Talk:Space tether

Rotovator

People in industry call these tethers momentum exchange tethers. Rotovator isn't used. I myself have never heard the term and I work with in-space propulsion. In-Space Propulsion --Belchja 02:46, 29 July 2005 (UTC)[reply]

Folks, thanks for your work in explaining this tricky subject. But I think this article needs a lot more work. It is the first article I have accessed in Wiki which I could not understand.

I have a BS in engineering and have been reading science books for decades, so I think I am at least average in knowledge about this stuff, compared to your reader base. But you use a lot of terms where you dont define what they mean.

Rotate and Culivate make Rotavator, the English Rotavator Rotary Tiller, which existed long before thether propulsion was thought of.. Gregorydavid 06:38, 10 August 2006 (UTC)[reply]

Tidal forces

I am having trouble understanding the tidal forces point and I think the article could do with a word of explanation in this respect. Surely the vertical alignment of the masses is simply a case of this being the lowest potential energy configuration? Paul Beardsell 16:50, 6 May 2004 (UTC)[reply]

You're right, it is precisely because it's the lowest potential energy configuration; since the tidal forces are the gradient of the potential, this is the same as saying that the tidal forces pull it into this configuration. It sounds like a fuller explanation is appropriate, although if the tidal force page has something, it could maybe be minimal. --Andrew 21:20, May 8, 2004 (UTC)

"rotovator" or "rotavator" ?

What's the proper spelling, "rotovator" or "rotavator" ? (Remind me to use the proper spelling at micrometeorites ) -- DavidCary 03:41, 8 Jul 2004 (UTC)

Rotavator is a registered trade name for a kind of agricultural machinery (short for rotary cultivator). Rotovator was probably spelt with the O to avoid confusion. --Heron 15:23, 27 Aug 2004 (UTC)

Clarification requested

I'm a getting a little confused over the terminology being used in regards to the subject of space elevator and I was wondering if it would be possible to clear some things up. First, I would like to point out a few items that seem confusing to me:

See non-rocket spacelaunch. It has a nice brief description of tethers related to LAUNCHING vice orbital maneuvering.KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]

The space elevator article puts space elevators in the class of spacecraft propulsion, which is correct, but does not mention the tether propulsion category of that class.
The tether propulsion article talks about several flavors of tethers, including the so-called beanstalk and something called the rotovator, but makes no mention of skyhooks which is a widely used term for tether propulsion.
1. I'm working on this. I hope to have it a little clearer soon. KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]
The space elevator article correctly mentions the terms 'beanstalk' and 'skyhook', and the skyhook article correctly identifies two kinds of tether propulsion: the 'space elevator' and something called hypersonic skyhooks. However, the 'hypersonic skyhooks' link merely points to the tether propulsion article, which itself does not mention 'hypersonic skyhooks', but instead describes an identical concept as rotovators.
The space elevator article also correctly identifies a space elevator as a geosynchronous orbital tether, but provides no link to this subject.

Now, that being said, here is my attempt to sort it all out:

The term 'space elevator' is equivalent to the terms 'beanstalk' and 'geosynchronous orbital tether'.
1. I agree, I had not previously heard of geosynchronous orbital tether, but it is technically accurate. 66.177.175.227 02:46, 23 June 2007 (UTC)[reply]
2. I do NOT agree. A space elevator / beanstalk is a special case of a synchronous tether, that is, one that reaches the planetary surface and is part of a LAUNCH process. It is possible to have a synchronous tether that boosts from low to high orbit (and vice-versa) without being part of a launch.KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]

The term 'skyhook' is equivalent to the term 'tether propulsion', but is not exclusive to the terms 'space elevator', 'beanstalk', or 'geosynchronous orbital tether' as these are all equivalent terms for only one form of 'tether propulsion'.
1. Do NOT agree. The term 'skyhook' as used in orbital or satellite discussions is equivalent to a tidally locked tether. KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]

