Fibre for space elevator

[YNM]

9 hours ago, magnemoe said:

You can get away with it an theory called active support, have an inner ring spinning far faster than orbital speed who hold the structure up. Problem with this is that happen then some of the magnetic bearings fails or some other fail mode, it would be an train wreck who would be very visible from the moon. 
Pretty sure this has to be far more massive than an space elevator to as it just has to be an long wire.

If a space elevator cable was to broke it'd also start to rain down on us regardless.

Structural failures are always far more frightening, unlike the explosion of a few million gallon of fuel (in both controlled and uncontrolled manner). I guess it's not surprising we always use a large 'safety factor'.

Edited October 31, 2018 by YNM

[KerikBalm]

15 hours ago, cubinator said:

Are we talking a Von Braun-style ring, or a planet-circling ring? Because I think the latter would be easier to build with a space elevator anyway.

I'd strongly disagree, space elevators are ridiculously hard to build, everybody acts as if one could just wave a wand a poof it into existence if we have the material. The entire 35,000+ km cable will fall down to the earth until its attached to a counterweight past GSO. You'd have to put a spool of 35,000+ km cable into GSO, and lower one end (that is going to then start moving faster and ahead in orbit) as you push the counterweight to a higher altitude, requiring thrust and stabilization on each end, for the entire ~35,000 km journey of the one end down to Earth and while its being attached... not easy by any stretch of the imagination, and you need to get a lot of mass up to GSO first.

In contrast, an orbital ring will be just a bit larger than the circumference of Earth, so ~40,000 km in total. Considering that a space elevator needs to go past GSO, its going to end up being roughly the same size in total (~40,00 km long). The ring can be built entirely on Earth (with some stretchy-ness or expansion joints in it since the circumference at ~300km above the ground would be nearly 5% more). Then you simply spin up the ring and it will start to float into the sky, pulling the cables taught. The problem of course is that the cables essentially attach to a maglev train that runs across the "ribbon" or "cable" of the ring at above orbital velocity. Ideally you'd design it so that if the maglev failed it would fall off the ring without touching it (and you'd of course have backup systems), since you could use multiple ground stations, you'd be able to lose multiple of them before the ring became unstable.

If done this way, you'd need to construct a planet encircling track on the ground to launch the ring, in addition to the ring... but that's just one way to build it, and at least its a plan. There's really no plan to how to actually construct a space elevator, given that the entire thing will want to come crashing down the entire time its under construction. How to keep it up there and stable while its being built is still something people are fuzzy on.

I'll take the structure of the same size, with actual ideas about how it could be built, that can be built with current materials, that can have multiple redundant systems and service points all over the globe, over a structure of roughly the same size, that cannot be built with current materials, with no plans how to build it (just the idea that if it were somehow built, it would then be stable), that can only service one point on the globe, with very little possibility for redundancy, and very long transit times to orbit.

 

13 hours ago, magnemoe said:

Problem with an planet circling ring with tethers down to the ground is that the ring can not rotate at orbital speed. You can get away with it an theory called active support, have an inner ring spinning far faster than orbital speed who hold the structure up. Problem with this is that happen then some of the magnetic bearings fails or some other fail mode, it would be an train wreck who would be very visible from the moon. 
Pretty sure this has to be far more massive than an space elevator to as it just has to be an long wire.

Look at the distance to GSO, and then the circumference of the Earth, it doesn't need to be far more massive at all. The "ring" needs only be a cable or ribbon as well (no need for inner and outer rings, just a ribbon around the earth, with spokes ending in maglev trains). It would be fairly easy to design the trains such that they hang from the ring and if magnetic suspension is lost, they fall from the ring without destroying it. Then you have something falling straight down from 300km, not a cable tens of thousands of km long falling across a huge swath of the earth.

The reason a space elevator needs such a high tensile strength is because the cable needs to be massive to support its own weight (ideally, its tapering, and it would be thickest at around GSO). The ring can be much thinner, assuming its made out of similar materials.

[cubinator]

 I see. I was thinking of a ring that went at GEO, which would be much bigger. The atmosphere would cause problems for your ring as it is spun up, but otherwise it seems physically possible, given current materials and unlimited budget.

[mikegarrison]

19 hours ago, KerikBalm said:

I'd strongly disagree, space elevators are ridiculously hard to build, everybody acts as if one could just wave a wand a poof it into existence if we have the material. The entire 35,000+ km cable will fall down to the earth until its attached to a counterweight past GSO. You'd have to put a spool of 35,000+ km cable into GSO, and lower one end (that is going to then start moving faster and ahead in orbit) as you push the counterweight to a higher altitude, requiring thrust and stabilization on each end, for the entire ~35,000 km journey of the one end down to Earth and while its being attached... not easy by any stretch of the imagination, and you need to get a lot of mass up to GSO first.

