On Sun, 27 Jun 2004 10:22:24 -0800, "Ron Webb"
wrote:

Arthur C Clarke said that the space elevator would be built "about 20 years
after everyone stops laughing." I think we
have a while to wait yet (heh heh).


My concerns are also practical. The things I have read sound like a bunch of
folks who have the theory analyzed, but
don't really want to confront the real world details. The math has been
worked out in great detail by a cadre of folks
who have been working on this for many years. Just because something is
impossible doesn't stop folks from designing it.
http://www.space.com/businesstechnol..._020327-1.html

Agreed, there are plenty of examples out there supporting your case.
On the other hand, it's been proven time & again that Man Will Never
Fly, rendering this whole newsgroupe & many years of postings a bit of
a waste-

Realisticly there is much research & real world experimentation to be
done before the concept is either proven or disproven. Hopefully
they'll offer me a job helping out with that .


anything with the kind of strength we're talking about here,
under that amount of tension ain't gonna be much bothered by the
occaisional blow.


The tensions are unimaginably high at the hub, but at ground level (at the
ends of the tether), they are zero. A typhoon
would be a BIG problem.

I don't think they're un-imaginably big, just a number followed by a
lot of zeros. Everything I've read on the subject says that the
ground anchor would be under tension for stability purposes. However,
consider this- Not being sure of the elastic modulus of a Nanotube &
Glop composite cable, I'll make the un-warranted assumtion it's not
much. So even if the anchor is under little or no tension any big
wind that comes along will have to deal with the inertia of the
cable's portion lying outside of the atmosphere before causing a
deflection. Hurricane winds are limited mostly to the troposphere,
~30,,00 feet at the equater if I recall correctly. I'm pretty sure
there are large suspension bridges around with an equivalent wind load
to 30,000 feet of space elevater that withstand typhoons just fine.


Then there are the electrical effects. A carbon nanotube cable will
conduct
electricity pretty well. Some claim it's a room temperature
superconductor
candidate. A tropical lightning strike can be several million amps and
this
cable will be a pretty good lightning rod. There's the induced voltages
too. The normal atmospheric potential gradient is several million volts
per
meter.

Last I checked deltaV/m was more liket ~200V.

A dV/dM of 200V per meter, in anything approaching a superconductor could
give nearly infinate
current (I=V/R as R approachs 0 --- Ohms law). That is what burned the
Italian tether. I have not seen
any data on how they plan to avoid this fate. I am sure they have a plan - I
just haven't seen it.

Lightening would do bad things to it I am sure.


The potential for high voltage is certainly there, but consider that
the source impedance of the atmospere is rather large untill actual
ionization occurs, which would limit the currents available. Possibly
large, but not unlimited. Also, AFAIK, superconducting effect breaks
down once a certain current desity is reached in the conductor,
another limitation. TANFL.

As for lightening, I'm from the school of thought that says a
conductor stuck up into the atmosphere actually discourages
lightening, drains away all those pesky ions in the neighborhood.
leaving a nice insulating volume of poorly conducting air around it.

I heard the speculation about nanotubes being superconductors a few years
ago back before they
were able to produce them in decent quantities but haven't heard anything
since, If they really are I
think somebody would have noticed by now.

I agree. It probably isn't a superconductor, although there might be a way
to make it into one.


To bad though, a superconducting space elevator would be a
neat way to generate "free" power, a-la the NASA/Italian experiment
with a tether a few years back.

The power is there, using it to power the tram would be downright elegant,
but you sure can't ignore it - ask the Italians.

I thought the whole idea of the Italian tether Was to generate power-
wasn't the problem with it a stuck cable reel?

And the next detail is bullistic damage. It forms a ribbon, very thin but a
meter or so wide (tapering). If a piece of
space debris were to blow a hole in it half way up, sufficiently large to
cause a failure, the consequences
would be amazing, as the upper part went winging off into space, and the
lower part came crashing to
earth with a lot of mass and residual velocities of up to 17,000 mph.

Again, I agree with you on the space junk problem, I did hear
recently that the problem has been considered, can't remember what the
take on it was though. But consider this, it wouldn't be just the one
cable, I believe the design would be four or more cables tied
togeather every so often that would tend to minimize the potential
for (hind quarters of a cat mounted on a wooden plaque) from smaller
pieces of junk. The larger pieces' orbits are pretty well defined,
they'd have a good idea of what could cause major damage well ahead of
time. What to do about it? Damned if I know- I guess maybe they're
not gonna give me a job afterall .

I do remember from the same article that the problem of a broken
cable coming down isn't thought to be a major concern. Due to the
density of the cable & it's profile in theory once it hit the
atmosphere, instead of falling down @ 17,000 + change MPH as you (& I
as well) thought, it would end up "fluttering" down, coupla feet per
second range. IIRC the simulation showed it would all end up
reasonably close the the anchor site. Still a big mess to clean up
though-


Bottom line - the advances in carbon nanotube manufacture show promise. It
used to be said that this thing needed to be made
from "unobtainium" - now it is not quite so unobtainable (but still not
exactly available either.) but there are still many problems,
and more than one look to me to be showstoppers for now.


You can never tell what's gonna show up & bitechya on the ass when you
start something as new as this, always unforseen problems there are.
Can't say that I see what they are right at the moment though-

Howard.