Still More on Space Elevators

parseexception writes "The NYTimes is reporting Not Science Fiction: An Elevator to Space. Not a whole lot of technical detail but good read. It is interesting to see alternatives to current space technologies being explored."

Comments

Happy Google linky no reg

[borgboy]

Not Science Fiction: An Elevator to Space

Fountains of Paradise

[Henry V .009]

Clarke's Fountains of Paradise is a wonderful book. It presents the basic physical problems of a space elevator and some ways to solve them (and it has a great plot too).

Right now, as Clarke envisioned, carbon nanotubes are the only engineering material that could be used to make this elevator. The basic technological question is whether we will ever be able to make cheap and long nanotubes. Despite the enthusiasm for a space elevator in the NYT article, we really have no idea how to do that. And any cost estimates like "$6 billion" are very immature without that kind of detail.

Re:Fountains of Paradise

[Birger Johansson]

Space elevators are one of three basic "space tether" concepts, and it would be logical to start with more humble objectives.

The simplest form of a space tether is essentially a high-tech version of that bronze-age weapon, the sling. A tether for sending satellites to higher orbits, or spacecraft on lunar or planetary trajectories can be achieved with *existing* materials, like kevlar or spectra.
This form of tether concept is being investigated by NASA.
A second form of space tether -the hypersonic skyhook- is described in Zubrin's and Schmidt's book "Islands In the Sky". In the example with a hypersonic skyhook whose lower end is travelling at 5 km/s relative to the surface of Earth, the total mass of the system need only be 20 times that of the load. The launcher need only travel at the same speed as the end of the tether, and -here is the tricky part- the cargo module then has to manouver for some hair-raising seconds to hover at ca. 0.5 g in order to dock with the end of the skyhook. After that, it can travel up the tether like a elevator car, and the tether can interact with the Earth's magnetic field to regain the kinetic/potential energy lost simply by running a current through a cable.
The difference between a minimum velocity of 5 km/s and 8 km/s (low-earth orbit) is enormous in terms of the launcher/payload mass ratio, and could make single-stage to orbit (SSTO) rockets viable with *current* technology.

The space elevator is the third and most technically difficult form of space tether, and it should be attempted last. It can only be built by bulk production of carbon nanotube materials and this is still far into the future.

Re:Predictable joke #1 of X

[borgboy]

Far more predictable:
Cost to transport all the IP SCO claims to own via space elevator? $100M
Cost to transport all the moral fibre SCO execs currently posses? $.02
Cost to transport unprotected SCO execs?

Priceless.

(Cant buy me love, but 10 minutes of bliss is another story)

Cool

[Bob Vila's Hammer]

This is the best thing for the success of the project. All it really needs is publicity. The Space Elevator is a solution to so many problems with current space technology that it should be operating right now.

The good publicity drumming up the imaginations of people in this country can only get this thing built faster. The technology needed for the ribbon is almost perfected. In 5-10 years or so (maybe after all this terrorism and war bs has ended) this thing might really get off the ground and the more people who know about it and want it to happen the better success the project will have.

Re:Cool

[Flingles]

"this thing might get off the ground"

pun intended? :)

Re:Cool

[Bob Vila's Hammer]

haha, absolutely. :-)

All we need is for the ball to get rolling...

[LinuxMan]

Once we have a number of people going into space, and it builds a travel industry around it, suddenly we will see lots of innovations like the space elevator, etc. become not only practical but quite cheap. If only the government used computers, computers would cost a fortune, but since there is such competition, we have computers that sell for $300 or less now. If that same concept is applied to space travel, there WILL be competition, and there WILL be new better, cheaper ways to get up there, which may or may not involve the space elevator.

My theory has always been that once we have a hotel on the moon, due to it's gravity, people will not have as much of a hard time adapting to it for their vacation, more hotels will open up in competition. Initially, a lot of money would be spent setting up, but the costs of everything would continue to go down. Before we knew it, we would have faster propulsion technologies, better gravity-like technology, and we would be off to Mars for our next resort, resulting in even better competition for innovation.

space elevator

Re:All we need is for the ball to get rolling...

