Naive questions about a space elevator

I have two naive questions about a space elevator to which I haven't
seen clear answers. Both have to do with the bottom 0.5% of the cable:
the part in the atmosphere. (I hope this isn't too off-topic for a
_policy_ newsgroup...)

1) How is the cable expected to handle tropical storms? Is it
believed that such a structure could ride out hurricane force winds
without turning into Galloping Gertie? Or is it assumed that, since the
elevator will touch down either on the equator or very near to it,
hurricanes won't be an issue, since they normally don't form closer than
about +/- 300 miles from the equator (due to lack of Coriolis effect)?

The 300 mile "restricted zone" for hurricanes sounded good until I
realized massive storms can migrate to the equator, even if they can't
form there, and even if they're doomed by crossing the equator they still
might be able to cause significant havoc to a ground station on the
equator.

One issue with riding out strong winds, of course, is that the
tension vector is almost straight up, even if the the cable has been
pulled far off to one side: at the top of the atmosphere we're already
more than 99 percent of the way down. This would seem to suggest that
the cable will not be very "stiff" in response to horizontal
wind loading.

2) What's the current story on the atmospheric E and B fields? I seem
to recall a shuttle experiment with a tethered satellite failed due to
high electrical tension along the cable. Now, as I understand it,
that was most likely due to the earth's B field (which the shuttle
cuts across at high speed), which would presumably not be an issue
for something stationary WRT the Earth's surface. But the atmosphere
also has a significant (vertical) E field. I've seen speculation
(elsewhere) that this would be a problem for an elevator; I've speculated
privately that this could be a great resource for an elevator to tap (if
the voltage isn't too impossibly high). Does anyone here know the correct
story on this? Is it even an issue?


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In article ,
sal wrote:

I have two naive questions about a space elevator to which I haven't
seen clear answers. Both have to do with the bottom 0.5% of the cable:
the part in the atmosphere. (I hope this isn't too off-topic for a
_policy_ newsgroup...)

1) How is the cable expected to handle tropical storms? Is it
believed that such a structure could ride out hurricane force winds
without turning into Galloping Gertie? Or is it assumed that, since the
elevator will touch down either on the equator or very near to it,
hurricanes won't be an issue, since they normally don't form closer than
about +/- 300 miles from the equator (due to lack of Coriolis effect)?

The 300 mile "restricted zone" for hurricanes sounded good until I
realized massive storms can migrate to the equator, even if they can't
form there, and even if they're doomed by crossing the equator they still
might be able to cause significant havoc to a ground station on the
equator.

One issue with riding out strong winds, of course, is that the
tension vector is almost straight up, even if the the cable has been
pulled far off to one side: at the top of the atmosphere we're already
more than 99 percent of the way down. This would seem to suggest that
the cable will not be very "stiff" in response to horizontal
wind loading.

2) What's the current story on the atmospheric E and B fields? I seem
to recall a shuttle experiment with a tethered satellite failed due to
high electrical tension along the cable. Now, as I understand it,
that was most likely due to the earth's B field (which the shuttle
cuts across at high speed), which would presumably not be an issue
for something stationary WRT the Earth's surface. But the atmosphere
also has a significant (vertical) E field. I've seen speculation
(elsewhere) that this would be a problem for an elevator; I've speculated
privately that this could be a great resource for an elevator to tap (if
the voltage isn't too impossibly high). Does anyone here know the correct
story on this? Is it even an issue?

The biggest question is, "How do you impart and remove velocity gaing up
and coming down?" The energies simply don't match!

In article ,
sal wrote:

I have two naive questions about a space elevator to which I haven't
seen clear answers. Both have to do with the bottom 0.5% of the cable:
the part in the atmosphere. (I hope this isn't too off-topic for a
_policy_ newsgroup...)

No, it's fine. I'll answer the first onefrom my understanding, with the
caveat that I'm not an expert and it's been a while since I've read the
studies. (Which, BTW, you might Google for and read yourself.)

1) How is the cable expected to handle tropical storms?

First, it's positioned on the equator, where large storms are rare, and
second, it's mobile. The cable anchor is on a large moving platform
which can be moved out of the way of oncoming storms (if any), as needed.

2) What's the current story on the atmospheric E and B fields?

Sorry, haven't any serious clue, except that I recall it was looked at
and thought not to be a problem.

HTH,
- Joe

On Mon, 06 Nov 2006 03:46:14 +0000, Orval Fairbairn wrote:

In article ,
sal wrote:

I have two naive questions about a space elevator to which I haven't
seen clear answers. Both have to do with the bottom 0.5% of the cable:
the part in the atmosphere....

[ snip ]

The biggest question is, "How do you impart and remove velocity going up
and coming down?" The energies simply don't match!

Well, yeah, you need to add a delta-v of about 5,000 MPH to get to
geosynchronous orbit. And since the cable is so long, the tension is
essentially vertical at each point even if the cable wanders around a lot,
so you don't get a lot of help with horizontal thrust from the cable
unless it's seriously bent -- one would not expect the cable to act very
"stiff". This suggests (to me) that thrusters of some sort might still be
needed, which would reduce the win of the cable but probably won't
entirely eliminate it.

