Talk:Space elevator/Archive 3

message: Metadiscussion:this whole talk page has become a colossal mess.
(User:Bryan Derksen)
Boy, hasn't it. How about wiping archiving the whole thing and starting over, with a set of disputed topics, e.g., ==marginal_cost/kg via an Edwards skyhook==, ==... via an advanced elevator==, ==what is the minimum traffic level needed to justify an elevator?== How about taking longer turns, with sceptics and boosters posting on alternate days?
But it seems to me that webpages are suboptimal for presenting multiple involved discussions with multiple participants. People insert and/or append responses, and the ::: thing is quickly overloaded. How about moving the discussion to  sci.space.tech  or some such? I'd be much easier to keep the threads and subthreads straight, and we could get some new participants, with maybe some new perspectives. It's not true that all knowledge is contained in wikipedia (not yet, anyway).
--wwoods 08:06, 12 May 2004 (UTC)
replies: text: I very nearly did that yesterday, I had the page nearly cleaned off into /Archive 2 in edit windows but I chickened out before pressing "save page" because I didn't want to be quite that bold. :) However, since I take this as a "seconding" of the idea, I think I feel more confident in doing that now. I'm going to move the bulk of this talk into Archive 2, and if there are unresolved issues that get moved with it I suggest that the proponents of those issues please restate them in a more organized way. Also, in future when this page gets too long let's be more careful about how it gets trimmed down; we shouldn't be snipping out bits and pieces of discussions, instead leaving or removing them whole-cloth. Bryan 15:27, 12 May 2004 (UTC)
replies: text: By all means include a general comparison with rockets here. But don't spend whole paragraphs exploring the details of rocketry economics behind those general figures, instead say "for a detailed breakdown of these costs, see rocketry" and put them over there. If there's disputes over those details, then talk:rocket would be the appropriate place to work them out. Bryan 00:26, 13 May 2004 (UTC)
thread_title: Metadiscussion:''this whole talk page is not as messy as it used to be.''

message: I moved all the discussion of economics to Talk:Space elevator economics. -- DavidCary 18:51, 1 Jun 2004 (UTC)
thread_title:  Economics: Where did these numbers come from? 

message: One of the candidate versions of the article now says that an orbital launch to 180km at 51 degrees inclination is a "LEO transfer orbit" - an unusual term!  An orbit at 180km altitude is a good LEO.  If any part of it is higher than 180km then that is even better.  But the 51 degrees refers to the angle at which the satellite crosses the equator - it does not make the orbit elliptical.  Paul Beardsell 00:54, 25 May 2004 (UTC)
thread_title:  LEO transfer orbit 

message: What's the solution to this?  On the face of it we have mutually exclusive solutions.  The focus in the Economics section is the Edwards proposal, an up-only elevator, but the proposed solution (in this article) to the Van Allan belt problem is portable shielding shifted about on an up-down elevator.  An up-down elevator reduces by a large factor the payload capacity (it seems difficult or impossible to have multiple climbers and decenders simultaneously - difficult enough that Edwards did not want to solve it) and thereby the economics are significantly affected.  Maybe we send cargo only and the astronauts go by rocket?  Paul Beardsell 13:49, 25 May 2004 (UTC)
thread_title:  Van Allan belt problem 

message: I moved the economics section to space elevator economics and replaced the section in this article with a short summary. This is not an attempt to get rid of the current dispute by hiding it in some obscure place. I moved it because 1)  the section was getting very detailed, and 2) this article is too long and should preferrably be partitioned even further. The fact that the economics dispute can now get a talk page of its own is merely a fortunate side effect :)
Feel free to improve my summary. It was a quick work and doesn't do as good a job as it should do. But try to keep excessive details out of it. The separate article can now be expanded to include every conceivable cost detail, this summary should be kept brief and abstract so a casual reader can get a good overview without getting confused. I find the current economics article rather confusing to read myself. Fredrik 22:46, 27 May 2004 (UTC)
thread_title: Economics detached from main article

message: ::Hmm. In all fairness you'll note that space elevator designs currently rely on currently non existent [ carbon nanotube ] cables, whereas rocket cost reductions mainly rely on launching more often, using better engineering and architectures, using actual past experience; and avoiding being part of a national program as they usually don't care so much about money. Wolfkeeper
I moved discussion of carbon nanotubes to the Talk:carbon nanotube article
-- DavidCary 18:51, 1 Jun 2004 (UTC)
thread_title:  carbon nanotubes 

