5 ms·

2 GPa is 2GN/m^2, or 200kN/cm^2, which supports 20Mg. Taking silk density as 1 g/cm^3, that is 20M cm, or 200,000 m, or 20km. Silk density is actually ~1.37 g/

by ncmncm 4y ago

2 GPa is 2GN/m^2, or 200kN/cm^2, which supports 20Mg. Taking silk density as 1 g/cm^3, that is 20M cm, or 200,000 m, or 20km. Silk density is actually ~1.37 g/cm^3, so derate the above to 14.6 km. IOW, a silk cable can support 14.6 km of rope hanging below it. Not close to strong enough for a Space Elevator.

telchar 4y ago
You lost a zero in there, so more like 146km.
- Gravityloss 4y ago
  Examples here, one can sort by breaking length: https://en.wikipedia.org/wiki/Specific_strength https://en.wikipedia.org/wiki/Specific_strength (2 GPa) / (1400 kg/m^3 x 10 m/s^2) = (2e9 N/m^2)/(1.4e4 N/m^3) = (2e5/1.4) m = 140 000 m = 140 km P = F/A, F = M x g, M = A x L x rho L = M/(A x rho) = (F/g)/(A x rho) = (F/A)/(g x rho) = P / (g x rho)
- ncmncm 4y ago
  Oops, tx. Seemed low.
- cstross 4y ago
  Not enough for a space elevator on Earth, but definitely in the range you need for very long suspension bridges -- think in terms of railway bridges over the English Channel, the Straits of Gibraltar, the Irish Sea, and (with causeways/island hopping) the Bering Straits.
dekhn 4y ago
That's still only the very first step to building a Space Elevator. And, the product, if it had those properties (when deployed in the wild) would find more profitable use in terrestrial applications. We will be burning rocket fuel to get to space for the foreseeable future. Better to launch enough to build in-space processing facilities if you're really committed to dual-homing humanity or making space travel more cost effective.
tb_technical 4y ago
Space elevators are lame. Launchloops and orbital rings are where it's at! :D
- ncmncm 4y ago
  Orbital rings like circular skyhooks that are much easier to schedule pickup on, and have much friendlier structural stress characteristics, seem not to be written about as much as they ought to be. If by orbital ring you mean a band girdling the planet and rotating well above orbital speed, magnetically coupled to and supporting stationary structures that reach ground level... I don't see any value in discussing those in this century.
Maursault 4y ago
> Not close to strong enough for a Space Elevator. I thought space elevators were deployed with space tethers, which are held taut by the planet's rotation and centrifugal force. While mutant silk worm silk still may not be strong enough for a space elevator, it shouldn't depend on the weight of the rope hanging below it but instead the on the centrifugal force pulling in the opposite direction, away from the planet's surface.
- ncmncm 4y ago
  Space elevators depend utterly on extreme material strength to be possible at all, and on centrifugal force only to stay taut. They could be practical with known materials on Mars, but there is nothing there worth building one for.
  - Maursault 4y ago
    You are sidestepping the fact that in your OP you demonstrated and argued that the silk could not be used for space elevators because it could not support its weight beyond 14.6km, but I argued it would not need to since that weight will be countered by centrifugal force, so whether the silk can be used for a space elevator has nothing whatsoever to do with supporting its own weight, rather, it must support the centrifugal force. If the silk only need be kept "only" taut, its weight will be perfectly balanced against centrifugal force, so in effect the silk will be weightless. But my suspicion is the tether would necessarily need to be kept taut with extra force, not "only" taught, but with force to put stress on the silk tether pointing outwards and upwards. I have no idea how one would calculate the required centrifugal force, but perhaps you'll do us all the favor and determine whether or not the silk would be strong enough without being distracted by the weight of all the silk, which is irrelevant due to it being cancelled by centrifugal force.
    - ncmncm 4y ago
      The correct figure, as noted elsewhere, is 146 km. But literally the only thing holding up almost all of the span from geosynchronous orbit down to the ground is the pure strength of the cable. Centrifugal force would act usefully mainly on parts of the cable that extend out past geosynchronous orbit, to support the whole structure through tension in the cable. The cable inside that orbit would absolutely not be weightless. Its weight per unit mass is of course lower close to geosynchronous orbit, but most of the cable is very far from it.

by ncmncm 4y ago

telchar 4y ago

You lost a zero in there, so more like 146km.

