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#32
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In article ,
Tim McDaniel wrote: you want the insulation external, because that lets you use MLI, which is superb insulation but works only in vacuum "MLI"? http://www.acronymfinder.com/ suggests "Multi-Layer Insulation", but not what's so special about insulation having multiple layers (putting on a sweater only works in a vacuum?). I gather that the abbrev. is a little imprecise. It's just a name. MLI is many layers of thin metallized plastic, held apart either by light, fluffy netting or (sometimes) by corrugating the plastic and running the corrugations in each layer at right angles to those in the adjacent layers. Typically the layers are also perforated here and there to help vent trapped gas. Conduction through MLI is minimal, there is no convection in vacuum, and the many layers of shiny surfaces make it quite difficult for radiated heat to work its way through. It's amazingly effective, and very light; it sees extensive use on spacecraft. MLI has a few problems. First, as noted, it works well only in high vacuum. The slightest hint of gas between the layers greatly increases its conductivity. Spacecraft MLI can take noticeable time -- hours or even days -- after launch to reach full effectiveness. It's useless on Mars, thin though the Martian atmosphere is. Second, edges are a problem. MLI conducts heat moderately well *along* the surface, so you have to watch what edges might be in contact with. Third, anything that compresses MLI greatly increases conduction. MLI jackets for equipment have to be precisely tailored and slightly oversize so they'll stay loose everywhere, seams and edges have to be done very carefully to avoid compressing the layers, and careful handling and installation is needed. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#33
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Henry Spencer wrote: In article , Tim McDaniel wrote: you want the insulation external, because that lets you use MLI, which is superb insulation but works only in vacuum "MLI"? http://www.acronymfinder.com/ suggests "Multi-Layer Second, edges are a problem. MLI conducts heat moderately well *along* the surface, so you have to watch what edges might be in contact with. How is it as a radiation shield? multi layers gives the secondary radiation places and room to spatter? Harmon |
#34
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"Ian Stirling" wrote:
I'd argue slightly about the 3PSI, you can go a fair bit lower if your goal is "won't die in 5 minutes without exertion". No, the situation under discussion was normal breathing pressure for work, not "emergency requirements". At the lung wall at 37C is 47 torr (780 torr = 1 atmosphere = 14.7 PSI) of water vapour. This can't be reduced, and is a hard limit (barring hypothermia). About 15 torr of CO2, at normal metabolic rates. People (nutters) have climbed everest without supplemental oxygen. These cannot be used as a standard. Mountain climbers spend a significant fraction of their lives "at altitude", so gain some of the same benefits as the South American natives living normally at altitude, and their lungs are arguably (and documented to be) more efficient as a result of long practice breathing thin air. At the top, you're looking at 276 torr, of which about 1/5 is O2. So, at the lung wall, we have 276-62 torr = 214 torr of atmosphere, or 43 torr of oxygen. So, for pure O2, 43+62 = 125 torr or 2.35 PSI will get you the same oxygen saturation as on the top of everest. No, if I understand what you're claiming, you can't do the math that way, you _still_ have to overcome the displacement effect within the lungs of the water vapor and CO2 enhancements with a "higher than final desired lung interior level" external O2 pressure. snip I think you're missing the point that if you put 3 psi of oxygen _inside_ the human body, you have to put 3 psi _outside_ the human body as well, or the person just explodes. Err, no. If you try to hold your breath, your lungs rupture. If you don't, you'r fine until you die from lack of oxygen (about a minute until you need more than CPR). Again, you are confusing the "emergency" situation with the "working day" situation. So far as I can determine from a very muddled description, the OP was claiming one could, during the working day, breath 3PSI oxygen in the suit while supplying only 1-2 PSI counterpressure, from inert gases, in the containment vessel, and do that using a "flight suit", which in its normal incarnation supplies extra pressure to the legs to counteract G forces and keep blood in the brain, but does not somehow magically supply an overall external counterforce to the internal overpressure. The worker would indeed "explode", just as warned. Human skin is not strong enough to withstand pressure differentials of such sizes, nor is it strongly enough attached to the underlying flesh to stay attached when "inflated", thus the common technique of skinning animals with an air pump. HTH xanthian. -- Posted via Mailgate.ORG Server - http://www.Mailgate.ORG |
#35
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In article .com,
