> 2. If one designed an O'Neill habitat to be as maintenance-free as
> possible, one might be surprised how much could be achieved. If we
> want to talk about somebody moving in and then undergoing a
> technological collapse, one might manage to keep going for
centuries,
> if the timers and motors associated with tilting the mirrors for the
> day/night cycle could hold out that long.

This is an interesting line of thought. Suppose that "zero
maintenance" (even in the event of a technological collapse) over the
course of hundreds of years were a hard design criterion for an orbital
settlement. How would we design such a settlement?

First, "no moving parts" comes to mind. Obviously the habitat itself
needs to spin to provide artificial gravity, but it could do so pretty
much indefinitely in the absence of any frictional processes. But this
means that we can't have two cylinders connected at the hubs, O'Neill
style, as those connection points could wear out. Instead, we'd want a
single structure that spins as a solid piece, except maybe for a
docking hub -- since in the case of a technological collapse, nobody's
going to be using the docking hub anyway.

That puts serious constraints on the shape of the habitat. A cylinder
isn't dynamically stable unless it's so short as to really be more of a
disk than a cylinder. (And rotational stability is also on the
must-have list; it'd be a disaster for the colony to start tumbling.)
A torus or barbell is much better.

Now, what about lighting? I'm generally a proponent of artificial
lights. These don't need any moving parts, but they do need a power
source. Solar cells degrade over time, and solar-thermal schemes
involve moving parts. My instinct is that if the solar power system
were carefully designed for longevity, it could probably work, but it
wouldn't be easy.

The alternative is natural lighting through windows (plus chevron
shields). Windows and mirrors degrade over time, too. This would
reduce the lighting level and also make it more diffuse. I'm not sure
how quickly this happens, or what could be done about it.

As for a day/night cycle, regardless of where the light comes from, the
safest thing is to probably not have one. Just leave the lights on all
the time. Otherwise, you risk the cycle grinding to a halt at night,
and that'd pretty much be the end of life in the habitat.

Rejection of waste heat... that's hard to do without moving parts. You
usually want fluids moving around, transferring heat from the interior
out to the radiators, but that involves pumps and valves and whatnot.
A zero-maintenance heat rejection system would be another engineering
challenge.

Finally, there's the question of how big an ecosystem has to be to be
stable without careful maintenance. I don't think we know the answer
to this, but bigger is certainly better. So for surviving the collapse
of civilization, you'd probably want to build as big as you possibly
can.

It's a fun exercise, at least. Anybody else have ideas to contribute?

Best,
- Joe

Joe Strout -- joe@...

Thermionic generators need no moving parts, a rotating
cylinder, rotating about the long axis, like a dumb
bell, would be stable indefinetly, heat dumping via
thermionic transfer would also last. As far as counter
rotating cylinders or docking hubs are concerned,
magnetic bearings would have no wear, but the magnets
might lose integrity over time. Micro meteor wear and
tear could be a problem, unless there was a massive
rocky shield surrounding the rotating system.
Yeah, lots of possibilities for stories there.
Eco system degradation might impell the inhabitants to
assume a great deal of responsiblity for manageing the
eco interaction. Some good story possibilities there
as well,,,

Gary 7

> Thermionic generators need no moving parts

That's true! However, I'm not sure how much this helps. A thermionic
converter (AIUI) needs a large heat differential over a short distance
(or else it needs a working fluid to transfer the heat). So we have
the same problem -- the heat is in the colony interior, and the
radiators are way out there.

However, I suppose you could use some solid material (probably a metal)
with a high thermal conductivity to transfer the heat. If that metal
happened to be thermionic, so much the better. The cool thing about
this approach is that you not only get rid of the waste heat, you get
some electricity out too. (Though of course it's no good if that
energy just stays in the colony... ultimately it all has to be radiated
away anyway.)

> a rotating cylinder, rotating about the long axis, like a dumb
> bell, would be stable indefinetly

Well, true, but what a waste of space! You'd be living only on the end
caps; the vast majority of the volume would be unusable... UNLESS you
put in multiple decks, and just accept that they will have lower
gravity levels. Which, come to think of it, is probably fine. But
this geometry pretty much necessitates the use of artificial light, I
think.