Gravityloss 4y ago
Examples here, one can sort by breaking length: https://en.wikipedia.org/wiki/Specific_strength https://en.wikipedia.org/wiki/Specific_strength (2 GPa) / (1400 kg/m^3 x 10 m/s^2) = (2e9 N/m^2)/(1.4e4 N/m^3) = (2e5/1.4) m = 140 000 m = 140 km P = F/A, F = M x g, M = A x L x rho L = M/(A x rho) = (F/g)/(A x rho) = (F/A)/(g x rho) = P / (g x rho)
ncmncm 4y ago
Oops, tx. Seemed low.
cstross 4y ago
Not enough for a space elevator on Earth, but definitely in the range you need for very long suspension bridges -- think in terms of railway bridges over the English Channel, the Straits of Gibraltar, the Irish Sea, and (with causeways/island hopping) the Bering Straits.

dekhn 4y ago

That's still only the very first step to building a Space Elevator. And, the product, if it had those properties (when deployed in the wild) would find more profitable use in terrestrial applications. We will be burning rocket fuel to get to space for the foreseeable future. Better to launch enough to build in-space processing facilities if you're really committed to dual-homing humanity or making space travel more cost effective.

tb_technical 4y ago

Space elevators are lame. Launchloops and orbital rings are where it's at! :D

ncmncm 4y ago
Orbital rings like circular skyhooks that are much easier to schedule pickup on, and have much friendlier structural stress characteristics, seem not to be written about as much as they ought to be. If by orbital ring you mean a band girdling the planet and rotating well above orbital speed, magnetically coupled to and supporting stationary structures that reach ground level... I don't see any value in discussing those in this century.

Maursault 4y ago

> Not close to strong enough for a Space Elevator. I thought space elevators were deployed with space tethers, which are held taut by the planet's rotation and centrifugal force. While mutant silk worm silk still may not be strong enough for a space elevator, it shouldn't depend on the weight of the rope hanging below it but instead the on the centrifugal force pulling in the opposite direction, away from the planet's surface.

ncmncm 4y ago
Space elevators depend utterly on extreme material strength to be possible at all, and on centrifugal force only to stay taut. They could be practical with known materials on Mars, but there is nothing there worth building one for.
- Maursault 4y ago
  You are sidestepping the fact that in your OP you demonstrated and argued that the silk could not be used for space elevators because it could not support its weight beyond 14.6km, but I argued it would not need to since that weight will be countered by centrifugal force, so whether the silk can be used for a space elevator has nothing whatsoever to do with supporting its own weight, rather, it must support the centrifugal force. If the silk only need be kept "only" taut, its weight will be perfectly balanced against centrifugal force, so in effect the silk will be weightless. But my suspicion is the tether would necessarily need to be kept taut with extra force, not "only" taught, but with force to put stress on the silk tether pointing outwards and upwards. I have no idea how one would calculate the required centrifugal force, but perhaps you'll do us all the favor and determine whether or not the silk would be strong enough without being distracted by the weight of all the silk, which is irrelevant due to it being cancelled by centrifugal force.
  - ncmncm 4y ago
    The correct figure, as noted elsewhere, is 146 km. But literally the only thing holding up almost all of the span from geosynchronous orbit down to the ground is the pure strength of the cable. Centrifugal force would act usefully mainly on parts of the cable that extend out past geosynchronous orbit, to support the whole structure through tension in the cable. The cable inside that orbit would absolutely not be weightless. Its weight per unit mass is of course lower close to geosynchronous orbit, but most of the cable is very far from it.