Yes, this should be obvious, but a space elevator needs to be built from orbit, not from the ground. As you are building it you need to keep careful control over the c.g. and the the orbit it is in so that you work it around to the correct spot in the sky and then keep it there. Once the cable starts getting long enough to dip into the atmosphere, you will need to worry about aerodynamic forces, so by that time it would be best if it is already in its final geostationary position. And keep in mind that as you extend the cable down you also have to move mass up too, so that the overall c.g. remains geostationary.

[KerikBalm]

4 hours ago, mikegarrison said:

a space elevator needs to be built from orbit, ... keep in mind that as you extend the cable down you also have to move mass up too, so that the overall c.g. remains geostationary.

Yes, that's basically what I said:

Quote

and lower one end ... as you push the counterweight to a higher altitude

Lower one end, raise the other, keep the center of mass the same (GSO). Keep in mind though, that as you lower one end, its going to start to get ahead of the other end (they'd be going the same velocity, but it would be on the "inside track"), and the counter weight going up would start to fall behind.

Now for such a long structure, you would also benefit from https://en.wikipedia.org/wiki/Gravity-gradient_stabilization , but as you'd also be actively extending stuff, there would be a lot of swaying and oscillation. You'd have to limit the rate at which you extend it to a pretty slow speed, so it would take a long time to cross those 35,000 km.

17 hours ago, cubinator said:

 I see. I was thinking of a ring that went at GEO, which would be much bigger. The atmosphere would cause problems for your ring as it is spun up, but otherwise it seems physically possible, given current materials and unlimited budget.

Nah, in that case, the cables from the ring to the ground would need to be just as strong as a standard space elevator.

As for atmospheric drag, it would be minimal friction with the sides, there would be no "frontal" drag because its a ring. On a spaceplane you want to minimize the frontal area/maximize the sectional density... on a ring that circles back on itself, this goes away completely. if you could make its surface very smooth, the drag would be very very small. It would be actively stabilized via the maglev trains connecting to the Earth, so the residual drag at about 300km would be a minimal increase in power consumption. The maglev trains/stations  would ideally be solar powered. They'd be up where solar power is not weather dependent. Ideally you could also transmit power around the ring (but each station should also have a backup power reserve for the nighttime cycles.

You could also build two stations at the poles, and have each station high enough to be in constant light (although given our axial tilt, that would have to be quite high, still not up to GSO).

Active stabilization may sound a bit scary, but a space elevator would need maintenance too (and payload rate would be very limited and have to be controlled to keep the sway under control). The massive increase in structural strength margins and possibility for much more redundant and much more cargo capacity make it worth it, IMO.

Long carbon nanotubes can be great for many things, but don't think that getting them means that we get a space elevator. We essentially have the tech that we need to make something even better than a space elevator... we just don't have the will to embark on a project to build 40,000 km+ structures just to send stuff to space... carbon nanotubes won't change that.

We can't even build a launch loop (which is like a partial orbital ring)

[YNM]

On 10/31/2018 at 2:46 PM, KerikBalm said:

Then you simply spin up the ring and it will start to float into the sky, pulling the cables taught.

I was thinking slowly unspooling them out in the target orbit, by having one end at a higher orbit and the other kept at the intended orbit. Every now and then a 'station' is released that will serve as the elevator up towards the orbital ring (they'll drop cables down), they will bring the section of the cable down towards the intended orbit.

A lot of things to loft in the first place but a lot less scarier than having a huge thing going up through the air across the globe.

40 minutes ago, KerikBalm said:

we just don't have the will to embark on a project to build 40,000 km+ structures just to send stuff to space...

For now. And for a good foreseeable future.

I'd call it by 2070 though. (either that or we'd be all dead by then.)

On 10/31/2018 at 2:46 PM, KerikBalm said:

... circumference at ~300km above the ground...

Thanks, I'll try and see it.

[James Kerman]

If I understand both ideas correctly, these structures would be unable to maneuver to avoid orbital debris and would also need to be engineered to withstand high speed collisions.

Edited November 1, 2018 by James Kerman
grammar

[Green Baron]

Theoretically, you can have satellites in GSO where the elevator's platform is fantasized to be, but not above where the counter weight is or below, or they must be in resonance with the rotating elevator. So, either elevator or satellites.