[Ruy-sun]

NASA is trying to artaficaly boost competion in the usa's space indevers. NASA is hold a contest for the first group of people using a 3 million dolor buget. (not sure the about of prize money) to reach space with 3 people for the competion they do not have to go all the way in to orbit just very close. The consept behind the compation is to see what new ideas are made. and if one of the groups gets a good shutle (or whatever) and patents it then that would boost technologies, but also the tourist base, and the need for other organizations to join in on a second space race. The idia for this compition is the same that lead to the first person to crose the atlantic in a plane; only then the prize was 50k ... god nows what this is. well if it worked once... lets just cross are fangers. :)

Ps. i thank these facts are right im not sure i can't seem to find the artical i had on this if i do i will repost with corect names and such

Space elevator old news?

[GeoGreg]

Hasn't the confectionery industry already done this? I hope these NASA folks know to watch out for Vermicious Knids.

Space elevator news

[Quaelin PoD]

I help run a space elevator news / portal site:

LiftWatch.org

We've got links to this story and many more... plus reports on the recent SE conference in Santa Fe, discussion forums, image galleries, etc. Check it out!

I've put in a request... hopefully our headlines will be added as a slashbox here soon.

This "Space Elevator" seems cool and all, but

[the_Upsetter]

Funny... I can't think of a single thing to say on this topic.

Not one thing.

Re:This "Space Elevator" seems cool and all, but

[torpor]

My middle mouse button thanks you, sir.

From signage found only on California elevators...

[mbstone]

"If the elevator gets stuck, do not become alarmed. There is little danger of the elevator running out of air. Then again...."

kind of funey

[Ruy-sun]

I have thought of this beffor. It's intresting though what made me thank of it is in the anime Armitage: Dule Matrix they use, one and the finaly fight takes place there. hmmm. I wonder if the scientist who came up with this idea saw Armitage too?

Re:kind of funey

[zero_offset]

You'll be pleased to know there is a simple answer to your question: No.

Balance

[CowboyRobot]

In addition to the other MAJOR engineering challenges that this project would require, how will the satellite at the other end of the elevator stay in place? I understand that if it has the proper mass and is at the proper altitude it will orbit directly above the base station - but what happens when you deliver a 13-ton payload there? Won't all that additional weight cause the satellite to swing forward?

Re:Balance

[Patrik_AKA_RedX]

A 13 ton payload doesn't add much mass to a multi Mton rock. I guess they'll need to do a few course correction every nth time some cargo goes up.

Re:Balance

[tiled_rainbows]

I've read stuff suggesting that you could have the elevator going up to Geostationary orbit, but then continuing past that point to a big rock or something further out. The big rock would be dragged round at geostationary-orbit-speed but, being further out, would have a centrifugal pull on the cable, keeping it taut. You might want to have the elevator cars going 'up' to this counterweight (although once you passed the geostationary point it would feel more like you were going down) as it would have a weak 'gravity' pulling away from the Earth, which would be pretty cool.

Then again, I could well have totally misunderstood the wohle thing and this post merely serves to exhibit my shameful ignorance of physics.

Re:Balance

[Anonymous Coward]

In a word: no. The mass of a satellite does not influence its orbit, just like a space shuttle's orbit doesn't suddenly change when it grabs hold of a satellite.

Re:Balance

[Anonymous Coward]

No.

Orbital paths are dictated by velocity and altitude.

Any objects,regardless of differances in mass, at a given velocity and altitude will orbit along the same path.

Re:Balance

[barawn]

Well, close.

Orbital paths are dictated by velocity and altitude of the center of mass.

Move the center of mass, you change the orbit of the object. So the point that he made is valid, if not for the reason he thought (or you thought that he thought).

This isn't a problem in this case, as others have pointed out, for many reasons. First, it's a trivial mass compared to the elevator mass. So it's not going to significantly change things at all.

Second, even slightly altering the center of mass like this would do won't cause an orbital change, because the elevator is fixed at one point - the anchor. It'll cause a torque, which will make the elevator try to lean. The anchor will hold it in place (it's only a *small* torque), transferring the torque against the Earth's rotation, slowing down the Earth's rotation (trivially, of course).

So, you get an oscillation, which can be damped by the base, and has already been modeled and simulated. It's not a problem, and no, it will not wobble and collapse like the Tacoma Narrows bridge.

Re:Balance

[barawn]

The shuttle's orbit would change if it grabs a satellite, unless the center of mass of the extended "satellite/shuttle" system is at the same altitude as the shuttle was originally.

