PDA

View Full Version : Apollo vs Orion used air



samkent
2008-Nov-26, 01:17 PM
I know that Apollo used pure O2 at about 5 psi and scrubbers to remove the CO2. Did they vent anything overboard in the process?

Is Orion using nitrogen to reduce the fire hazard? Does that require a higher psi to maintain the needed percentage of O2? Will they vent anything?

01101001
2008-Nov-26, 05:19 PM
I know that Apollo used pure O2 at about 5 psi and scrubbers to remove the CO2.

Perhaps you have a citation for that which you know? It doesn't jibe with my recollection of what was used in practice, for either launch (mixture) or cruise (pressure). It's been a long time. Maybe I've forgotten.

JohnD
2008-Nov-26, 05:37 PM
See: http://www.ocii.com/~dpwozney/apollo2.htm (which includes a comtemporary reference)

After the Apollo 1 tragedy (truly a tragedy as the gods had given fair warning in a series of fire incidents, that >90% O2 at 16psi would kill more people) NASA used 60% O2 in N2 for launch, and 5psi cabin pressure in orbit. The LM used 3.5psi and the suits 3.8psi.

I can't find an explanation for why such high O2 conc was used. At 5psi, 60% O2 provides a partial pressure of 155mmHg (20Kp), to be compared with Earth air where the partial pressure of O2 is 160mmHg(21kP), so why did they give them 90-100%?

In a decompression incident from the capsule at any height from which survival was possible, the 'bends' would be the least of the stronauts' problems!

John

samkent
2008-Nov-27, 05:38 AM
Was anything vented during flight? Other than minor leakage throught the seals.

JonClarke
2008-Nov-27, 07:25 AM
See: http://www.ocii.com/~dpwozney/apollo2.htm (which includes a comtemporary reference)

After the Apollo 1 tragedy (truly a tragedy as the gods had given fair warning in a series of fire incidents, that >90% O2 at 16psi would kill more people) NASA used 60% O2 in N2 for launch, and 5psi cabin pressure in orbit. The LM used 3.5psi and the suits 3.8psi.

I can't find an explanation for why such high O2 conc was used. At 5psi, 60% O2 provides a partial pressure of 155mmHg (20Kp), to be compared with Earth air where the partial pressure of O2 is 160mmHg(21kP), so why did they give them 90-100%?

In a decompression incident from the capsule at any height from which survival was possible, the 'bends' would be the least of the stronauts' problems!

John

Pure oxygen was used because it was simpler from an engineering perspective.

No need to carry nitrogen, just oxygen.

No need to monitor partical pressure of O2, just toal pressure, so simpler instrumentation.

It also means lower pressures can be be used, which means lighter pressure hulls. Lower pressure also means less leakage.

Pure oxygen also eliminates the need for prebreathes before EVA. And faster depressurisation-recpressurisation cycles.

All round the arguments for O2 are very cogent, and still are.

The problem was prelaunch plugs out tests where overpressures were needed. These were hazardous, but the risks could have been minmised had more attention been paid to eliminating combustibles. High pressure O2 environments were extensively used in the medical, aerospace, and diving industries with minimal risk if the proper proceedures were followed.

Jon

JonClarke
2008-Nov-27, 07:27 AM
Was anything vented during flight? Other than minor leakage throught the seals.

As far as I know, depressurisation during EVAs was achieved by venting the cabin atmosphere.

Jon

joema
2008-Nov-27, 05:58 PM
...I can't find an explanation for why such high O2 conc was used...In a decompression incident from the capsule at any height from which survival was possible, the 'bends' would be the least of the astronauts' problems!
John
To reiterate what JohnClarke said, a single-gas environment control system was lighter, simpler, more reliable.

A mixed-gas system would require astronauts to pre-breath pure O2 for hours before an EVA. Apollo featured many EVAs, so a pure O2 cabin avoided this.

