Apollo 13

[OperaTenor]

I think what he meant was kind of shorthand for constantly exerting energy for maintaing a constant attitude in low earth orbit to counter the effects of gravitational pull (kick me if I'm wrong). I can see that as accelerating, so to speak.

[Shapley]

Barfle,

An object in geosynchronous orbit falls to earth at the same rate as its forward momentum carries it around the earth. An object closer to the earth falls to the earth at a higher rate of speed, and thus its foward momentum is not sufficient to overcome gravity. It must accelerate at a constant rate in order to overcome the force of gravity. Since the direction of the object is constanly changing, the constant acceleration does not result in a change of position relative to the earth.

At least thats the short version. The long version involves green cheese and crossing over to the dark side.

V/R
Shapley

[BenODen]

[SNIP! Shapely beat me to the punch on the geosync explantion.]

Some day energy might be cheap enough to keep a short elevator up by providing thrust to counteract gravity, but for now we need a 22,000 mile elevator, probably more, since there's a lot of weight below geosynchronous orbit... The more I think about it, the more I get confused by the physics of a space elevator... I'll have to read up on the elevator physics sometime..

-Benito

<small>[ 09-23-2003, 12:39 PM: Message edited by: Benito Of Denver ]</small>

[Shapley]

Benito,

The big problem with the elevator is the cables. Not the cable material, which could be resolved as you noted, but the end treatment. Somehow the stresses on the cable have to be transferred to the earth and the satelite. You're talking some pretty big bolts, plus the mechanism to tranfer the stresses to and from the bolts. Some poor engineer is going to have an interesting project!

V/R
Shapley

[BigJon]

Ooh! Ooh! Ooh! I'll take the job. I love designing stuff that people say can't be done. (And a few things that shouldn't be done.)

Stress Analyst
BigJon

[Shapley]

Bigjon,

Okay, you've got the job. The pay starts at zip, but you can work your way up to the big zip pretty quickly, if your work is neat.

V/R
Shap

[barfle]

Ooookay, here's a very short (and necessarily incomplete) physics lesson.

A satellite orbits the earth because its angular velocity around the earth matches the gravitational attraction. That is to say, that as the object falls, the surface of the earth falls away at the same rate. Another way of looking at it is by comparing the satellite in orbit to a rock on a string. If you sling the rock around on the string, the tension on the string does to the rock what gravity does to the satellite. The angular momentum (centrifugal "force") keeps the rock from falling.

Satellites in low earth orbit do not require any additional accelleration to maintain orbit - all that's necessary is that they be outside the friction of the atmosphere. Those satellites we have intended to recover or at least return to earth (orbital Mercury, Gemini, and Apollo capsules, space shuttles, and even Mir) had to be slowed down in order to drop their orbits to the point that the atmosphere captured them. Let me repeat that - NO additional accelleration is required to maintain orbit. The satellite is coasting in an almost frictionless environment. Low earth orbit satellites generally have an orbital period of around 90 minutes. If they get much lower (in order to have a shorter period), they get bogged down by the atmosphere, or even the surface of the earth.

The ONLY thing that's special about a geosynchronous orbit is that it's period is precisely one day. So, as the earth rotates on its axis, the satellite orbits around it and stays over the same point on the ground.

If a satellite is further than the geosynchronous diameter, then its period is greater than a day. The moon, whose period is a little longer than four weeks, is an example of this.

[barfle]

If done correctly, the only forces on the ends of the cables of a space elevator would be the weight of the cables and the cars and cargo they carry, and the force required to lift the cars and cargo.

There is no cable tethering, for example, the satellite that distributes HBO's signal to the various cable companies. Simply tying a string between two objects that are stationary with respect to each other would not result in a tension on the string.

Of course, that string will have mass, and that mass results in weight for the portions that are not at orbital velocity (which is nearly all of it), so that has to be dealt with, but particularly on the bottom end, there's no real need for huge anchors.

