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12 Feb 2014 - Commercial Mining with a Lunar Elevator

Posted by: charlesfradley - Fri Jan 31, 2014 3:54 pm
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12 Feb 2014 - Commercial Mining with a Lunar Elevator 

Should the lunar elevator be our top priority in space development ?
Yes 17%  17%  [ 1 ]
No 33%  33%  [ 2 ]
Unsure 50%  50%  [ 3 ]
Total votes : 6

12 Feb 2014 - Commercial Mining with a Lunar Elevator 
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Post Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator   Posted on: Wed Feb 19, 2014 10:51 pm
The bobbins would be part of the counterweight mass. I think you would want at a minimum 3 strands to the ribbon to prevent loss of the entire thing. I imagine a small robot that would constantly traverse the cable, inspecting it an making minor repairs.


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Post Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator   Posted on: Thu Feb 20, 2014 2:01 am
JamesG wrote:
The bobbins would be part of the counterweight mass. I think you would want at a minimum 3 strands to the ribbon to prevent loss of the entire thing. I imagine a small robot that would constantly traverse the cable, inspecting it an making minor repairs.


My understanding from the talk and the pdfs was that the "cheap" version would have 6 strands in rectangular form to reduce the chances of more than one cable being sheared by a micrometeorite at the same time tho I would have thought a hexagonal one would be better for that purpose than a rectangle but maybe it eases some other aspect of construction / functionality?. But I was imagining paired bobbins on each strand so that when one failed on average once every 3 months they could wind the broken cable in at both ends when it snapped to avoid it being a cause and creation of shrapnel at both ends if it hit in an uncontrolled manner.

But you are right they would be part of the counterweight effective mass so if one snapped and was wound in the other bobbins with intact strands would have to wind in or out extra cable to balance the weight distribution and keep the central mass in the L1 point. But I think the bobbin system would give the overall system more flexibility and more importantly have a chance of safely containing the catastrophic fail of strands that are known to be likely on a regular basis. But could be coped with with enough cable redundancy and a robot strand splicer as long as there was some spare cable on each bobbin.

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Post Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator   Posted on: Thu Feb 20, 2014 2:39 am
There isn't much risk of high energy release if one, some, or even all of the strands are cut. Yeah there is a lot of mass under tension, but it's a lot of mass spread over a very long distance. That is the spooky thing about the physics of "bean stalks", they are mass and gravitationally balanced. a cut end will just start to slowly (but exorbitantly) drift "out" or "down", with the loose strands pilling up. I guess you would want to keep it from making a mess, and that would be the utility of reels.

Another thing, a Lunar Elevator isn't the same kind of animal as an Earth or Mars one. It isn't held by centrifugal force, but by gravitational attraction (why it has to be at L1). This also reduces the energies (I think, I didn't do the maths).


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Post Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator   Posted on: Thu Feb 20, 2014 7:44 pm
JamesG wrote:
There isn't much risk of high energy release if one, some, or even all of the strands are cut. Yeah there is a lot of mass under tension, but it's a lot of mass spread over a very long distance. That is the spooky thing about the physics of "bean stalks", they are mass and gravitationally balanced. a cut end will just start to slowly (but exorbitantly) drift "out" or "down", with the loose strands pilling up. I guess you would want to keep it from making a mess, and that would be the utility of reels.


I think you are right that they start slowly and it gives you time to come up with a fix if you have stuff that can move faster than the gravitational acceleration and you are actually helped by being in effective geostationary orbit for an Earth based system for any mass above the cut point as it would just move out to a higher orbit. But below that point you are you are constantly turning potential energy into kinetic energy for the entire mass of the cable even if its averaged out a bit as different parts of the cable would be in different strength gravitational fields so there would be an element of the bits in the weaker field still holding up that in the stronger field as it dropped. But the whole bottom part of cable would be accelerating downwards at an increasing rate of acceleration as the top part was gradually removed from the weaker part of the field. For an Earth space elevator the bottom cut bit would I think wrap around the earth with the end hitting the ground probably away from the elevator unless it was very unlucky at the point of snapping but the end would hit like a glorified cracked whip and I would not like to be in the general vicinity of where it hit the ground.

JamesG wrote:
Another thing, a Lunar Elevator isn't the same kind of animal as an Earth or Mars one. It isn't held by centrifugal force, but by gravitational attraction (why it has to be at L1). This also reduces the energies (I think, I didn't do the maths).


Yes the Lunar one is based upon gravitational balancing so should overall have less energy stored in its structure from an orbital point of view but because of the gravitational balancing if it was cut the bottom bit would I think drop all the way down to the same point below the elevator right next to the point of attachment. And even worse from our point of view or at least those who live on or near the equator could have 2 spinning whiplash points heading towards them accelerated by the Earth's gravitational force on hundreds if not thousands of tons of mass from a height of a quarter million miles again turning potential energy into kinetic energy. An interesting spectacle to watch from a distance if the problem was not dealt with in the time available.

Again I have not done the calculations but it seems to me any multi ton mass dropped from thousands of kilometres up is going gain considerable kinetic energy on its way down even if the averaged out G-forces are less than a sixth of a G for the Moon side and less than a full G for the bit on the Earth side. As we have an atmosphere I think its probable that the cable would burn up in an interesting effect but the counter weight and any robots/elevator carriages may get thru to make interesting holes in the ground. :wink: :twisted:

I suppose if the cable was thin enough air resistance on a huge surface area against density might slow it down for the cable bit just making a mess around the equator on the Earth side but it would not help in the vacuum on the Moon.

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Post Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator   Posted on: Thu Feb 20, 2014 8:55 pm
Naw, air resistance will slow the "ribbon" down as it falls into the atmosphere. Air resistance likewise will prevent any "wrap around the world whipping". Also remember as it falls it gets lighter as more of it is on the ground. So yeah it will make a mess, but mostly just from a seemingly endless coils and loops of cable falling out of the sky and getting all over the place.

While space elevator collapse would be annoying and expensive to replace, it would be a slow-motion catastrophe. Unless you are one of the unfortunates riding it when it let go!


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Post Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator   Posted on: Fri Feb 21, 2014 12:46 am
JamesG wrote:
Naw, air resistance will slow the "ribbon" down as it falls into the atmosphere. Air resistance likewise will prevent any "wrap around the world whipping". Also remember as it falls it gets lighter as more of it is on the ground. So yeah it will make a mess, but mostly just from a seemingly endless coils and loops of cable falling out of the sky and getting all over the place.

While space elevator collapse would be annoying and expensive to replace, it would be a slow-motion catastrophe. Unless you are one of the unfortunates riding it when it let go!


That's nice to know for use on planets with atmospheres I keep forgetting that the ribbon idea has taken over from the version promoted in Arthur C Clarkes fountains of paradise where IIRC they thought of using a special artificial diamond (this was a long time before buckyballs and CNT's were identified and made) as the cables and it would be quite thick and massive high up and tapered down to a few centimetres at the Earth end if you wanted to be able operate a man capable version.

I suppose that the ribbon version might actually act as a big long thin parachute for the payload elevator if it did not spin the end of the ribbon into a cord on the way down so even the unfortunates riding it might survive with a bit of luck.

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Post Re: 12 Feb 2014 - Commercial Mining with a Lunar Elevator   Posted on: Fri Feb 21, 2014 2:05 am
Yeah the idea has evolved abit over the decades. Really the mode and effects of a collapse would depend on where it failed.


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