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I agree that MCP suits make a much better choice for surface explorations almost anywhere, if you make them vacuum-protective underwear, and put whatever outer garments over them to match the job at hand. Not only are they less restrictive, they are lighter, and they are more amenable to in-the-field first aid, as Kbd512 so astutely points out just above.
The problem with MCP-done-as-elastic is difficulty donning and removing the suit. But the principle behind it is simple mechanical compression applied to the skin. The very first example of this principle was the "partial pressure" suit of the late 1940's and throughout the 1950's, into the early 1960's. Those were not elastic, and used air pressure in tubes to pull them tight against the skin. Hands and feet were uncompressed, adequate for a 10 minute bailout from above 70,000 feet.
Why not combine the two technical approaches that embody the same fundamental principle? Use breathing gas pressure in tubes to pull the elastic fabric tight against the skin. Being elastic it conforms better, evening-out the achieved compression far better. You have to have the breathing gas at a suitable pressure anyway inside the helmet and tidal volume bag. Feed it to the tightening tubes as well.
Now you have a lightweight, supple "skinsuit" as vacuum-protective underwear (!!!) that you can sweat right through for cooling, over which you can wear any needed outer garments for insulation and protection, and which is now easy to doff and don. Plus it is easily launderable. Your backpack reverts to only an oxygen supply and a radio.
So why isn't this already a done deal? Maybe because it is yet to be used as another reason not to go? Or maybe the "old space" suit maker vendors aren't through making scads of money on ever-more-ridiculous gas bag suits? Or maybe both?
You come in through a more-or-less standard airlock, do a gas blowdown to remove loose dust, pressurize, go into a changing room to doff the suit and bag it for the laundry, shower, dress, and then go on inside. Put the changing room as a separate compartment adjacent to the airlock itself, but as the only way on inside. Why is this a difficult conceptual issue?
As for gas contamination, what you do is blow a tiny bit of habitat air into the open-outside airlock to purge the Martian atmosphere volume with an airlock volume of real air. Then you close the outer door and pressurize with air. At ~6 mbar, that's only a tiny mass of air you lose. On the moon or an asteroid, there is no need to purge. Or in space, for that matter. You are having to manufacture habitat air anyway on a continuous basis over time.
I'm not sanguine where the nitrogen will come from, but the oxygen derives from electrolyzed Martian water. Using something near 21% O2 79% N2 at near 0.5 to 1 bar lets people live pretty much in the atmosphere that they evolved to be in. That pretty much takes care of unexpected adaptation issues or medical effects, including pregnancy and children.
Why is this still a conceptual problem? For use later as another excuse not to go? Or because no "old space" contractor has figured out how to profit from it? Or both?
GW
Last edited by GW Johnson (2017-11-23 11:11:40)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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It's not so hard to imagine something like the reverse of what you're proposing, where a system of tubes can be inflated to loosen up the suit to make donning and doffing easier. This seems like it would impede motion somewhat less.
I'm all for the use of an air shower to remove dust from the suit.
-Josh
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Hi Josh:
I was just trying to make it (1) as foolproof as I could, and (2) as close as possible to exactly what we have built before. The idea is to build a partial pressure suit with capstan tubes, but implemented with the elastic fabrics tried experimentally as a "space leotard" in 1968.
Use multiple layers whose count varies about the body in order to tailor as even a compression as possible. No tailored fabrics required, just plain commercial materials, and commercial sewing techniques. If the breathing oxygen is "on", the suit is tight. If "off", it is loosened. Simple.
The elastic leotard experiments were done with a breathing oxygen pressure of 0.25 atm, as opposed to NASA's current "standard" of 0.33 atm. It could even be lower, perhaps 0.15-0.20 atm. Using 0.20-0.21 atm with pure oxygen pretty much gets you the same oxygenation as sea level air, if you ignore the water vapor displacement effect of wet lung tissues at body temperature. Unlike the stiff gas bags, the wearer does not have to "fight" the suit to do tasks or move about, so there's no need to compensate by over-oxygenation.
GW
Last edited by GW Johnson (2017-11-23 15:09:12)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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The elastic leotard experiments were done with a breathing oxygen pressure of 0.25 atm, as opposed to NASA's current "standard" of 0.33 atm. It could even be lower, perhaps 0.15-0.20 atm. Using 0.20-0.21 atm with pure oxygen pretty much gets you the same oxygenation as sea level air, if you ignore the water vapor displacement effect of wet lung tissues at body temperature.