Harmon Everett wrote: you want the insulation external, because that lets you use MLI... How is it as a radiation shield? multi layers gives the secondary radiation places and room to spatter? It's pretty much worthless as a radiation shield (against high-energy particles, that is, as opposed to thermal IR), because it just doesn't have enough mass. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#36
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Harmon Everett wrote:
Henry Spencer wrote: In article , Tim McDaniel wrote: you want the insulation external, because that lets you use MLI, which is superb insulation but works only in vacuum "MLI"? http://www.acronymfinder.com/ suggests "Multi-Layer Second, edges are a problem. MLI conducts heat moderately well *along* the surface, so you have to watch what edges might be in contact with. How is it as a radiation shield? multi layers gives the secondary radiation places and room to spatter? Harmon Not especially great, spaces between is irrelevant for (most) secondary particles, it's mass and composition which is important. For micrometeorites, it's not too bad, but not especially great, you really want greater stand-off distances between layers for optimal mass use. |
#37
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Earl Colby Pottinger wrote: For a 100m diameter sphere if I got it right = (50 * 50 * Pi) * (50 * 2) * 2 / 3 = 523,599 cubic meters of gas. That is a lot of mass. Might as well make it bearthable then. Earl Colby Pottinger Absolutely! Eventually! But it doesn't have to be breathable right away for it to be useable - or leasable. That amount of volume gives us plenty of immediate real estate to lease. Bits of it have to be breathable, but once the external skin (actually a double wall, so the inside diameter is 94 meters) is up, constructing internal rooms is much easier. You have places to store materials and equipment you aren't using right then, while you are working in the airless parts, you don't have to worry about drifting away, or dropping tools. The 100 meter sphere gives room to develop a 94 meter diameter rotating wheel inside the structure, that if rotates at 2 RPM, gives about 2/10 of a g accel. We are also going to want to keep strengthening the outer walls to 4 or 5 times necessary, to give us a hefty safety margin. Once the inside can support a breathable atmosphere, strengthening the outer walls further will be much easier. Harmon |
#38
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wrote: Harmon Everett wrote: wrote: wrote: Well, it'd be easier, but the resulting structure isn't something I'd expect to pass any safety inspection, at least not until it was much thicker than a shell made of continuous, woven fibers. I was thinking the additional layers of fibers would be continuous woven fabric. Unless the interior of the sphere is completely empty (of revenue-generating modules), you're not going to be able to install a continuous-fiber fabric shell. You're going to have install patches, which goes right back to the problems of hand lay-ups. Which is why I was planning on the double shell design. The interstice between the two will be mostly unobstructed. Probably regular belt loop connecting points here and there that could be temporarily unhooked to unfold another layer of fabric blanket. The second layer would be thicker and more capable of being pushed against with the squeegee, and so on. Speaking as a materials engineer, I *still* wouldn't trust a composite shell laid up by hand in that fashion even if you could press and roll it firmly. And each layer would add to the radiation protection and micrometeorite puncture protection. No, the micrometeorite protection won't increase noticeably with a slight thickening of the hull. Thick, solid materials are poor means of stopping micrometeorites. Why limited to the strength of the binding resin? Because the fibers would not be interlinked between layers and between patches. I don't understand what you are thinking about when you say patches. I'm imagining large area blankets that stretch from pole to pole, and probably run twenty meters wide or so, and who's to say we can't run some sort of sewing machine up and down a couple of layers, quilting them together before they get doused with resin? The resin would be the only source of strength in those areas. As I understand it, the tension would be trying to pull the skin apart along the length of the fibers - so the fibers would be carrying the load, wouldn't they? Why would the resin be the only source of strength? What am I missing? Correct. And in the years it takes you to slowly hand-assemble your giant sphere, those customers might as well be operating in a normal space station, since the enclosing sphere isn't doing much for their modules. In fact, they could go to the competitor who doesn't increase rates to pay for a big, non-profitable sphere. A small station isn't big enough to have enough tenants to hold the rates down. If you only have a couple of tenants, they have to be able to pay for the entire station and all the launches it will require. If you have 50 tenants, the development costs can be spread around all of them. If you want that sphere to do something useful, you'll want to build it quickly to the point it can house occupants without separate, vacuum-rated modules. This painstaking