And, of course, if the counterweight breaks off, the cable will not fall
straight down -- that 5000 MPH delta-V will assure that it winds itself
around the Earth once or twice as it falls, which could be interesting
from the POV of the insurance company. But again that's one of the
obvious in-your-face problems which needs to be addressed -- it's one of
the first things most people think of when they hear about an S-E, I
suspect.


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sal wrote:

I have two naive questions about a space elevator..

They're taken up -- not settled, but considered -- in Brad Edwards'
NIAC Phase II report ( www.liftport.com/files/521Edwards.pdf ) and in
"The Space Elevator."

Storms: the prevailing ideas are a combination of (1) equatorial
location -- as you note -- with low incidence, (2) a moveable base
that could do some avoidance, and (3) possibly a true cable rather
than a flat ribbon for the bottom ~25 km. Nothing would eliminate risk
entirely, but it might be brought down to tolerable odds: we have a
lot of tall buldings and bridges that wouldn't survive an F5 tornado,
either.

Vertical E field: to the extent they can guesstimate properties of a
bulk material from what is known about CNTs, it looks like currents
that could be induced by E variations with height are orders of
magnitude too small to be a problem. By the same token, all the
recurrent schemes for running power to climbers through the ribbon,
tapping the E variations, or recovering energy from descending
climbers look impossible by orders of magnitude.

If you want to worry about E-fields, think lightning. While it would
be rare just as storms are in the likely locations, there's no
question it could take out the cable. My own speculation is that if by
bad luck the base *did* end up under a threatening cell, it would be
nice to have a battery of cheap rockets with trailing wire that could
be sent up from rafts a few miles out as "sacrifices."

All this is moot, of course, without the CNT or CNT-based material. I
think it's sinking in among space elevator fans that (1) the material
is a lot less of a slam dunk than Edwards assumed five years ago, and
(2) funds for tackling all other SE questions are going to remain a
trickle until the prospect of the material firms up. You can
brute-force or clever-finesse a lot of the challenges, but if the
material isn't strong/light enough, beanstalk SEs just won't happen.

Monte Davis
http://montedavis.livejournal.com

sal wrote:

you don't get a lot of help with horizontal thrust from the cable
unless it's seriously bent -- one would not expect the cable to act very
"stiff"

It doesn't need to be. You're acquiring that GEO orbital velocity over
more than a week; the "sideways" acceleration is small, and easily
supplied from the restoring force that's trying to keep the ribbon
vertical and taut. People keep thinking about the flimsy part we
build, and forgetting the six-sextillion-ton flywheel at the base...

But again that's one of the obvious in-your-face problems which
needs to be addressed

And was, years ago. A beanstalk SE simply isn't going to happen in
the foreseeable future unless the cable can be much, much, much
lighter than the multi-billion-ton versions in Red Mars, Fountains of
Paradise, and Web Between the Worlds. IIRC, Edwards' baseline is ~800
tonnes for the finished ~100,000-km cable. Do the math: the mass per
m^2 is comparable to newsprint or plastic food wrap.

I have lots of doubts about space elevators, but jeez, I wish we could
get past the Red Mars disaster scenario.

Monte Davis
http://montedavis.livejournal.com

On Mon, 06 Nov 2006 14:55:05 +0000, Monte Davis wrote:

sal wrote:

you don't get a lot of help with horizontal thrust from the cable unless
it's seriously bent -- one would not expect the cable to act very "stiff"

It doesn't need to be. You're acquiring that GEO orbital velocity over
more than a week; the "sideways" acceleration is small, and easily
supplied from the restoring force that's trying to keep the ribbon
vertical and taut. People keep thinking about the flimsy part we build,
and forgetting the six-sextillion-ton flywheel at the base...

Yes, and what's more a rough BOT calculation indicates that far more than
half the energy which needs to be put in is "pure lift". (In fact I make
it something like 90% change in potential energy, 10% change in kinetic
energy, to get to geosynchronous orbit -- but I suspect my numbers may be
kind of wrong; too many places where it's easy to misplace a factor of 10...)


But again that's one of the obvious in-your-face problems which needs to
be addressed

And was, years ago. A beanstalk SE simply isn't going to happen in the
foreseeable future unless the cable can be much, much, much lighter than
the multi-billion-ton versions in Red Mars, Fountains of Paradise, and
Web Between the Worlds. IIRC, Edwards' baseline is ~800 tonnes for the
finished ~100,000-km cable. Do the math: the mass per m^2 is comparable
to newsprint or plastic food wrap.

I have lots of doubts about space elevators, but jeez, I wish we could
get past the Red Mars disaster scenario.

Yes, well, I kind of figured those two issues had been beaten to death
already, which is why I didn't raise them in the OP.


Monte Davis
http://montedavis.livejournal.com

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sal wrote:

Yes, and what's more a rough BOT calculation indicates that far more than
half the energy which needs to be put in is "pure lift".

Yep. Ground to GEO takes about 58 MJ/kg, and 53 of that is
potential... very different from the case for LEO, where kinetic
dominates.