Apollo was originally going to have 3.3 psi pure oxygen in suits. That would allow 10% pressure loss, leaving oxygen astronauts could easily breathe. 1 standard atmosphere is 14.69595 psi. Earth's atmosphere is 20.946% O2, so partial pressure O2 is 3.0 psi. That means humans can breathe 3.0 psi pure oxygen just like sea level. Boulder Colorado has 2.54 psi partial pressure O2. Dr. Paul Webb did several experiments. He had access to a pressure chamber simulating 50,000 feet, so chamber pressure was only reduced to 60 mm mercury, which equals 1.16 psi. He also built an arm chamber, which reduced pressure to 5-8 mm Hg (0.09668 to 0.15469 psi). The sleeve developed 100 mm Hg (1.93 psi) counterpressure. He concludes by recommending a suit with 170 mm Hg (3.287 psi) counterpressure.
Apollo A7L (Apollo 7-14) and A7L-B (Apollo 15-17, Skylab, Apollo-Soyuz) had operating pressure of 3.7 psi.
The white spacesuit used on Shuttle and now ISS is EMU. It's operating pressure is 4.3 psi. So GW is right.
The paper in question. I received a photocopy from the journal of Aerospace Medicine. I scanned it, retyped and formatted using Microsoft Word. I kept formatting as close to the original as possible, including font, italics, text size, bold, and all images embedded in their original locations. All spelling and punctuation is original. The Space Activity Suit: An Elastic Leotard for Extravehicular Activity
As another piece of trivia, Dr. Webb did the experiments in 1967, his first contractor report to NASA was written in 1967, submitted the paper for publication in December 1967, it was published in the April 1968 issue.
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To get the numbers truly right, you have to displace the air pressure in the lungs by the equilibrium vapor pressure of water vapor at body temperature. This humidity inside the lungs derives directly from the the moisture evaporating from the wet lung tissues. That is how one calculates the in-lung partial pressure of the air, which you can then split into partial pressures of the oxygen and the nitrogen with their volume percentages.
What you want in your suit is the same wet, in-lung partial pressure of oxygen as for the altitude you believe is tolerable breathing Earthly air. For a pure oxygen suit, use the necessary wet in-lung partial pressure of oxygen plus the same equilibrium water vapor pressure at body temperature. That sum is your suit pressure requirement for the very same oxygenation level.
It is only 0.26 atm in a pure oxygen suit to match the oxygenation you get from sea level air on Earth. 0.20 atm in the pure oxygen suit corresponds to Earthly air at 10,000 feet, where USAF demands its pilots to go on oxygen. 0.18 atm in the suit corresponds to Earthly air at 14,000 feet, where the FAA requires civilian pilots to use oxygen regardless.
You can survive at even lower suit pressures, but it is unlikely you would be cognitively functional. On the other hand, there is simply no need for the excess oxygenation of NASA's "standard" of 0.33 atm, except to help overcome the work-induced oxygen deficit of having to overcome the enormous resistance forces of the stiff, restrictive gas balloon suits they insist on using.
By the way, the old partial pressure suits used from the late 1940's into the 1960's for high-altitude pressure breathing are actually an early, incomplete form of an MCP suit. Achieved compression was rather non-uniform, and the hands and feet were left uncompressed. Blood pooling into undercompressed body parts let to fainting within about 10-15 minutes, while the uncompressed hands and feet would only start swelling after about 30 minutes. So, these suits were intended for only 10 minutes protection during a high altitude bailout above 70,000 feet. They were used in the early rocket X-plane work to that altitude, and in the U-2 to near 90,000 feet.
Somewhere about 1959 somebody tried a foam that expanded as air pressure dropped as the basis for an MCP suit. Then Paul Webb did it with tight elastic fabrics in the late 1960's far more successfully. Some years later still Dava Newman at MIT did an MCP suit based on exotic tailored fabrics with anistropic stiffness properties.
The idea I had today was to combine the tensioning capstans of the partial pressure suit with the simple elastic fabric layers of Dr. Webb's leotard design. Deflating the capstans let you relax the garment for easy doff and don, while the elastic fabric evens-out the compression, and lets you tailor it to the part of the suit by selecting the right number of layers. When the suit oxygen is "on", helmet, tidal volume bag, and tensioning capstans are all pressurized.
I did post this over at "exrocketman" today, too.
GW
Last edited by GW Johnson (2017-11-24 00:59:18)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW,
This is just a thought, but the way in which English riding boots are donned is by using boot hooks or pulls. The same thing can be done with a MCP suit using woven eyelets (actually more like the pocket where a drawstring goes than an eyelet or tab) in the material. I can think of a way to expand or stretch the material using flexible plastic rods. Conceivably, this could even be done one-handed so no assistance is required to don a suit.
I assume everyone here has seen releasable zip ties. Same concept, except instead of a thin piece of plastic, you use a flexible plastic rod with a blunt tip and reversed teeth. You fish the rods through the eyelets in the material and then push the rod back through a locking mechanism to expand the material. It's just a reversed zip tie. Whereas zip ties are designed to prevent unintended opening, this device prevents unintended closing. Anyone who has ever threaded a rod through a nylon tent knows how this works, but we're using teeth or a friction locking mechanism to prevent the fabric from collapsing. Perhaps teeth aren't even necessary and a friction lock will do.
