If I give you 100% oxygen to breathe, without changing the ambient pressure (say by providing a tight-fitting face mask and high oxygen flow), you'll gradually wash all the nitrogen out of your tissues. The partial pressure of dissolved nitrogen in your blood and well-perfused organs falls very rapidly, but the gas continues to trickle out of poorly perfused tissues (fat, connective tissue) for many minutes.
But you don't get the bends. The reason nitrogen bubbles don't form under these circumstances is because the ambient pressure is maintained. Any dissolved nitrogen that "tries" to form a bubble can only do so by producing a gas pressure higher than ambient (which is transmitted hydrostatically from the atmosphere to all your tissues). But ambient pressure is, in this scenario, still one bar, which is higher than the 0.8 bar (and falling) partial pressure of dissolved nitrogen in your tissues. So nitrogen bubbles can't form.
But if I were suddenly to transport you from a one-bar air environment to a 0.35 bar oxygen environment (as in a spacesuit), the hydrostatic pressure in your tissues would now be lower than the partial pressure of nitrogen dissolved in your tissues: gas would be able to come directly out of solution by forming bubbles, rather than having to find its way out by diffusion to the circulation and lungs.
In practice, then, depressurization turns into a race between the loss of dissolved nitrogen via the circulation and lungs, and the falling ambient pressure. If ambient stays higher than the dissolved partial pressure, then you don't bend. If ambient drops significantly below the dissolved partial pressure, bubbles can form. Hence the need for decompression tables and prebreathing, in order to ensure that ambient pressure is always safely above the dissolved partial pressure in even poorly perfused tissues.

Grant Hutchison

BigDon
2008-Feb-21, 09:56 AM
What almost never gets airtime in the recruitment commercials is how many former Navy SEALs are permenantly incontinent from nerve damage due to the bends. I'm told it's one of the first thing that goes. Now there is a sacrifice for your country.

Spend as much time in a VA hospital neuro-ward as I have and you learn these sad little things.

mugaliens
2008-Feb-21, 03:21 PM
I haven't seen this mentioned anywhere in this thread (unless I missed it), but aren't the spacesuits only pressurized to a percentage of pressure at sea level? If they didn't, then they would have a lot of trouble bending their arms when doing an EVA (the space suits stiffen up like balloons when filled to 1 atmosphere). Isn't that the reason for the O2 prebreathing?

Spacesuits with CV (constant volume) joints have been around for years, which minimizes or totally negates the resistance against bending joints.

The main reason behind O2 pre-breathing is simply that there's no need to use nitrogen (or any other gas) for EVA, and doing so has a weight penalty. Pure O2 at a reduced pressure is the cheapest way to go, and reducing the pressure requires the astronauts to prebreath.