> As far as counter
> rotating cylinders or docking hubs are concerned,
> magnetic bearings would have no wear, but the magnets
> might lose integrity over time. Micro meteor wear and
> tear could be a problem, unless there was a massive
> rocky shield surrounding the rotating system.

Yeah, pretty much anything with moving parts I think is going to fail
sooner than things without them.

> Eco system degradation might impell the inhabitants to
> assume a great deal of responsiblity for manageing the
> eco interaction. Some good story possibilities there
> as well,,,

Yeah, and I suppose some amount of eco system management could be done
in a low-tech society... but it would be hard. I suspect that keeping
a small ecosystem stable really requires active, high-tech intervention.

Best,
- Joe

Joe Strout -- joe@...

Yeqah Joe, the whole idea of a rotating cylinder as I
described it is based on my perception that malls have
ceilings only 20 or 30 meters high, yet give an
impression of vast open space. Designing such a
structure with the habitation surrounding an atrium
style overhead, with multiple levels would be visually
appealing yet provide pleanty of living area,,,and you
could look thru the entire length of the cylinder,,,

GAry 7

Instead, we'd want a
single structure that spins as a solid piece, except maybe for a
docking hub -- since in the case of a technological collapse, nobody's
going to be using the docking hub anyway. I don't think a monolithic habitat rotating as a unit with no sliding parts would even require us to drop the docking hub. Spacecraft could dock in the same manner that we saw in 2001: A Space Odyssey. Of course I guess thatassumes a closable hatch such that the docking bay could be pressurized...
The alternative is natural lighting through windows (plus chevron
shields). Windows and mirrors degrade over time, too. But not, I would think, at the same rate as PV cells. One might arrange the radiation shields such that they would also not permit straight-line paths for micrometeoroids to the mirrors or windows.
This would
reduce the lighting level and also make it more diffuse. I'm not sure
how quickly this happens, or what could be done about it. You mentioned story possibilities. Seems like somebody mentioned a cyberpunk story set in a habitat that had been neglected for so long that the windows had fogged from micrometeoroid sand-blasting.

Rejection of waste heat... that's hard to do without moving parts. I remember O'Neill, when talking about the Amish living in a space habitat, mentioned the fans blowing air through the heat radiators as something they might have to make an exception for. Still, I suspect that a totally passive heat rejection system, while very inefficient of materials, could still probably be engineered. Finally, there's the question of how big an ecosystem has to be to be
stable without careful maintenance. I don't think we know the answer
to this, but bigger is certainly better. So for surviving the collapse
of civilization, you'd probably want to build as big as you possibly
can.
Makes sense to me. For example, if we were talking about some kind of "Banks Orbital" which had the same surface area as Mars or Earth, and somebody was persisting in expecting it to require some kind of ecological maintenance that they would not concern themselves with in the case of a terraformed Mars, or the Earth, I would suspect a bias. Regards,
Mike Combs

Of joe@...

> I don't think a monolithic habitat rotating as a unit with no sliding
> parts would even require us to drop the docking hub. Spacecraft
> could dock in the same manner that we saw in 2001: A Space
> Odyssey. Of course I guess that assumes a closable
> hatch such that the docking bay could be pressurized...

One more reason for building such a passive-tech habitat large. Then it
will rotate slowly, and incoming ships (during the high-tech phase) will
have less of a problem.

>> The alternative is natural lighting through windows (plus chevron
>> shields). Windows and mirrors degrade over time, too.

> But not, I would think, at the same rate as PV cells. One might arrange
> the radiation shields such that they would also not permit straight-line
> paths for micrometeoroids to the mirrors or windows.

