[ExI] The L5 Society ( was: EP and Peak oil.)

[hkhenson]

At 10:48 PM 4/7/2008, John K Clark wrote:
>Me:
> >>Rubber, and you'd only need to use it for a few hundred
> >> miles of its 22000 mile length.
>
>hkhenson" <hkhenson at rogers.com>
>
> > That's not a lightweight solution
>
>Who cares, nanotubes are strong and you only have to do it for a short
>distance.

There are no nanotube cables yet that are strong enough for a space 
elevator.  The long rumored 20 GPa is not strong enough.  40 GPa is 
marginal, you have to get into the 50 GPa range or higher.

The taper ratio is exp (p [in kg/m^3]/s [in Pa-N/m^2] x 4.832 x 10^7 m^2 s^-2)

The density for nanotube cable is about 1300 kg/m^3.  1300/(20 x 
10^9) x 4.832 x 10^7 = 3.1408, exp 3.1408 is about 23.  This means in 
the step-taper design that two cables leaving the earth would grow to 
46 at GEO, not counting the departure of step taper from ideal or the 
mass of the free spinning pulleys.

At 40 GPa, the ratio is about 5, so 2 cables would grow to 10 at 
GEO.  At 50 the ratio is about 3.5 so 2 cables would grow to 7 or 4 
would grow to 14.  This is just the cable holding itself up, no 
margin for payloads going up the cable.

Rubber has a dialectic strength of around 500 volts/mil, and a 
density close to water.  (I had to dig out my Handbook of Physics and 
Chemistry since I could not find these quickly on the net.)  500 kV 
would be an inch, close enough to 2.5 cm, twice that for a million 
volts, and twice again to go from radius to diameter (with no safety 
factor).  Giving some space for the wire in the middle, the minimum 
cross section would be about 100 sq cm or .01 sq meters.  So every 
hundred meters would take a ton of insulation, ten tons per km, or 
2000 tons if you just had to insulate only 100 km for two wires.  (If 
one gets nicked, the other has to carry the entire voltage stress, so 
I don't share the insulation between them.

The rough designs of a moving cable elevator gives a best case mass 
of about 50 times the daily through put, but only about 1/20th (100 
tons) of the loading is in the sensitive lower 1000 miles.  So as a 
rough estimate, the cable mass to hold up 100 km of rubber insulation 
alone would be in the range of 2 million tons.

> > plus the fact that atomic oxygen will eat the rubber.
>
>Well regardless of that observation I tend to think that criticism doesn't
>forever destroy the possibility of space elevators ever carrying power and
>rendering the concept physically imposable, as mythical as perpetual motion.

It isn't mythical, but it's sure on the remote side of the ocean of 
engineering possibilities.

> > Building power sats from the ground is a possible step into space where
> > space industry/colonies are not.
>
>I don't quite know what you mean by that.

"While we all may agree that a beanstalk would be nice,
we need to be cautious about proposing is a a way to
facilitate SBSP. The following is a story I've told
before, but it's worth repeating:

"I was once at a Congressional hearing where Peter
Glaser described the SPS, and then Gerry O'Neill got
up and proved (he said) that the SPS was hopelessly
uneconomic unless it was built from materials from the
Moon. The Chairman thanked both of them for their
presentations, and said he was especially grateful to
Prof O'Neill. He  might have been willing to consider
Peter's fantastic vision, but Gerry had shown that
another fantasy, lunar industrialization, was a
prerequisite. Gerry had saved the Committee from
wasting any more time on this subject, since a fantasy
built on a fantasy was not worth thinking about.

"Unfair it may be, but any talk about the space
elevator as a precursor to SBSP will turn the giggle
factor into a belly laugh, and prevent any funding of
either."

Phil Chapman (April 5, 2008)

> > Do you remember who I am?
>
>Yes I do, you're the fellow who started the L5 society and from that I can
>only conclude you are a better man than me. I mean that Keith, I hope you
>don't think my arguing with you over this small point as any sign of
>disrespect.

I don't have any problem with you arguing over points small or 
large.  But when they involve statements that should rest on physical 
reality, I really wish you would at least state the problem as you 
understand in terms of numbers.  If you don't know how to set up or 
solve problems like the mass penalty for insulating cables, ask

Best wishes,

Keith