forming composit space station skin in situ

wrote:
Harmon Everett wrote:

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.

Speaking as a person who does a lot of composite fabrication, nor would I.

To give some idea of the problems, first, the layup would be terribly
uneven, no matter how skilled the operatives. That means there would be
places where the layers are seperated by thick layers of resin - this a)
uses a lot of heavy resin to no purpose, and b) makes the finished composite
much weaker - the resin cracks and layers, allowing the tension to be taken
on only one layer of reinforcement, which will fail if it's not strong
enough. There are other failure modes too.

Slightly worse, there will also be gaps between layers which are filled with
air rather than resin; and worst of all places where the fibres are not
impregnated with resin.

Hand layup under those conditions also means that the fibre/resin ratio is
far higher than optimum - optimum is about 30% resin or less, but it's hard
to get ratios of less than 70% or so with hand layup. The extra resin does
not contribute to the strength of the composite, and the larger gaps between
fibres actually weaken the composite.

Temperature control would be a bitch - mixed catalysed resins can only be
worked reliably in a very small temperature range.

Adhesion of extra layers would be a problem too, few resins like to stick to
hardened versions of themselves, and the best composites are made in one
piece, or if that's too difficult then a strict timing is used when applying
extra layers, so that the layers are only partly set when the next layer is
applied - usually less than 12 hours between layers, not really practical in
your scheme I suspect.




To get round some of the problems you could use a UV hardening prepreg.
That's fibre preimpregnated with resin in a semi-solid rubbery form, which
is formed into flexible sheets. These are melted together (either by heat,
solvents, or adding extra resin, though heat is best), and set when UV light
shines on them.

However, just using prepreg isn't enough, you would want to vaccuum-bag it.
On Earth that involves laying up the resin and fibre or prepreg, covering
with a mylar bag and sucking the air out, letting the air push the layers
together. Often you use a layer of mylar with tiny holes in it next to the
composite as well, with a layer of fabric (or of plastic with channels in
it) between the mylar layers, to ensure the air gets sucked out from the
middle of the piece.

Plenty of vaccuum available in space, the problem is the air. I use about
10psi for my best work - but perhaps 3-4 psi would be sufficient with a
suitably chosen prepreg.


You would have some UV opaque sheets to ward off the sunlight, and spread
the prepreg out where needed, let it get hot and melt together eg by shining
extra sunshine on it with mirrors, bag, then remove the UV opaque material
and let the sunshine set the resin.

You would then need to replace the UV opaque layer for long-term UV
protection, as most resins are badly affected by UV (there are exceptions,
such as some straight-chain polyurethanes, but these are expensive and hard
to use).

Still doesn't solve the multiple layer adhesion problem, but perhaps some
clever chemistry might help enough there. You are still talking about a long
and expensive development and testing program though.






A large pressuriseable and _depressuriseable_ volume might be more useful
for building work. You would need air pumps, air storage tanks, and a long
zipper to get things in and out of it. No need for 100m sphere though, I'd
guess, something smaller would be enough.



Might I recommend my scheme as an alternative source of living space? Every
flight puts a 10 meter long 4.2 meter diameter insulated tank into orbit.
That's about the size of a small house, fifteen times a day, 300 days a
year.

Or the same volume as a 100 m sphere every 14 months, fully pressurised and
divided into usefully-sized compartments. For free.


--
Peter Fairbrother

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".

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.
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.
About 1.2PSI or so is the pressure at which you're about as well off
as you are holding your breath in normal atmosphere.

Thanks - I was just thinking of a new scenario - a lunar or orbiting
workshop:

Astronauts perform EVA from the workshop to fetch a faulty machine, but
once they get back in, they can unstrip their suit (or the gloves and
helmut at least), fix the machine, suit up, and take the machine back
out. They probably want to do this a few times in the space of a six
hour shift.