The primary mirrors will need to face the Sun. Of course they may be
shielded from other directions, like in a telescope, to minimize
micrometeoroid incidence. Presumably the micrometeoroid flux directly from
the Sun is significantly less than from other directions
However, the primary mirror (the one that is "first in line" as regards
the sunlight) needs be pointed always at the Sun. If the habitat rotates
monolithically this requires the primary mirror to be a plane one set at 45
degrees to the spin axis (as envisioned for the Stanford Torus), which needs
be perpendicular to the habitat's ecliptic (plane of orbit with respect to
the Sun). This again would require some sort of technological mechanism to
rotate the primary mirror to track the Sun as the habitat circles it, and if
in a tech-collapsed future this mechanism jams the barbarians will be stuck
in much the same way as the people in your "Journey to Alfahsfere".
Alternatively we might have an Island Three-type arrangement, with
magnetic bearings that have been engineered not to lose magnetism in the
lifetime of the Solar System, and perhaps a long pole of sorts to make the
twin habitat gravity-gradient stabilized in an orbit directly around the
Sun - so that periodic adjustments to its attitude need not be made with
some sort of thrusters.

>> Rejection of waste heat... that's hard to do without moving parts.

> I remember O'Neill, when talking about the Amish living in a space
> habitat, mentioned the fans blowing air through the heat radiators
> as something they might have to make an exception for.

One design, which might presumably be mass-intensive and otherwise
expensive to build but after that might be less expensive in maintenance
would have a double-hulled habitat with an all-metal outer hull - perhaps
with a full radiation-proof thickness to allow inspection and maintenance
between the hulls, and also to reduce the risk of a meteoroid puncturing the
outer hull - which in a tech-collapsed society would be disastrous. If air
is pumped through the inter-hull space then the outer skin of the hab would
be the waste heat radiators. This surface would be negligibly lit by the
Sun if the spin axis pointed always towards it and a screen were set before
the southern (Sun-facing) end cap. Their area would be sufficient, as also
intuitively makes sense as this is, within order of magnitude, the case on
Earth, where the planetary surface both receives the sunlight and reradiates
the waste heat. In the tropics the radiating-to-sunlit surface ratio is pi
(sunlit surface is effectively the Earth's diameter while the radiating
surface is its circumference), while in such a hab it would be two (sunlit
days, reradiation round the clock - provided even our low-tech society could
operate the sunscreens to provide night). Reducing the difference between
the Earth case and the habitat case would be the lack of greenhouse blocking
of the waste heat radiation off the habitat. Of course occasional sweeping
would be required to keep the floor of the inter-hull clear of dust and
grit, which would insulate the outer hull and reduce its efficiency as a
radiator.
Of course great fans would be required to blow the heated air through the
inter-hull, as warm air would naturally rise towards the axis and therefore
need to be forced downwards. Reintroducing the cooled (and therefore heavy)
air at valley level rather than near the axis would reduce the power
required of the fans. They could be powered by amorphous PV cells, which
IIUC are considerably less efficient than the other ones, but which do not
degrade from radiation and sunlight. The sunscreens to provide night would
also have to be placed on the inside, and perhaps operated by hand.

Alternatively one might imagine a design in which the air is cooled by
radiators near the axis, while of course being heated at the valley floor.
Then the buoyancy of heated air would naturally cause the required
circulation, perhaps with some baffles to negate the Coriolis effect of the
rising air. Perhaps the radiators, which would have to be thick-walled in
order to reduce the risk of puncture as well as co-rotating with the habitat
to avoid rotating pressure seals, might be shaped like a long leaf, doubling
as the pole-along-the-axis required for gravity-gradient stabilization. Hot
air rises to the axis, traverses a pipe that is insulated like a stove pipe
through the radiator to its far end, and is then cooled by contact with the
radiating surface. It drifts back towards the habitat, continually cooling,
until like a mistral wind it katabatically cascades down into the habitat to
be heated again.

In writing a novel about a tech-collapsed society it is not necessary to
presume that they regress to a stone-age stage. One might easily imagine a
society that is isolated from all other societies, losing the ability to
produce high-tech gadgets such as electronics and reaction engines for use
beyond the habitat, while retaining the ability to use stored metals to
produce electric engines and maintain and replace space suits of a
sufficiently rugged design so that they can fix problems outside the
habitat. Such a society would probably retain knowledge of its history, of
course, unless the regression were caused by some sort of cultish revolution
in which knowledge of the past were suppressed along with non-essential
technologies and sciences. Let's say a combination of your "Condemned to
Repeat It" and "Journey to Alfahsfere". :-)

Jon Lennart Beck.