forming composit space station skin in situ

(Henry Spencer) :

In article .com,
wrote:
...you're going to need hundreds of tons of [gas]...

What volume are you considering?

For any big unobstructed volume at any breathable pressure, gas mass is
going to be serious. Air at 1atm weighs about 1.25kg/m^3 -- rather more
than people think.

And I didn't say a pressure suit would
be unnecessary, I think the *space* suit would be unnecessary. In
between the shells, the workers would probably need something like a
flight suit pressure suit, but not the full bulk of the space suit.

Unfortunately, the big problem of working in spacesuits is the stiffness
resulting from suit pressurization. Getting rid of the outer
thermal/micrometeorite protection would help only a little.

I'm not sure what you mean by "flight suit pressure suit", but note that
the suits worn (for example) for shuttle ascent are *emergency* suits,
which get their lighter weight and greater *unpressurized* flexibility
partly by accepting that they will be uncomfortable and very difficult to
work in when pressurized.

Does anybody know what the specs for such a pressure suit, that would
deal comfortably with, oh, 1/100 of an atmosphere would be?

It would have to be essentially a full spacesuit.

...4,188 cubic meters of air at Denver air
pressure, oh, half stp, or so...

Uh, no, sorry, Denver pressure is much higher than that. Even if we make
it Quito instead -- nearly twice as high as Denver -- air pressure and
density are still about 75% of sea level. (And if you're doing physical
work at that pressure, you'll want higher than normal oxygen content, as
anyone who has been to Quito will tell you...)

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

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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 |

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

"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

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 |

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.

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

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

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

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.