Many people misunderstand the "efficiency" of a SE and the importance
of the "free" kinetic energy from the earth's rotation. The raw
efficiency of the Edwards scheme -- with conversions from source to
laser to photovoltaics to electric motors pulling climbers up the
cable -- is *much* less than the efficiency of a rocket. The advantage
comes not from that, but from putting all the energy into lifting
payload and structure, not propellant.... and from leaving the
heaviest, highest-power propulsion components on the ground.

The "free" rotational energy comes into its own out beyond GEO, where
the far end of a 100,000-km SE could sling payloads with major
delta-v. What you pick up on the way *to* GEO is just a pleasant
little bonus; most of that task, as you say, is pure lifting.

Monte Davis
http://montedavis.livejournal.com

I like the space elevator a whole lot. Imagine practically no
throw-away
mass vs when you do it with rockets, you're doing fine if you wind up
with 1% of the mass you started with. Further, if a booster fails, the
whole thing comes crashing down; vs, using an elevator, it simply stops.
Inconvenient, maybe, but I know which transportation method I'd
prefer for getting out of this Terran gravity well.

I've been to some talks about space elevators. The platform can sit
on some large ships, like oil tankers; when one of the ships needs
overhaul you pull it out and slip in the replacement standing by. One
of the ship's engines could provide the power. My imagination runs
like this:

1) Goggles, everyone!

2) A great big engine cranks up somewhere nearby.

3) Nobody looks at the elevator, which starts rising quietly. (Beware
reflected laser light!)

4) Maybe not impressive, but deeply satisfying from my point of view.

If an airplane flies into the elevator cable, the most expectable result
is two pieces of airplane falling from the sky.

A ribbon cable can be renewed by continually adding on one side, and
removing on the other.

The problem I do not hear anyone talking about is, how do you
consistently and efficiently grow nanotubes up to a few cm long? If
we can learn to do that, seems to me, the rest of it is (relatively)
easy. My guess for the #2 major problem: achieving an adequate
energy density of the laser light that powers the elevator's motors,
without melting the hardware.

Re airplanes again, have you thought what you might do with such
a laser if you spotted a known hostile airplane coming up over the
horizon? The laser will have to be gimballed in any case, to track
the elevator through the cable's swinging across some of the sky.
Just make the gimbals with a wider range.

My guess for the #3 major problem: destructive people who have no
contributions to society but they will run around breaking things.

I've seen a book around, and I had a peek into it. Space elevator
theory (i.e, mathematical). It outlines where the physics pinches if
you want to do a space elevator. I think one of the Liftport principals
wrote it.

Cheers -- Martha Adams

"sal" wrote in message
news
I have two naive questions about a space elevator to which I haven't
seen clear answers. Both have to do with the bottom 0.5% of the
cable:
the part in the atmosphere. (I hope this isn't too off-topic for a
_policy_ newsgroup...)

1) How is the cable expected to handle tropical storms? Is it
believed that such a structure could ride out hurricane force winds
without turning into Galloping Gertie? Or is it assumed that, since
the
elevator will touch down either on the equator or very near to it,
hurricanes won't be an issue, since they normally don't form closer
than
about +/- 300 miles from the equator (due to lack of Coriolis effect)?

The 300 mile "restricted zone" for hurricanes sounded good until I
realized massive storms can migrate to the equator, even if they can't
form there, and even if they're doomed by crossing the equator they
still
might be able to cause significant havoc to a ground station on the
equator.

One issue with riding out strong winds, of course, is that the
tension vector is almost straight up, even if the the cable has been
pulled far off to one side: at the top of the atmosphere we're already
more than 99 percent of the way down. This would seem to suggest that
the cable will not be very "stiff" in response to horizontal
wind loading.

2) What's the current story on the atmospheric E and B fields? I seem
to recall a shuttle experiment with a tethered satellite failed due to
high electrical tension along the cable. Now, as I understand it,
that was most likely due to the earth's B field (which the shuttle
cuts across at high speed), which would presumably not be an issue
for something stationary WRT the Earth's surface. But the atmosphere
also has a significant (vertical) E field. I've seen speculation
(elsewhere) that this would be a problem for an elevator; I've
speculated
privately that this could be a great resource for an elevator to tap
(if
the voltage isn't too impossibly high). Does anyone here know the
correct
story on this? Is it even an issue?


--
Nospam becomes physicsinsights to fix the email
I can be also contacted through http://www.physicsinsights.org

In article uGn5h.1593$fk2.466@trndny02, Martha Adams
wrote:

The problem I do not hear anyone talking about is, how do you
consistently and efficiently grow nanotubes up to a few cm long? If
we can learn to do that, seems to me, the rest of it is (relatively)
easy.


The number of people talking about growing longer nanotubes is probably
orders of magnitude larger than the number of people talking about
space elevators. The number of space nuts is much smaller than the
number of people who build cars, airplanes, buildings, and practically
everything else that can benefit from good strength to weight ratios.

The cover story of this month's Nature Nanotechnology is about
producing 2 mm nanotubes in large quantity.
http://www.nature.com/nnano/index.html

--
David M. Palmer (formerly @clark.net, @ematic.com)