If the shuttle approached the satellite from along its orbital path (the path of its velocity) or the path perpendicular to it, then it wouldn't change (in front of/behind it, or left/right of it). If it approached along its radial vector, it would change (above/below it).

If it grabbed along the radial vector, the shuttle and the satellite would start rotating around each other slowly, around the center of mass. Depending on when the shuttle let go (and the original orientation of the satellite/shuttle - i.e., which was higher, which was lower), it could be boosted into a higher orbit, or lowered to a lower orbit, or returned to its original orbit.

This is actually the whole idea of orbit-changing space tethers.

I hope

[cbmeeks]

to be the first to stand in that elevator...and press all the buttons so that the next guy will have to stop at every port before the 60,000 mile trip is over. hehehe cb

escape velocity?

[eggoeater]

So how does escape velocity play into this? If we can leave the earth without reaching an escape velocity then (on a very theoretical level) could we ride a space elevator out of the event horizon of a black hole? I realize there's a million other reasons why we couldn't get near a black hole, hawking radiation, strong gravity would rip us apart, etc. But you could also apply this argument to other things, like going into Jupiters' upper atmosphere which has bucko gravity. I was taught escape velocity was unavoidable to get out of a gravity hole.
-Steve

Re:escape velocity?

[Anonymous Coward]

Escape velocity is just a way to climb out of the earth's "gravity well" with a little velocity to spare. You're still doing that, just using a different method.

Besides, reaching earth orbit doesn't require escape velocity -- you haven't actually "escaped" the earth as long as you're in orbit around it. You'd need an extra nudge to do that.

Re:escape velocity?

[Planesdragon]

So how does escape velocity play into this?

At a very basic level.

Remember that the Earth rotates--right now, you're moving at, what, about 2,000 mph? As you go straight up, relative to a point on the surface, your speed increases because you're still completing one rotation / day. Once you get up to geostationary orbit, you've got (at least) the necessary escape velocity. (I also suspect that e.v. goes down as altitude increases, but I'm not certain.)

As for the black hole--if you managed to place a cable of a material that the black hole wouldn't shred in the right location and homehow managed to survive both your passage through the event horizon and whatever the black hole does to you after that, you could, theoretically, raise yourself out of a black hole.

The energy that it would take to ascend the indistructable space elevator would be rather high, of course. And they'd be worse the deeper into the hole you were.

Re:escape velocity?

[barawn]

Remember that the Earth rotates--right now, you're moving at, what, about 2,000 mph? As you go straight up, relative to a point on the surface, your speed increases because you're still completing one rotation / day. Once you get up to geostationary orbit, you've got (at least) the necessary escape velocity. (I also suspect that e.v. goes down as altitude increases, but I'm not certain.)

No. Escape velocity is what you get when you set kinetic energy = potential energy. Kinetic energy is T = 1/2 mv^2, potential energy is U = GMm/r. Set the two equal, and poof, you get escape velocity.

If you have escape velocity, you are no longer in orbit. At all. You will always be moving away from the Earth. Technically we call this a "parabolic orbit" (the C3=0 orbit), but it's not what people normally consider an orbit - that is, you'll never come back to Earth.

If you're at geostationary orbit, you don't have escape velocity, because, well, you're still orbiting the Earth, now aren't you? GEO is still shy of escape velocity by something like a few km/s (2 or 3-ish, I think, I can't remember. Don't correct me if I'm wrong and it's 6 or 7, though it really can't be - it's too late for me to use real numbers).

Escape velocity depends on your altitude, yes. v_esc = sqrt(2GM/r), where r is your altitude above (the center of) the Earth.

There is no way you could build a space elevator out of a black hole. It's not possible - not for material science reasons, or practical reasons, or anything - it is simply not possible to build *anything* that could cross the event horizon of a black hole. The intermolecular forces required would be infinite - literally, infinite. Unbounded.


The energy that it would take to ascend the indistructable space elevator would be rather high, of course. And they'd be worse the deeper into the hole you were.

You're right it'd be high. Infinite. And the deeper you got, the worse it'd be: infinity+1, infinity+2, infinity+3..

I'm kidding, of course. Again, you can't. It's infinite. Period. The integral is unbounded.

Re:escape velocity?

[Planesdragon]

Ah. So geo. orbit isn't escape velocity, it's just about exactly shy of escape velocity. Gotcha.