Here's a commercial product that is the basis for the general idea:
Imagine that the teeth were reversed such that you could push back on the tip of the clamp jaw, because the "jaw" was actually a flexible plastic rod completely separate from the locking mechanism, so it would "stay" or hold open using the locking mechanism. When the lock is released, the suit clamps around the wearer's body and the rods are pulled free of the suit. A skilled user could don an entire suit in about ten minutes or less, especially if the rods were already fished through the suit. The same process would assist with suit removal.
Although air tubes woven into the fabric could also assist with the task of fitting the suit to the wearer's body or expanding the fabric for donning, I think the original Space Activity Suit worked well and was simpler in operation, even if complex manufacturing was required and it was a pain to put on and take off because zip ties were not used. Ty-Rap was invented around 1958, but maybe Dr. Webb wasn't aware of the technology. In any event, we now have laser body mapping and computer controlled fabric milling machines to mill and sew fabric in ways that simply didn't exist back then. Seamless fabrics are still expensive to manufacture because the machines are expensive, but that's what we need.
Pregnant women, small children, and the elderly can be moved using rescue balls, if required. EVA is not a task for people who are not completely mobile and strong.
I've only put a few minutes of thought into this, so maybe it won't work or will tear the fabric. It seems like a simple solution, but maybe it won't work for reasons I haven't thought of. Feel free to poke holes in the idea. Incidentally, Dava Newman's group from MIT already came up with a solution for applying even pressure over the entire body that's woven into the fabric and it appears to work. I've no idea about long term durability, but presume they've tested it to perfection, as is the NASA way. She seems obsessed with NASA pressurization standards rather than simply performing testing to determine what humans can withstand without injury. There may be a good reason for that. If it prevents them from coming up with a working solution because the technology to do what she wants to do doesn't exist, then maybe she should approach the problem by first determining what human physiology can withstand without injury.
Maybe Rob Dyck could comment since he knows so much about plastics.
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Hi Kbd512:
I honestly don't know if some sort of expansion by rods would prove practical. I suppose it could. The zip tie and cable clamp technologies were pretty new in the early to mid 1960's. I would suspect that Paul Webb wanted to stay with older, more tried-and-true technologies, allowing only one new one: panty hose stretchable material. Which actually derived from women's stretchable nylon hose in the early 1950's, thus really not so new after all.
My Dad told me before he died that he knew Paul Webb. Webb was their go-to adviser at Chance Vought for high-altitude escape human factors. Dad made use of that connection designing the F-8 Crusader, which could reach altitudes exceeding 50,000 feet.
Dava Newman was relatively-underfunded at MIT to study MCP suits for NASA, which is why she had to use NASA's 0.33 atm criterion. Her designs easily worked at 0.2 to 0.25 atm breathing pressures, but could not reach 0.33 atm. NASA eventually hired her for a while, but did not let her work on MCP suits. To the best of my knowledge (1) no one is working MCP anymore, and (2) Dava is no longer with NASA.
Someone should be working MCP. I'm too old (and retired) to do it anymore, plus it's quite far from my experience and training, which was aerothermodynamics and propulsion. I just had the idiot idea to use the partial pressure suit tensioning capstans as a doff/don aid, while using the elastic leotard idea to even-out the achieved compression far better.
So far, I don't see why it wouldn't work, and I don't see why we cannot use something closer to 0.25 atm. Probably not below 0.20, although it might actually work pretty well even down to 0.18 atm.
I'm trying to figure out what the capstan pressure should be to achieve compression in the suit. Since the actual movement is small, the material stiffness will have to be high, which suggests multiple layers of the stretchable fabric. That's pretty much the same thing Webb did: his suit was 6 to 7 layers on the torso and extremities. Somewhere in one of his reports, he presents data on how much lower the extremity compression can be, without causing pooling problems.
I'm not sure just how low the compression in the gloves and booties can be, except that no compression is not acceptable. We know about that from Joseph Kittinger's balloon jump from above 100,000 feet circa 1960. His right-hand glove failed to pressurize, exposing that hand to fairly hard vacuum for hours. It swelled up to painful uselessness within about 30 minutes. After landing, the re-compressed hand un-swelled within a few hours. No permanent damage.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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To the best of my knowledge ... (2) Dava is no longer with NASA.
She was Deputy Administrator of NASA. Wikipedia descrbes the position "The Deputy Administrator of NASA serves as the agency’s second in command and is responsible to the administrator for providing overall leadership, planning, and policy direction for the agency." Her employment ended the day Donald Trump was inaugurated. I'm sure there's a story there.