The term 'hypersonic skyhook' and 'rotovator' are equivalent to the term non-synchronous orbital tether I understand why you want this catagory, but no one IMHO will think to put "non-synchonous orbital tether in a search box, plus we already have too many names for much the same thing without adding another. This term describes a system that is very similar to a 'geosynchronous orbital tether', but which is different in several very important aspects.
1. Rotovators we can build now need to be hypersonic with respect to Earth's surface, but a 100,000 kilometer rotovator could briefly move less than 60 miles per hour = 100 kilometers per hour at closest approach to Earth surface. Rotovators close to the Moon or other airless bodies can also move at much less than hypersonic speed. At least one group considers bolo a prefered name for shorter rotovators. 66.177.175.227 02:46, 23 June 2007 (UTC)[reply]
2. Skyhooks and rotovators are fundamentally different. One is tidally locked, the other rotates rapidly. The mathematics used to describe them are significantly different and use different material terms.KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]

Given the above, I would like to suggest that the subject be re-organized in this way:

A 'space elevator' is one form of 'tether propulsion', also known as a 'skyhook', which in turn is a category of 'spacecraft propulsion'.
There are two forms of 'space elevator', one is a 'geosynchronous orbital tether' also known as a 'beanstalk', and the other is a 'non-synchronous orbital tether'.

I make this suggestion because it is my POV that the 'non-synchronous orbital tether' concept is just as valid as the 'geosynchronous orbital tether' and that both should be clearly described under one subject heading of either 'space elevator' or 'tether propulsion', however this would raise several questions:

Should 'skyhook' be standardized as 'tether propulsion' and should the skyhook article be removed and/or incorporated into the tether propulsion article?
Should the terms 'hypersonic skyhook' and 'rotovator' (both of which I have never seen before) be standardized as 'non-synchronous orbital tether'?
1. No. Tether propulsion is a MUCH broader issue than skyhooks which are one small subset thereof.KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]

Should the space elevator article be expanded to include both 'geosynchronous orbital tether' and 'non-synchronous orbital tether', or should the space elevator article be renamed as 'geosynchronous orbital tether' and another article added to describe the similar but also very different 'non-synchronous orbital tether'?
1. Neither. I am working on a clarification of the sets and subsets of tether propulsion. Please permit me the time to complete.KitemanSA (talk)
If the latter, then shouldn't the term 'space elevator' also be standardized as 'tether propulsion' with links to 'geosynchronous orbital tether' and 'non-synchronous orbital tether' articles? If not then should term 'tether propulsion' be standardized as either 'skyhook' or 'space elevator'?

For reference I cite the following websites which describe both geosynchronous and non-synchronous orbital tethers, and although they seem to use the terms 'skyhook' and 'space elevator' interchangeably the latter has clearly overtaken the former in recent popular usage:

http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/papers/scable.pox

http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/papers/endsky.pox

http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/1982.articles/elevate.800322

Jerry mcmanus 20:55, 8 Jul 2004 (UTC)

A space elevator is a fixed structure, extending from the surface to beyond GEO. So called because from the bottom it looks and acts like an elevator system in a very tall building.

A beanstalk is just an informal term for the same thing, with an allusion to the fairy tale of a climbable column reaching up beyond the sky.

A rotovator is quite different; an orbiting structure with one or two long cables extending out from its center of gravity. It rotates on a horizontal axis at a rate such that the tip(s) of the cables are momentarily at rest when closest to the ground, Visualize a bicycle wheel rolling down the street and subtract the rim and all but one or two spokes. As seen from the ground, the tip drops down nearly vertically, payload is attached and/or detached, the cable then rises back to space.

A hypersonic skyhook is sort of a hybrid: a more-or-less vertical cable system, with a center of gravity orbiting well below GEO. The bottom tip is moving at hypersonic speed relative to the atmosphere, so it can't extend down close to the surface, but significantly slower than an object orbiting at that height. A merely suborbital rocket can rendezvous with the cable and transfer payload.

A skyhook is any kind of cable system for getting from ground to space, but I think it fits the orbiting systems best. The image is of a crane hook dropping down from the sky, picking up a payload, and pulling it back up.

I was with you to here. A skyhook is a tidally locked tether.KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]

Tether propulsion can also cover rotating tethers used the transfer payloads in space, e.g. from Earth orbit to Lunar orbit.

It can also cover a number of other uses like endo-atmospheric tethers.KitemanSA (talk) 19:06, 7 August 2008 (UTC)[reply]

None of these are spacecraft, exactly, but...

