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lyner

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« Reply #25 on: 15/12/2007 11:56:38 »
Phew! that was a lot of thoughts.
Here is my fundamental objection - based on energy considerations. these are usually the clincher.
I think that what you are suggesting is some kind of resonant cavity - a bit like the mirrors in a laser. The energy sloshing back and forth would produce pressure each end due to momentum change. Fair enough, but the energy density in the  beams would be very high and there would be an enormous level of absorption by the atmosphere. In fact, any energy which wasn't actually transferred to Kinetic Energy of the vehicle would be dissipated (conservation of energy argument). Until the vehicle is going very fast, the majority of energy is wasted. This is the same as happens for a rocket at launch -zero efficiency at the instant of takeoff. The rocket only works because it soon is going quite quickly. Your proposed system would have to produce the same sort of acceleration if you wanted to avoid much higher losses.
The idea has some of the same aspects as the Ion Drive; particularly good for long term low power propulsion.
 

another_someone

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« Reply #26 on: 15/12/2007 14:01:12 »
Fair enough, but the energy density in the  beams would be very high and there would be an enormous level of absorption by the atmosphere.

Very early on, I did state that as one of my concerns also, but the comment then was the levels of atmospheric absorption in the RF region was negligible; but no doubt at the time we had not gone into detail.  Whatever the issues of power density in the atmosphere, those problems would also have to be managed by the transmitters and reflectors, which will also have to manage similar power densities (which was my concern about maintaining efficiency within those).

Again, I would guess that minimising the frequency of the radiation used (so long as it remains high enough to keep refraction effects sufficiently low) would help reduce atmospheric absorption.  Also, maximising the base area of the launch vehicle would help reduce the power density (as well as allowing for lower radiation frequencies to be used).

I am not sure why you think that the efficiency would rise when speeds increase.  From the component perspective, rapid acceleration would present problems, since the components will only work effectively over a narrow range of frequencies (particularly the meta-material), and high speeds would introduce Doppler shifts.  At high speed, but low acceleration, the Doppler shifts could be adjusted for.

The difference between this and an ion drive is that the ion drive is a reaction drive, using high velocity mass ejection.  There is no inherent velocity (if one ignores the speed of light) which I can see as being an optimum velocity for the system to work (excepting trying to minimise Doppler effects).

The other point you make about long term, low power - again, quite contrary to what would be possible simply because over long duration (if by that one means, long distances), you would be moving beyond the range of where the ground station transmitters could track.

That we are inevitably looking at low rates of acceleration, and thus longer launch times, I would agree with; but the question is whether the launch times can still be kept to a realistic timescale (even 6 hours to reach space is not unreasonable).  I am just not convinced that efficiency will improve with velocity (my gut feel would in fact have been the opposite - but then, as I said, I don't even know where to start the maths to show this).
 