There is no way you could build a space elevator out of a black hole

Given everything we know, yes. But "it's impossible" has never stopped hypothetical science fiction before

Re:escape velocity?

[Wardish]

Something you said interests me.

"
There is no way you could build a space elevator out of a black hole. It's not possible - not for material science reasons, or practical reasons, or anything - it is simply not possible to build *anything* that could cross the event horizon of a black hole. The intermolecular forces required would be infinite - literally, infinite. Unbounded.
"

My understanding (and flawed it probably is), is that the event horizon and the singularity are quite different things. I'll agree what you state is likely true in reference to the singularity but as for the event horizon I must disagree.

Please correct me if I'm wrong but the event horizon (EH from now on) is simply that boundary where the escape velocity exceeds the speed of light. And for a large enough black hole (perhaps a local group sized one), one where the region of the EH had sufficiently low tidal stress's to allow materials we know about to exist, then you could indeed move something through the EH. After all we don't need to approach or exceed any escape velocity to move to a higher orbit. And the energy requirement although huge is not by any means infinite.

Once again, that is my understanding and it wouldn't be the first time I've been in error...

Thought experiment: Your orbiting a black hole where the orbital velocity is 99.9% of light speed. (The previously mentioned really really big black hole so tidal forces aren't tearing anything apart.) You extend a line toward the black hole and another equally away from the black hole (space elevator style) So the whole thing is still orbiting at your point. Now the lower end gets to the EH... it's still traveling at 99.9% of light speed. It's still orbiting at the center point of the entire structure. Meanwhile the rest of the universe has aged a bit more than you...

As for the singularity, you're likely correct, but you really can't know. The rules change when you get close and at the point of the singularity, well, we've really no idea. It's literally out of our universe and we have no idea what the rules are on the other side, and that's assuming there is another side or even that there is only a single other side...

Another thought experiment: What would it look like for an object that repeatedly moved back and forth through the EH. BTW every read any Frederik Pohl? Been a few decades myself but might be time to revisit some of his novels.

Re:escape velocity?

[barawn]

My understanding (and flawed it probably is), is that the event horizon and the singularity are quite different things. I'll agree what you state is likely true in reference to the singularity but as for the event horizon I must disagree.

They are. The event horizon is not a true discontinuity in space. To an observer moving inward, nothing weird happens at the event horizon. The singularity is a discontinuity in space, and an indestructible 'observer' would see something weird happen at the singularity.

However, the event horizon does separate space into two regions that cannot communicate with each other - that is, the light cones of the two regions are physically distinct. There are certain coordinate systems where this is extremely obvious, like Kruskal-Szekeres coordinates (see this nice discussion on different coordinate systems for more info).

Please correct me if I'm wrong but the event horizon (EH from now on) is simply that boundary where the escape velocity exceeds the speed of light.

You're right, but you're not right. Gotta love that. The whole "escape velocity exceeds the speed of light" is a useful mnemonic to remember the Schwarzschild radius, but it is *not* where the event horizon comes from. This is an unfortunate coincidence of physics, that "bad" Newtonian gravity can give the "right" GR answer. Ah, well. The event horizon is where the denominator blows up in the Schwarzschild/Kerr/Neumann/Kerr-Neumann equation, in Schwarzschild coordinates. In non-pretentious speak, the equation describes the geometry of space around a black hole (normal, charged, rotating, or charged-rotating), and Schwarzschild coordinates are "normal" coordinates: 1 time coordinate, 3 spherical coordinates.

Now, there's nothing special about those coordinates, and that's why the event horizon isn't a real discontinuity in spacetime (though it *is* a causal discontinuity).

Let's restrict ourselves to Schwarzschild black holes only - the other ones have weird shaped event horizons (donut-shaped) and ring singularities, and weird things like that.

The "Minkowski metric", the metric of normal flat spacetime, looks like this: ds^2 = -dt^2 + dr^2 + r^2 do^2. You might have seen something similar in the flat space form: ds^2 = -dt^2 + dx^2 + dy^2 + dz^2 - this is the "4-dimensional Pythagorean theorem": if you ignore the "dt^2", which is relativity, the other part says that the distance between three points (squared) is the sum of the squares of the displacements along each axis.

However, note that time is negative, and space is positive: you can switch that (space negative, time positive) as all that matters is "ds^2", but there is still a sign difference between space and time.