Both contraptions, elevator and ring, are unstable and subject to high mechanical forces, mainly tear from the own weight and the counterweight and coriolis. Neither regards earth's axis precession or tidal forces from moon, sun and other planets. Neither is stable and needs correction. Both are exposed to all sorts of pebble that buzz around and cross between GSO and earth surface every now and then. And i am not sure if we have enough obtainable metal in the earth's crust to build a ring. But, its scifi so everything is possible :-)

And btw., is the material, even if it could be produced industrially, really strong enough to support its own + counterweight ? So, all in all, where are the news :-) ?

[KerikBalm]

7 minutes ago, James Kerman said:

If I understand both ideas correctly, these structures would be unable to maneuver to avoid orbital debris and would also need to be engineered to withstand high speed collisions.

Well, only if their attachment points to the ground are mobile as well. A cool thing about the ring is that it can be entirely within the area of low orbit where stuff doesn't stay up there very long. Also as its rotating at close to (above) orbital velocity, impacts velocity is due to lateral relative speed.

In contrast, at lower altitudes, the elevator is essentially stationary while stuff may be whizzing by at up to 8km/second (or higher if it highly eliptical orbits)

32 minutes ago, YNM said:

I was thinking slowly unspooling them out in the target orbit, by having one end at a higher orbit and the other kept at the intended orbit. Every now and then a 'station' is released that will serve as the elevator up towards the orbital ring (they'll drop cables down), they will bring the section of the cable down towards the intended orbit.

That's not going to work. The stations would have to be in orbit, and then when they drop the cables down, they'd be dragging cables through the atmosphere at nearly 8km/second... unless you mean that the ring is completed:

Then the stations decelerate while the ring accelerates, and then once the stations are stationary, they drop the cables.... until those cables are secured, the whole thing is dynamically unstable, and you'd need a lot of thrusters on the stations to actively stabilize it without a ground connection. On the plus side, they only have to drop cables a few hundred km, not 35,000 km like with an elevator, I suppose it could be done very fast. If the response time of  the thrusters is really fast, it wouldn't take too much propellant (the longer one waits, the more thrust needed, so we'd ideally want things like nanosecond response times to minimize propellant mass needed).

Quote

A lot of things to loft in the first place but a lot less scarier than having a huge thing going up through the air across the globe.

Why is it scarier to have something at 50m and ascending, vs a huge thing going up in pieces, ending up either way as a huge thing at 300km around the globe?

The stabilization system keeping it at 300km would work pretty much the same as when its being lifted up in the first place. The only added complication is the surface track that it would rest on before it starts to lift (irrelevant once it actually starts lifting), and the expansion system as its radius increases (with only 5% expansion needed, elasticity may be enough, without any need to engineer in expansion joints/telescoping segments/whatever).

Quote

For now. And for a good foreseeable future.

I'd call it by 2070 though. (either that or we'd be all dead by then.)

That is both very optimistic, and very pessimistic.

I don't think we'll see a infrastructure for space access by 2070, and I hope we won't all have killed ourselves by then.

[kerbiloid]

People laugh at a 30 km long maglev along a high mountain ridge to launch a reusable HTHL SSTO powered with a bimodal aerospike, but seriously discuss a 264 000 km long orbital ring with L:D ~26 000 000 attached to the Earth with cableways...
Of course every radial cableway will be just ~3 600 000 : 1.

Please, somebody tell about a L:D=10⁷ : 1 rigid structure.

Another suggestion: let's make a kilometer-wide parachute, push it from ground with laser and land as a chute. _{Just a joke.}

Edited November 1, 2018 by kerbiloid

[Green Baron]

I think we are all just playing in our minds.

Nobody seriously believes that anybody can put the mass of a continent into space and run a biosphere on it. Or an atmosphere worth of gases and an ocean worth of water on Mars. Or build a 50.000km long cable with a terrace and McDonalds and attach an asteroid on the the other end.

Or not ?

Edited November 1, 2018 by Green Baron

[YNM]

4 hours ago, James Kerman said:

If I understand both ideas correctly, these structures would be unable to maneuver to avoid orbital debris and would also need to be engineered to withstand high speed collisions.

Depends on the material choosen and the type of collision being most probable.

I'm pretty sure if they really want to make one they will look into it. I did a curious calculation on the page before though, and the answer is still a no.

4 hours ago, Green Baron said:

is the material, even if it could be produced industrially, really strong enough to support its own + counterweight ?

