Beanstalks...

So, what are the arguments pro and con for having
the high end of the beanstalk anchored to some Big
Honking Rock ... or just a garden variety space station, as
the far point anchor, given that the beanstalk's
stationkeeping

I'm of the "big honking rock" fan club. The big honking rock doesn't
need to be rock - it can be a metallic asteroid or some other valuable
asteroid. If travel on the orbital elevator gets cheap enough, it might
be worthwhile to deliver ore (or refined metals) from the anchor rock
to Earth.

As for making up momentum, putting the anchor rock beyond
geosynchronous should result in the Earth's rotation replacing any
momentum lost by the beanstalk.

Mike Miller

"delt0r" wrote:

There will almost certainly be a sizable counterweight and it only
really makes sense when its quite a nit past GEO. The first reason to
have a counterweight is dynamic stability, to keep everything from
shaking itself apart. Its huge so destructive resonate modes will be
measured in hours.

Hmm, as with Niven's _Ringworld_,
a beanstalk may need active
countermeasures to keep it from
yanking itself out of orbit.

Would the much mentioned concept
of using the earth's electromagnetic
fields for marching satellites
around also work for a beanstalk, so
that lengths of powered cable up and
down it's length could be tuned to
counter and thus damp the resonances?

xanthian.

Fun off-topic factoid: about 32 years
ago, I was driving a research ship to
support el Nino research by putting
down and picking up deep ocean buoys
in 4000 meters of water. A "many
metric tons"-stressed Kevlar(tm)
cable that long is a very low period
musical instrument, in the passing
tidal current flows my passengers were
studying. The deep infrasound it created
turned out to attract huge schools of
huge tuna, visible through the clear
water 100 meters down as if they were
fleets of trucks doing synchronized
maneuvers, which infrasound-tropic fish
the crew then gleefully harvested with
hook and line, to barbecue on deck
for lunch.

Think of that next time you hear some
stranger humming, or a cellist getting
in tune.


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delt0r wrote:

Iain McClatchie wrote:
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.

Yes, the are often called LEO elevators IIRC, but 767 with rocket
assist?. Unfortunatly they still require quite good materials anyway
to get a decently low speed (orbit velocity does not go down that
quikly with height). You could run a GEO (GEO as in period and
inclination) space elevator were the end still sits 1000km up or a
little more. This could a simpler way to avoid debris, while the dV is
still very modest for a rocket.

Zubrin refers to it as a hypersonic skyhook.

The important characteristic for a material is "characteristic
velocity, U = SQRT(Strength / Density).

http://www.islandone.org/LEOBiblio/SPBI1MA.HTM has a table. Kevlar has
a U of 2.2km/s, and nanotubes in theory 15km/s according to this.
According to Zubrin, kevlar is 1.2 to 1.6.

A skyhook with a material with U of 2km/s, and tip velocity relative to
Earth of 5km/s would have to mass 11 times its payload.

The only thing that is a bit niceer on the specific strength of
materials it rotavators.

A problem with elevators and skyhooks is the need for an elevator with
power source. This means that cargos would have to spend days
travelling through the radiation belts. Rotovators get round this
problem by flinging cargos through the radiaton belts in a matter of
10s of minutes. They also need no elevator mechanism, and can be
lighter than an equivelant skyhook.

wrote:
Zubrin refers to it as a hypersonic skyhook.

Its not in the atmosphere, so this is a bad name for it (what is
hypersonic speed when there is no speed of sound!). My understanding
was that a hypersonic skyhook was infact a rotovator that had one end
dipping into the atmosphere.

But i don't want to get into the names of things. Lets just agree that
names are not well defined at this point.

I also agree that rotovators make a lot more sence. But still end up
big by todays stardards and space junk is a big problem.

Greg

wrote:

A problem with elevators and skyhooks is the need for an elevator with
power source. This means that cargos would have to spend days
travelling through the radiation belts.

"Days"???

Why "elevator"? Once you have a beanstalk, and can thicken it pretty
much indefinitely using itself as a hoisting mechanism and (limited
only by keeping its mass below the order of planetary size masses,
so it isn't flinging the Earth itself around in its orbit) there really
isn't
much reason _ever_ to stop increasing its strength/diameter by using
"free" power to run "cheap"(*) bulk-up loads of nanotube fiber up the
beanstalk to help it gain mass) think of it as the base stratum for
some (very vertical) construction, and run a railgun up it, whose
mass could by then be trivial w.r.t. the mass of the beanstalk. Even
half a G of continuous acceleration/deceleration gets stuff from here
to there quite promptly, IIUC.

xanthian.
(*) Granted, after a while, you're going to need to mine a gas giant
for
the needed carbon for the nanotubes, or you'll come up short one
ecosystem.

Ian Stirling wrote:
A non-tapered beanstalk on earth is not possible with any known material.

That's not actually true. Blaise Gassend has shown that a non-tapered
beanstalk is possible with nanotube material strength of about 65 GPa
(which is the same target strength needed for a conventional
beanstalk). A non tapered beanstalk has much less capacity though.

But there is a big advantage during construction of a non tapered
beanstalk- it's actually possible to create a loop out beyond GEO and
back to the ground and spin it using a motor on the ground using
untapered fiber. It turns out that construction is faster than the
normal construction approach using laser powered climbers. You can
exponentially increase the cable thickness from the ground; so
basically you would be using carbon nanotube to lift more carbon
nanotube, and with efficient/cheap mechanical power supply from the
ground.

on Wed, 15 Mar 2006 19:40:25 -0000, Ian Woollard sez:
` Ian Stirling wrote:
` A non-tapered beanstalk on earth is not possible with any known material.

` That's not actually true. Blaise Gassend has shown that a non-tapered
` beanstalk is possible with nanotube material strength of about 65 GPa
` (which is the same target strength needed for a conventional
` beanstalk). A non tapered beanstalk has much less capacity though.

Well, if "known material" is read as "known to be manufacturable
to the required length" then AFAIK the statement is true.

` But there is a big advantage during construction of a non tapered
` beanstalk- it's actually possible to create a loop out beyond GEO and
` back to the ground and spin it using a motor on the ground using
` untapered fiber. It turns out that construction is faster than the
` normal construction approach using laser powered climbers. You can
` exponentially increase the cable thickness from the ground; so
` basically you would be using carbon nanotube to lift more carbon
` nanotube, and with efficient/cheap mechanical power supply from the
` ground.

I'm trying to visualize what you are describing here. A pair of
cable ends at the earth mounted next to each other and rotated
about a common centre? Or a cable pulled through a pulley like
a hoist? I assume the latter as the former doesn't make any sense
to me, though "spin" doesn't quite seem like the right word for
that - it would be parallel lines rather than a ring.


--
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Pete Vincent
Disclaimer: all I know I learned from reading Usenet.