Now, past the event horizon, in the Schwarzschild metric, radius is negative, and time is positive. That is, your radial distance from the singularity becomes timelike, and your time since crossing the event horizon becomes spacelike. In this case, that means that all worldlines lead to the singularity once you pass the event horizon. It's like saying "you can't move backwards in time" - same thing. Inside the event horizon, you can't move outwards. Period. End of discussion. Game over, player one.

(The astute reader may conclude that maybe you can, in fact, move backwards and forwards in time once you're inside the event horizon. Well, maybe. That fact is outside the realm of our discussion. :) )

Anyway, the best way to realize that nothing can "come out" of the event horizon is to view the event horizon from infinity. Imagine that you send a probe in, designed to emit one beep every picosecond (10^-12 second). As the probe gets closer to the event horizon, time dilates dramatically, so every picosecond for the probe takes incredible amounts of time for an observer at infinity - hours, days, months, years, etc. The other way to think about thi

Re:escape velocity?

[Wardish]

Wheeeee!

I'm having way to much fun with this. Thank you.

I was having real trouble thinking about this until you mentioned that spacetime itself is moving/accelerating. Then it clicked into place.

When you drop that bit of line into the EH (event horizon) you're fine and dandy to that point. But every elementary particle that crosses that threshold is gone. I would suggest that pulling the line back up would be just as easy as letting it down. But it would be cleanly cut with a very exact curvature. At least that is my guess... Since any sublightspeed communication is not going to happen across the point where spacetime itself exceeds lightspeed the forces that would hold/connect particles inside the boundary to one outside the boundary would be cleanly severed.

Would I be right in guessing that the movement of spacetime towards a black hole is a true phenomena for any gravitational source? Hmmm, That also clicks into place references to rotating black holes and even the possibility of a naked singularity... Nope, still can't see a naked singularity, quite. Depends on some interesting answers.

Is spacetime moving because of gravity or is gravity the result of spacetime moving?

If you have two black holes orbiting each other such that the EH's intersect there is a plane where the gravity of the holes counteracts each other and a volume on either side of that plane where spacetime will be moving much less than light speed (if gravity is the cause and not the effect).

Now to access these areas you will need another large mass that can slow the in fall of spacetime and produce an entryway, talk about a balancing act... Exceed the maneuvering capability of the mass in the direction of either hole and It's a long drop.

Now as to a naked singularity. I would have to know what happens when one black hole enters the EH of another.

Wheeee! Now I'm wondering if you can get eddies in spacetime.

I better quit now, having way to much fun but it's eating up the work day and they might have a hard time paying me for this much fun.

And plenty of otherthings to think about. Such as if spacetime is moving, in relation to what?

Other questions occur, might be silly but... Where does spacetime come from? Is a coordinate system inexhaustable? Where does it go?

Thank you once again.

Re:escape velocity?

[barawn]

I was having real trouble thinking about this until you mentioned that spacetime itself is moving/accelerating

I don't remember saying that... that's definitely not true. The Schwarzschild solution has an event horizon even if it's static.

The problem is that you've got to worry about reference frames quite a bit here. For the observer at infinity, space and time seem infinitely stretched out as you approach the event horizon.

For the observer falling INTO the black hole, nothing weird happens at all near the event horizon - it still takes finite (small!) time to reach the singularity.

(note: I guess in some sense you *could* consider spacetime to be "moving" near a black hole. The math might even work out exactly the same! the problem is that it tries to simplify the discussion too much: nothing 'magic' happens at the event horizon. Time (from an infinite observer's point of view) is dilating continuously as you approach the event horizon. Once you cross it, that's when strange things can happen - you can start receiving images/messages/information from the 'future', etc., but no matter what, you are heading to the singularity.)

When you drop that bit of line into the EH (event horizon) you're fine and dandy to that point. But every elementary particle that crosses that threshold is gone. I would suggest that pulling the line back up would be just as easy as letting it down. But it would be cleanly cut with a very exact curvature. At least that is my guess... Since any sublightspeed communication is not going to happen across the point where spacetime itself exceeds lightspeed the forces that would hold/connect particles inside the boundary to one outside the boundary would be cleanly severed.

Ah, reference frame issues. To you, you never see anything cross the event horizon. Nothing at all. All you see is things ever increasingly slowly approach the event horizon. It takes infinite time for the last photon that the object crossing the event horizon emits to reach you.