[Shapley]

As you stated, the farther an object is from the earth, the slower its velocity needs to be in order to match its rate of descent due to gravity. Satelites orbiting at altitudes lower than 22000 miles do so at a rate faster than geosynchronous, or else they would fall to the earth. If they were to be maintained in geosynchronous orbit at their present altitude, they would require continuous acceleration at an angle tangent to the earths surface.

For our elevator to work, it has to be geosynchronous, unless we want to drag the cable around the earth continuously. I suppose while we're at it, we could build a track around the earths perimeter, which the cable could follow in.

Anchorage would be very much a necessity. The pull on the cable would be much greater on the end closest to the earth. In addition, wind and friction would place drag on the cable, all with the purpose of pulling it out of the sky. The mass of the satelite would need to be sufficient to overcome these factors. A cable stretched between two objects will be pulled to the greater of the objects. If a cable of uniform density were run from here to Jupiter, it would fall to Jupiter, unless it were anchored firmly to both. This, of course, would require the orbit of Jupiter to be in synch with the earths, which it is not. If it were firmly anchored to both, it would likely pull earth towards Jupiter, unless the orbits of both could be adjusted to compensate for the additional mass of the cable.

The only way to achieve a net force of zero on the end of cable is to pull it to a height that the mass of the orbiting end of the cable exerts an upward force on itself exactly equal to the downward pull from the earth.

Benito, add to your task job the task of figuring out what elevation that is.

V/R
Shapley

[OperaTenor]

Originally posted by Shapley:

net......zero

V/R
Shapley

Isn't NetZero an ISP?

[Shapley]

OT,

Yes. They used to be free. But figured out they weren't making any money (I wonder how long it took them to figure that out?). Now they charge for connections, but didn't want to change the name to net-for-less, or net-cheap, or net-discount, or whatever.

V/R
Shapley

[barfle]

Shapley, if your understanding were correct, then only satellites that had no thickness could orbit.

The elevator doesn't have to be geosynchronous, but it would be a lot simpler if it were. Consider simply a very tall building. None of it is in orbit, yet the elevators work fine.

Now make it so tall that the top of it is above the atmosphere (a hundred miles high or so). You could do away with all the rocketry to get altitude, and simply provide enough horizontal velocity to keep the object from falling into the atmosphere. Mind you, none of the building or its elevators are in orbit. People at the top of the building would have very nearly their normal weight.

Now make it so tall that the top of it is at the geosynchronous orbit altitude. Anyone at the top of the building would be weightless, but the rest of the building still exists, and it's still, well, still, as far as the surface of the earth is concerned. The rest of the building is not in orbit, but it's supported and held in place by the rest of the structure. The only wind friction would be from the normal weather, not from the elevator (and building) moving through the air.

If you have a cable between two object that have no relative motion between them, why would there be any tension on the cable at all? A string tied between two billiard balls won't pull the balls together (OK, a little, until the string rests on the table). If you hang a string that's the right length so it touches the ground, there's no tension on the lower end of the string. How can there be? And simply making the string longer and its support higher won't change that one iota. You're right about what it would take to have zero tension on the bottom of the cable, but that's what any design goal would be anyway (at least from engineers who went to the same school I did). By your reasoning, my very tall building would need to be held down to the ground, for fear it would fly away (where would it go?)

In practical terms, yes, there would need to be a massive anchor for the cable at the top end, adequate to resist the weight of the cable, the car, and the cargo via its orbital velocity. Actually, the center of mass of the entire system (anchor, cable, car, and cargo) should be at the geosynchronous orbit point.

[Shapley]

OT,

Maybe they should have kept the cost at zero and changed the name to NetLoss.

It reminds me of the supposed quote from the CEO of General Motors: "We lose money on every car we sell, but we make up for it in volume."

V/R
Shapley

[Selma in Sandy Eggo]

Clearly, we need a skyhook. I shall order one immediately. That'll anchor the upper end to the space station at 22,500.

The lower anchor will be no trouble at all. We'll just run the cable around a pulley outside the terminal building and back up the other side to the skyhook.

If you guys can get the cable strung, I'd say we're good to go, here. Alternate strategies, in case of cable problems - we could either get the Enterprise to help out with the transporters, or borrow ferry dragons from Dragonhold.