--wwoods 00:14, 9 Jul 2004 (UTC)

Tether propulsion also covers electrodynamic tethers, which are sort of like cut-rate magnetic sails. Also, there's a class of rotating tether I've seen referred to as "bolos" that don't fit perfectly into the divisions above; it's a rotating tether located away from planetary surfaces, used for changing the momentum of spacecraft in transit. Bryan 06:03, 9 Jul 2004 (UTC)

Since they can't tap the Earth's angular momentum and rotational kinetic energy, the orbiting tethers need some other means of reboosting themselves after lifting a payload up from the Earth. Their advantage is that, being in orbit, they can use high efficiency (=> low thrust) methods, e.g. the geomagnetic motor or ion jets. The best method of all is traffic in the other direction; plain dirt from the Moon could deliver a huge amount of impulse to LEO...

"Bolos" is what I was thinking of in the "tether propulsion" above, but I didn't remember that term. A way to move stuff around while leaving your engines behind, so you don't care about their mass or propellant.

[This should be doubly indented, but for some reason it's not.]

--wwoods 07:43, 9 Jul 2004 (UTC)

Ah yes, for some reason when I read that the first time I pictured something completely different. My bad. BTW, your paragraph looks doubly indented to me. I use the "classic" skin, perhaps there's a bug in one of the others? Bryan 08:13, 9 Jul 2004 (UTC)

It's indented right now, but I'll swear it wasn't in the preview. Weird. Let's see... And that post isn't -- again -- in the preview, though this one is. I also use the Classic, so it's not that. "Curiouser and curiouser" --wwoods 08:44, 9 Jul 2004 (UTC)

So, if I understand correctly, the subject breaks down like this:

Space Tethers. What is currently labeled as 'tether propulsion' and which breaks down into three categories:
- Geosynchronous Orbital Tethers. AKA 'space elevator' AKA 'beanstalk'
- Non-synchronous Orbital Tethers. AKA 'skyhooks' which include:
  - Electrodynamic Tethers.
  - Hypersonic Tether.
  - Rotovator.
- Free Space Tethers.
  - Bolos.

If this is a valid breakdown, should the current articles be edited to reflect this, or is the currrent tether propulsion article good enough? Jerry mcmanus 21:09, 9 Jul 2004 (UTC)

I believe that by 'electrodynamic tether', Bryan was referring to a method of moving an orbital tether, not to a separate type. And I find it easier to think of the space elevator as a stationary structure, held up by centrifugal force, than as a body in orbit. Otherwise, yeah. --wwoods 08:22, 10 Jul 2004 (UTC)

As I understand it the space elevator is only stationary because it's center of mass is in GEO. In fact, the Edwards plan calls for the launch platform to be mobile in the Pacific in order that it may be moved to avoid space debris. The mass of the platform acts as a counterpart to the counterweight above GEO, but it is the center of mass in GEO that is doing all the work, and so a space elevator need not actually be attached to the surface of the Earth, or even strictly stationary. This is a common misconception caused by the unfortunate comparison of the space elevator to a 'ball on a string'. I believe this is also the reason that earlier references used 'space elevator' and 'skyhook' interchangeably, the portion of a space elevator below GEO is essentially just 'dangling' down to the surface of the Earth, like a hook from the sky. However, for the sake of clarity, I can see an argument for changing 'geosynchronous' and 'non-synchronous' to 'geostationary' and 'non-stationary' respectively.

Jerry mcmanus 18:30, 10 Jul 2004 (UTC)

It's stationary because it is tied to the ground, more-or-less. The center of gravity has to be a little above GEO, putting tension on the cable, if it's to have any payload capacity. In the Edwards design, the base station's weight is partly supported by the cable and partly by its buoyancy. Also there has to be enough of a connection to [the Earth] for the cable to transfer angular momentum from the Earth to the payload as it is lifted or you lose the greatest advantage of this type of tether. I haven't gone through the Edwards plan in detail, but there'll have to be various anchors that the cable can be moored to while operating, or it'll have to use its motors to keep from being pulled steadily westward.

--wwoods 20:23, 12 Jul 2004 (UTC)

I should point out that there are tethers that do not rotate at all and really can't be called skyhooks - in fact all the tethers currently built are of this type. We should make sure that the ultimate classification includes, for example, the use of long tethers to tidally control the orientation of satellites. --Andrew 23:37, 12 Jul 2004 (UTC)

robert l forward

Shouldn't there be a link to 'Robert Forward' somewhere in this entry? -anon(not at my comp and can't remember password)

comments only

tethers are exteremely hard science. A one meter length as release makes for the t.