If you try to pull upwards on something, you're asking the molecules in that "something" to communicate your upward force to the molecules they're attached to, and likewise, the downstream particles need to communicate their downward force (preventing the upstream particles from moving away, producing tension) upward. This "upward" communication will take an increasingly long amount of time, which means that the "stretch" between adjacent molecules will increase as you get closer to the event horizon. At some point, this stretch will be great enough that the bond will just break. This does -not- occur at the event horizon, but before it. The deeper the particle falls in, the less likely you're going to be able to get it out, because the energy required continues to go up, and at some point, the energy required will be more than you can convey to the particle.

And plenty of otherthings to think about. Such as if spacetime is moving, in relation to what?

Well, as I've said above, spacetime itself isn't really moving. But, there are cases where it does (like, the expansion of the universe). In those cases, it's moving in relation to a flat coordinate system, at infinity.

Other questions occur, might be silly but... Where does spacetime come from? Is a coordinate system inexhaustable? Where does it go?

Spacetime is spacetime - it's a coordinate system. The coordinate system deforms, yes, and "the minimum amount it can deform" is up for grabs. Can you increase the volume of spacetime? Sure - all you need to do is stretch it, which is quite easily doable. It doesn't "come from" or "go" anywhere, because it "isn't" anything. It's just a coordinate system. It's how long it takes light to get from one place to another one.

Picture two "squares" of 2D spacetime. In one square, put a black hole, in another put nothing. The length of each size of the square is 1 light-year. Now, crossing the square diagonally for the square

It's all about balance!

[Wardish]

The elevator would be build from geostationary orbit. An asteroid isn't required as a counterbalance but it does make things more convenient.

Without an asteroid you would need to send a foot of cable away from earth for every foot you send toward earth. With a counterbalance you get to stay MUCH closer to the balance point (geostationary orbit) which has many conveniences.

When you send up an elevator and it gets closer and closer to the balance point the mass on the other side (counterbalance) has to be adjusted to match, either the entire thing could be winched in (bad method in my mind) the small amount needed to maintain the systems balance or some mass in the counterweight would need to be moved around to do so (my personal favorite).

For example if you've a 20,000 kilogram elevator and it moves up the cable 10,000 meters then a counterbalance that was (for example only) 20,000,000 kilogram's would have to move 10 meters toward the balance point to maintain the status quo. Or you could move 100,000 liters of water from a tank in the bottom of the counterbalance to a tank at the top which would be 2000 meters above the bottom tank. Please keep in mind that these numbers are examples, the real numbers for such a facility would be a bit more impressive and the real mechanisms would be more complicated.

The counterweight is also the most logical place to put the terminus facilities as that's the handiest real-estate in the area and even has significant apparent gravity. So since you have mass at/in the counterweight that moves around (us and/or equipment and supplies) then you would need to constantly rebalance anyway.

Actually I would suggest some liquid movement tanks high and low on the counterweight to maintain the balance without actually having to shorten the cable/distance to the balance point. Or you could have some type of elevator/freight system moving large plugs/masses of rock up and down to do the job. Better yet, both, having a dual system (or even ternary) would make sense considering the importance of balancing the system...*GRIN* I would do a great deal of work making it as foolproof as possible cause goodness knows we keep making the fools.

Well, enough on this, time to go home.

Re:It's all about balance!

[barawn]

Actually I would suggest some liquid movement tanks high and low on the counterweight to maintain the balance without actually having to shorten the cable/distance to the balance point. Or you could have some type of elevator/freight system moving large plugs/masses of rock up and down to do the job. Better yet, both, having a dual system (or even ternary) would make sense considering the importance of balancing the system...*GRIN* I would do a great deal of work making it as foolproof as possible cause goodness knows we keep making the fools.


Actually, no...

Think about this for a second.

Picture an object in a circular orbit. Gravity is F = GMm/r^2, and the centripetal force required to keep it in that orbit is F = mv^2/r. So, so long as v^2 = GM/r, everything's happy.