The entire LEM cabin had to be evacuated and re-pressurized several times for later lunar missions. An O2/N2 cabin would require carrying a lot more gas to the moon for that, and weight was a premium -- every ounce mattered.

The low pressure pure O2 environment enabled longer survival if a cabin breach happened. The Apollo CM/LM environmental control system could compensate for a 5 inch diameter hull puncture for 2 minutes, which wouldn't be possible with a higher pressure two-gas system.

A 5 psi pure O2 environment isn't that dangerous from fire -- esp. in zero G, provided care is given to non-flammable materials.

However during the Apollo 1 "plugs out" test, the cabin was at about 16 psi pure O2. This was needed to over-pressure the interior vs the 14.7 psi ambient atmosphere. While 5 psi pure O2 isn't that dangerous a 16 psi pure O2 environment is quite dangerous from a fire standpoint.

samkent
2008-Nov-28, 04:32 AM
As far as I know, depressurisation during EVAs was achieved by venting the cabin atmosphere.

No I mean through the normal day. Like divers used air bubbles to the surface. Is there a constant release of atmosphere from the cabin as fresh O2 is entering from the tanks?

JonClarke
2008-Nov-28, 07:24 AM
No I mean through the normal day. Like divers used air bubbles to the surface. Is there a constant release of atmosphere from the cabin as fresh O2 is entering from the tanks?

No. CO2 was scrubbed and pressure topped up by O2 reserves.

Vallkynn
2008-Nov-28, 10:34 AM
I know that Apollo used pure O2 at about 5 psi and scrubbers to remove the CO2. Did they vent anything overboard in the process?

Is Orion using nitrogen to reduce the fire hazard? Does that require a higher psi to maintain the needed percentage of O2? Will they vent anything?

Apollo 1 used pure oxygen just like Mercury and Gemini, but that was a cause for the Apollo 1 fire so at launch the cabin atmosphere would be at sea-level pressure with of 60% oxygen and 40% nitrogen, changing over to 100% oxygen at about 2 psi during the first 24 hours of the trans-lunar coast.

JohnD
2008-Nov-28, 01:30 PM
I read and understand the reasons given above for the pure O2 in the capsule. But those are engineering reasons and ignore the chemistry/physics reasons not to, let alone the human risk ones.

The gods really did try to warn against it. From that link I posted above:

"Ralph René, in his book NASA Mooned America, provides a list of government-sponsored testing that resulted in oxygen fires. René extracted this information from Appendix G in Mission To The Moon by Kennan & Harvey. Here are some tests on that list:

"September 9, 1962 - The first known fire occurred in the Space Cabin Simulator at Brooks Air Force Base in a chamber using 100% oxygen at 5 psi. It was explosive and involved the carbon dioxide scrubber. Both occupants collapsed from smoke inhalation before being rescued."

"November 17, 1962 - Another incident using 100% oxygen at 5 psi in a chamber at the Navy Laboratory (ACEL). There were four occupants in the chamber, but the simple replacing of a burned-out light bulb caused their clothes to catch on fire. They escaped in 40 seconds but all suffered burns. Two were seriously injured. In addition an asbestos 'safety' blanket caught fire and burned causing one man's hand to catch fire."

"April 28, 1966 - More Apollo equipment was destroyed as it was being tested under 100% oxygen and 5 psi at the Apollo Environmental Control System in Torrance, CA."

"January 1, 1967 - The last known test was over three weeks before Grissom, Chaffee & White suffered immolation. Two men were handling 16 rabbits in a chamber of 100% oxygen at 7.2 psi at Brooks Air Force Base and all living things died in the inferno. The cause may have been as simple as a static discharge from a rabbit's fur ... but we'll never know."