I was thinking you'd have an Everest Atmoshpere, about 1/3 bar. Up the
oxygen content a little, and then provide the mechanics with an oxygen
bleeder just under the nose. Would that work?

Then they can get in and out of the space suit with no need for
decompression, but still perform siginificant amount of equipment
repair, and have the occassional break.

Would that work?

Below this, you get rapid de-oxygenation of the blood as it passes
through
the lungs.

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).

I remember what the scuba diving instructor said - if you have to make
an emergency ascent, make sure you open your mouth say "Oh
Shiiiiiiiiiiiiiiiiiit" all the way up.

wrote:

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

Ooooh. Energia. Like that could be resurrected any time soon. Sigh.
Harmon

As a thought - if you placed your Hydrogen tank in a lunar polar
crater, and put MLI between it and the surface, but had no insulation
where the tank could only see black space - what would that do to the
cooling requirements.

Likewise, a hydrogen tank in Earth Orbit, with three disks of MLI. One
of these blots out the sun, one the Earth, and the other the moon. The
MLI discs are some way from the tank, so that only a small part of the
IR they emit will hit the tank. The tank can only "see" three discs,
and lots of black sky.

Would these concepts work?

In article . com,
wrote:
I was thinking you'd have an Everest Atmoshpere, about 1/3 bar. Up the
oxygen content a little, and then provide the mechanics with an oxygen
bleeder just under the nose. Would that work?

You'd be better off to just make it pure oxygen like Apollo, or oxygen
with a little bit of nitrogen like Skylab. Then no supplementary oxygen
would be needed.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

wrote:
As a thought - if you placed your Hydrogen tank in a lunar polar
crater, and put MLI between it and the surface, but had no insulation
where the tank could only see black space - what would that do to the
cooling requirements.

Likewise, a hydrogen tank in Earth Orbit, with three disks of MLI. One
of these blots out the sun, one the Earth, and the other the moon. The
MLI discs are some way from the tank, so that only a small part of the
IR they emit will hit the tank. The tank can only "see" three discs,
and lots of black sky.

Would these concepts work?

In principle, in high earth orbit.
In LEO, the earth is nearly 180 degrees.

Peter Fairbrother wrote:
wrote:
Harmon Everett wrote:

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.

Speaking as a person who does a lot of composite fabrication, nor
would I.

Thanks for leaping in with the voice of experience! Mine is limited
to patching my canoe, and a rather misguided attempt to patch a big
hole in some drywall with fiberglass.

To give some idea of the problems, first, the layup would be terribly
uneven, no matter how skilled the operatives. That means there would
be
places where the layers are seperated by thick layers of resin - this
a)
uses a lot of heavy resin to no purpose, and b) makes the finished
composite
much weaker - the resin cracks and layers, allowing the tension to be
taken
on only one layer of reinforcement, which will fail if it's not
strong
enough. There are other failure modes too.

Slightly worse, there will also be gaps between layers which are
filled with
air rather than resin; and worst of all places where the fibres are
not
impregnated with resin.

Is it that much of a problem in this circumstance? The force across
the fiber/film layer - from inside of the sphere to the outside, is
just the air pressure at that point isn't it? The reason we are using
the fiber blanket is to take up the tension along the length of the
fibers -warp and woof. That doesn't depend on the uniformity of the
resin application.


Temperature control would be a bitch - mixed catalysed resins can
only be
worked reliably in a very small temperature range.

I've wondered what those conditions would be at the film layer,and how
they could be controlled.

Adhesion of extra layers would be a problem too, few resins like to
stick to
hardened versions of themselves, and the best composites are made in
one
piece, or if that's too difficult then a strict timing is used when
applying
extra layers, so that the layers are only partly set when the next
layer is
applied - usually less than 12 hours between layers, not really
practical in
your scheme I suspect.

No. The time between different layers could be months.