lyner

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« Reply #27 on: 15/12/2007 18:40:23 »
I did a sum!
The momentum change (impulse) which is available a burst E of em energy (whatever the frequency) hits a reflector is 2E/c.  That's not a lot. It would mean only 2.5 Ns from 1kWh of energy(that's 3.6MJ) - that would accelerate 1kg to 2.5m/s.
At every reflection, you would get 2.5m/s more - less a fraction which was lost. It would reduce exponentially. The doppler shift wouldn't be relevant as the system is not tuned; it is not a standing wave - just energy sloshing up and down.
You would need to have as many return paths as possible, in order to squeeze as much as possible out of your beam.
Atmospheric absorption at below 20GHz is around 0.005dB per km when it's not raining and well above sea level  (you could choose when and where to launch). Each return path would be up to 200km long, say, so that would mean a loss of 1dB. After 3 bounces, the impulse would be halved but the total impulse from all bounces would probably amount to 5 or six times the amount from a  single bounce (it would drop exponentially). This assumes perfect focusing and aiming for each reflection.
Using as high a frequency as possible would allow you to minimise 'optical' losses but you have to keep lower than 20GHz because the absorption goes up steeply (H2O vapour).
You would be very lucky to make a system of microwave optics that could contain the wanted power with as little as 1dB of loss, bearing in mind that Jodrell Bank (76m diameter) would have a beamwidth of about 0.1 minutes of arc (very roughly) which would have a 3dB beamwidth at 100km of about 25m. Both reflectors would need to be about as big as the Jodrell bank one and very good, optically. I must admit, that sounds almost 'do able' and could be done adaptively to compensate for refraction in the atmosphere.
Perhaps, allowing for absorption and for optical losses, you could improve my origina figure for the impulse to 10Ns, downhill with the wind behind you.
All the above tells you that you would need 3.6MW to provide a force of 10N. Just to  support a reasonable payload ( 1000kg minimum) you would require a force of 10,000N which would need 3.6GW of power. You would need more force that this (quite a lot more) so that the journey time would be reasonable; the 3.6GW needs to be switched on all the time - quite an electricity bill if the journey takes an hour.
I wonder how that compares with rocket fuel needed to launch a  modest payload.

Someone please check my sums - I'm supposed to be cooking the dinner!!
 

another_someone

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« Reply #28 on: 16/12/2007 06:40:37 »
First, minor issue, you say that the return path is 200Km - this is at the end point of the launch, so average return path would only be half of this (and even at the end point, some part of that return path will be beyond the atmosphere).  The negative issue is to ask how the absorption are effected by the ionosphere.

3.6GW of power would require a very large nuclear power station dedicated to supplying power to the system.

You mention Jodrell Bank as an example, but you don't mention what frequency this 0.1 minute of arc is for - that is a key feature.

I do agree that higher frequencies will reduce diffraction effects, but it will also require much higher precision to maintain an accurate focus, and any imprecision could more than lose any advantages of improved diffraction.  Since the ground stations are using synthetic aperture, is is easy to expand the effective aperture to overcome and diffraction problems well down to sub-gigahertz levels; but the lower the frequency, the easier to maintain accurate phase relationships to create proper focus (although very long baseline synthetic apertures would increase the length of the return path of the signal at low altitudes - but then, it would probably make sense to increase the number of groundstations coming on line as the launch vehicle gains altitude).

The situation with the launch vehicle is more complex, since clearly having a launch vehicle with an antenna (even an actively adapted antenna) of a hundred kilometre baseline is clearly not an option.  On the other hand, maintaining phase coherence is still critical, and the larger the wavelength, the easier this remains.

There are other reasons why I consider a very large antenna, and thus one better suited to longer wavelengths, an advantage to the launch vehicle, is to minimise the power per unit area.  A circular antenna of 100m diameter has an area of 754 m2, which, for 3.6GW of power, would mean power density of 458KWm-2.  That is a lot of cooking power in not that large an area.  Ofcourse, if one pushes that out to a 1Km circular base for the launch vehicle, then one starts to get a far more manageable 4.5KWm-2 (still a lot of cooking power, but not nearly as bad as 0.45MWm-2).
« Last Edit: 16/12/2007 07:56:11 by another_someone »
 

Offline Pumblechook

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« Reply #29 on: 16/12/2007 13:37:53 »
I think the 76 Metre dish at JB has a 3 dB beamwidth of 18 arcmin at 1GHz and 8 arcmin at 2.4 GHz.   It will be inverse prop to the frequency and the diameter of the dish.  I would expect 0.1 deg (6 arcmin) to be achieved at about 3.2 GHz. 