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So far, I don't see why it wouldn't work, and I don't see why we cannot use something closer to 0.25 atm. Probably not below 0.20, although it might actually work pretty well even down to 0.18 atm.
I'm trying to figure out what the capstan pressure should be to achieve compression in the suit. Since the actual movement is small, the material stiffness will have to be high, which suggests multiple layers of the stretchable fabric. That's pretty much the same thing Webb did: his suit was 6 to 7 layers on the torso and extremities.
I have argued to reduce suit pressure to 3.0 psi. To keep the principle that the suit can lose 10% with partial pressure O2 still being the same as astronauts are used to, reduce cabin pressure to 2.7 psi O2. Apollo and Skylab used 5.0 psi total pressure with 60% O2, 40% N2. That works out to 3.0 psi pp O2, 2.0 psi pp N2. That's the same partial pressure as KSC; no need for a Mars mission to do that. As I pointed out, Boulder Colorado has 2.54 psi pp O2, so setting Mars cabin partial pressure to 2.7 means it's still higher than Boulder.
This is important because it dramatically affects how MCP suits work. Dr. Paul Webb found silicone pads were not necessary in the palm and back of the hand. Mitchell Clapp worked on an MCP glove in the 1980s, intended for use with the existing MCP suit. He hoped to use those gloves for space station construction. So he increased pressure to match the MCP suit: 4.3 psi. Doing that required the silicone pads. These are flat portions of the human body, to ensure smooth pressure on the body from elastic fabric. The pads are plastic bags filled with the same liquid silicone that women's breast implants used to be filled with. The test subject would wear the gloves and place his hand in a glove box, which was pumped down to vacuum. The test subject was a master student, and a member of the Mars Society. He reported that the gloves were comfortable while his hands were in vacuum, but until he put his hand in the glove box, the glove hurt. That is, wearing just a glove with nothing else MCP on his body, the unbalanced pressure was so uncomfortable it hurt. Dr. Webb did not report his test subject reporting this, but Dr. Webb's suit used multiple layers and covered the whole body, so strong unbalanced pressure was not an issue.
Keeping pressure low is absolutely critical to make an MCP suit work. Dr. Webb argued for 170 mm Hg (3.3 psi), which was the same pressure the Apollo A7L suit team was initially working with. I argue to sneak it down a bit, to make donning and doffing more convenient/comfortable, but also to reduce counterforce on joints while wearing the suit outside on Mars.
Last edited by RobertDyck (2017-11-25 16:21:01)
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Some of Webb's work was done at 190 mm Hg = 0.25 atm = 3.67 psia. Some was done at 170 mm Hg = 0.224 atm = 3.29 psia.
I was showing the same wet in-lung PpO2 as sea level air with suit oxygen at 0.26 atm = 3.82 psia = 197 mm Hg. But if there is no suit stiffness to overcome, there is no need for even sea level oxygenation. I used the 10,000 foot USAF oxygen requirement. For that altitude, one has the same wet in-lung PpO2 with suit oxygen at 0.20 atm = 2.94 psia = 152 mm Hg. That really eases the MCP feasibility and comfort problem.
If you lose 10% of that by leakage to 0.18 atm = 2.65 psia = 137 mm Hg, your wet in-lung PpO2 is the same as air at 14,000 feet. That is the FAA limit beyond which civilian pilots must go on oxygen. That's plenty, really.
Those numbers are why I think less-than-3 psia oxygen suit pressure is fine, as long as the suit is non-restrictive! If you are working really hard to overcome suit stiffness just to move or do a task, then you really do need more wet in-lung PpO2. It's those idiotic stiff, bulky gas balloon suits that drive the NASA requirement for 0.33 atm = 4.85 psia = 251 mm Hg.
I did some studies of pure oxygen suit pressures versus habitat atmosphere pressures and compositions. Those are also posted over at "exrocketman". About the max allowable oxygen concentration is 30% by volume, limited by acceptable fire hazard. 40% is just too much of a fire hazard.
At 30%, habitat atmosphere pressures can range from around 8 psia to a full 14.7 psia, and still match relatively low altitude places where people live on Earth. I could not meet the pressure ratio criterion for eliminating the pre-breathe time, but the time to actually don the MCP suit should be roughly comparable to the necessary pre-breathe time. The lower suit pressures do help that a bit.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Here's the NASA Contractor Report on Dr. Webb's work, for those who are interested, courtesy of Mars Society Canada:
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That's his second contractor report, from November 1971. It has an updated design, with an air bladder vest. They found their 1967 design produced some difficulty breathing. The torso bladder covered the chest and upper abdomen, covered by a non-elastic fabric. This meant when the chest and diaphragm expanded to inhale, it compressed the air bladder, squeezing air out as lungs sucked air in. As the chest and diaphragm contracted to exhale, the bladder expanded, taking in air as lungs expelled air. This produced constant volume on the covering non-elastic fabric. To ensure the same pressure inside the lungs as outside the chest/abdomen, the bladder was connected to the helmet. Rebreathers used for diving call this a counter lung. It eliminates restriction to breathing.
