0697: "Tensile vs. Shear Strength"

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Re: "Tensile vs. Shear Strength" Discussion

Postby Eternal Density » Wed Feb 03, 2010 10:26 pm UTC

Arancaytar wrote:Never fear though; even though the elevator is gone, we will still get our manned mars mission:

Free! Free!
A trip
To Mars
For 900
Empty jars

Burma-Shave

So get shaving, NASA. :P

Surely for a trip
to Mars
You'd need over
9000 jars.
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Wed Feb 03, 2010 10:49 pm UTC

tpow wrote:
squall_line wrote:It took about 35 posts for the word to show up, 34 posts before it showed up again, and now another 3 posts before someone (this guy, right here) points out that

Spoiler:
There is NO SUCH THING as CENTRIFUGAL FORCE




Thank you.

http://www.xkcd.com/123/




Wait.
So can I say "centrifugal" or what?
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Re: "Tensile vs. Shear Strength" Discussion

Postby snowyowl » Wed Feb 03, 2010 11:00 pm UTC

Re centrifugal force: I don't know about you, but when dealing with objects in geostationary orbits I like to use a rotating frame and polar coordinates. But whatever floats your boat (or space elevator for that matter). In this context, po2141, you can use the words "centrifugal force". Comic 123 has already been linked to, so I have now been ninja'd. Also, I fear any further discussion of centrifugal force will send us off-topic again, so let me say that in an inertial frame, which is best for most physics and mechanics problems that I have encountered and is the standard situation assumed by your high-school physics teacher, centrifugal force does not exist. Hope that clears it up a bit.

Re space elevators: Yes, I think the cable would just dangle there, but how do you propose to reattach it to the anchor? I'm sure I read on Wikipedia (so I'm probably wrong :) ) that even a single atom out of place removes about 30% of the tensile strength of carbon nanotubes, due to the principle of "a chain is only as strong as its weakest link".

Re Randall get out of my head: I just realized this is the 100th update since I started reading xkcd. Doesn't time fly... Is there a thread for boasting about how long you've been on xkcd for? I need the hardcore fans to take me down a notch :mrgreen:
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Wed Feb 03, 2010 11:35 pm UTC

Re: centrifugal force
Gotcha, rotating reference frame=centrifugal ok(ish). Everything else= an illusion caused by centripetal force.

Re: missing atoms
Thats pretty much correct, try it yourself with a molecular modeling kit.

Re: dangling cable
wind?

Re: nanotubes again
make your own carbon60 and nanotube forest! Burn some toast!
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Re: "Tensile vs. Shear Strength" Discussion

Postby not baby Newt » Thu Feb 04, 2010 12:38 am UTC

squall_line wrote:There is NO SUCH THING as CENTRIFUGAL FORCE

The fact that it doesn't exist is only important when explaining what it is. Once everyone know what it is and that it doesn't exist, it's a handy shortcut to what might otherwise be fairly longwinded.

(in my fairly humble opinion)

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Re: "Tensile vs. Shear Strength" Discussion

Postby TheGrammarBolshevik » Thu Feb 04, 2010 12:43 am UTC

Like kind of a lot of things in physics, there can be "such a thing" as centrifugal force without it being a literal concrete Thing.
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Re: "Tensile vs. Shear Strength" Discussion

Postby mouseposture » Thu Feb 04, 2010 1:14 am UTC

My first thought was Haldeman, rather than Burma Shave

Stuck on this lift for hours, perforce ...
This lift that cost a million bucks.
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Re: "Tensile vs. Shear Strength" Discussion

Postby axilog14 » Thu Feb 04, 2010 2:16 am UTC

atchius wrote:
navigatr85 wrote:Everyone's been saying that the use of the word "shears" in this comic is a pun. Maybe I'm being a little too nit-picky here, but I wouldn't call it a pun. The word "shears" is directly etymologically related to the concept of "shear strength" in physics. If I understand correctly, the concept of "shear strength" in physics was actually named after the cutting tools known as "shears". The term "shearing" in physics is simply a detailed description, at a microscopic level, of what shears and other cutting tools do.

If two words are so closely etymologically related, I wouldn't call their usage a pun. For example, if I say:

"I want to drink something, so let's go get some drinks."

....then that's not a pun. I'm using two different definition of the word "drink," but those two definitions are very closely related. If I say:

"A dog's bark is worse than its bite. An apple tree's bark tastes worse than its fruit."

...then that IS a pun, because the two definitions of "bark" are unrelated.


Thank you! That's what I was thinking.

