Review: "Concrete Space Colonies"

D. J. Sheppard wrote about "Concrete Space Colonies" in
Spaceflight, January 1979.
He has some really good arguments which make the concept
worth study.

His designs got an excellent write up with good illustrations in:
The Illustrated Encyclopedia of Space Technology, 2nd ed. by
Kenneth Gatland, 1989, Orion Books, ISBN: 0-517-57427-8
(Thanks to Mike Combs)

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It's not on the 'net, so all I can do is quote & paraphrase it, and
maybe send a fax copy to anyone interested. (For posting to a
website, after I contact the publisher for permission?)

The recipe for a concrete colony would use about 1/3 to 1/2 the
amount of metals as the typical "metal" design, which is typically
90%+ rock.
In addition, the metals are in the form of cables. This is the
easiest way to use metals, requiring the least fabrication
equipment.
Common civil engineering allows 6 times the stress for cables
as for any equal mass of plating or beam of metals.
Cables are also easier to test/replace before & after installation.
Compare the level of labor & difficulty of moulding the
bricks/segments, and that of fitting & welding the myriad metal
struts, plates, and beams of a metal design.
(I hate to think of doing repair work on a torus, while the shell is
spinning by a few meters away -actually you're hanging
underneath, and the rock shield is non-spinning underneath
you...)

Sheppard compared the difficulty of maintanence of the metal
colony, with concrete; Metal pressure vessels have a tendancy to
go "pop!" when punctured, as cracks spread violently from any
impact that penetrates. There's nothing to stop these cracks,
except expensive, uncertain, and complex methods.
Concrete would also exhibit a "leak before failure" quality that is
lacking in metals, unless expensive & complex methods are
taken.
In a concrete structure, the cables apply a permanent crush,
forcing sections together. The total squash applied to the
structure is enough that tension forces can be applied up to test
& safety levels, without actually applying tension to the concrete.
In failure, the compression locally goes to zero, and the blocks
separate slightly, while many of the cables in the area still have
resilience left.
Prestressed concrete can be thoroughly shot through with
cracks without losing its compressive strength.
No similar capability for gradual failure applies for a metal
pressure vessel.

He also wrote convincingly about the comfort & confidence
qualities of concrete.
Metal is cold, noisy, slippery, alien. Rock is accoustically
excellent (damping vibrations naturally, instead of resounding as
metal does). Rock allows for architectural variation, and interior
decorations -all of which are more easily altered than a metal
frame/plate structure would be. Repairs & alterations can be
made with concrete/plaster fairly easily & fail-safe.

The article compared various construction methods. Concrete is
the only method accepted for things which are very big, and must
last a long time.
The largest mobile structure at the time of writing was a concrete
oil rig of 500,000tons, while some dams are 20 million tons. The
largest mobile metal structure is barely 100,000 tons, while
aerospace techniques are nowhere near as well accepted for
such large things.
Many rock structures have held together for millennia, while
metal shipbuilding is barely 150 years old, and aerospace
construction is in its infancy in comparison. (It's unfair to use the
pyramids in comparison -being rock structures under pure
compression- but even cathedrals and bridges of rock last
hundreds of years)

Sheppard proposed a building block method of construction;
A 13 X 17 X 1.6 meter brick, massing 1000 tonnes was
described. Assemble these into a ring (sector of the torus) under
temporarily loose tension, and assemble sectors together for
the circumference of the torus. Cables only hold minimal tension
during assembly, until they all are "tuned" together.
(Note in Shepard's figures:
~176 lbs/cubic foot (2.83 tonne/cu meter). Approx 4.53 tonnes/sq
meter of surface area as shielding mass. This agrees with the
older figures from the Ames summer studies.)

To be fair, while the article did tout many advantages of
concrete/sintercrete construction, he did say that there was little
or no experience with moveable structures and mobile
construction facilities for such huge concrete forms & such.
We also know little about fused rock space manufacturing
methods, though they should still be simpler than traditional
ship/aerospace methods. We'll have to do R&D on it, but the
challenges should be easier than it would be with traditional
metal construction.

Rock should be good for any construction where mass is not
critical (just about any space built thing like antenna platforms,
SSPS, large ships and such. For ships, mass is typically a
problem, unless you're thinking of something with performance
to spare, like Orion/Daedalus or anything where shielding is
needed along with structure -though even Orion or a magsail
can't carry full shielding from cosmic radiation)

Yes, concrete has some gasseous absorbtion qualities, and it
will need to be sealed. (Plastic &/or ceramicrete "paste"?)
Metal structures also have some similar problems; oxidation,
sealing and painting (and repainting) the myriad parts, and he
did mention the horrific effects of explosion & fire on metal.
Aluminum is absulutely ruled out, being unforgiving in
impact/fragmentation, as well as fire effects.

Shepard mentioned that the "window" areas of the torus are
usually the weakest part. If metal ribs ran across wondows, and
are put under comperession the windows are no more prone to
catastrophic failure than any other area.
Alternatively, thick glass panes could be laminated, and spaces
left between layers so the entire structure could be under
uniform compression.
He took this farther into speculation, and mentioned that it may
be possible to make blocks of optical quality glass of equal
thickness, so the "windows" could in fact be an identical brick
unit, with no other means of blocking outside radiation needed.
He wrote wistfully of the unknown possibility of making the entire
colony hull out of optical glass, with no other shielding or window
considerations needed.

Another article about concrete uses in space colonies was
different. (unknown source, here)
A ring section of the torus circumference was cast as a whole.
The inner 1/3 (roof) was thicker, with the windows being slits
with a zig-zag << path through the thickness (light passing down
and bouncing through through this "<<" representation.
Mirror on the surfaces of the channel, and several panes of
glass let light through, while ray particles had to pass through
the thickness of rock, with no straight path to the interior.
Certainly more elegant than the clumsy external "chevrons".
Seems much more secure & simple.