In 2005 we invited Dr. Paul Webb to give a presentation at the Mars Society convention. I spoke with him briefly at that convention. I asked about sweat from the chest under the bladder. He replied that wasn't a problem, that sweat would wick through the fabric layer between skin and the rubber bladder, evaporating at the edge of the bladder where exposed to vacuum.
By the way, I maintain the Winnipeg website. I got that contractor report from the NASA technical report server.
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I keep missing this discussion about MCP suits because it's hidden under "Airlocks." Maybe we should have a new MCP Suit thread so titled?
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At 30%, habitat atmosphere pressures can range from around 8 psia to a full 14.7 psia, and still match relatively low altitude places where people live on Earth. I could not meet the pressure ratio criterion for eliminating the pre-breathe time, but the time to actually don the MCP suit should be roughly comparable to the necessary pre-breathe time. The lower suit pressures do help that a bit.
Using 3.0 psi pure O2 in a suit, and 2.7 psi partial pressure O2 in the habitat, that allows 10% pressure loss yet O2 remains what astronauts are used to. Maximum N2 for zero pre-breathe is 3.0 * 1.2 = 3.6 psi partial pressure N2. Keeping the ratio of N2 to Ar the same as Mars atmosphere, producing N2 and Ar is easy. We can harvest them from Mars without the need to separate them. Viking 2 reported 2.7% N2, 1.6% Ar, so keeping that ratio we get 2.1333 psi Ar. Adding that up we get 2.7 + 3.6 + 2.1333 = 8.4333 psi total pressure for the hab.
You could reduce hab pressure further. For example, the Apollo spacecraft and Skylab used 5.0 psi total pressure with 60% O2, 40% N2. That means 3.0 psi partial pressure O2, and 2.0 psi pp N2. Or you could reduce O2 in the hab. Boulder Colorado has 2.54 psi pp O2 outdoors; higher altitude cities have even less. You could reduce O2 to 2.5 psi pp in the hab, but really not lower.
The reason I was able to get 8.4333 psi total pressure in the hab is a tri-gas mix. US Navy divers use tri-gas for deep diving. They use 4-gas for even deeper diving, adding helium as another gas. Each gas has a maximum partial pressure. And the ratio of total suit pressure to partial pressure of the diluent gas in the higher pressure environment is 1.2 maximum. Again, that maximum, you could reduce it. Shuttle used 1 full atmosphere pressure, with 4.3 psi suit pressure, so required 17 hours of pure oxygen pre-breathe to flush nitrogen out of the blood before decompression. These lower partial pressures are for zero pre-breathe.
You could also update Mars atmosphere composition. I cited Mars atmosphere data from the Viking 2 lander from the late 1970s. Viking reported 95.32% CO2, 2.7% N2, 1.6% Ar, 0.13% O2, 0.07% CO (carbon monoxide), 0.03% water, 0.00025% Ne, 0.00003% Kr, 0.000008% Xe, 0.000003% O3 (ozone). Modern atmosphere measurements from Curiosity rover are a bit different, but it's mostly more CO2. Content of CO2 varies with weather, so it depends which day you take the measurement. But current reported data from Curiosity is 95.97% CO2, 1.93% Ar, 1.89% N2, 0.146% O2, 0.0557% CO. Using that ratio of N2:Ar and keeping N2 at 3.6 psi, we get 3.676 psi Ar. That's significantly more; in fact that's so much more that we have to worry whether that exceeds partial pressure maximum for zero prebreathe time. Limiting Ar to suit pressure * 1.2, the new habitat air is: 2.7 psi O2 + 3.6 psi Ar + 3.525 psi N2 = 9.825 psi total. That keeps both N2 and Ar at or below 3.6 psi. Again that's maximum, you could reduce it.
I don't have a study of maximum pp of Ar, just for N2. I know there's a maximum pp Ar, but at this point can only assume it's the same as N2. The US Navy must have done a study since they use tri-gas and 4-gas for divers.
If anyone is concerned with breathing a tri-gas mix (O2/N2/Ar) then I have to point out you're breathing a multi-gas mix right now. Argon comes from Earth's atmosphere. Earth has: 78.0845 N2, 20.946% O2, 0.9340% Ar, 0.0407% CO2, 0.001818% Ne, 0.000524% He, 0.00018% CH4, 0.000114% Kr, 0.000055% H2. So at sea level such as Miami Florida or KSC where pressure is 1 atmosphere, you're breathing 0.13726 psi partial pressure Ar.