Now, let's suppose that something grabs onto the elevator, say, something that's 10K pounds (pounds, because pounds is a force, and that simplifies things). So, now, the force that's pulling on the object is an F > mv^2/r. But it's still *moving* with v^2 = GM/r, so it can't simply be "pulled downwards" - instead, it starts to *lean forward*. All you've done is increased the centripetal acceleration, which means all you're going to do is bend the velocity vector such that it *starts* to head towards you. If it were a satellite, you'd be putting it into a lower elliptical orbit that still keeps that point-of-increased-a_c as its perigee. Since its an elevator, it starts "falling down" - leaning forward.

OK, now send the object up the elevator. As it goes up, it's now *torquing* the elevator (because the radial vector - perpendicular to the velocity vector - is no longer in the same direction as the force vector, so there's a torque), trying to make it lean forward.

Once the mass gets beyond geosynchronous orbit, it's now shifted the center-of-mass of the elevator above GEO, and it's now trying to make the elevator lean *backward*.

See a pattern? Sending a load up the elevator will induce an oscillation in the ribbon. It won't make it slowly fall, unless you send loads constantly to less-than-GEO and let them go, and don't bother trying to actively damp the oscillations at the base.

Yah, you could just actively damp the oscillations. The elevator starts leaning one way, you pull the elevator the other way. The two oscillations cancel each other out. No balancing act needed.

(Yes, you can keep a satellite in the "wrong orbit" - imagine if the satellite was too close to the Earth, such that it tried to fall. Now imagine constantly blowing on the underside of the satellite, such that the sum of your force and the force of gravity equals the necessary centripetal force. Poof. Satellite in an orbit again. That's what active damping would do).

Re:It's all about balance!

[Wardish]

*grin* Yes indeed. I was actually trying to minimize the apparent complexity while still providing a reasonable explanation.

The inside of a spinning structure can provide a very odd and confusing appearance to simple physics. However, if you back up a little so your frame of reference is at rest to the spinning globe what your describing starts to make sense to people.

The track of an ascending elevator car follows a nicely curved path out from the surface to geosynchronous orbit. Even though from the perspective of the elevator they are just rising vertically they are actually slowly moving from a point that is moving around a circle at 1000 miles an hour (very roughly) at a radius of 4000 miles to a point that is moving around a circle of 26000 miles at 6800 miles an hour. The point that was 26000 miles above you has traveled 40,000 miles while you moved toward it and you have traveled most of that extra distance as well. So if you move something out in such a manner you can see that there is always a horizontal component that results in the leaning you're describing. (Be happy were not dealing with much in the way vectors away from the orbital path, that gets squirrelly fast.) (For interesting fun and games, play catch with a gyroscope that is up to speed...)

That also should answer the questions of another poster about how this all relates to escape velocity, it's not escape velocity but the concept should be apparent. It is however a change in orbit from a supported orbit, such as you referred to and which describes any resting mass on the surface unless it's at the poles, to an unsupported geosynchronous orbit.

Now if we want to address this in more detail (yep I'm going to avoid as much of the math as I can, I'm very rusty...) we would have to address this horizontal component. Perhaps something as simple as an ion thruster on the elevator car that supplies the necessary vector to balance things out.

Another point for people to get a grasp on is that it's not exactly a ride to the penthouse of an apt. complex. Hmmm, lets see for a trip of 22,000 miles in a reasonable amount of time and considering most of it is outside any noticeable atmosphere an average speed of several thousand miles an hour should be no problem. (I'm guessing that an actual physical contact with the elevator wouldn't be necessary, magnetic would be my choice.)

Ok, back to dealing with the imbalance that would cause your "lean". Fair warning, lots of guess's, assumptions, and downright neighborhood arithmetic coming...*GRIN* So you should be able to make the trip in 6'ish hours. That means a change in orbital velocity from 1000 miles and hour to 6800 miles an hour gives you very roughly about 1000 miles/hour change per hour which works out to the neighborhood of .012 m/s/s acceleration needed to counter the "lean". Lets see, drag out the old number of 20,000 kg elevator and that's 240 newtons. Now a quickie check of the net for ion thrusters gives us... Ok, Boeing has a deep space thruster that supplies 165 millinewtons. It's 25 cm, their 13 cm unit is 18 millinewtons. If it keeps scaling that nicely 240 newtons would be on the order of a 3 meter thruster grid. *chuckle* (pulling it out whole cloth now) Although I expect there are better or more efficient alternatives...

Ok, enough of this, I've work to do... I have enjoyed the thinkfest though..

Re:It's all about balance!