 20 m dish at 10 GHz has a 3dB beamwidth ~ 0. 1
« Last Edit: 16/12/2007 20:22:42 by Pumblechook »
 

lyner

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« Reply #30 on: 16/12/2007 19:55:05 »
Quote
You mention Jodrell Bank as an example, but you don't mention what frequency this 0.1 minute of arc is for - that is a key feature.
I built  this in,assuming  2cm wavelength or 15GHz - but that's a bit optimistic; it would be more like 2 minutes to the first zero if you needed to get the sidelobes low enough; you can't splash the odd megawatt in awkward directions! Thanks Pumblechook - your figure looks more correct!  We should do better than JB with beamshaping, nowadays, I should hope but it's a serious task to avoid losing a chunk of your power due to pointing and focusing errors.
A-S, Your idea of a synthetic aperture - using multiple ground stations (?)- is likely to have large side lobes and you can't afford that, in this case. Interferometry is fine for resolving distant structures but that isn't  really the same problem as directing all the power in one place.
I'm not sure the maximum power which can be produced at this frequency these days - a few kW, probably. The Amplifier could be fairly efficient as it would only involve cw. Combining multiple amplifiers is a popular technique but the losses mount up as you double up and double up. You would not do better than 50%, tho' ; double your power supply!

As far as choice of frequency is concerned, lower frequencies require bigger and bigger antennae and I am sure that mechanical considerations would favour the smallest possible; hence 10GHz+ would be my choice. Higher frequency means  easier focusing  because the beamwidth is limited by diffraction.
The power flux is pretty huge - I had not even considered the absorption in the reflectors - they would be pretty red hot! Both reflectors would have to be of similar size. Wow - a 1km dish floating up to the sky; what sort of windage would that represent? You could use it as a kite for the first 15km. Perhaps that's part of the solution - tow it to the stratosphere first.
One thing bothers me about this idea, however. If it were feasible, why hasn't someone made a very small scale, lightweight model yet? Power or efficiency wouldn't have  been a problem.  Are there any published examples of 'radiation levitation'?
 

Offline thebrain13

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« Reply #31 on: 16/12/2007 22:17:54 »
Figuring out ways to get things into outerspace is irrelevant, we allready know how to do that. Figuring out ways to get things in outerspace cheap is the issue, because if we dont do that, we will always remain stuck on earth.

The idea of a space elevator is a million times better than any idea that blasts an object up there using whatever type of fuel. The reason is because all the physically push it up there with this or this idea would be impractical even if you could do it. Since we know it would cost tons of money.

A space elevator on the other hand would be diffifcult to build, but it would be  fantastic if you could. It would allow space exploration to be cheap. And that would benefeit all mankind.
 

another_someone

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« Reply #32 on: 17/12/2007 03:15:05 »
Figuring out ways to get things into outerspace is irrelevant, we allready know how to do that. Figuring out ways to get things in outerspace cheap is the issue, because if we dont do that, we will always remain stuck on earth.

Agreed - but that is what I thought we were discussing.

The idea of a space elevator is a million times better than any idea that blasts an object up there using whatever type of fuel. The reason is because all the physically push it up there with this or this idea would be impractical even if you could do it.

That is a very glib statement.

Since we know it would cost tons of money.

And a space elevator wont?

The point is, whatever system we use will not be cheap, but we want to move to maximise the re-usability of components, and minimise fuel usage.

There is a minimum amount of fuel you cannot get away from using, and that is the fuel that represents the energy required to move a mass from Earth into orbit.  What we want to avoid is to minimise the cost of having to carry more fuel than we need, by not having to lift the fuel with the launch vehicle (i.e. to try an make sure that most of the fuel used remains on the ground, while most of the fuel used is only that which is used to lift the launch vehicle and payload itself.

A space elevator on the other hand would be diffifcult to build, but it would be  fantastic if you could. It would allow space exploration to be cheap. And that would benefeit all mankind.