Seconded on the thank you. I'm not American so I had trouble figuring out the Burma Shave reference at first, then got REALLY confused when everybody was talking about the pun I couldn't find. :(

All in all I found this a silly but cute strip. I thought the "disappears" rhyme was a bit painful, but that last line was just priceless. And as usual the alt-text made the comic, scientific accuracy be damned. Suspension of disbelief, people! :P
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Re: "Tensile vs. Shear Strength" Discussion

Postby TheGrammarBolshevik » Thu Feb 04, 2010 3:01 am UTC

It's perhaps a pun on the other kind of /ʃiːɚ/.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Mavrisa » Thu Feb 04, 2010 4:14 am UTC

kirrus wrote:Had a discussion yesterday with a friend (comic writer, nascent sci-fi writer) who thought that steel would be sufficient for a standard space elevator..

Is there an article I can link him to about space elevators, which includes math, but is sufficient for (UK) A-level maths understanding?

While a mathematical solution was provided that I would probably agree with if I had taken the time to understand it, I read a wonderful article about space elevators a while ago in spectrum magazine. Here's a link to the online article. I found it fascinating.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Sprocket » Thu Feb 04, 2010 5:26 am UTC

BURMA SHAVE!
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Re: "Tensile vs. Shear Strength" Discussion

Postby phillipsjk » Thu Feb 04, 2010 7:45 am UTC

With all the talk of puns I assumed it was in the title: Shear vs sheer.

Edit: how is a "burma shave" reference a pun?
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Re: "Tensile vs. Shear Strength" Discussion

Postby Pfhorrest » Thu Feb 04, 2010 11:16 am UTC

axilog14 wrote:I thought the "disappears" rhyme was a bit painful

Why?
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Re: "Tensile vs. Shear Strength" Discussion

Postby Notere » Thu Feb 04, 2010 1:41 pm UTC

Everyone's been saying that the use of the word "shears" in this comic is a pun. Maybe I'm being a little too nit-picky here, but I wouldn't call it a pun. The word "shears" is directly etymologically related to the concept of "shear strength" in physics. If I understand correctly, the concept of "shear strength" in physics was actually named after the cutting tools known as "shears". The term "shearing" in physics is simply a detailed description, at a microscopic level, of what shears and other cutting tools do.

If two words are so closely etymologically related, I wouldn't call their usage a pun. For example, if I say:

"I want to drink something, so let's go get some drinks."

....then that's not a pun. I'm using two different definition of the word "drink," but those two definitions are very closely related. If I say:

"A dog's bark is worse than its bite. An apple tree's bark tastes worse than its fruit."

...then that IS a pun, because the two definitions of "bark" are unrelated.

Stop Having Pun, Guys.

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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Thu Feb 04, 2010 2:16 pm UTC

Stop Having Pun, Guys.


punaccetable...
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Re: "Tensile vs. Shear Strength" Discussion

Postby Claren » Thu Feb 04, 2010 3:05 pm UTC

This strip reminded me of what I always think of when I think of a space elevator, and one of the biggest problems with actual implementation others have raised: what a target the SE makes !

Seriously, how could you even -begin- to defend such a fragile thing from an opponent bent on destroying it ?

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Re: "Tensile vs. Shear Strength" Discussion

Postby Coffee » Thu Feb 04, 2010 3:19 pm UTC

Claren wrote:This strip reminded me of what I always think of when I think of a space elevator, and one of the biggest problems with actual implementation others have raised: what a target the SE makes !

Seriously, how could you even -begin- to defend such a fragile thing from an opponent bent on destroying it ?


Er... spray it with "Bitter Apple" and set up a catnip-scented scratching post as an alternative?
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Re: "Tensile vs. Shear Strength" Discussion

Postby Eikinkloster » Thu Feb 04, 2010 4:25 pm UTC

Claren wrote:This strip reminded me of what I always think of when I think of a space elevator, and one of the biggest problems with actual implementation others have raised: what a target the SE makes !

Seriously, how could you even -begin- to defend such a fragile thing from an opponent bent on destroying it ?