To get round some of the problems you could use a UV hardening
prepreg.
That's fibre preimpregnated with resin in a semi-solid rubbery form,
which
is formed into flexible sheets. These are melted together (either by
heat,
solvents, or adding extra resin, though heat is best), and set when
UV light
shines on them.
Unfortunately, once you crack the UV protective barrier, the prepreg
will harden, whether you get the blanket spread out in time or not.
Other than a process which takes place automatically - such as the
original inflation and hardening, which could be done automatically,
there isn't any real way to guarantee that the workers won't be
interrupted by something more important, like a puncture or something
before they get the blanket spread out and rolled into place.

Still doesn't solve the multiple layer adhesion problem, but perhaps
some
clever chemistry might help enough there.
The different layers don't stick together very well? If they function
as separate shells, what do we lose? The outer film is flexible and
you can't stress against it very much. the first inside layer hardens
much stiffer, and after it hardens you can roll against it with a
controlled amount of force without worrying too much about going
through it. The third layer you could probably really reef against.
So the successive layers are serving as forms for the next layer as
much as they are interconnecting 3 dimensional webbing.

A large pressuriseable and _depressuriseable_ volume might be more
useful
for building work. You would need air pumps, air storage tanks, and a
long
zipper to get things in and out of it. No need for 100m sphere
though, I'd
guess, something smaller would be enough.

We've got a ISS, its not big enough to lease out volumes to independent
organizations. A space station needs to be big enough to be able to
bring in enough income to support itself.


Might I recommend my scheme as an alternative source of living space?
Every
flight puts a 10 meter long 4.2 meter diameter insulated tank into
orbit.
That's about the size of a small house, fifteen times a day, 300 days
a
year.

Or the same volume as a 100 m sphere every 14 months, fully
pressurised and
divided into usefully-sized compartments. For free.
You mean the Shuttle External Tank? It ends up in space with lots of
internal stuff that needs to get refitted, and cut out, and fuel that
gets nasty if you don't dispose of it well. Once you have the 100
meter sphere, you have a place big enough to work on a shuttle tank
and do the refit is a controlled environment easily.

Harmon
Let's light this candle - Alan Shepard

Harmon Everett wrote:
Is it that much of a problem in this circumstance? The force across
the fiber/film layer - from inside of the sphere to the outside, is
just the air pressure at that point isn't it? The reason we are using
the fiber blanket is to take up the tension along the length of the
fibers -warp and woof. That doesn't depend on the uniformity of the
resin application.

Yes, it does depend on uniform resin application. Non-uniform resin
application means fibers laid out of true and loads transferred to the
resins. You can have cracking and failure of the matrix where air
pressure tries to force the fibers into a spherical shape.

No. The time between different layers could be months.

That's a lot of revenue being spent on months of expensive labor.

The different layers don't stick together very well? If they
function
as separate shells, what do we lose?

Not so much, but if the layers are not sticking together well, then
it's likely the different parts of a single layer are not sticking
together too well, and that's a real problem.

Keep in mind that your idea of subleasing station volume during
construction involves building a space station like the ISS inside the
big shell. All those subleased internal volumes have to be airtight,
climate controlled, and otherwise protected from space until the 100m
shell is a human-safe environment.

In other words, you'd be building two stations simultaneously, and only
one of them is making money.

This is why I keep suggesting either a) assembling the shells on Earth
and launching them in one piece, or b) assembling them in a smaller,
full-pressure station where you can have the equipment to decently
weave together the strength shell. Hand lay-up is really not the way to
do this for economic and engineering reasons.

You mean the Shuttle External Tank? It ends up in space with lots of
internal stuff that needs to get refitted, and cut out, and fuel that
gets nasty if you don't dispose of it well.

The ET's fuel (hydrogen and oxygen) are volatile substances that can be
dealt with easily: you open vents and let solar heat boil them into
space.

Alternately, you attach a scavenger system (oft proposed for shuttle ET
conversions) and collect the 7-ish tons of very useful residual
hydrogen and oxygen in the tank.