See my post in previous page - the material they're saying to have 'found' is still 18 GPa short of the absolute theoretical minimum. (also there's a paper that suggest the most likely loading - if you know a more accurate loading mechanism I'd be interested).

Not to mention they haven't made one strand that's kilometres and kilometres in length. If you want to clamp them to connect the strands then the clamp structure itself will add to the weight and will be subject to twice the stress - so needs a material twice stronger.

4 hours ago, KerikBalm said:

unless you mean that the ring is completed:

Then the stations decelerate while the ring accelerates, and then once the stations are stationary, they drop the cables.... until those cables are secured, the whole thing is dynamically unstable, and you'd need a lot of thrusters on the stations to actively stabilize it without a ground connection.

Yeah, that was what in my mind, but it does sound odd.

4 hours ago, KerikBalm said:

Why is it scarier to have something at 50m and ascending, vs a huge thing going up in pieces,

A failure will be immediately catastrophic. Plus I'm not sure it wouldn't buckle at all.

4 hours ago, KerikBalm said:

That is both very optimistic, and very pessimistic.

Yeah. Well probably not 2070 - any year China is dominating space I'd say.

By that year we'd be all client states.

3 hours ago, kerbiloid said:

rigid structure.

It's not rigid. Suspension bridges aren't rigid, high-rises aren't rigid. They're designed with sway in mind, esp. in an earthquake. Or wind.

But the sway is indeed limited and there are damping systems that quickly absorps the vibration.

3 hours ago, Green Baron said:

I think we are all just playing in our minds.

Kind of. I personally just want to see how impractical is it.

Edited November 1, 2018 by YNM

[kerbiloid]

1 hour ago, YNM said:

It's not rigid. Suspension bridges aren't rigid, high-rises aren't rigid.

How great is their L:D?

What is the landmass the space ring is attached to?

If a space ring/lift changes it length due to stress or heating for 0.1%, how stable is its orbit?

If a stress wave runs along the ring/lift, how great will be the amplitude and how long would the system stay properly rather than bend and curl?

Btw the part of the ring in the Earth shadow will be shrinking, due to cold while the other part expanding due to sunlight.

Edited November 1, 2018 by kerbiloid

[KerikBalm]

The part of the ring in shadow will be in shadow less than 45 minutes (and how much less depends on how much over orbital velocity it is spun up).

The ring would have a very small "foot print". It could service any landmass.

Its orbit isn't stable, that's the point of the ground stations, it does require active stability (but not propellant once connected to the ground)

[kerbiloid]

3 minutes ago, KerikBalm said:

The ring would have a very small "foot print". It could service any landmass.

No, I mean: any bridge is attached to a landmass. But a ring/lift is just floating in space, it isn't anchored.

And it has L:D as "a thousand" bridges attached one by one.

[YNM]

9 minutes ago, kerbiloid said:

How great is their L:D?

According to the calculation I did earlier, L:D for space elevator is about 1:47,000,000.

Akashi-Kaikyo's bridge midspan has a thickness of 14 m and a length of 1,990 m - so about 1:142. However, it is a steel lattice structure and not a girder box.

The 1940 Tacoma Narrows bridge that ultimately failed has a midspan length of 853 m and a deck thickness of 2.4 m, giving 1:356.

There's the Çanakkale bridge, under construction, that has a midspan of 2,023 m and a deck thickness of 3.5 m - that's 1:578.

I should note that these structures has to resist bending.

15 minutes ago, kerbiloid said:

If a space ring/lift changes it length due to stress or heating for 0.1%, how stable is its orbit?

It's a gravity-dominated structure - so the stresses remains mostly constant. Temperature heating behavior will have to be determined on the material itself - I was thinking that the cable can be spooled and unspooled from the counterweight slightly.

28 minutes ago, kerbiloid said:

If a stress wave runs along the ring/lift, how great will be the amplitude and how long would the system stay properly rather than bend and curl?

This was one of my own questions as well, as so far I only found formulations for the loading due to gravity. To the best of my knowledge the only way out is for active stationkeeping to be employed on the counterweight. As for their bending I'll have to ask for the formulation.

4 minutes ago, KerikBalm said:

it does require active stability (but not propellant...)

It requires continuous energy feed even if no payload is being raised.

[kerbiloid]

A stress wave with 24h period probably should present.

Also when a carriage arrives/starts, it pushes a stress wave along the ring and the lift.

[YNM]

14 minutes ago, kerbiloid said:

But a ring/lift is just floating in space, it isn't anchored.

They're anchored, but in a manner that they'd rotate accordingly.