The space elevator would not be cheap to build (ok, that is a one off cost); but neither would maintenance of it be cheap - so there remains ongoing cost.  There are, as I have said, also serious safety issues concerning it, and what is more, you could only build a very few of them (and they would all have to be situated exactly on the equator).  Even the issue of how you deal with the precession of the Earth's rotation has somehow to be managed.
 

another_someone

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« Reply #33 on: 17/12/2007 07:17:46 »
The momentum change (impulse) which is available a burst E of em energy (whatever the frequency) hits a reflector is 2E/c.  That's not a lot. It would mean only 2.5 Ns from 1kWh of energy(that's 3.6MJ) - that would accelerate 1kg to 2.5m/s.

I make the numbers:

2 x 3.6x106/3x108 = 2.4x10-2 not 2.5x100

Makes something of a slight difference.
 

lyner

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« Reply #34 on: 17/12/2007 11:54:28 »
Yes, you are right!
Owch. Back to the drawing board.
 

lyner

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« Reply #35 on: 17/12/2007 11:59:27 »
Space elevator:
Quote
(and they would all have to be situated exactly on the equator). 
Why? The tether would just sway around a bit (very slowly) would that matter? You could adjust the length, continuously, to get the period right.
 

another_someone

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« Reply #36 on: 17/12/2007 13:51:20 »
Space elevator:
Quote
(and they would all have to be situated exactly on the equator). 
Why? The tether would just sway around a bit (very slowly) would that matter? You could adjust the length, continuously, to get the period right.

The whole point of a space elevator is that it is tied to a geostationary satellite, which means a satellite in equatorial orbit (just like all of our present communications satellites).

All the suggestions I had seen for a space elevator previously, as far as I recollect, made the assumption it was tied to a geostationary satellite.

As I understand it, the reasoning for this is:

a) Assuming the geostationary satellite is sufficiently massive, it can retain the overall structure under tension, which is much easier to manage than trying to maintain internal rigidity against all the forces that will be acting of the structure otherwise.

b) Any object at the space end of the elevator will be in natural orbit, and so it makes docking and undocking from elevator terminus a fairly easy manoeuvre - otherwise you have to rapidly move from a natural orbit around the Earth (which is what most satellites will naturally be approximating to) to match the unnatural orbital speed of the docking station at the space end of the elevator.

As for adjusting the length of the tether - even a 1% change in length would mean you would need somehow to cope with a massive amount of material to manage.  What are you going to do with a space one or two kilometres of extra tether?  You could dig a 1Km deep hole beneath the base to push it down, or you could look towards a telescoping tether.  Telescoping, while it is the most flexible mechanism, would now mean you are dealing with a hollow structure, and one that has different diameters along its length in order to telescope one bit into another.

Whatever mechanism you use for altering the length, it has to be able to maintain integrity under all the forces applied to the elevator.  Furthermore, this change in length has to be in response to external forces - what happens to all of the vehicles that are half was up the elevator (or, maybe even worse, have just got on the bottom of the elevator that is now descending)?

As for swaying around a bit - how?  Are you going to put a massive gimbal on the base of the tether (that is again a lot of force to apply to the pivots on the gimbal), or are you simply going to flex the whole structure (and thus the structure cannot be rigid)?
 

Offline thebrain13

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« Reply #37 on: 17/12/2007 21:41:55 »
When an elevator moves up the tether, the centripital force counteracts the gravitational force. The elevator gets the centripital force from the earths angular momentum. In otherwords yes the elevator requires "fuel" to travel up the tether, but that fuel is the earths angular momentum, which we have plenty of. These other designs use conventional type fuels.

And if we get good at building these we could design a pulley system that could lift any object into outerspace without applying any fuel at all (minus the earths angular momentum)

Also, when an elevator moves upwards, the tether applies a force which puts it into orbit. If you blast anything else upwards you would have to apply additional energy to put it into orbit, or accelerate it past the escape velocity.
 

another_someone

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« Reply #38 on: 17/12/2007 23:37:00 »
When an elevator moves up the tether, the centripital force counteracts the gravitational force. The elevator gets the centripital force from the earths angular momentum.