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As far as I'm concerned, this thread (and specifically the posts by midoshi) is authoritative as far as atmosphere limits are concerned.
In general, I tend to think a Martian hab atmosphere should be composed as follows:
21 kPa O2
12 kPa N2
12 kPa Ar
4 kPa CO2
1 kPa H2O (40% humidity at 22 C)
Total pressure: 50 kPa (500 mbar/.5 atm/7.25 psi)
This accords with the following limits:
1) Adequate (Earth sea level!) Oxygen levels, but not so high as to create excessive flammability hazard
2) Elevated CO2 within adaptable and safe levels promotes faster plant growth and acts as a fire retardant due to high heat capacity
3) Nitrogen level of 12 kPa (1.75 psi) probably does not require prebreathe for EVA
4) Nitrogen and Argon levels are equal, in accordance with the best available data on their relative proportion in the Martian atmosphere
5) 40% humidity is a confortable level for most people
6) Total pressure is high enough for comfort but at roughly half of Earth standard makes pressure containment easier
Now we just need to build a sealed, low pressure vessel and stick someone inside for a few months to see what happens. I'd volunteer!
-Josh
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I disagree with your CO2 figure. On Earth, human tolerance to CO2 is quite high:
From Wikipedia, with a reference to an actual medical document: Hypercapnia
This shows 1% CO2 in Earth's atmosphere does not cause symptoms.
1.5% to 2.5% over a month causes mild respiratory stimulation.
3.0% over a month, or 3.5% over a week causes moderate respiratory stimulation.
4.0% over a week or 4.5% over 8 hours causes moderate respiratory stimulation, exaggerated respiratory response to exercise.
5.0% over 4 hours or 5.5% over 1 hour causes prominent respiratory stimulus, exaggerated respiratory response to exercise.
6.0% over ½ hour or 6.5% over ¼ hour causes prominent respiratory stimulus, exaggerated respiratory response to exercise, beginnings of mental confusion.
7.0% over 0.1 hour (6 minutes) causes limitation by dyspnea and mental confusion.
Another website said 10% or more is fatal. And 2% will smell stuffy.
1 standard atmosphere = 101.32501 kPa. So 4 kPa = 3.95% at 1 atmosphere. After a week that will cause "moderate respiratory stimulation, exaggerated respiratory response to exercise".
I propose keeping CO2 down to 1 kPa (0.145 psi) partial pressure, because that's equivalent to 1% at 1 standard atmosphere. And perspective: the maximum limit I am recommending for normal operation is 25 times the pp of CO2 in Earth's atmosphere. What you recommended is 100 times.
Last edited by RobertDyck (2017-11-26 14:00:30)
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Hey RobertDyck,
I respect your disagreement, but don't have any response of my own. I would however suggest you give that thread a read-through. Midoshi's posts are deeply researched and well-sourced. Incidentally he is a real planetary scientist who, last I heard, worked on the MAVEN mission.
For my part, I can't stand by anything I said in 2008, as usual
-Josh
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I respect your disagreement, but don't have any response of my own. I would however suggest you give that thread a read-through. Midoshi's posts are deeply researched and well-sourced. Incidentally he is a real planetary scientist who, last I heard, worked on the MAVEN mission.
The argument that I'm nobody and someone else is somebody will only get me upset. No, I won't respect that argument. You will notice I cite medical references regarding CO2 exposure. No, increasing to partial pressure equal to 4% of Earth's atmospheric pressure is not safe. I already posted the medical symptoms of doing so.
As for oxygen, maintaining partial pressure equal to the beach beside the Kennedy Space Center is not necessary, and not desired. Dropping pp O2 so it is lower than sea level but still higher than Boulder is definitely workable. Workers adapt, it takes 6 weeks to adapt to high altitude, but keeping it higher than Boulder is so mild that I don't think anyone would notice any effects. Do you notice when you fly to a Mars Society convention in Boulder? And a 6 month journey to Mars is 4 times the time required for astronauts to adapt. And that's if they don't adapt before launch. And again, the pp O2 I'm recommending is more than Boulder.
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The studies referenced by Midoshi suggest that humans can acclimatise to such CO2 levels. Obviously you're going to get problems if you put someone straight into such an atmosphere. Though I would probably set the limit at 1-2 kPa - that's saturation level for plant growth, but humans won't have to worry about acclimatising to it.
Use what is abundant and build to last
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The argument that I'm nobody and someone else is somebody will only get me upset. No, I won't respect that argument. You will notice I cite medical references regarding CO2 exposure. No, increasing to partial pressure equal to 4% of Earth's atmospheric pressure is not safe. I already posted the medical symptoms of doing so.