[barawn]

...*GRIN* So you should be able to make the trip in 6'ish hours. That

6 hours? No way. 2 weeks, nominally.

The whole reason a space elevator wins over a rocket is because you can avoid air resistance by going slow. In 6 hours, you might as well be using a rocket. You're talking about 60,000 km, so roughly 10,000 km an *hour*.

There's another problem with a 6 hour trip, too - the speed of sound in the elevator produces about a 7 hour natural vibration. A 6 hour trip means that you're actually travelling supersonically up the elevator (supersonic with respect to the cable).

Anyway, with a 2 week trip, you can easily see that even a very gentle push (I think it's in the "few newtons" range) can counteract the slight lean of the elevator.

Re:It's all about balance!

[Wardish]

Granted. A bit more time would probably be a good idea, and especially while in the atmosphere. However the portion of the trip inside a noticeable atmosphere is a tiny fraction (1/600th).

Once you're outside the atmosphere there are no outstanding reasons for not moving faster. That is why I mentioned magnetic drive/coupling instead of any physical connection. And of course carbon nanotubes can be configured to carry power, it should also be relatively simple to configure part of the elevator as a track for a magnetic linear accelerator. No inefficient rocket power required (although to be honest I'm not sure what the efficiency of a linear accelerator is).

As long as you have access to a constant power supply it's useful to have a constant acceleration/deceleration an if you do so your midpoint velocity will be impressive and your time for the trip will be much shorter. 6 hours may or may not be a reasonable figure, I would have to do way more math than I'm currently willing to do to figure that out. For instance I wouldn't want to go very fast inside the atmosphere for the obvious reasons. Outside I would like to maintain a decent apparent weight for the passengers as well as constantly reducing it so it will match the environment at the terminus station when they dock. So the midpoint turnaround and switch from acceleration to deceleration wouldn't be anywhere near the middle. The actual numbers would be interesting though... *chuckle* The acceleration numbers would make an interesting compound curve.

Keep in mind that even though we are using an elevator we still must pump in the energy needed to move the mass into orbit, it's just this is a much more efficient method.

Hmmm... I read some pulp sci fi recently that had a very good example of magnetic transfer/acceleration/deceleration etc... Ah... I remember "The Duke of Uranium" by John Barnes Not exactly the best read but worth it for some interesting tech ideas.

Last but not least, I do want to thank you for an interesting discussion. I find real thinking to be a rare pleasure these days and I do enjoy it.

Re:It's all about balance!

[barawn]

Once you're outside the atmosphere there are no outstanding reasons for not moving faster. That is why I mentioned magnetic drive/coupling instead of any physical connection. And of course carbon nanotubes can be configured to carry power, it should also be relatively simple to configure part of the elevator as a track for a magnetic linear accelerator. No inefficient rocket power required (although to be honest I'm not sure what the efficiency of a linear accelerator is).

Damage to the ribbon is a good reason. However, avoiding the Van Allen belts is a better reason to move quicker once you're a bit farther up, so point. However, counterpoint is that you don't torque the ribbon as heavily once you're up at a higher point, as you're shifting the center of mass less.

However, the best reason for not moving faster is, of course, because you don't really have the power. It's silly to carry the power with you (reduction of launch mass), so you beam it up from the ground. I've a feeling that this will *stay* the main method for scaling a space elevator for a long time to come, simply because it's the easiest, and the most launch-mass efficient.

Also, the nanotube power carry thing is a bit of a red herring. There are two problems:

1) The elevator won't use continuous nanotubes - it'll be using composite nanotube fiber, which, of course, cannot carry power, as it's just glue with a bunch of really weird things called 'nanotubes' inside it, and the glue probably won't conduct.

2) Even if they end up using continuous nanotubes (maybe multiwall nanotubes, as it appears you may want to fuse them), nanotubes conduct power along their axis, and a continuous string of nanotubes would have no way of conducting power off-axis. Plus, when you're talking a length of 60,000 miles, the nanotubes would have to superconduct (which they don't - they do weird things with electrons, but it doesn't look like true superconductivity) - even the best conductor known to Man would appear to be a gigantic resistor when it's 60,000 miles long.


Last but not least, I do want to thank you for an interesting discussion. I find real thinking to be a rare pleasure these days and I do enjoy it.


Whew! I normally call lack of thinking death. Interesting discussion is the only reason to wake up in the morning. :)