9 minutes ago, kerbiloid said:

A stress wave with 24h period probably should present.

Also when a carriage arrives/starts, it pushes a stress wave along the ring and the lift.

Yeah, but I'm still not sure for the calculation. As the horizontal and vertical forces remains constant I guess the tolerance being allowed will also depend on how strong the cable is (and so the maximum angle we can let the cable go). If someone can give me the calculations on where the coriolis force is going to be strongest, how strong is it (it's fine even in acceleration) and what's the self-weight tension there I could try seeing it. Or you can try apply the LRFD standards yourself and see does it exceed the maximum gravity load at GSO height (if it does then it becomes the new limit).

 

If anyone hasn't been sure yet, I'm not saying these things are possible - they're very much not yet possible, and remains so for a good time into the future.

Until we see more utilization of carbon-based materials in our everyday structures building these behemoths is a huge no. And even then when that happens we might find more details that needs to be addressed, such as their aging or failure behavior.

Edited November 1, 2018 by YNM

[kerbiloid]

1 minute ago, YNM said:

They're anchored, but in a manner that they'd rotate accordingly.

Either they will be anchored, so any difference in the rotation speed will tear out the lift either from the land or from the ring.
Or the lifts are threads just hanging down from the ring. Then they are not anchored.

In any case there is no massive support structure carrying the lift and the ring, it can be wriggling as a whip.

[YNM]

6 minutes ago, kerbiloid said:

In any case there is no massive support structure carrying the lift and the ring, it can be wriggling as a whip.

I think the idea being for a ring is that :

- You have a cable that's freely orbiting the earth

- You have stations on thr cable that's moving wrt the cable but stationary wrt the earth's surface.

So it's like having something going on the cable going backwards such that they become stationary wrt earth surface.

Then from these stations you have a lift down to surface.

Or I don't know.

[KerikBalm]

The ring is definitely anchored. The stations move with respect to the ring (exerting force on it, anchoring it) while remaining stationary over the ground.

Any stress-wave is not going to have a 24 hour period, as the ring would just be in LEO, and orbiting faster than orbital velocity, giving it a period of under 90 minutes.

There would be at least 2 stations (so down to 45 minutes between stations), ideally many more (for redundancy and greater stability, or less "wriggling as a whip"). To avoid wear and friction, the station would ideally not be in physical contact with the ring, but rather interacting via electromagnetic forces - the lack of physical contact does not mean its un anchored, it would be coupled to the stations electromagnetically, and those would be anchored to the ground mechanically.

Kerboloid, you're really going to have to make it clear if you're talking about a classical space elevator, or an orbital ring, because often it seems like your comments are meant to apply to both (ie: ring/lift) when they are very different designs with very different challenges.

[kerbiloid]

9 minutes ago, KerikBalm said:

Any stress-wave is not going to have a 24 hour period

Ring radius  = 42. Earth radius = 6.4.
This means a permanent heating cooling of  ~0.15 of the ring.
The Earth shades 6.4/42*24 = 3.7 hours of the ring.
So, maybe not exactly 24h, but a stress wave caused by the temperature deformation would be permanently generated and passing along the ring disturbing its proper shape.

9 minutes ago, KerikBalm said:

orbital velocity, giving it a period of under 90 minutes.

90 min is LEO.
GSO has a 24h period.

9 minutes ago, KerikBalm said:

There would be at least 2 stations (so down to 45 minutes between stations), ideally many more (for redundancy and greater stability

How big are the stations?

9 minutes ago, KerikBalm said:

Kerboloid, you're really going to have to make it clear if you're talking about a classical space elevator, or an orbital ring, because often it seems like your comments are meant to apply to both (ie: ring/lift) when they are very different designs with very different challenges.

(kerbiloid)

I mean both because they have similar problem of enormous elongation.
Any relative error becomes a huge absolute error.

Though the ring looks by an order of magnitude more unstable.

Edited November 1, 2018 by kerbiloid

[KerikBalm]

The ring is not at GSO for the nth time!

Ring radius could be as low as 6.4 Mm

Edited November 1, 2018 by KerikBalm

[Green Baron]

A subterranean orbital ring through hills and mountains ? Decimeters AMSL ? I do not understand what you mean ...

Edited November 1, 2018 by Green Baron

[tater]

For a ring isn't the important material issue the breaking length (assuming it's spun up to 1g)? For even exotic (and impossible to mass produce at the moment) carbon types (nanotubes, graphine, etc) the breaking length at 1g is something like 6000-6500 km---so that's the largest ring circumference, right?