NASA subjected Grissom, White and Chaffee to over 90% pure oxygen at over 16 psi in a test with live electrical circuits and switches being thrown, and with a hatch that took more than three minutes to open, resulting in the fatal Apollo 1 fire. "

I have not read the book quoted, and Mr.Rene may be a 'hostile witness', but that is a damning list of ignored forewarnings, all occuring in the NSAS environment and contemporray with the Apollo program. I wonder if the 'spam-in-the-can' attitude of NSAS engineers had peristed from the Mercury days.
John

slang
2008-Nov-28, 02:33 PM
I read and understand the reasons given above for the pure O2 in the capsule. But those are engineering reasons and ignore the chemistry/physics reasons not to, let alone the human risk ones.

To decide upon a certain course of action after weighing advantages versus risks does not mean that those risks are ignored. Risk assessment with 20/20 hindsight is easy.


I have not read the book quoted, and Mr.Rene may be a 'hostile witness', but that is a damning list of ignored forewarnings, all occuring in the NSAS environment and contemporray with the Apollo program.

It might seem damning, but I'd like to see those incidents compared to all other NASA-related fire incidents in that time-frame, where no oxygen-rich atmosphere was involved. People like mr. Rene tend to be very good in cherry picking data.

joema
2008-Nov-28, 04:32 PM
The gods really did try to warn against it. From that link I posted above:..
If the "gods" were trying to send a messages via events, they should have written a letter instead. There is little about the listed 5 psi O2 accidents indicating a fundamentally unsafe fire risk.

Rather they indicate 5 psi pure O2 in 1G without proper control of flammable materials can be a fire risk.

Every Apollo mission flew a pure O2 cabin in space. After the Apollo 1 fire, they'd never have done that if it wasn't safe.

There's no question a 16.5 psi pure O2 cabin is very dangerous. Solid metals can burn like wood, and many materials can spontaneously combust without an ignition source.

After the fire there was significant flammability testing of the spacecraft interior using various materials. Ultimately they found an approx. 5 psi pure O2 cabin was safe, provided proper attention to materials flammability:

http://www.astronautix.com/craft/csmecs.htm

A key Apollo philosophy was "complexity breeds failure". Therefore designers strove to avoid failures by designing in simplicity, rather than rely on additional systems to solve problems. That was one reason for staying with a single-gas control system.

Also, on a lunar mission you're a long way out. You can't quickly deorbit like today's shuttle missions. Maximum simplicity and reliability were vital. Every design decision had to be evaluated against the full spectrum of various contingency situations, not just fire risk.

There were various plausible situations where a higher pressure O2/N2 cabin would have killed the crew. Imagine a micrometeroid cabin puncture, smoke in the cabin, or an Apollo 13-type explosion which punctures the cabin -- anything which requires quickly donning space suits and cabin depressurization. The Mir collision and fire are two examples which nearly required this.

With a 5 psi pure O2 cabin, you don the suits and are OK. With a mixed gas cabin, you're dead from the bends. The Apollo suits were pure O2 at 3.8 psi.

JonClarke
2008-Nov-28, 08:21 PM
What evidence is there that you would be dead from the bends from a pressure drop of only ~1 atm breathing air?

Remembering some very rusty diving medcine I recall you need a certain overpressure of dissolved N2 before the bends can happen Since people have spent day and weeks at 2 atm pressure without decompressing at the end (in the Conshelf 1 and 2 experiments, for example) the bends risk from decompressing a 1 atm cabin filled with air should be small.

But space medicine people are always hyper cautious.

Mike_c130
2008-Nov-28, 09:42 PM
Since fliers exposed to low pressures due to altitude can develop various dissolved gas problems including the bends, it stands to reason that an astronaut could do so as well.

I personally know an individual who suffered an injury due to nitrogen bubbles while in an altitude chamber (starting at ground level). Don't think it was specifically the bends, but there are other problems you can get, depending on where the Nitrogen is when it comes out of solution...

Mike

JonClarke
2008-Nov-28, 10:33 PM
Since fliers exposed to low pressures due to altitude can develop various dissolved gas problems including the bends, it stands to reason that an astronaut could do so as well.