patchwork method does not seem good for business. I dunno, Mike. Whenever I start wondering whether I'm being stubborn about the 100 meter diameter just to be stubborn, all sorts of reasons show up to argue for it. I used to be a framing carpenter. I got to be pretty good at walking the rafters as we were installing them. You know, the 2 x 4's set on edge at a 30° angle, hanging out in the air 3 stories above the ground. So, while I could do it all day, I was never really comfortable. The knowledge that the slightest lapse of attention, or slip, would send me carreening irretreivably into a disaster was always there, and you can never really relax and it is very exhausting. I imagine it is the same for an astronaut in a space walk. Sure, they are tethered, and watched. But the slightest inattention, or slip, could spell disaster. The cluster of grapes design, with 50 meter spheres would need a lot of spacewalks to connect them together. And the connections between them would suffer a lot of torquing and bending that just isn't there with the 100 meter sphere. And I think the double shell design is really the way to go for ease of construction, and ease of repair, and if you string 8 of the 50 meter spheres together, with double shells each, you only get 356,816 cubic meters of inside the inner shell space, rather than the 434,892 cubic meters of the 100 meter sphere. I'm probably biased in my imagining how things would work, but in my imagining, I imagine that it would be way easier to work inside between the shells of the 100 meter sphere, than outside of either the 100 meter, or the 50 meter shells. And once the 100 meter shell gets fully pressurized - in a couple of years, even in the cluster of grapes concept we are going to want to keep adding layers to the exterior shells to develop a safety margin able to deal with 4 or 5 times the tension that the shell is actually under. That process, of redundantly adding more layers to the external shell, is going to be even easier with the 100 meter concept, than with the cluster of grapes design. It won't take Boeing much to strap some SRBs onto its Delta IV heavy and give you a 40-ton payload. With that, you can launch entire 0.5mm shells into orbit in a single bound. If you go larger, up to a 100- or 120-ton launcher, you can put the full pressure shell into orbit in a single launch. Then its just a matter of spraying on foam insulation, inside and out, and laying down some non-structural liners and insulation. You get me the big launcher and I'll gladly go with one launch of the full pressure shell! Harmon Let's light this candle! - Alan Shepard |
#39
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Harmon Everett wrote:
As I understand it, the tension would be trying to pull the skin apart along the length of the fibers - Length and width of the fibers. The tension will be along the both directions of the skin. so the fibers would be carrying the load, wouldn't they? Why would the resin be the only source of strength? What am I missing? Anyplace where the fibers are not thoroughly connected to other fibers, the resin will take the load. It's easier to make a continuous strength layer on Earth than in space. A small station isn't big enough to have enough tenants to hold the rates down. Correct. And by building the sphere in a patchwork fashion over the course of several years *while* housing tenants in separate interior modules, you're not only putting the tenants in a small space station, but you're making them pay for the expensive construction of the sphere. So, you're hitting them with bills twice times over: First, by making them pay for habitable sections of the sphere during the construction phase. That's basically the same as building a whole new space station out of smallish modules. Second, by making them pay for an outer shell that isn't generating money (it's the inner modules that are generating money), but is demanding hundreds of expendable rocket launches and years of construction. So, I'm not necessarily advocating that you build a small sphere only, I'm recommending that you build the big sphere *quickly* and *cheaply* before it's a burden to your customers. Something requiring hundreds of rocket launches to assemble is not going to be built quickly and cheaply. You get me the big launcher and I'll gladly go with one launch of the full pressure shell! Growth options for the Delta IV Heavy: http://www.boeing.com/defense-space/...th_options.pdf That would use relatively off-the-shelf components. Depending on the number of launches involved, it still might be cheaper and quicker to resurrect something like the Saturn V or Energia. Mike Miller, Materials Engineer |
#40
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It's like a multi layer vacuum flask. Even a single layer vacuum flask
is a superb domestic insultator. But a multilayer vacuum flask would collapse under atmospheric pressure (a bit like neoprene at 100m water depth). I guess when launched the corrugated layers trap air - if assembled in a vacuum they'd be crushed. The air then needs to escape. If it takes days to be effective, then it's less suited to keeping the crogenic propellant cool for the week or so between launches. |
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