No offense was meant. If anything, the implication was that I am nobody. Both you and midoshi have researched this topic more deeply than I have. However, given that you and he are making different claims I do try to come to some conclusions.
In this case, he seems to have looked at a broader range of studies and to have suggested results that expand on the ones you are suggesting without contradicting them outright, while remaining within available evidence and looking deeper into causal mechanisms.
That he is a planetary scientist working with NASA and you are at present doing this research out of personal interest is mostly irrelevant to me. The way I look at it is that on the internet people are either credible or not credible. You and midoshi are credible. You have established credibility through your posts, while midoshi has established it through both his posts and credentials. I assume you are familiar with him, but others reading this thread may not be.
If you are still upset that I believe his arguments over yours ¯\_(ツ)_/¯.
-Josh
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As far as oxygen levels are concerned, 21 kPa is definitely not necessary. I was recently in the Andes, where the total atmospheric pressure is about 60-65 kPa. At 21% O2, that means 12-13 kPa of O2 rather than 21 kPa at sea level. While you definitely felt it (stairs and inclines were killer) I don't doubt I could have adapted to it.
On the other hand, as long as your chosen atmosphere isn't excessively flammable, why wouldn't you want an Earth Sea Level normal amount? Our settlers are going to face plenty of hardships, but this need not be one of them.
-Josh
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Agreed Robert. The "argument from authority" (to use its philosophical category) is no argument. We all know of august bodies who have been devastatingly wrong about things. We have to look at the evidence as best we understand it.
JoshNH4H wrote:I respect your disagreement, but don't have any response of my own. I would however suggest you give that thread a read-through. Midoshi's posts are deeply researched and well-sourced. Incidentally he is a real planetary scientist who, last I heard, worked on the MAVEN mission.
The argument that I'm nobody and someone else is somebody will only get me upset. No, I won't respect that argument. You will notice I cite medical references regarding CO2 exposure. No, increasing to partial pressure equal to 4% of Earth's atmospheric pressure is not safe. I already posted the medical symptoms of doing so.
As for oxygen, maintaining partial pressure equal to the beach beside the Kennedy Space Center is not necessary, and not desired. Dropping pp O2 so it is lower than sea level but still higher than Boulder is definitely workable. Workers adapt, it takes 6 weeks to adapt to high altitude, but keeping it higher than Boulder is so mild that I don't think anyone would notice any effects. Do you notice when you fly to a Mars Society convention in Boulder? And a 6 month journey to Mars is 4 times the time required for astronauts to adapt. And that's if they don't adapt before launch. And again, the pp O2 I'm recommending is more than Boulder.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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I am not up to dealing with the numbers here but I do know that you can adapt to and live quite happily in the high Andes but not be able to reproduce in the high Andes unless you are indigenous to that area (ie come from a long line of people who have lived up there). So this might be partly a definitional problem relating to how well one can function: you may be able to function (in the sense of looking after yourself and pursuing an active lifestyle) but (a) not be able to reproduce or (b) not live as long as you would otherwise have done.
Last edited by louis (2017-11-26 18:03:15)
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I'm trying to design a total system that works on Mars. Work by Mitchell Clapp showed the pressure of an EMU suit makes MCP difficult. You have to keep pressure low for MCP to work. Raising habitat pressure requires raising suit pressure as well. If there's too much difference, then oxygen pre-breathe is required. If hours of oxygen pre-breathe is needed just to walk outside, that makes settlement impractical. Apollo suit designers started with a design goal of allowing 10% pressure loss while maintaining oxygen partial pressure at the level astronauts are used to, so I tried to stick to that design goal. Janice Wynter is an aerospace engineer and member of the Mars Society. She works for a company designing business jets, not a NASA contractor, but I respect her work. She pointed out MCP suits are machine washable. Apollo astronauts had problems with contamination of spacesuits after walking on the Moon, and data so far from Mars indicate different problems. Mars won't have sharp abrasive fines like the Moon, instead rounded fines, but it does have a lot of fines. Mars has wind-blown fines, so it will get in everything. Apollo astronauts had difficulty closing their suit zipper because it was clogged by fines; rounded or sharp will be the same. Satellite data from MGS indicates possibly of asbestos, although I haven't read that from any lander or rover. What has been found on the surface is perchlorates. All this creates contamination problems. A machine washable spacesuit solves most of them.
The liquid cooling garment of Apollo and EMU suits works, but it's heavy. MCP suits control temperature by astronaut sweat. A bottle of drinking water is a lot simpler, lighter, and simplicity makes it more reliable. MCP suits have greater range of joint motion, less joint counterforce. It has sufficient dexterity to climb a cliff face of a vallis. Geologists want exposed rock in hard-to-access locations, because that's most interesting. Dexterity of an MCP suit allows all that. So I'm trying to do what it takes to ensure a Mars suit is MCP.
