# Space elevator vs Moon elevator

Discussion in 'Architecture & Engineering' started by icarus2, May 30, 2010.

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1. ### cluelusshusbund+ Public Dilemma +Valued Senior Member

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Well for now im lookin at it as if the cable does not stretch... woudnt the cable have to be elastic for what you'r talkin about,???

3. ### BlindmanValued Senior Member

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Yes. That would be related to the speed of sound through the cable. So maybe 1000mps (meters not miles) so that would give a delay of around 390000seconds or four and a half days. The cable would have to stretch at least 15000km to accommodate this delay.

5. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Name a material that doesn't stretch.
Especially over the length we're contemplating.
For ordinary everyday use engineering can (and often does) treat materials as inelastic, but over 384,00 km even minuscule elasticity will lead to whipping.

7. ### cluelusshusbund+ Public Dilemma +Valued Senior Member

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Im takin what Blindman said one step at a time... an the firs thang im tryin to determin if this is corect:::

Blidman---"But as it (falls) moves toward the earth it will accelerate in a vector not perpendicular to the earth as it is attached to the moon which is in orbit."

I visualize the cable stayin perpindicular to earf.!!!

It seems like earfs gravity woud keep the cable ponted toward the center of earf... but the cable woud not stay perpindicular to the moon... dew to the rotation of the moon stayin constent durin its elicptical orbit.!!!

8. ### BlindmanValued Senior Member

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Dywyddyr: But because of the speed of sound is so low in the cable it would require near 4% change in length. Thats a lot.

9. ### BlindmanValued Senior Member

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cluelusshusbund: the earth end would be in partial free fall thus will accelerate as it gets closer to the earth. Remember even though the end of the cable is not at orbital velocity for its hight but if the earth where a point it would still be in a very distorted orbit.

10. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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What would require a 4% change in length? I don't follow you.
If there's any elasticity then there's a lag in movement. Small percentages add up tremendously over ~400k km.

11. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Why?
The cable cannot be perfectly rigid therefore there'll be lag in movement pulling it away from perpendicular.

Plus the slight problem of: if the cable extends closer to the Earth than geostationary orbital distance* then whatever is closer will be pulled Earthwards and whatever is outside that distance won't.

* Which would be at some "point" on the cable dependant on the velocity of the Moon's orbit (talking off the top of my head here, it's 4:30 AM - I'm not going to work it out).

Last edited: Aug 13, 2010
12. ### BlindmanValued Senior Member

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As you pointed out the pull of the moon would be delayed. This delay is Dependant on the speed of sound in the cable. If the speed of sound in the cable is 1kmps this adds up to 4.5 days of delay. In that time the moon will move an average of 15000km too and away from the moon. Thats about 4% of the cables lenght.

13. ### cluelusshusbund+ Public Dilemma +Valued Senior Member

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Originally Posted by cluelusshusbund
I visualize the cable stayin perpindicular to earf.!!! ”

For esample... attach a small steel ball to the end of a rubber band an hold the other end of the rubber band up in the air... an then place a strong magnet under the steel ball... no mater if you move you'r hand farther away or closer to the magnet the rubber band will pont strate toward the magnet.. an as you move you'r hand in an eliptical orbit aroun the magnet... the length of the rubber band will change but the rubber band will stay strate an keep pontin toward the magnet.!!!

14. ### BlindmanValued Senior Member

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Dont forget that the moon does not have a perfect orbit. It varies by as much as 40000Km from earth. Try your magnet experiment but move closer at one side of your ellipse segment.

15. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Still don't follow. Why is the delay dependant on the speed of sound? (Speed of sound in materials varies with elasticity AND density).
And which material are you using for 1 kps?
ANY elasticity will result in a lag. And unless very carefully engineered and counter-balanced may well be self-reinforcing.
A millisecond on the first orbit, two on the next...

16. ### cluelusshusbund+ Public Dilemma +Valued Senior Member

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Yes my esample includes an eliptical orbit.!!!

17. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Really?
Truly?
You've defeated inertia?
There's no lag whatsoever?
Or is there a tiny (not noticeable, possibly discountable) one?

18. ### BlindmanValued Senior Member

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Dywyddyr: if you tug on a rope the fastest that pull can move along that rope is the speed of sound in that rope. I picked 1kmps to make calcs easy. Nothing currently exists to do the job.

Clueluss. Your magnet experiment can not be used as and example as you need to scale the magnetic force to the speed of your orbits. if your doing it by hand your many magnitudes of velocity below the orbit speed of the magnet.

19. ### DywyddyrPenguinaciously duckalicious.Valued Senior Member

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Ah, got you. I couldn't see where the 1 kps came from. (It IS 5 AM here...).

20. ### BlindmanValued Senior Member

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Speed of sound is dependent on the material. I just used a easy number but could be as high as 10kmps..

21. ### cluelusshusbund+ Public Dilemma +Valued Senior Member

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I thank the prollem i have is wit the term "whip about"... which i visualize as a bak an forth motion... cause i thank the earf end of the cable will always be movin in a "forward" directon but at constently changin speeds dew to the moons orbit bein eliptical.!!!

So the earf end of the cable woud increase in speed as the moon goes from apogee to perigee... an that perod of the moons orbit woud be spread over (about) 7 days... an it sure woud be interestin to know the strongest g-force which woud be created at som pont durin that 7 days.!!!

Surly sombody here woud know how to figer that out.???

22. ### BlindmanValued Senior Member

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Well I cant seem to work it out without a simulation. I have looked at at various parts of the problem and I think it would be a very worth while endeavor if there is profit on the moon.

The cable would provide a very energy efficient means of moving material from the moon to earth. For most of its length it will have Earth pulling gravity on which move elevators down using the energy in the moons orbit and earths gravity. They may well be under brakes thus providing more energy. Gases and liquids could even be siphoned from the moon.

Going up to the moon via cable is a little energy expensive. But if the net mass movement is to the Earth, it should need no energy.

One other problem is that the orbital plane of the earth end would also vary and an Earth space cable would be on another plane. Optimal transfer between the two ends would be cyclic. Masses would be accumulated at the ends of both cable and released every few months to keep energy cost down. All transfers will require orbital change.

So a moon to earth orbit (near geo stationary) cable can provide free transport to and from the moon. Energy extracted from the moons potential energy.

Building one is somewhat difficult. I like the boot strap method. Instead of creating one massive cable with counter weight to establish a moon cable which can then be used to extend toward earth.

I would start with a micro cable, may be a few millimeters or less in diameter. Then when that is connected to the moon and suspended toward earth (not all the way), the free energy available can the be used to transport material up the cable to increase its mass and strength. But we would need an established moon base where all the materials and processing can be supplied. Not cheap.

23. ### j.colfaxRegistered Member

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wow. it's like the cast of big bang theory in here.