Mike Miller, Materials Engineer

Harmon Everett opined

You mean the Shuttle External Tank? It ends up in space with lots of
internal stuff that needs to get refitted, and cut out, and fuel that
gets nasty if you don't dispose of it well. Once you have the 100
meter sphere, you have a place big enough to work on a shuttle tank
and do the refit is a controlled environment easily.

All that internal stuff is free metal that can be made into something usseful at
a later time.

The fuel is H2, which can be vented and O2 which is the basis for your
breathable working enviroment.


-ash
Cthulhu in 2005!
Why wait for nature?

Harmon Everett wrote:
Peter Fairbrother wrote:

To give some idea of the problems, first, the layup would be terribly uneven,
no matter how skilled the operatives. That means there would be places where
the layers are seperated by thick layers of resin - this a) uses a lot of
heavy resin to no purpose, and b) makes the finished composite much weaker -
the resin cracks and layers, allowing the tension to be taken on only one
layer of reinforcement, which will fail if it's not strong enough. There are
other failure modes too.

Slightly worse, there will also be gaps between layers which are filled with
air rather than resin; and worst of all places where the fibres are not
impregnated with resin.

Is it that much of a problem in this circumstance? The force across the
fiber/film layer - from inside of the sphere to the outside, is just the air
pressure at that point isn't it? The reason we are using the fiber blanket is
to take up the tension along the length of the fibers -warp and woof. That
doesn't depend on the uniformity of the resin application.

Yes, it is a problem.

The composite structure helps to even out the loads on the reinforcement.
This requires even and thorough dispersal of the reinforcement in the
matrix. This evening-out prevents all the load being put on part of the
composite and breaking that part - then the load goes on a new part, and
breaks that, and so on until the whole fails.

It also requires that any layers be well bonded together and that they not
delaminate - for instance, if you had just two layers, one flat and one with
a wrinkle, any tensile forces will only be on the flat layer, the wrinkled
layer will not contribute. The overall strength will be halved.

The matrix also provides some extra tensile strength, esecially when the
bond thickness is near the optimum. This requires a ratio of resin to
reinforcement of usually around 30% for maximium strength _from a fixed
amount of reinforcement_. Adding more resin actually weakens the final
structure, even if the amount of reinforcement is constant.

The time between different layers could be months.

Leaving the economics aside, that means that the layers will not bond
together easily. They could delaminate, which would be a big problem.



Unfortunately, once you crack the UV protective barrier, the prepreg will
harden, whether you get the blanket spread out in time or not. Other than a
process which takes place automatically - such as the original inflation and
hardening, which could be done automatically, there isn't any real way to
guarantee that the workers won't be interrupted by something more important,
like a puncture or something before they get the blanket spread out and rolled
into place.

The UV barrier goes on the outside of the sphere. It is only "cracked" when
you want to harden resin repreg which has already been spread out and
heated. It is in place all the rest of the time.




One more problem I should have mentioned, resins stink, and most are toxic
until set.



A large pressuriseable and _depressuriseable_ volume might be more useful for
building work. You would need air pumps, air storage tanks, and a long zipper
to get things in and out of it. No need for 100m sphere though, I'd guess,
something smaller would be enough.

We've got a ISS, its not big enough to lease out volumes to independent
organizations. A space station needs to be big enough to be able to bring in
enough income to support itself.

I wasn't thinking of using the volume as a space station, but as a
construction and repair bay, just part of a station (or a garage/workshop
attached to a station).




You mean the Shuttle External Tank? It ends up in space with lots of internal
stuff that needs to get refitted, and cut out, and fuel that gets nasty if you
don't dispose of it well.

The tanks are 4.2 m dia x 10 m long foam insulated steel (yes, steel) LH2
tanks. Leftover LH2 is scavenged, the inside is cleaned by warming and
opening to vaccuum, and the slosh baffles can either be removed or left in
place as floor supports.


--
Peter Fairbrother