The point where the centrifugal force of the Earth's rotational speed matches the force of gravity is at an altitude of 35,786 km, which is why all our geostationary satellites are parked at that altitude.

If a satellite were to gain sufficient centrifugal force at ground level to lift off from the Earth, then none of us would be sitting here, but we would all be floating out in space.

 

lyner

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« Reply #39 on: 18/12/2007 12:07:43 »
However you get up into orbit you still need to supply enough energy to add to the potential of your craft. However, using a mehanical system, you only need to supply this plus enough to overcome friction losses. This would mean an efficiency well over 50%, compared with the efficiency of a rocket system which is very low.
Why all this worry about forces and controlling the position? The structure - tether plus space station would be massive and control could be achieved easily by a large mass, secured beyond the station with an adjustable length of tether. This would advance or retard the phase of the orbit to keep the tether more or less vertical. Any side to side wobble would be irrelevant. The time constant for any control would be in the order of many hours and would involve very little energy use.
Power would be supplied for all these functions by electric cables.

Of course, in addition to getting strong enough materials, there would be the problem of getting the thing installed. I suppose you would have to start with a big satellite in orbit an, somehow, trail a line with a vehicle on the end. This would have to ' fly' the end down to the Earth's surface. I haven't yet figured out the forces involved. Has anyone got a link discussing the practicality of the idea? It could involve a lot of initial energy input.
 

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« Reply #40 on: 18/12/2007 14:05:34 »
Not sure I would 'fly the end down' post construction.

Start with a very massive satellite (something that makes the current ISS seem puny, and that puts all discussion of resupplying the space station into a position of superfluity), and place this in geostationary orbit.  Then start building the then down from there.  As the tether is being built down, move the satellite itself to a slightly larger orbit, so as to retain the centre of mass in the geostationary orbit.

The problem starts to happen when you reach the upper regions of the atmosphere, as at that point you will start to experience weather, and this will be applied along a massively long single ended lever.

Ofcourse, by this time, you have already had to move about 36,000 Km of structure into space, which one would expect to include both structural components and electrical conductors (and having to transmit electricity over 36,000Km would amount to something that exceeds the longest power line laid down anywhere on this planet, and it would have to be a very thick power line in order to minimise resistance).  Incidentally, running power (at least hundreds of kilowatts) along a conductor of 36,000Km would generate quite a substantial magnetic field as well.
 

lyner

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« Reply #41 on: 18/12/2007 18:32:49 »
As far as I can see, the best thing would be to send a 'thin'  tether, first and then haul up the parts for a thicker one, once the system is up and running. The CM of the whole thing would, as you say, need to be at the geostationary distance so the final space station would have to be pretty massive - or  on the end of a long extension beyond the geostationary distance.

This would be an unbelievably massive project involving serious quantities of energy investment but, if you really want to get a lot of people and things up there and away, it could be the most economical arrangement. You could imagine it taking generations of engineers and  hundreds of conventional launches to get it built.    Materials from the Moon might be cheaper to use for that high orbital position.
 

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« Reply #42 on: 18/12/2007 19:50:39 »
I would worry not only about the energy requirements, but the requirements for raw materials for a structure that could wrap itself around the circumference of the Earth one and a half times.

Bearing in mind also that for any civil engineering project, the final structure represents only a small percentage of the supporting infrastructure required to build it.

To build this kind of structure I would guess would require a significant percentage of the Earth's available resources, and as you say, it could take many generations of engineers to build it.  As you say, we could maybe mine a fair amount of the material from the Moon, or the asteroid belt; but that then requires us to build up interplanetary mining and materials processing facilities of sufficient capacity first.

