Thread: Cable to the Moon

1. Cable to the Moon

Ignoring for the moment the rotation of both the Earth and the Moon. If you strung a cable between the two bodies how much tension are we talking about?

If it were at all possible, we could have a seriously cheap way of transferring mass between the two systems. The mechanism would be like a true elevator, with a counterweight and a pulley.

2. I don't think you're necessarily talking about any tension at all--assuming you can get past the probably insurmountable problem of daily rotation of the Earth. You can leave the line slack. The moon is in a stable orbit and can hold its own position (within reason). You don't have to pull on the rope.

3. We should also ignore:
- the fact that we don't have any cable materials strong enough to support its own weight even for a few tens of miles in the Earth's gravity
- the fact that the Earth and Moon vary in distance about 23,000 kilometers during every Month.

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Originally Posted by Trebuchet
I don't think you're necessarily talking about any tension at all--assuming you can get past the probably insurmountable problem of daily rotation of the Earth. You can leave the line slack. The moon is in a stable orbit and can hold its own position (within reason). You don't have to pull on the rope.
You are forgetting about the effects of gravity which will put the cable under a lot of tension

5. Yeah, I'm also assuming a constant Earth-Moon distance for now.

There'll be tension. The part of the cable between Earth and L1 will want to fall to Earth, and the part between L1 and the Moon will want to fall to the Moon.

So my idea is to lift a payload from Earth to L1 by dropping a counterweight from L1 to Earth. The transfer of PE has to be connected somehow, but doesn't necessarily have to be a cable and pulley system. The payload at L1 then becomes a counterweight for raising payload of the Moon. And so on...

The interesting thing is that the counterweight gains weight as it falls, while the the payload loses weight as it rises. So you could stick another payload onto the system.

6. It's an interesting idea. Would a Pluto-Charon cable be theoretically easier to implement by magnitudes? It has much smaller eccentricity of the orbit, much smaller orbital radius and gravitational lock for both objects - so not having the problem of daily rotation.

7. Originally Posted by PraedSt
Yeah, I'm also assuming a constant Earth-Moon distance for now.

There'll be tension. The part of the cable between Earth and L1 will want to fall to Earth, and the part between L1 and the Moon will want to fall to the Moon.
For an ordinary beanstalk, the lower part is from geosynch to earth -- 36,000 kilometers. For this beanstalk, the lower part would reach from L1 to Earth -- 320,000 kilometers. There would be a lot more tension than an ordinary beanstalk.

For a beanstalk 380,000 kilometers in length, the cross sectional area is even larger than an ordinary beanstalk. Thus even more vulnerable to impacts from debris.

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Originally Posted by Trebuchet
the probably insurmountable problem of daily rotation of the Earth.
I wonder how high a tower you'd have to build at the south pole so as to keep the moon always above the horizon from the tower's top?
A simple gimbal mechanism would work from there.

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With moderate advances, and lots of money, we can build a Dr. Hugh Edwards type space elevator near the center of the portion of the moon that faces Earth. The ribbon passes, approximately though L1 and extends toward Earth another 100,000 kilometers, with a tiny counter weight at the Earth end. Making it longer = counterweight closer to Earth, requires CNT = carbon nano tubes with great specs, as the gravity of Earth, is quite significant even half way to the Moon, so the ribbon is under much tension due to it's own weight.
With strong enough material the counter weight will be about 50 kilometers above Earth's surface at the Moon's closest approach. If made much longer it will start to wrap around the Earth one day each Lunar month which will make the tension even greater. The counter weight will move though Earth's upper atmosphere at about 1000 miles per hour, which will produce only modest air drag at that altitude. The counter weight could be a modest station that could be reached from Earth's surface with about 1/10 th the energy required for reaching LEO, on that one day per month. So yes, a cable from the Moon almost to Earth's surface may soon be possible. For the Earth elevator, the laser propulsion needs a range of about 36,000 kilometers, going toward the moon. The lasers on the Earth and Moon need about 6 times that range to propel the elevator car (in either direction) to L1. Possible with CNT with good specs, due to the lower speed of a long Moon elevator.
As antoniseb noted, the counter weight would be up to 23,000 miles from Earth's surface the other 26 days of each Lunar month. Results will vary from month to month as the Sun and planets, change the closest approach of the moon a bit. Both LEO and GEO satellites will colide with the ribbon rarely, so we likely do not want to build a long lunar space elevator.
There are more details in the forum at www.liftport.com which is having serious problems with spammers, so could be gone soon. edit: I checked; the form is gone, but may be back. Michael Lane is now thinking Moon Elevator, before Earth space elevator. Neil
Last edited by neilzero; 2011-Dec-01 at 02:52 PM.

