Beanstalks...

A couple of questions on this space technology:

1)I was under the impression that carbon nanotubes, if manufacturable
at reasonable lengths (~ a foot?) would make it possible to construct
a beanstalk. Then I came across references saying that NO physical
material would be able to take the stresses involved on Earth (though
a moon or Mars beanstalk was possible).

Which is true? Or are there different beanstalk designs which have
orders of magnitude difference in the calculated forces, and are there
reasons that the higher-stress version would be used?

2) Assuming you're building a beanstalk, what methods are likely to
be used to anchor it?


--
Sea Wasp
/^\
;;;
Live Journal: http://www.livejournal.com/users/seawasp/

In message
Sea Wasp wrote:

1)I was under the impression that carbon nanotubes, if manufacturable
at reasonable lengths (~ a foot?) would make it possible to construct
a beanstalk. Then I came across references saying that NO physical
material would be able to take the stresses involved on Earth (though
a moon or Mars beanstalk was possible).

A lot of materials could be used, the taper factor required (the bit of
the cable at geosynchronus altitude generally needs to be thicker than
that at either end) makes the gathering of sufficient material to be
problematic. An untapered cable needs a tensile strength comparable to
the theoretical limit.

2) Assuming you're building a beanstalk, what methods are likely to
be used to anchor it?

Something along the lines of an oil rig has been suggested for early
versions. Have a read of http://en.wikipedia.org/wiki/Space_elevator to
get started, but note the section on the tensile strength required
doesn't agree with section on calculating the taper.

Anthony

Sea Wasp wrote:

A couple of questions on this space technology:

1)I was under the impression that carbon nanotubes, if manufacturable
at reasonable lengths (~ a foot?) would make it possible to construct
a beanstalk. Then I came across references saying that NO physical
material would be able to take the stresses involved on Earth (though
a moon or Mars beanstalk was possible).

You don't need long lengths of fiber.
As you increase the fiber length, over a few nm, it starts to steeply
rise, till you get to a few um, when it's not really rising much more.
Consider cotton - the fibers are nowhere near a foot long, but it's
strong enough.
The rate at which it rises depends on the matrix which the nanotubes are
in.

Which is true? Or are there different beanstalk designs which have
orders of magnitude difference in the calculated forces, and are there
reasons that the higher-stress version would be used?

A non-tapered beanstalk on earth is not possible with any known material.

A tapered one is possible, given certain constraints.

The optimum tether is (pretty much) one with a given tension on the
anchor point on the ground, with the material at (say) 90% of nominal breaking
strain.
As you rise up the cable, the cable has to support more of its weight
in addition to the tension on the anchor, so needs to increase in crossection.

(the tension on the cable is decreased when a load goes up it, and cannot
fall below 0 if you want it stable)

When you do all the maths, it turns out that the first bit of the cable
tapers quite steeply, then the taper reduces as gravity falls off, with the
cable reaching a maximum thickness at GEO (about 40000Km) and then
gradually tapering out to a small counterweight.
The further out you put the counterweight, the lower the mass of the whole
system.

The key to all of this is the taper ratio.
(from memory), the maximum diameter is about 1.5 times the minimum diameter
for theoretically optimal nanotubes.
For 25GPa, it's 10 times, and for 12GPa, 100 times, and for 6 (the best of
current non-nanofibers) it's about 1000.

Elevators become theoretically practical (IMO) when the total payload that
can be moved by the elevator in a year or two is about the same as its mass.

If this can't be done, then the massive capital investment of launching it
will only be repaid in dozens of years.

If it can, then you end up with an elevator that can double in payload every
few years, scaling up fairly rapidly to truly enormous sizes.

There are other designs, but unfortunately, none gets significantly better
than this - starting the tether from a balloon at 100Km only decreases the
mass by a vanishingly small amount for example.

Which is true?

It depends on what you assume about the properties of carbon nanotubes.
It's one thing to make nanotubes a foot long. It's another to make a
structure that maintains near-perfect nanotube properties for over
22500 miles.

2) Assuming you're building a beanstalk, what methods are likely to
be used to anchor it?

For an Earth beanstalk?

An asteroid with a mass much greater than the beanstalk mass at an
altitude above the beanstalk's center of mass (i.e., above
geosynchronous orbit).