When I was part of the Mars Homestead project, most of the other guys wanted high pressure in the hab. They assumed 2/3 atmosphere, but had no basis for it. A couple of them wanted that for plants. But they weren't aware of research by the University of Guelph that found plants can grow in lower pressure than humans can tolerate, and it doesn't even slow plant growth. The trick is plants require ever greater amounts of water the lower pressure is. But in a sealed greenhouse that water condenses to be recycled, so it doesn't really "consume" water. Still, I tried to find a way to support MCP spacesuits while appeasing their design for higher pressure hab. I had assumed the same pressure as Skylab, and Robert Zubrin's books made the same assumption. Why change it? But these guys wanted higher pressure, so I tried to appease them.
NASA spacesuit guys have been obsessed with gas bag suits ever since President Nixon slashed funding to NASA, and Dr. Paul Webb's project was one of those cancelled. The guys left appear to be trying to justify their job, rather than working to find the best solution. And each new suit designed uses ever higher pressure. We know why: Shuttle and ISS use 1 atmosphere of pressure. The closer you get to zero pre-breathe time the better. But they're attacking the problem from the wrong side. The solution is not to increase suit pressure, the solution is to reduce habitat pressure. Ideal is to reduce it to equal Skylab.
But even the A7L suit had a problem. They had initially started with 3.3 psi based on a contingency of 10% pressure loss and Apollo spacecraft partial pressure of oxygen equal to the beach beside the KSC launch site. A lot of the complexities of donning and doffing an MCP suit simply go away with lower pressure. So dropping pressure to 3.0 psi bypasses a lot of design issues holding up MCP development. But if you want to keep the 10% contingency, that requires dropping habitat O2 pp to 2.7.
On the original Mars Society forum (1998 on), one individual posted results of an Air Force study. It found strong fighter pilots in their prime, and after high altitude training, could remain conscious and operate the aircraft with 2.5 psi pure oxygen. At 1 atmosphere pressure, partial pressure O2 could drop to 2.0 psi, but minimum pp O2 is higher with lower total pressure. They found pilots could remain conscious and operate the aircraft for up to 30 minutes with 2.0 psi pure oxygen; but again that's a fighter pilot or test pilot, in his prime, with exercise, and using the "high G manoeuvre" to keep blood flowing to his brain, and after weeks of high altitude training. And 30 minutes was the maximum. This is why I recommend slightly higher pp O2, and slightly higher pressure.
Plants require nitrogen. They don't actually fix nitrogen directly from air, legumes have a bacterium grow symbiotically. The plants provides water and carbohydrates to the bacteria, the bacteria fixes nitrogen to ammonia. Some plants can use that ammonia directly, others require another bacteria to convert it to nitrite, and yet another to convert that to nitrate. But regardless, the system required nitrogen gas. So a greenhouse needs sufficient nitrogen. That's the reason for keeping pp N2 close to maximum.
Habitat structure suffers increased stress from increased pressure. You can reduce stress by dropping pressure. But if you use regolith for radiation shielding, you need sufficient pressure to offset weight from the regolith on the roof.
You could use 2.7 psi O2 + 3.42 psi N2 = 6.12 psi total. That would sneak back N2 by 5% from the maximum limit for zero pre-breathe for suit pressure of 3.0 psi. Remember exceeding the limit could result in the bends. This mixture doesn't include any argon. If you bring breathing gas from Earth, that's easy. Argon causes timber of human voices to drop, so an O2/N2 gas mix wouldn't have that problem. A science habitat would only have a layer or two of sand bags on the roof, not 2.4 metres of water soaked regolith. So this lower pressure may work for a science mission.
Mission plans from Old Space contractors don't even appear to address any of these concerns.
Last edited by RobertDyck (2017-11-27 15:10:07)
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Seems like we're in roughly the same place as far as atmospheres are concerned. Converting your numbers, you're suggesting 18.6 kPa O2, 23.5 kPa N2 (plus probably 1 kPa of H2O, which is basically unavoidable, and 1 kPa of CO2, using your proposed number above). Total pressure 44.1 kPa. Your 23.5 kPa of N2 is virtually identical to my 24 kPa of N2+Ar, and you definitely expressed openness above to a mixture of N2 and Ar as inerts.
It's probably not a coincidence that we're coming up with basically the same answers, since we're working with basically the same constraints. O2 needs to be within roughly 15-21 kPa, and the high end is probably better but not strictly necessary; CO2 should probably be somewhat higher than Earth to make plant growth faster and to make it easier to regulate; O2 should be somewhat less than half the total pressure in the hab; and Nitrogen (or a mix with argon) needs to balance out the mixture. Total pressure near half an atmosphere or so as a result of all these, better for suits and pressure containment.
-Josh
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