another “ask the expert” response…
http://itss.raytheon.com/cafe/qadir/q62.html
You guys find the coolest stuff…
(kicks at airlock panel)
JRDelirious wrote:
The novel was Earthlight, which Clarke wrote in the 1950s (before Sputnik 1, I believe). The novel was primarily about a war between the Earth and the moon, and the moon guys had this Way Cool gun that fired a bolt of white-hot molten iron.
Speaking of rapid pressurization changes and science fiction authors: I just finished reading Ben Bova’s novel Moonrise, which was set on (surprise) the moon. The astronauts’ space suits were pressurized to about 5 PSI, containing a box of about 75% oxygen and 25% nitrogen; but indoors inside their moonbase, they breathed normal sea-level air (14.7 PSI, 20% oxygen, 80% nitrogen). According to Bova, if the astronauts didn’t pre-breathe pure oxygen for about an hour before getting into their space suits, the low air pressure in the suits would give them the bends from all the nitrogen in their blood. Is this true? Is a 9.7 PSI pressure differential enough to give you the bends if you’ve been breathing normal air?
Arthur C. Clarke liked the idea of guys in vacuum. Not only does it show up in Earthlight and 2001: A Space Odyssey, he used it in The Other Side of the Sky and at least talks about it in The Fountains of Paradise. The movies usually get it wrong, as noted, but so do some books. Pierre Boulle got it wrong in A Graden on the Moon and Martin Caidin (a pilot, who should know better) got it wrong in Four Came Back
This is OT but it addresses some of the question regarding pressure posted:
The human body is remarkably resistant to high pressure but as with anything there are limits. I have always been told that the limits for diving are 300 feet without special measures being taken. I think the record for a free dive is well over 400 feet. The problem comes with squeezing your chest cavity. At some point the pressure will cave in your chest. However, there is a phenomenon known as ‘blood shift’ where your body will take blood out of your extremities and pump it into your chest cavity. Blood, as a liquid, can’t be compressed thus saving your chest from implosion. There are other problem though…at these extreme depths your heart may beat erratically. In short it is quite dangerous and only a few people have attempted such a feat.
Please note that this is regarding a free dive where a breath is taken on the surface and then you descend. If you use scuba equipment the air entering your lungs is pressurized thus counteracting the effects of pressure and is where issues such as the bends and embolisms enter the picture.
Sure…this should be possible. I’d need my dive tables in front of me to figure this out but it works something like this:
As you dive a scuba tank provides pressurized air into your lungs. The deeper you dive the more pressure is entered into your lungs. The diver doesn’t notice anything since the pressure out (from your lungs) is balanced by the pressure in (from the surrounding water) trying to collapse your chest. Basically you always feel like you would breathing on the surface.
All of this pressure is provided by extra air pumped into your chest and air is largely nitrogen. Here’s the trick…the volume of air in your lungs remains constant but the mass increases. That’s to say there is more gas per given volume than there is at the surface. Your blood now picks up, say, 10,000 atoms instead of the 1,000 it is used to. The pressure keeps it all nice and compact so your blood happily hangs on to it.
After your dive you start rising to the surface. Your blood has all of this extra nitrogen in it. If you come up slowly, or stop at decompression points, the blood passes through your lungs and the excess is exhaled. If you come up too fast the gas expands and at some point your blood cells can no longer hang onto it…it has become a little bubble of nitrogen floating in your blood stream. These bubbles tend to collect in your joints and cause extreme pain leaving the unlucky person all bent up hence the name.
Dive tables tell a person at what point they need to be concerned about the bends. Generally, under 33 feet (1 atmosphere of pressure or double the pressure at sea level) it isn’t much of a concern simply because scuba tanks don’t contain enough air to keep you down long enough to saturate your blood. The deeper you go the less time you can stay down before you require some sort of decompression. For sport divers this usually means a few stops on the way up where you hang out for a few minutes allowing normal respiration to get rid of extra nitrogen. For some professionals who go real deep or stay down for extended periods they get into a decompression tank and may stay there for several days (yes…days).
