Apollo 13

[barfle]

I know how the footprints described in the article are generated. I don't know why a geosynchronous satellite wouldn't be positioned over the equator, but I can accept that it's due to the mission of the satellite. Those figure-8 tracks were on globes when I was in elementary school (in the 1950s). I believe I was in the fifth grade when I realized what that big "8" was on the globes.

I also realize that a true circular orbit would be impossible to attain, so the communications satellites that are in geosynchronous orbit were designed to allow for a moderate amount of movement. I'm sure calculations were done to determine where the minimum cost point was in the tradeoff between the coverage of the satellite and the allowable error from a circular orbit. Note that the language I used was "significantly different from circular." As noted in the article, the differences quoted are in the tenths of one percent, so I would generally have trouble calling those significantly different from circular.

Seasonal changes are caused by the fact that the axis of the earth's rotation is not perpendicular to its orbit around the sun. With very minor differences, that axis always points in the same direction (think "Polaris"). Yes, over many centuries, precession is significant, but that has nothing to do with seasons as we sense them. I believe that's what you said, but if so, I'm not sure why you brought it up, since it's not a factor. What is a factor is that space isn't as empty as we might think, and various forces act on those satellites and cause them to move around some. That's why they have little rockets on them - to push them back where they belong. And the life of the fuel supply is JUST as important as the life of the electronics!

[haggis]

good Space blog

[Shapley]

Barfle,

I didn't get to finish my thoughts on this yesterday, as I had an appointment to keep.

I also noted that the difference of a few kilometers is not a significant amount in the overall distance, until we tie the satelite to the ground. Similarly, the lateral motion north and south (or even east and west, atttributal to minor variations in velocity) of the original position.

If we think of a balloon on string. As a child walks along with it, it is, for these purposes, in orbit (I know it doesn't meet the definition, I only mean that it is moving parallel to the earths surface for the period of my explanation). If a lateral force, such as wind or the air currents from a passing object are introduced, the balloon shifts in the direction the force moves it. However, the tether limits lateral motion, so the balloon moves in a arc, downward as well as laterally.

My aircraft carrier would be similarly moved, because its mass is not that great. The relocated asteroid you mentioned (keep in mind I haven't read the book) would have a significantly greater mass, and thus would be less likely to be pulled downward by the cable. It would more likely exert a force in excess of the cables strength. The cable would either part or the end treatment would fail, depending on the design of each.

Some ocean-going tug boats employ an auto-tensioner to pay out or haul in the hawser as needed to maintain proper tension on their tows, so such a device could be designed in this instance and employed.

Nothing in my posts is designed to suggest that the project is not practical, I merely wished to point out that Benito's project will be full or difficulties (or opportunities, if you're an optimist). I understood your post to suggest that the end reaction on the cable could be maintained at nearly zero, and I was suggesting that that did not seem plausible to me.

Regarding my signature: It only suggest what is required for wisdom. I never said I was wise.

V/R
Shapley

[haggis]

China plans manned space mission

[barfle]

As were my conjectures as to why the nonotube ribbon was so long, my conjectures as to the physical properties of the ribbon are also simply conjectures, with no specific knowledge of the material involved.

All materials have elasticity. Although the string in your child's balloon analogy has very little elasticity, if it were sixty-odd thousand miles long, I would venture that you would be able to stretch it quite a noticeable distance. I am certain that the same is true of the carbon nanotube ribbon, although to what extent I have no idea. That being said, though, I would be surprised if its elasticity couldn't absorb the variations a geosynchronous satellite's orbit (particularly one that is easily supplied with additional fuel).

Although it would be impractical to have the ribbon have zero tension, and expect to maintain that, I cannot imagine a design goal that wouldn't have it be as low as possible while still maintaining control over a three-foot ribbon that extends through the entire depth of the atmosphere. I don't know what level of tension that is.