That's what I was thinking. A Space Elevator only made sense before 9/11.
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Thu Feb 04, 2010 4:38 pm UTC

uh-oh...
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Re: "Tensile vs. Shear Strength" Discussion

Postby FriedSushi87 » Thu Feb 04, 2010 5:57 pm UTC

I live near Disney and they have this balloon ride. You pay money to go 300 feet up in the air in a hot air balloon that is tethered to the ground similar to the comic. The entrance to the thing is 5 feet from the balloon. I'd imagine someone could easily conceal some bolt cutters or something and send 50-100 people flying in the wind across Disney World

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Re: "Tensile vs. Shear Strength" Discussion

Postby project2051 » Thu Feb 04, 2010 9:43 pm UTC

If you want to take out a space elevator cutting it at the bottom couple of feet really isn't the way. Most elevator scenarios have the Earth end of the cable in a kind of free floating socket, just to keep the end stable rather than actually holding it down. You wold be much better off cutting it at or near the Geo-synchronous point, thus your counter weight or upper half of the cable goes flying off into space and the lower portion comes down and creates some lovely havoc. Not that I'm promoting causing havoc in any way.

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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Thu Feb 04, 2010 11:35 pm UTC

Im sure there are more efficient ways of causing havoc on earth that dont involve going into space first. Having said that, there would probably be quite a large economic fallout from disabling a space elevator.

And if you think about it, the bottom end would probably be surrounded by a customs-like terminal, just like an airport - planes are quite fragile too, as demonstrated at various points.

A large portion of the cable (dropped from geosync) is at a sizeable portion of orbital velocity, and with the cable being made out of carbon, alot of it will burn up.

I know, I know, it'd still be bad. But neither the good bits nor the bad bits of futuristic tech are ever as bad or good in as high a magnitude as we commonly envisage. They used to think we'd have robot servants and jetpacks by now, what do we have? Well, we certainly don't have flying robot armies thats for sure...at least thats what they tell us...
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Re: "Tensile vs. Shear Strength" Discussion

Postby rocketrat » Fri Feb 05, 2010 12:59 am UTC

po2141 wrote:Well, we certainly don't have flying robot armies thats for sure...at least thats what they tell us...

http://en.wikipedia.org/wiki/Predator_drone
Also, I swear there is an alt text somewhere that mentions these. Can't find which comic.

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Re: "Tensile vs. Shear Strength" Discussion

Postby project2051 » Fri Feb 05, 2010 1:36 am UTC

rocketrat wrote:
po2141 wrote:Well, we certainly don't have flying robot armies thats for sure...at least thats what they tell us...

http://en.wikipedia.org/wiki/Predator_drone
Also, I swear there is an alt text somewhere that mentions these. Can't find which comic.



http://xkcd.com/652/

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Re: "Tensile vs. Shear Strength" Discussion

Postby quadmaster » Fri Feb 05, 2010 2:24 am UTC

'flying robot armies.' Yes, welcome to the future. Certainly, there would have to be a no fly zone for miles around the elevator. Does anyone here know the plan for protecting it from space junk?
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Re: "Tensile vs. Shear Strength" Discussion

Postby project2051 » Fri Feb 05, 2010 3:17 am UTC

In the "Mars" trilogy the author use oscillations of the cable along it's length to flex it out of the path of objects. The Mars elevator had a regular one for the passing of one or both of the moons, and with a little forewarning using attitude thrusters along the cable they could move it out of the way of most objects. It's been awhile since I read the books so I'm a little fuzzy on the details.

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Re: "Tensile vs. Shear Strength" Discussion

Postby Mavrisa » Fri Feb 05, 2010 4:51 am UTC

quadmaster wrote:Does anyone here know the plan for protecting it from space junk?

In the article I posted, it mentions using a radar array to track objects larger than a few millimeters across. The belt would just be shifted from the ground in order to avoid them... I'd have to read it again, but I think there was another method mentioned...
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Fri Feb 05, 2010 9:43 am UTC



Damn...Touche
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Re: "Tensile vs. Shear Strength" Discussion

Postby Eikinkloster » Fri Feb 05, 2010 1:57 pm UTC

po2141 wrote:A large portion of the cable (dropped from geosync) is at a sizeable portion of orbital velocity, and with the cable being made out of carbon, alot of it will burn up.


I don't get this. If it is geostationary, it is stationary also in relation to the atmosphere, isn't it? It will be subjected to winds, but not to orbital velocity in relation to anything it gets in contact with.
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Re: "Tensile vs. Shear Strength" Discussion

Postby nahkaimurrao » Fri Feb 05, 2010 2:47 pm UTC

okay, if the ribbon is cut, it will NOT just hang there. Kind of the whole point of the elevator is to put the center of mass, likely a counterweight, PAST the geostationary orbit, resulting in a super strong force pulling the tether outward, only countered by its attachment to earth. This will result in unbelievably high tensions (sorry don't have the maths but check out other guy's previous calculations for steel but solve for tension). If this ribbon is 'cut' the resulting release of energy, I can only imagine will likely be on the order of a megaton explosion. so yeah.....