And t means the length of General Relativity mass difference.

A size of long line literally makes a boost to higher altitude!

A third body was added to General Relativity :)

A tide is not required to make the small boost. A tide boost is a difference of lengths of tether system of changing orbits.

HELP REQUESTED What about torsion tethers that propel circling aircraft or drive machines

The tether rotates and drives the propeller of the circling aircraft. E.g.: http://www.google.com/patents/about?id=Rp0rAAAAEBAJ&dq=3705720 Such is a torsion tether. Then another tether that propels is the endless-loop tether that drives pumps or generators. These two tether propulsion families are to be treated in what article? Discussion is invited. Thanks. Joefaust (talk) 05:19, 12 February 2011 (UTC)[reply]

References

The second reference (Bacon 2005) is very confusing. Does it refer to the source James A. Bacon, who often works with these technologies? Here is one of his articles: http://www.richmondcatalyst.com/Issue2_Kevlar.asp —Preceding unsigned comment added by 199.211.157.2 (talk) 11:29, 5 December 2009 (UTC)[reply]

Electric Sail

I'm not sure that really belongs in the page summary, and is under a different meaning of "tether". The other techniques mentioned in this article relate to two bodies being attached by a tensile cable, creating a new centre of mass which can be manipulated for certain results (generally whilst in orbit, although tethered formation flying is not, but still involves two or more bodies being tethered together). Meanwhile, electric sails are the propulsion method of a single craft, providing thrust through an electrically charged wire (forming a sail) being pushed by the solar wind. There is no "tether" in such as the body is not attached to anything. 160.5.185.13 (talk) 15:28, 29 October 2013 (UTC)[reply]

Practical materials - Misleading tag.

The reference cited for the list, states those materials' strength are appropriate only for Lunar tethers, and that for Earth, the stronger carbon nanotubes are required.

Quote from the the cited reference [1]:

The table below shows some candidate materials for lunar space elevators, with density, stress limit, and the breaking height (the longest cable that can be suspended in 1 g). Lunar space elevators require much lower material strengths than the Earth space elevator, which will require carbon nanotubes (shown in Table 1 for comparison). All these materials, save the carbon nanotubes, are available now.

Listing all these materials in this Wikipedia article is misleading because it gives the impression they are up to the job on Earth, which they are not. Cheers, --BatteryIncluded (talk) 13:21, 21 April 2014 (UTC)[reply]

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Gravitational gradient stabilization - Technical difficulty?

I might be getting the wrong impression, but I believe this section could use better wording, however I feel like I should ask before editing it.

Why is this point marked as a "technical difficulty"? I believe that it must be referring to the fact that synchronizing the movement of the tether in such a way is complicated, but it gives off the impression that the fact that both ends of the cable are at different lengths and speeds is the problem in on itself, when it is in fact the point. — Preceding unsigned comment added by Tairetsu (talk • contribs) 08:17, 2 July 2017 (UTC)[reply]

History/pop culture

I came across what I think may be an extremely early discussion of a non-anchored (that is, non "beanstalk") tether in a short story at Gutenberg.org, from the magazine "If Worlds of Science Fiction" April 1956; "Atom Drive" by Charles L. Fontenay. http://www.gutenberg.org/files/32067/32067-h/32067-h.htm

I'm not sure that this goes in "history", or in a pop culture reference section, which currently does not exist, but I thought it pretty interesting, as it far predates any discussion I am aware of (excluding Tsiolkovsky's original elevator idea), and I bet few others coming from the technical side know of it. Fontenay was a newspaper editor and SF writer, not an engineer that I can discover, so I wonder if he got the idea somewhere else, or had sufficient physics education to come up with the idea himself. Anyway, I leave it for anyone who feels it worthy to insert it in the article, but I leave this note here, as I am certain that tethers have a great future ahead of them, and at some point such history will be of considerable general interest.173.180.151.52 (talk) 07:20, 10 January 2018 (UTC)an occasional anonymous contributor, mostly of minor grammar and spelling repairs[reply]