Personally, I think the project itself would be a white elephant, but the very act of constructing it would create such massive investment in space infrastructure that it may actually provide the impetus for a substantial shift in the way we operate in space.  Maybe the building of a 36,000Km vertical bridge may not be of that much value, but to prove we were able to build any structure of that size in space would allow us to consider building planet sized space ships.
« Last Edit: 18/12/2007 19:57:09 by another_someone »
 

lyner

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« Reply #43 on: 18/12/2007 22:26:16 »
This tether idea is growing bigger before our very eyes. It may, in fact, not need to be particularly big. If it is to be strong enough to support itself in tension, it would depend upon the material of which it is made- not on the thickness, although some taper might help in this respect.
 

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« Reply #44 on: 18/12/2007 22:48:34 »
This tether idea is growing bigger before our very eyes. It may, in fact, not need to be particularly big. If it is to be strong enough to support itself in tension, it would depend upon the material of which it is made- not on the thickness, although some taper might help in this respect.

It needs to perform two functions.

It needs to provide traction in order to be able to lift the car up into space (or control its descent from space).  This means it needs to be strong enough to hold the car (even against any weather, or other forces it may encounter), but must also provide a surface against which traction can be applied.

Secondly, it must deliver power to the car to facilitate its lift.  The most obvious way seems to be to deliver electrical power, but the other alternative would be simply to provide a mechanical lifting system.

If we are looking at a mechanical lifting system (such as a pulley and rope), then in theory we could do away with any permanent structure altogether; we could simply drop a flexible which rope down from 36,000Km, attach it to the top of the car, and start lifting.

One problem we will have with any tension based system (whether permanent or transient) is that as the car is pulled up, so the satellite will be pulled down, and no matter how heavy the satellite is, it will still need to have the energy caused by the reduction in orbit to be recompensed, and that energy must be obtained from somewhere.  It may be possible that the energy could be replaced in situ from solar energy, but it would have to be a pretty massive solar panel that will supply enough energy to do that.
« Last Edit: 18/12/2007 22:51:03 by another_someone »
 

Offline thebrain13

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« Reply #45 on: 19/12/2007 00:14:29 »
anothersomeone, as long as the centripetal force pulling on the tether as a whole is greater than the gravitational force pulling it down, you wouldnt need to "resupply" the counterweight with energy, the earths angular momentum would do that by itself, granted you didnt try to pull up too much weight at one time.

And what is your basis for saying it would take up a significant portion of the earths resources?  thats a rather glib statement if you ask me. You do know this cable is not going to be as wide as a stadium right? The current design suggests a cable 4 inches across. Its estimated that a space elevator able to pull up 20 tons at a time could be built in 3 years, and cost under 10 billion.

The secret that makes it possible is the newly discovered carbon nanotubes that are very light and very strong. This material is now being produced in the tons by firms in japan and the u.s.

Also once the structure is in place you could build a pulley system that would allow you to pull an object up without applying any energy outside the earths angular momentum. Because remember the centripetal force pulling upwards is greater than the gravitational pull downwards(im talking about as a whole not on the surface of the earth(angry face pointed at anothersomeone)), that means if you had more weight on one side of the moving cable than the other, the extra weight would cause any object to move up, all by itself.
 

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« Reply #46 on: 19/12/2007 02:07:23 »
And what is your basis for saying it would take up a significant portion of the earths resources?  thats a rather glib statement if you ask me. You do know this cable is not going to be as wide as a stadium right? The current design suggests a cable 4 inches across. Its estimated that a space elevator able to pull up 20 tons at a time could be built in 3 years, and cost under 10 billion.

That would amount to about 29,000m3 of carbon - more so if one has a pulley system, with multiple ropes.  I suppose in the whole scheme of things, 60,000m2 of material is not that horrendous.


The secret that makes it possible is the newly discovered carbon nanotubes that are very light and very strong. This material is now being produced in the tons by firms in japan and the u.s.