No matter how great the mass at the top end, it will be affected by the tension on the cable. That's why I noted earlier that the center of mass of the system - cable, car, cargo, and top anchor - should be at the geosynchronous altitude. In Clarke's book, the asteroid was positioned very close to geosynchronous altitude, the mass of the cable, car, and cargo being trivial in comparison. Note that the ribbon wasn't there to tie the asteroid to the earth. It would stay where it was put without the cable, since it was in geosynchronous orbit.

To be sure, the project, if it goes ahead (and ohh, how I hope it does!) will be full of challenges. But orbital mechanics are extremely well understood. There need be very little tension on the ribbon beyond that required to hold up its own weight, plus the weight of the car and cargo. The anchor on the top can be balanced so that it does not need to have significant tension beyond that amount.

[Shapley]

Barfle,

RE:<<As were my conjectures as to why the nonotube ribbon was so long, my conjectures as to the physical properties of the ribbon are also simply conjectures, with no specific knowledge of the material involved>>

My only knowledge of the properties of carbon is that it leaves a heck of a mess on the roof of my cave. I've tried different types of wood on the fire, to no avail. Can you recommend a good cleaning compound?

V/R
Shapley

[BenODen]

*gasp* CARBON! On your ROOF! Some of that will be bucky balls and some of it Nanotubes, so we weil be over to collect it forthwith! Leave no cave unscrubbed, the director said!

Well, now that we're all in agreement, bigDummy can begin construction. It needs to be just the right size to make all the physics work. Chop chop! I expect to be able to ride by Tuesday! (Maybe not next Tuesday, but hopefully the Tuesday after, or some successive Tuesday! )

Some day!

-Benito

[piqaboo]

B o' D,
LOL!!

Chimney sweeps become the new millionaires!

[haggis]

Another good news commercial industy space story the "X-Prize" cometetion is pretty neat.

[BenODen]

Yahmon, good stuff!

To me, $100,000 is way too steep for a 15 minute ride, but it gives hope that there's downward pressure on the price of space flight. There may be two leaders that have the best chance of getting the x-prize, (my "money"'s on Rutan's group...) but I think there's a good chance that other competitors will still press on after the prize is given away, which would be good for competition of course.

It'll be a long time, I know, but $10,000 for a low earth orbit or so would get me in an instant.

-Benito

P.S. An included docking would rule too. Set to Strauss, of course.

<small>[ 09-29-2003, 02:45 PM: Message edited by: Benito Of Denver ]</small>

[dai bread]

Tell me , all you physicists and engineers, isn't it the centre of mass of the whole cable/platform system that has to be in geosynchronous orbit?

[barfle]

dai, I'm pretty sure I said that at least twice. However, it's only true if the tension on the bottom of the cable is zero. In order for it to dangle and touch a stationary position on the ground, the center of mass of the system would have to be at the geosynchronous orbit altitude.

[Shapley]

Revisiting Old Issues on this thread, here's an item of interest:

http://www.snopes.com/science/greatwal.htm

V/R
Shapley

[BigJon]

This link http://tinyurl.com/p6yb goes to an article on the NASA website describing nanotubes and the potential uses in space.

BigJon

[Shapley]

BigJon,

I owe you an apology. I kept referring to Benito doing the calcs on this project. I was rummaging through the thread and see that it was you tht volunteered for the job! My most humble apologies.

So, have you got those preliminaries worked up yet?





V/R
Shapley

[BigJon]

Originally posted by Shapley:
BigJon,

I owe you an apology. I kept referring to Benito doing the calcs on this project. I was rummaging through the thread and see that it was you tht volunteered for the job! My most humble apologies.

So, have you got those preliminaries worked up yet?

No apology needed.

One harsh lesson I learned (again) at my current job was; don't start designing until you fully understand function as well as form. I recently designed a part that fit all the form requirements perfectly, but since I'm new here, I didn't understand the function of the related parts, so when assembled, my part didn't work.

I decided to do my own brainstorming. I believe we would need to install force limiting devices so neither end would have to be overly heavy duty. What if the nanotube ribbon was longer than it needed to be to reach the orbiting body? The ribbon would extend freely beyond the body and a there would be a pit with a take-up reel under the earth attachment. Then tractive rollers (picture great grandma’s wringer clothes washer on a grand scale) would pay out or retrieve the ribbon to maintain minimum tension in the ribbon. Should the net effect of the many dynamics be that ribbon tends to drift towards one end, the orbiting body would crawl along the ribbon with its own tractive rollers until the ribbon started falling back to earth or paying out as needed.