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Re: "Tensile vs. Shear Strength" Discussion

Postby Coffee » Fri Feb 05, 2010 2:58 pm UTC

When it comes to gedanken experiments like this one doesn't it really all come down to the math?
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Fri Feb 05, 2010 2:59 pm UTC

Eikinkloster wrote:
po2141 wrote:A large portion of the cable (dropped from geosync) is at a sizeable portion of orbital velocity, and with the cable being made out of carbon, alot of it will burn up.


I don't get this. If it is geostationary, it is stationary also in relation to the atmosphere, isn't it? It will be subjected to winds, but not to orbital velocity in relation to anything it gets in contact with.


It is stationary w.r.t the atmosphere while it is hanging, but it is still moving very quickly w.r.t the centre of mass of the earth. I used an analogy for the opposite effect several posts ago - just like the ballerina bringing her arms and legs close to her body increases her rate of revolution, preserving angular momentum - as the cable falls its rate of revolution around the planet will increase, dramatically.

Consider a one-meter portion of cable at the top - it is in geostationary orbit. it will be pulled downwards, if disconnected from the anchor, by the weight of the cable that is not in orbit. But it will not have its orbital velocity decreased, so by the time it reaches the atmosphere, it is traveling sideways very quickly indeed.


In fact, this raises another question in my head -
The top of the cable is in geosync orbit.
The bottom is not in orbit at all.
With a smooth increase in "orbitness" (orbitidy? oh dear...) to the top.
As the cable falls, the top will want to retain its angular momentum and thus will try to whip round the earth faster and faster as it falls (note, not actually increasing in speed, just rate of rotation around the earth) while the lower portions dont really want to - it is going to try and wrap itself around the earth, tensioning the cable.

Perhaps this might rip it into pieces?

In fact, the quickly moving upper portion of the cable could drag the whole cable into a lower-than-geosync orbit not even reaching the earth at all. The centre of mass of the cable (assuming even thickness) will be moving around the earth at about 11000km, at a velocity (relative to earths centre of mass) somewhat lower than orbital-at-11000km. But probably at a velocity sufficient for an orbit of some lower altitude, no?


@nahkaimurrao
Im considering a cable with an anchor at geosync - possible with more cable extending beyond geosync to counterbalance the tension of the lower portion - with the lower portion being severed close to the anchor. I'll confuse myself if I start thinking of another setup!

Im also ignoring any elastically stored energy.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Prometheus » Fri Feb 05, 2010 3:08 pm UTC

snowyowl wrote:Re space elevators: Yes, I think the cable would just dangle there, but how do you propose to reattach it to the anchor? I'm sure I read on Wikipedia (so I'm probably wrong :) ) that even a single atom out of place removes about 30% of the tensile strength of carbon nanotubes, due to the principle of "a chain is only as strong as its weakest link".


I'd imagine that there would have to be some massive crosslinking between the fibers. On a more macro scale, I'd imagine there would be bundles, not unlike a rope or steel cable. On a nano scale, perhaps the fibers can be engineered with 'attachment points' so the individual fibers link together into a larger 3D atomic structure, rather then just being held together with Van der Waals force (a concept that I believe is currently being called nano-yarning.)
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Re: "Tensile vs. Shear Strength" Discussion

Postby Eikinkloster » Fri Feb 05, 2010 5:12 pm UTC

po2141 wrote:Consider a one-meter portion of cable at the top - it is in geostationary orbit. it will be pulled downwards, if disconnected from the anchor, by the weight of the cable that is not in orbit. But it will not have its orbital velocity decreased, so by the time it reaches the atmosphere, it is traveling sideways very quickly indeed.


I think I got you now. What is, however, pulling the cable downwards, to begin with? Your scenario makes sense if we have a top anchor and a bottom anchor, and we letting the bottom anchor go down. but the bottom anchor will not go down by it's own gravitational pull, because it is as in orbit as is the top anchor. You'll have to have a rocket pulling the bottom anchor down.
If you direct the pulling rocket directly downwards, your scenario ensues. You won't do that, though. You will have the rocket in such an angle that it will cause the bottom anchor to move straight downwards. I'm not sure how to calculate such an angle. But it will be between straight down and straight backwards. I think it should be straight backwards, but my kinematics are completely rusty by now. velocity = angular velocity * radius, so if you cut the velocity by half, you'll cut the distance from the anchor to the center of the Earth by half too.
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Re: "Tensile vs. Shear Strength" Discussion