message: Someone needs to fix the engineering discussion in the Cable section.  The feasability of constructing a space elevator isn't determined by the material's tensile strength, it's determined by the ratio between the material's tensile strength and density.  If someone discovers 120 GPa Unobtanium tomorrow, it'll still be useless for a beanstalk if it's as dense as gold.  One of the reasons people are so excited about carbon nanotubes is the combination of low density with high tensile strength.
:Yes, I agree. Since the "strength/density ratio" is the critical number, that's what we should be comparing between these materials. Another name for this number is the "characteristic length". Is it OK if I quote http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/1982.articles/skymes in the article ? -- DavidCary
 A nice intuitive way to express the strength to weight ratio
 of a material is called "characteristic length".  It is the length
 of material fashioned into a constant cross-section rope that can
 just support itself when hung from one end in a uniform one earth
 gravity field. (The formula is  tensile-strength/(density*1g)).
 Graphite, with its strong covalent carbon-carbon bonds is the best
 actually existing material. Its theoretical characteristic length
 is several thousand km.  A metastable metallic version of hydrogen
 that can exist at room temperature might be quite a bit better
 because hydrogen has much less dead weight, but its existence is
 only conjectured.
 .
 Later editions of the CRC handbook have a NASA originated
 table labelled "Mechanical and Physical Properties of Whiskers".
 The indicated whiskers are actual laboratory grown, millimeter
 length, single crystal rods of various substances, whose strength
 and density can be measured.  The measured properties give the
 following characteristic lengths:
 .
 Graphite whiskers	961 km
 Al2O3 whiskers		527 km
 Iron whiskers		162 km
 Si3N4 whiskers		455 km
 SiC whiskers		704 km
 Si whiskers		337 km
 .
 These numbers are about 1/5 to 1/10 of the theoretical limits for the
 substances.
 .
 By comparison we have todays engineering materials:
 .
        Bulk aluminum		  10 km
        Bulk iron		  11 km
        Bulk steel		  40 km
        Nylon			  88 km
        Fiberglass		  98 km
        Kevlar			 195 km
 .
 As stated in my previous message, a strength five times that of Kevlar
 would make earth elevator cables of varoius varieties possible. This
 means a characteristic length of about 1000 km; a bulk material with
 the graphite whisker strength above would do fine.
--
http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/1982.articles/skymes
: It would be really, really nice if the above materials: aluminum, iron, steel, nylon, fiberglass, kevlar, silicon (Si), graphite, sapphire (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC) -- as well as nanotubes -- had both the numbers we need for this calculation (tensile strength and density) in their respective articles. Unfortunately, most of them *only* list density; and nanotubes only lists tensile strength. -- DavidCary
::Tensile strength has a table. Assuming tensile strength of 60 GPa and a density of 1.5 g/cm³  gives a characteristic length of about 4,000 km. Another figure of merit is the characteristic velocity, sqrt(strength/density), which is not intuitive, but isn't tied to Earth's gravity either. Other refs:  --wwoods 23:00, 12 Jun 2004 (UTC)
replies: text: The optimal taper (ignoring saftey margin) is exponential; the characteristic length is the length over which the diameter must increase by a factor of e.  So even with constant gravity, you can get away with a few characteristic lengths before your cable becomes outrageously huge.  (I assume this is part of what went into the 4940 km above; at such lengths decreasing effective gravity can help too.) --Andrew 04:51, 14 Jun 2004 (UTC)
thread_title:  characteristic length 

message: The "Launching into outer space" describes drag on the cable (west ward0 when mass climbs.  But it doesn't explain how the cable katches up after imparting orbit velocity to the payload. Pud 23:59, 25 Jun 2004 (UTC)
replies: text: Yes it does. "This angular momentum is taken from Earth's own rotation." and "The horizontal component of the tension in the cable applies a tangental pull on the payload, accelerating it eastward. Conversely, the cable pulls on Earth's surface, insignificantly slowing it." - basically, the rotation of Earth pulls the elevator back up to its original momentum again, infinitesimally slowing down Earth's rotation in the process. I'll add the direction of the pull to the article, hopefully clarifying this. Bryan 00:29, 26 Jun 2004 (UTC)
replies: text: The centrifugal force acting on the counterweight at the end of the tether. Since the elevator's center of gravity is slightly farther out than geostationary orbit, the elevator acts a lot like a rope being held onto by a spinning figure skater; it "wants" to stick straight outward, and if it gets nudged to the side it will swing back out again on its own. Bryan 02:46, 26 Jun 2004 (UTC)

text: Sounds good, I'll work that in. Bryan 02:46, 26 Jun 2004 (UTC)
replies: text:  It won't necessarily return to the vertical--if there's a constant (net) flow of mass along the cable, the Coriolis force'll impose a permanent incline to the cable. Not much--it's like a rope being held by a spinning skater, along which ants are crawling. The tilt sets one limit on the throughput of the system, but it's not a problem, as long as the center of gravity isn't pulled down below GEO. Of course, by varying the movement of mass along the cable you can make it swing and shimmy; that's the way proposed to avoid some collisions.--wwoods 08:45, 26 Jun 2004 (UTC)
replies: text: Another good point to work in :) Bryan 16:52, 26 Jun 2004 (UTC)
thread_title:  Drag 

message: Moved discussion of various types of tethers, and what to name each one, to Talk:tether propulsion.
--DavidCary 08:52, 11 Jul 2004 (UTC)
thread_title:  Clarification requested 

message: There's been a factual accuracy dispute tag over at Space elevator economics for two months without any editing being done on either the article or the talk: page, does anyone know whether the dispute is still ongoing? I asked in that article's talk page too, but I figured this page was more likely to be bookmarked by interested people. Bryan 18:34, 25 Sep 2004 (UTC)
thread_title: Economics article factual dispute still ongoing?