What do you think? The control system needed to operate such devices should not be overly complicated, but I imagine the control range and resolution would need to be incredible.

BigJon

[barfle]

I have no idea what the surface friction of carbon nanotubes is, but somehow I wouldn't be surprised if they were pretty close to that of teflon, just because nature is a mother sometimes.

That would tend to mean that simply grasping the ribbon would be risking slippage. Maybe there would be some form of traction device, similar to a rack that could be attached to the ribbon, or if the ribbon exhibited adequate wear characteristics, a pair of gears, one on each side of the ribbon, to grip it with the teeth. The artist's concept shows something similar to tank or bulldozer treads (although they may be smooth) on both sides of the ribbon, undoubtedly squeezing it to provide adequate friction.

Of course, another distinct possibility is to use it just like an elevator, with the ribbon fixed to the car, with a counterweight. How about having the ribbon be a belt, with the propulsion provided from the ends? No engine to move up and down with each load that way.

[jmfryar]

I have NO clue if this has been mentioned before as I have not read all the pages...but...

NASA some time ago lowered a wire into the atmosphere from a shuttle with the idea that dragging it through the molecules there would generate static electricity that could be used to power satellites, shuttles and missions in low orbit (with appropriate thrust to compensate for the drag)

So much electricity was generated by the experiment that the cable burned away in seconds...

This is the image I have in my head when I hear this theory...how do you dissipate all the power generated by friction in the atmosphere as clouds, storms and other things slide by the stationary wire?

[BenODen]

Wow, good memory to even remember the experiment Mike. Some how I missed this semi-successful experiment. The last tether experiment like that I remember was cut short by a tangle while trying to deploy it... I found this url on the tether experiment that explains what happened better..

Friction wasn't the active force, but earth's magnetic field... For the most part, our elevator will be stationary to the magnetic field, except where we move things through it, traveling up the elevator, so we won't have the problem that tether had, but there's sure to plenty of problems with the ionosphere anyway. More fun for BigJon.

Interesting reel idea BigJon... Lets say we can get 150 thicknesses of ribbon in an inch. (perhaps overly optimistic) I've forgotten too much of my math to do the exact calculations, but even with a reel a a tenth of mile across, it'll be 40ish feet thick, minus a little, since each wrap is a bit longer. I guess that's not so bad, but the power needed to pull on the reel! AIE. 22,500 miles is a long ways!

On that other subject, space tourism, I just pre-lived docking in space. The Johan Strauss Jr.'s Blue Danube Waltz just played on B.Com. Maybe one day!

-Benito

[BigJon]

Originally posted by barfle:
I have no idea what the surface friction of carbon nanotubes is, but somehow I wouldn't be surprised if they were pretty close to that of teflon, just because nature is a mother sometimes.

Yes, I thought about that. I have a bit of experince with carbon fiber thread and it is very abrasive and doesn't have a high natural lubricity.

That would tend to mean that simply grasping the ribbon would be risking slippage. Maybe there would be some form of traction device, similar to a rack that could be attached to the ribbon, or if the ribbon exhibited adequate wear characteristics, a pair of gears, one on each side of the ribbon, to grip it with the teeth. The artist's concept shows something similar to tank or bulldozer treads (although they may be smooth) on both sides of the ribbon, undoubtedly squeezing it to provide adequate friction.

The definition of tractive roller is that it has enough grip built into the surface to prevent slippage. Also you can artifically increase grip by squeezing the rollers together.

Of course, another distinct possibility is to use it just like an elevator, with the ribbon fixed to the car, with a counterweight. How about having the ribbon be a belt, with the propulsion provided from the ends? No engine to move up and down with each load that way.

You'd still have the same problems of massive forces as you would need to pull the end rollers apart with sufficient force to keep the belt operating.

BigJon