10. Originally Posted by neilzero
With moderate advances, and lots of money, we can build a Dr. Hugh Edwards type space elevator near the center of the portion of the moon that faces Earth. The ribbon passes, approximately though L1 and extends toward Earth another 100,000 kilometers, with a tiny counter weight at the Earth end. Making it longer = counterweight closer to Earth, requires CNT = carbon nano tubes with great specs, as the gravity of Earth, is quite significant even half way to the Moon, so the ribbon is under much tension due to it's own weight.
With strong enough material the counter weight will be about 50 kilometers above Earth's surface at the Moon's closest approach.
Drop down 100,000 kilometers from EML1, and you'll be about 220,000 kilometers above the earth. Subtract 23,000 kilometers from this and you're still about 200,000 kilometers above earth's surface.

I suspect you lost a zero when doing your BOTE calculations.

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Hi Hop: Sorry I confused things. Yes EM L1 is much closer to the moon than to Earth and Kevar or equivelent requires serious tapering to get even 100,000 kilometers closer to Earth, So a moon elevator that long has little practiality, other than proving the concept, and traveling between the Moon and L1 = no customers at present. The great specs may not be available in 20 years, or ever, so probably, we won't build a moon elevator, even though Michael Lane thinks we can by 2017. The forum seems to be gone from www.liftport.com The forum may be back in 2012. Neil

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Hi Hop: Sorry I confused things. Yes EM L1 is much closer to the moon than to Earth and Kevlar or equivelent requires serious tapering to get even 100,000 kilometers closer to Earth, So a moon elevator that long has little practiality, other than proving the concept, and traveling between the Moon and L1 = no customers at present. The great specs may not be available in 20 years, or ever, so probably, we won't build a moon elevator, even though Michael Lane thinks we can by 2017. The forum seems to be gone from www.liftport.com The forum may be back in 2012. Neil

13. 1. The variation in the Earth-Moon distance is solvable. Assuming a change in 50,000km/month, this works out to approx 10m/s from each end of the cable. The winding period will probably be more difficult as you have to pack/roll it properly for the subsequent unwinding period.

2. The rotation of the Earth is also solvable, but the solution makes this architecture even more difficult. The end of the cable will have to hang well above the atmosphere because the relative speed will be in the supersonic region. What this means is that the climber will have to affect a docking with the tip of the cable at around Mach 2-3, 50-100km above the surface of the Earth. I don't know about the docking, but the required speed is certainly better than the Mach 25 required for LEO. So annoying, but possible.

However, this will also mean that for (1) above, you can only wind/unwind the cable on one location- the Moon, so the required rate will double to 20m/s.

3. The tension in the cable will indeed be a lot higher, but the inverse square law of gravity means that it won't get worse to scale.

14. Originally Posted by Wolf-S
It's an interesting idea. Would a Pluto-Charon cable be theoretically easier to implement by magnitudes? It has much smaller eccentricity of the orbit, much smaller orbital radius and gravitational lock for both objects - so not having the problem of daily rotation.
I presume so. However, they're not very massive so I presume other methods of transport would be cheaper. It'd be fun to build one though.