I personally know an individual who suffered an injury due to nitrogen bubbles while in an altitude chamber (starting at ground level). Don't think it was specifically the bends, but there are other problems you can get, depending on where the Nitrogen is when it comes out of solution...

Mike

That is the sort of information I was after. Do you have a reference?

Jon

joema
2008-Nov-29, 05:00 AM
What evidence is there that you would be dead from the bends from a pressure drop of only ~1 atm breathing air?...
You can definitely get the bends from rapid decompression from 1 atm. High-flying pilots are potentially subject to this. That's also why shuttle and ISS astronauts must pre-breath pure O2 for several hours before an EVA -- otherwise they'd get decompression sickness.

You're right, I'm not sure if it would be fatal. However the circumstances requiring rapid emergency donning of a space suit imply you'd need full physical and mental faculties to survive. If you were alive but debilitated by DCS, you might be unable to take necessary action.

I'll see what other info I can find...

JonClarke
2008-Nov-29, 06:09 AM
The current instance on using essentially Earth normal atmosphere for pressurisation is ging to cause real issues on the Moon and Mars. One of the reasons we go to the Moon and Mars is to explore, which means daily EVAs in many cases. This will be very difficult to do if we insist on one atm O2-N2.

Some sort of compromise may be needed, like Skylaab, with some N2 added but not enough to cause problems in the event of depressurisation.

mugaliens
2008-Nov-29, 01:58 PM
To decide upon a certain course of action after weighing advantages versus risks does not mean that those risks are ignored. Risk assessment with 20/20 hindsight is easy.

It's been my extensive experience that quite often, very clear warning information is ignored at the expense of "get the mission done" - itis. This is usually steamrolled by people in key positions with forceful, rather than flexible personalities. The latter tend to take detractions in stride, giving them their requisite consideration, and making changes where appropriate, usually early enough so that those changes do not materially detract from the mission, but rather, help improve the liklihood of overall mission success.

I'm in favor of the "you build it, you fly it" mentality. Let 'em die in their own deathtraps ; or let them succeed in the ones that are built to warrant success.

joema
2008-Nov-29, 02:06 PM
Re whether you can get DCS when quickly transitioning from 1 atm N2/O2 to 4 psi pure O2:

This can happen even after pre-breathing pure O2 for 1 hr, then ascending to 8,000 meters, where air pressure is still 10.9 psia:

http://www.chestjournal.org/cgi/content/full/121/1/173

"one hour of 100% oxygen prebreathing and 100% oxygen breathing during decompression is commonly used but is still accompanied by 50% of DCS symptoms at approximately 8,000 m"

If you were in a 1 atm N2/O2 cabin, had an emergency and had to rapidly don a 3.8 psi Apollo suit, this would equate to a going from sea level on earth to about 33,000 ft (where air pressure is about 3.8 psi): http://www.avweb.com/newspics/presvalt.gif

Your tissues and blood would be saturated with nitrogen. Exposed to the lower pressure it would come out of solution like an opened carbonated beverage.

It would be little different than a pilot in a high-flying jetliner experiencing rapid decompression, putting on an oxygen mask, and staying at 33,000 ft. He'd definitely get DCS, possibly be disabled or worse.

You likely would not die, but there's a wide range of physiological reaction. Some might be sick, others disabled, others dead.

Anyway it illustrates there are real risks to an earth-like 1 atm N2/O2 cabin environment. It's easy to look at the Mercury through Apollo use of pure O2 and think that's dangerous, just use a mixed-gas cabin. However each choice has it's own risks and costs.

Mike_c130
2008-Nov-29, 02:13 PM
That is the sort of information I was after. Do you have a reference?

Jon

I don't have a written reference for the injury, just a personal conversation with the individual back in about 1998. The injury did not permanently ground him as far as I know, but it was serious enough that he was considerably less enthusiastic about flying than he had been before the accident.