Whew…all of that brings us back to our astronauts. Things are reversed from a diver but the principles are the same. It isn’t so much the pressure differential but how fast it happens. If they slowly decreased pressure then no problems but making an abrupt 9.7 psi pressure change might be (your body doesn’t have time to exhale the excess nitrogen naturally). I’m sure if NASA has them do this then it either really is a problem or close enough to a problem to be on the safe side.
All of that said there is another problem divers experience on deep dives (200+ feet). While underwater you may not get the bends but you can get a thing called nitrogen narcosis. Remember, you still have all of that nitrogen in your system even if it isn’t in bubbles. Nitrogen narcosis has the effect of making you feel drunk. While in and of itself that isn’t a bad thing it is a bad thing 200+ feet underwater where good decision making is critical. Deep divers mix helium into their air mixture to avoid this problem.
One last issue with air pressure and your lungs is an air embolism. Remember that the air off of a scuba tank is pressurized in your lungs. As you ascend the air in your lungs expands. If you hold your breath the air sacs in your lungs will start to pop like little overinflated balloons. I said before that humans are resistant to pressure but I will qualify that statement. Your lungs are insanely wimpy when subjected to pressure differentials. Ever see those movies where the guy goes underwater and breathes through a tube to hide from bad guys? Guess what…it’s very difficult if not impossible. 3-4 feet of water is enough pressure to keep you from breathing that way. Try it if you don’t believe me (your chest needs to be 3 feet underwater…use as big a tube as you care to…doesn’t matter). Along these lines you can give yourself an embolism in less than 10 feet of water (and as little as 5 feet). Take a breath off of a scuba tank in the deep end of a pool, hold your breath and rise to the surface and you face some serious problems. Do NOT do this! Embolisms are quite serious and potentially fatal. In short…don’t mess with scuba equipment without at least some instruction beforehand.
I know all of that is all over the place but I hope it helps on the air pressure and lungs questions.
Originally posted by Stupendous man:
Rhythmdvl said:
Look again at what he said. He said it would take several hours. You are correct that normal metabolic rates add heat to the system. In fact, spacesuit design is such that it isolates the body from the extremes of space via white surface to repel sun energy and multiple layers of aluminized mylar to act as little thermos bottles and reflect radiation. Then the internal body heat is rejected via the cooling suit through the PLSS (backpack). However, it is important to note that the conditions described above are after the astronaut is dead. Kinda hard for metabolic rates to continue. Once the heart stops beating, and the blood stops circulating, everything else starts shutting down and the body stops producing heat. Now it does typically take a body a fair amount of time to cool down anyway, so heat loss will not be high, but eventually he will freeze. If you don’t retrieve him first. 
The membrane thin spacesuit is the Holy Grail of science fiction writers, with regards to space mobility. Here are the limitations on any space suit - requirements they must fulfill.
- Must provide breathing environment. This means oxygen at certain levels, may use a gas mixture or pure oxygen, whichever makes more sense. Must circulate oxygen and remove carbon dioxide.
- Must maintain pressure. This keeps the gases in blood solution and tissues from separating and causing the bends.
- Must protect against thermal extremes - -250 F (-157 C) to +250 F (121 C) is standard range, some objects get as hot as +350 F (177 C). Aiding is the fact that heat transfer is only via radiation and conduction (and evaporation/sublimation), but not convection. So you want to insulate against contact (boots, gloves, etc) to limit conduction. The current philosophy is isolate exterior and then control temps by regulating the removal of body heat.
- Protect against radiation - not just heat, but also electrons, protons, neutrons, alpha, beta, gamma, X-rays, cosmic rays, solar flares, etc. Some of this can be managed by controlling exposure by monitoring conditions and selective scheduling of EVAs.