Postby nahkaimurrao » Fri Feb 05, 2010 5:18 pm UTC

Would it be possible to construct a cable or wire or other structure out of mono-crystalline silicon and how strong would it be? I was watching silicon run the other day and was amazed at the giant silicon crystal hanging from a seemingly thin thread as I was quite ignorant of silicons tensile strength. According to http://en.wikipedia.org/wiki/Tensile_strength%20 monocrystalline silicon, as produced when making computer chips, has the second highest tensile strength (7 GPa) after carbon nanotube (11-63 GPa) about twice that of Kevlar (3.62 GPa). I was completely ignorant of silicon's tensile properties. Is it really that strong? Further down, is a list of annealed material strength. I am not sure what it means but Silicon is the only one even remotely in the GPa range up to 9 Gpa which is just under the lower bound of nanotubes (11 GPa). Is Silicon ever used for its tensile strength? It seems we are quite luck that our most abundant resource can have so much potential.

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Re: "Tensile vs. Shear Strength" Discussion

Postby Technical Ben » Fri Feb 05, 2010 5:44 pm UTC

I think it's around 50GPa for a space elevator. No idea why I think I heard that number. I don't know the maths or the physics. Just that, it's nanotubes, or nothing at all.
I think it's down to the desity, at 2.33g/cm-qubed, silicon is just too dense/heavy to make the right type of cable.
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Fri Feb 05, 2010 5:59 pm UTC

@Eikinkloster

Nono, I meant a cable from the earths surface to an anchor at geosync - only the very top of the cable is in orbit - the higher you are the faster you need to be travelling to maintain orbit, until you escape altogether. The lower portions of the cable are not rotating sufficiently fast about the earth to maintain orbit at their lower height and so their weight hangs off the anchor.

This weight can be countered by spooling cable from the anchor outwards, if it is straight, it will be rotating around the earth faster than orbital velocity at its heigher altitude and so will exert a "centrifugal" (NB: I know, but you get me right?) force on the anchor, balancing the weight of the lower length.

Angular momentum is the complicating factor here as it varies greatly from the bottom to the top of the lift. There is no "free" effort being provided by using a lift to get to orbit, you'd still need a comparable amount of kinetic energy to get there than you get from, say, a big ass-rocket. The anchor at geosync is moving at roughly 3km/s relative to the earths centre of mass, and so will any cargo have to be to reach it.

@nahkaimurrao
We run into the same problem with mono-crystalline silicon as with carbon, the theoretical tensile strength is much much greater than what can be achieved in practice due to flaws in the crystalline structure. Even if you make it as perfectly as you can, its like a jigsaw with a billionbillionbillion(...lots) of pieces, can you put them all in the right place? Impurities, even trace ones, also distort the structure, as will radiation (particulate or sufficiently energetic EM) over time.
Im also pretty sure that mono-crystalline silicon would have a similar appearance to strong glass ie: brittle. Whereas it doesn't take too much of a leap to compare carbon nanotubes to the fibres in something like nylon or, as you mention, kevlar.

Not that Im saying its impossible to achieve the kind of strengths we need, but as of today we still can't.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Coffee » Fri Feb 05, 2010 6:53 pm UTC

So if the same energy would be required to get something into orbit with or without the space elevator then what's the real advantage of making it? Just the cool factor?
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Re: "Tensile vs. Shear Strength" Discussion

Postby phillipsjk » Fri Feb 05, 2010 8:10 pm UTC

The article Mavrisa linked to pointed out you can reduce the weight of the spacecraft by 95% by not carrying any fuel with you. Rather than using a controlled explosion to gain the required energy, you can use solar panels and lasers.

Incidentally, the linked article also said a tether cut near the bottom would fly off into space.
Last edited by phillipsjk on Fri Feb 05, 2010 8:24 pm UTC, edited 2 times in total.
Did you get the number on that truck?

project2051
Posts: 178
Joined: Fri Jan 29, 2010 2:20 pm UTC

Re: "Tensile vs. Shear Strength" Discussion

Postby project2051 » Fri Feb 05, 2010 8:16 pm UTC

Coffee wrote:So if the same energy would be required to get something into orbit with or without the space elevator then what's the real advantage of making it? Just the cool factor?


Cost would be the main factor, a elevator should lower the cost of putting objects in space significantly.

http://en.wikipedia.org/wiki/Space_elevator_economics


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