This link (http://www.faa.gov/pilots/safety/pilotsafetybrochures/media/dcs.pdf) is to a Federal Aviation Administration (FAA) safety brochure on decompression sickness for pilots. It may have some more of the info you are looking for.

Mike

JonClarke
2008-Nov-30, 12:06 AM
I don't have a written reference for the injury, just a personal conversation with the individual back in about 1998. The injury did not permanently ground him as far as I know, but it was serious enough that he was considerably less enthusiastic about flying than he had been before the accident.

This link (http://www.faa.gov/pilots/safety/pilotsafetybrochures/media/dcs.pdf) is to a Federal Aviation Administration (FAA) safety brochure on decompression sickness for pilots. It may have some more of the info you are looking for.

Mike

Thanks Mike, that is most helpful.

Thinking this through....

From it I gather that DCS is unknown below 18,000 feet (5500 m), and rare even then. The pressure at that altitude is half sea level. This means that the pp of disolve nitrogen over and above ambient that can be sustained without DCS is 0.4 bars (I am a diver so I think in bars - take than anyway you will)

So a spacecraft atmosphere that has 0.4 bars pp N2 and 0.2 bars pp O2 should not cause DCS in the event of rapid decompression.

Jon

joema
2008-Nov-30, 02:51 PM
...I gather that DCS is unknown below 18,000 feet (5500 m), and rare even then. The pressure at that altitude is half sea level.
I think the article is saying DCS below 18,000 is rare assuming a controlled ascent to that altitude. IOW it's about going up in an unpressurized plane, not about rapid decompression at that altitude. In a spacecraft rapid depressurization requiring donning a pressure suit, the pressure loss rate could be much higher. Also the duration at the lower pressure would be longer. Likelihood of DCS varies based on altitude, rate of ascent, time at altitude and individual physiology.

One topic theme is "Apollo 5 psi pure O2 dangerous, they should have used N2/O2".

If they'd used a Soviet-style 1 bar (14.7 psi) N2/O2 cabin, consider the decompression parameters for a several-inch diameter hull puncture:

Loss rate (analogous to ascent): approx 5 minutes. During that time they'd don their 3.8 psi pure O2 suits. This equates to going from sea level to 33,000 ft in 5 min, or a sustained rate of 6,600 feet/min. Time at altitude would be indefinite, until problem was resolved or mission aborted.


...This means that the pp of dissolve nitrogen over and above ambient that can be sustained without DCS is 0.4 bars (I am a diver so I think in bars - take than anyway you will)...So a spacecraft atmosphere that has 0.4 bars pp N2 and 0.2 bars pp O2 should not cause DCS in the event of rapid decompression.


I think the max permissible pp N2 is lower than that, if goal is avoiding DCS from rapid pressure loss. Skylab used a 5 psi cabin, 70% O2 and 30% N2. This supposedly had low risk of DCS; not sure they pre-breathed O2 prior to EVA. However this equates to 0.24 bar pp N2.

Will try to look up more on this...

JonClarke
2008-Dec-01, 10:31 AM
I think the article is saying DCS below 18,000 is rare assuming a controlled ascent to that altitude. IOW it's about going up in an unpressurized plane, not about rapid decompression at that altitude. In a spacecraft rapid depressurization requiring donning a pressure suit, the pressure loss rate could be much higher. Also the duration at the lower pressure would be longer. Likelihood of DCS varies based on altitude, rate of ascent, time at altitude and individual physiology.

Agreed, there is no real cutoff but there is a point below which the incidence is so low as to be meaningless.



I think the max permissible pp N2 is lower than that, if goal is avoiding DCS from rapid pressure loss. Skylab used a 5 psi cabin, 70% O2 and 30% N2. This supposedly had low risk of DCS; not sure they pre-breathed O2 prior to EVA. However this equates to 0.24 bar pp N2.

Will try to look up more on this...

I am pretty sure that no prebreathing was needed on Skylab. Skylab was a good compromise, I think, although the crews did note that sound transmission was poor at such low pressures.

Jon