(I think we're closing in on either an old Clarke or an old Heinlein story line here. It's been so long I get them muddled.)

Bunch of old sf readers around here, aren't there.

Selma

[Shapley]

Barfle,

The original post:

Ben o' Den, have you read Arthur C. Clarke's book Fountains of Paradise? It deals with the development of a space elevator to lift just about anything from the ground up to a geosynchronous platform. Eventually the platform grows and becomes a ring all around the earth. Materials were from asteroids.

This concerns a platform (satelite) in geosynchronous orbit.

The top of a building is supported by the compressive strength of the structure below it. The idea of building a "city above the clouds" is not new, but is different from the original idea. here. There are different problems associated with the construction of a building that tall, the primary one being the slenderness ratio of the supporting column. Even if we build the structure to the point that those at the top are weightless, those at lower elevations, and the structure supporting them, are not. The compressive forces at the base are beyond comprehension (well maybe not for Benito, who lives for that sort of thing ).

Basically, were talking about building the Tower of Babel, or at least we're babbling about building a tower. The Tower of Babel was tapered upward, basically an oversized zigurat. If we build our structure 22,500 miles high, it would presumably be at least that wide at the base. We can build it of lightweigt material, assuming we have created one with a Young's modulus sufficient to handle the weight of 22,500 miles of itself piled above, but its mass would still be tremendous.

The problem then becomes that the the mass exerts its own pull on the bodies above it, thus the people at the top are not "weightless", since their proximity to the massive structure below them will give them weight. How much will be determined by the density of the material the tower is constructed from.

The elevator cable has no compressive strength at that length, so the only way to support it is to suspend it from above. Hence, our platform must be able to maintain it's position while anchored to the Earth. It's sort of like lassoing a satelite.

V/R
Shapley

[BenODen]

I'm sure even drag of normal wind blowing on 10 or 15 miles of cable (not much atmosphere beyond that, probably) will be taxing our engineering prowess. They're talking a newspaper thickness cable 3 feet wide. Space.com Space Elevator Story has the current thoughts. They don't mention wind, but I'm sure that if that gets at all broadside to whatever natural wind is out there, that could be a lot of drag and added tension on both ends. (Can't let this thing move around in the wind, even if it's only 'normal' winds can we?)

An interesting thing to note... 22,000 miles is only a bit shorter than the distance around the earth at the equator, and they're talking about a length three times that. NOW, if we could use carbon dioxide from the atmosphere to build the nanotubes we'd be all set, that's a lot of tons of carbon, and the environmentalists might talk about global cooling then!

Oh, and as for the compression and gravity effects for that HUGE tower? Nope, beyond fathoming for me too, at that height.. Maybe a Civil Engineer would be all over it... Probably it'd take an astronomer/Civ-E Hybrid.

Ah well, fun stuff to ponder. Way cool, beyond fathoming stuff.

-Benito

<small>[ 09-23-2003, 07:22 PM: Message edited by: Benito Of Denver ]</small>

[OperaTenor]

Shapley,

You are sooooo crackin' me up. Definitely nuclear humor applied to other areas.

Actually, I think if the tower was made of rice cakes we would solve the gravitational pull of the tower problem (after all, if you've eaten a rice cake, you'd know they have zero mass).

<small>[ 09-24-2003, 03:40 AM: Message edited by: operatenor ]</small>

[lliam]

I believe in space travel so strongly that, I've applied for a job. See sample application form below.

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[lliam]

I hope y'all watched the progress over the years of galileo?

Galileo End of Mission Status.
---------------------------------
The Galileo spacecraft's 14-year odyssey came to an end on Sunday, Sept. 21, when the spacecraft passed into Jupiter's shadow then disintegrated in the planet's dense atmosphere at 11:57 a.m. Pacific Daylight Time. The Deep Space Network tracking station in Goldstone, Calif., received the last signal at 12:43:14 PDT. The delay is due to the time it takes for the signal to travel to Earth.

http://www.jpl.nasa.gov/galileo/

[Shapley]

Lliam,

I hope they beamed the crew back onto the Enterprise!

V/R
Shapley