15. Originally Posted by neilzero
With strong enough material the counter weight will be about 50 kilometers above Earth's surface at the Moon's closest approach. If made much longer it will start to wrap around the Earth one day each Lunar month which will make the tension even greater. The counter weight will move though Earth's upper atmosphere at about 1000 miles per hour, which will produce only modest air drag at that altitude. The counter weight could be a modest station that could be reached from Earth's surface with about 1/10 th the energy required for reaching LEO, on that one day per month.
Sorry Neil, I didn't read this until after I'd posted. Yes, this is pretty much what I came up with.

So a moon elevator that long has little practiality, other than proving the concept, and traveling between the Moon and L1 = no customers at present.
We'll clearly need new materials, but customers from Moon-L1 wasn't the motivation behind this cable. I wanted to lift payloads off the Earth by dropping lunar mass onto it. Like a real elevator.

16. Originally Posted by PraedSt
3. The tension in the cable will indeed be a lot higher, but the inverse square law of gravity means that it won't get worse to scale.
The angular velocity is a tad less than a geosynchronous stalk. The so-called centrifugal force would be about 9/10 that of geosynch. Therefore for the 36,000 miles from earth to geosynch, the taper would have to be a little wider than the conventional bean stalk.

For the 190,000 kilometers from geosynch to EML1, the stalk would be even thicker than at geosynch. It would reach maximum thickness at EML1. You are increasing not only its length 5 fold, but also it's thickness. This huge volume would add a lot to the stress even if it is being pulled at only a fraction of a g.

The stress of a geosynch stalk is already pushing the limits of material strength, even for exotic materials like carbon nanotubes. What you need is Scrith.

edit: corrected arithmetic errors.

17. Originally Posted by Hop_David
The angular velocity is a tad less than a geosynchronous stalk. The so-called centrifugal force would be about 9/10 that of geosynch. Therefore for the 36,000 miles from earth to geosynch, the taper would have to be a little wider than the conventional bean stalk
It's much worse than that isn't it? Geosync is 360/day while my cable is 360/28days. I need superscrith.

18. Another idea. The eccentricity of the Moon's orbit changes the Earth-Moon distance by ~20m/s on average. If we had superscrith, we could place a piston in L1, and extract energy from this motion.

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What about electrical charge differentials between the Moon and Earth? We saw what happened to the Space Shuttle tether experiment!

20. Originally Posted by Solon
What about electrical charge differentials between the Moon and Earth? We saw what happened to the Space Shuttle tether experiment!
The cable doesn't have to be electrically conductive.

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Perhaps with some pully set up, the earths rotation and the moons orbit could lift objects moonward...

From Scott Lowthers site.

http://up-ship.com/blog/?p=11917

I wonder if this effect might be useful in some way...
Last edited by publiusr; 2011-Dec-04 at 08:33 PM.

22. That means my cable won't fall the moment it's cut! I'll have time for a controlled demolition

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A pulley and counterweight system increases the required strength of the ribbon, perhaps more than double, otherwise it does several useful things. Some one estimated 4 million ohms for the 91,000 kilometer Dr. Hugh Edwards type space elevator, so less than one million ohms seems unlikely for 360,000 kilometers of moon elevator. If the voltage difference, is one million volts, then one amp flows = one megawatt dissipated in the ribbon = 2.78 watts per kilometer = much less than one degree temperature rise, so the ribbon is safe, unless it is a superconductor = very unlikely, but corona discharge at an altitude of about 50 kilometers might interfere with docking at the Earth end of the cable.
Does every one agree that rolling in and out 50,000 kilometers from a giant winch on the Moon every 28 days is not a show stopper? I have no idea how to calculate the kWh.
To me it seems the centrifugal force would be considerably higher, since Earth rotates 28 (26?) times faster than the moon, but perhaps several times shorter, makes them about the same. The Dr. Hugh Edwards space elevator can also be almost 360,000 kilometers long which produces 28 times higher tip speed than the moon elevator, which can get us to the outer solar system quickly. Four times longer may only double the ribbon material strength requirement. Neil

24. Originally Posted by neilzero
Dr. Hugh Edwards type space elevator
What's one of those Neil? I've googled with no success.

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