- Protect against micro-meteoroids. These are very tiny dust particles that are flying at very high speeds, and can penetrate significantly, causing small holes in your pressure seal.
Thus the standard design using multiple layers of a variety of materials, each serving its own purpose. See
http://www.jsc.nasa.gov/pao/factsheets/nasapubs/wardrobe.html and http://www.apollosaturn.com/asnr/p223-228.htm .
The Shuttle/Station suit uses a hard torso to help protect the body, whereas the Apollo suits did not. That protects both micro-meteoroid and radiation. Any future suit needs to find ways to meet the above requirements. In order to be thin, light, and flexible, but also thermal, pressure, and radiation, and penetration protective, it would be difficult to find a material that would do all that.
Stupendous man said:
That seems a little high. From the link above:
That’s without the PLSS. The weight isn’t given, but I estimate it at about 50 lbs. So make it 160 lbs without astronaut. Estimates I’ve seen are about 300 lbs for astronaut and suit together. Maybe a little more, as that number is for Apollo suits, so 50 lbs seems right.
Also, it would be a complex chore to duct air through the whole suit to ensure proper flow. Water works better being pumped in the confined space and strange geometry of the human shape.
Close, but not quite right. The Shuttle and Station operate normally at 14.7 psia, so there would not be any fear of the bends if they stayed at that pressure. The reason they like a lower pressure is so it is easier to bend the joints of the suit. The suit is essentially a big balloon. The joints are designed to aid bending rather than fight it - constant volume joints, metal gimbals for wrists - but it is still an effort to move the suit. Lower pressure makes that easier, less stiffness. However, maintaining enough oxygen at a lower absolute pressure means using pure oxygen, not any mix gas (nitrogen). So they have to pre-breath pure oxygen prior to going out to remove the nitrogen from the blood. Also, they depressurize the shuttle and station to 10.2 psia some time prior to reduce time spent on masks.
Duration in the suit is limited by oxygen available and power to the battery that runs the PLSS. Depending on things like cooling rate the suit runs, how much effort is expended, etc, the EVAs can and have run 7 hrs. But that’s normally a stretch. Also, the crew gets tired, and there’s no food in it (there is water), and although there’s a catheter to urinate, the only way to crap is wear a diaper and get dirty. So that’s why the expendables were sized to run out when they do.
Bear_Nenno said:
All cells have cell walls. Whether they are thin membranes or thick cellulose is the difference.
tracer asks:
Most definitely. The real problem is not the absolute pressure difference, but the partial pressure difference. If they stayed at the same N2 levels and reduced overall pressure, the N2 wouldn’t leave the blood. Of course it would get hard to breath that way, and the oxygen would likely cause the same effect.
Current practice is to drop shuttle/station to 10.2 psia several hours before (like 12, I don’t remember exactly), and then they prebreathe for 2 hrs prior to getting into the suits. At one time there was a plan to make new, higher pressure station suits that worked at 8 psia. That would significantly reduce prebreathe time, as it would use an oxygen/nitrogen mix. But it hasn’t happened - budget cuts.
There have been numerous instances of folks experiecing the bends on commercial airflights, so yeah, I think it’s possible. Lest you start to panic, the victims are typically scuba divers who weren’t out of the water long enough. You’re supposed to leave at least 24 hours between any scuba dive, even one not requiring decompression while surfacing, and any flying. The typical sceanerio for bends-on-747 involves vacationers who were enjoying just one last dive on the last day of their vacation, followed by a flight the next to go back home. You see, the divers were OK for sea level pressure, but typical pressure on a commercial airliner is equivalent to 8000 feet above sea level.
Needless to say, if you did need to decompress from a dive you would stay on the ground even longer than 24 hours.
The military has pilots of certain very high altitude planes also pre-breathe an air mix containing little or no nitrogen prior to take off, as well. You don’t have to go all the way into space for nitrogen in the blood to be an important factor.