Re: beanstalks (was Re: Metallic hydrogen ...)

From: Ian Stirling (root_at_mauve.demon.co.uk)
Date: 06/27/04 

Date: 27 Jun 2004 16:13:21 GMT

In sci.physics "N:dlzc D:aol T:com \(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote:
> Dear Ian Stirling:
>
> "Ian Stirling" <root@mauve.demon.co.uk> wrote in message
> news:40de07ae$0$9698$ed2619ec@ptn-nntp-reader02.plus.net...
>> In sci.space.policy "N:dlzc D:aol T:com \(dlzc\)" <N: dlzc1 D:cox
> T:net@nospam.com> wrote:
>> > Dear Ian Stirling:
>> >
>> > "Ian Stirling" <root@mauve.demon.co.uk> wrote in message
>> > news:40dde11c$0$8790$ed2619ec@ptn-nntp-reader02.plus.net...
>> >> It depends on where you terminate the tether.
>> >> For example a 50GPa 1.3 density nanotube rope made into a tether.
>> >> If the tension at the base is 1Kg, then up to 48300Km (from the center
> of
>> > the
>> >> earth), the mass is 27Kg.
>> >> The tension is 30N, and the accelleration is .1m/s^2.
>> >> If you terminate here, you need a mass of 300Kg, for an overall mass
>> >> of 327Kg.
>> >> At 191000Km, the accelleration is 1m/s^2, and the tension is 4N, with
> the
>> >> tether mass being 84.4Kg below this. The counterweight only adds 4Kg,
>> >> for an overall mass of 87Kg.
>> >> Taking it right out to where the tension is 1/1000N, the mass is half
>> >> a gram, and the total 88Kg.
>> >
>> > You've descibed a tether system that can just support itself, with no
>> > elevator, no load. Each lofted kilogram will require slowly-decreasing
>>
>> Not quite.
>> The payload is the same as the tension.
>> When you put a payload on the bottom, the tension falls to near zero.
>
> The tension where? The tension at the anchor, as you've described the
> tether is simply supporting the tether. As you add an elevator, the net
> tension increases. As you add payload to the elevator, the tension
> increases again. The tension at the Earth end is not at issue.

Nope.
There is a preload of somewhat above the mass of the payload on the cable.
Say this is 2 tons.
If you put a car of 1.5 tons on the cable, then the tension at the earth
end drops to 0.5 tons, and gradually rises back to the 2 tons figure as
the car climbs.
<snip>
>> It depends on the average strength/density of the tether.
>> For the above tether, the mass of each car is some 1% of the total mass
>> of the system.
>> Payload maybe 0.5% or so per.
>
> So we can take up 0.5% of 327 kg per load (including the elevator)? That
> is a whopping 1.5 kilos... no room for batteries, drive motor, really not
> much of anything.

No, .5% of 88Kg, so 500g or so payload and 500g car, for a 1Kg
tether.

I'd hoped it was obvious that the 1Kg payload tether was only an example,
and the math is exactly the same for 1 ton, or 1000 tons.

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