Mike Miller

In article , Sea Wasp
wrote:

A couple of questions on this space technology:

1)I was under the impression that carbon nanotubes, if manufacturable
at reasonable lengths (~ a foot?) would make it possible to construct
a beanstalk. Then I came across references saying that NO physical
material would be able to take the stresses involved on Earth (though
a moon or Mars beanstalk was possible).

With a reasonable amount of tapering, carbon nanotube will do it. An
untapered nanotube elevator would not work.

No one would ever build an untapered elevator (until the day that
buying a 50,000 km spool of quarter-inch unobtanium rope is cheaper
than buying separate spools of 1/4, 3/16 and 1/8 inch rope and tying
the ends together).

Which is true? Or are there different beanstalk designs which have
orders of magnitude difference in the calculated forces, and are there
reasons that the higher-stress version would be used?



2) Assuming you're building a beanstalk, what methods are likely to
be used to anchor it?

Just about anything would work. The tension at the ground end is very
small (of order a few times the useful payload). You just have to
hold on to it. One idea is to tie it to a ship, which lets you move
the Earth end away from weather, and to set up a ~hundred km exclusion
zone to keep out terrorists, wayward airplanes, etc. which is easier to
do in mid-ocean than on land. You can even divide the tether at the
Earth end and tie it down in several places, for redundancy.


Good sources for information:
http://www.liftport.com/
http://www.spaceelevator.com/
http://www.tethers.com/

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

On Thu, 22 Dec 2005 13:01:50 GMT, Sea Wasp
wrote:


A couple of questions on this space technology:

1)I was under the impression that carbon nanotubes, if manufacturable
at reasonable lengths (~ a foot?) would make it possible to construct
a beanstalk. Then I came across references saying that NO physical
material would be able to take the stresses involved on Earth (though
a moon or Mars beanstalk was possible).

Which is true? Or are there different beanstalk designs which have
orders of magnitude difference in the calculated forces, and are there
reasons that the higher-stress version would be used?

2) Assuming you're building a beanstalk, what methods are likely to
be used to anchor it?


The requirements for a Space Elevator is currently being investigated
at the Institute for Scientific Research.

http://www.isr.us/research_es_se.asp

Paul C

Ian for 6 (the best of current non-nanofibers) it's about 1000.

Someone must have looked at having the base of the elevator up above
the atmosphere by now. You could have the base moving at 1000 m/s
relative to the equator, at a few hundred km altitude, which would make
a pretty reasonable target for a 767 with rocket assist.

The primary advantage is the lower orbit and thus the drop in length.
1000 m/s gets you a 30% drop in length. Since you get to higher v^2/r
values at lower altitudes, I would think it would improve the taper
value
quite a bit, to something more like 30-50 for 6 GPa material.

If people insist on having the bottom of the thing on the ground, the
orbiting mode might be a cheaper way of lifting all that mass.

Somebody wake me up when the taper value is around 10 for material
that is already made in bulk, with safety margins.

Someone must have looked at having the base of the elevator up above
the atmosphere by now.

http://members.aol.com/Nathan2go/SPELEV.HTM
http://www.strangehorizons.com/2003/20030414/rope.shtml

You could have the base moving at 1000 m/s
relative to the equator, at a few hundred km altitude, which would make
a pretty reasonable target for a 767 with rocket assist.

If you can get a 767 into vacuum, with safety margins, without building
a virtually all-new vehicle, let me know.

Mike Miller

Actually, building in both directions from LEO, with a platform at
either end and your weaving/loom system in the middle, you'll gradually
raise the orbit of the CG til it reaches GEO. All along the way, it
would be practical at reducing necessary delta-v to orbit, thus slowly
reducing launcher requirements and increasing launcher payload
capacities. Eventually you would have a platform just above the
atmosphere at the lower end of the cable, which SS1 type tourist
buggies could reach easily. If by that point the CG is at GEO, then the
platform is perfectly motionless wrt the earths surface below, and your
SS1-class vessel can put all of its delta-v into reaching whatever
altitude the platform is at, landing on it like an aircraft carrier,
and dropping off and picking up passengers and/or cargo. This is the
point at which things really start to get interesting.

Mike Lorrey wrote:
Actually, building in both directions from LEO, with a platform at
either end and your weaving/loom system in the middle, you'll gradually
raise the orbit of the CG til it reaches GEO. All along the way, it

However. this means lots of launches, or deliveries.
If you can possibly launch the tether in one lump - with a very small
payload - say a ton, and carry the rest of the tether up it, to
strengthen it till you hit 100 tons payload (for example), then you
don't need any launches at all.