In other words, we have no long term experience in how this material ages, under stress, when constantly flexed (as it would be through a pulley system), under the radiation of space, or under the thermal environments it is likely to be placed under.  We do not even yet know the toxicity of nanotubes.

http://en.wikipedia.org/wiki/Carbon_nanotube#Strength
Quote
Carbon nanotubes are one of the strongest and stiffest materials known, in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 GPa. Since carbon nanotubes have a low density for a solid of 1.3-1.4 g/cm,[17] its specific strength of up to 48,000 kNm/kg is the best of known materials, compared to high-carbon steel's 154 kNm/kg.

Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tube undergoes before fracture by releasing strain energy.

CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional or bending stress.


http://en.wikipedia.org/wiki/Carbon_nanotube#Defects
Quote
As with any material, the existence of defects affects the material properties. Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%. Another form of defect that may occur in carbon nanotubes is known as the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on the weakest segment of it in a similar manner to a chain, where a defect in a single link diminishes the strength of the entire chain.

Clearly, with some time to develop good manufacturing practice, one could reduce the defect rate to any arbitrary level, but how does one protect the material from developing defects in the field (through bending stresses, or because of radiation damage)?

Also once the structure is in place you could build a pulley system that would allow you to pull an object up without applying any energy outside the earths angular momentum. Because remember the centripetal force pulling upwards is greater than the gravitational pull downwards(im talking about as a whole not on the surface of the earth(angry face pointed at anothersomeone)), that means if you had more weight on one side of the moving cable than the other, the extra weight would cause any object to move up, all by itself.

OK, I am still trying to get to grips with what you are talking about.

Firstly, it seems the requirement is that the endpoint of the cable would be well above 36,000Km, since up until 36,000Km the centrifugal (as I understand it, only centrifugal forces pull upwards, and centripetal forces pull downward) will be less than the pull of gravity (for an object in geostationary orbit), but above 36,000Km, the centrifugal forces for an object that is geostationary will exceed the pull of gravity.

I am still trying to get my head around how you intend to use the extra centrifugal force to lift something off the ground?  The use of counterweights, where both weights are being pulled down, would work (if the weight that wants to come down is the greater than the weight wanting to come up - except the idea of this system is to take more up than you bring down).  The idea of counterweights, where the weight at the top wants to go up, and the weight at the bottom wants to come down, does not seem to make sense to me (unless you are going to let the upper weight fly out into space, and pull the lower weight up as it leaves?), so whatever it is, I think I will need a more detailed explanation.
 

Offline thebrain13

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« Reply #47 on: 19/12/2007 05:50:45 »
The pulley system would be a large oval running the entire length of the tower, on either side of the oval would be two wheels that would allow a large cable to move around the pulley in a big oval, like a conveyor belt. The cable wouldn't necessarily have to be made out of carbon nanotubes since they dont bend well, but you could probably use them by bending them over a large area say ten miles wide at the top and bottom. Also if you decided to not use carbon nanotubes you could alleviate the pressure on the less strong cable by creating many smaller conveyor belts, and then they could transfer their stress directly onto the underlying tower.

Now if you can picture my pulley device, I'll explain how it can lift objects into the air. To ramp up the affect, imagine that the tower was taller than 36000 km, lets make it 100,000 km. Although thats not entirely necessary it would make things more efficient. Now with this design, if you loaded up relatively equally waited cargo in somewhat regular intervals, the tower would "magically" lift up into the air, seemingly all by itself. The reason is because the cargo on the top of the pulley would pull about twice as hard as the cargo on the bottom would pull down. Of course once the cargo reached the top, you would have to release it. If you didn't the object would give all the energy it gained in its ascent back to the system.
 

another_someone

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Space Station Economical Resupplies
« Reply #48 on: 19/12/2007 07:19:34 »
OK, so as I understand it, you will not be raising a few large cargo batches, but lost of small ones, with at least 2 lost of cargo (probably far more) on the system at once.

The system will have to be restarted with a substantial input of energy every time you cannot fill it with sufficient cargo.  Since the system depends on running continuously, it cannot be allowed to stop at the top to unload, so the cargo must be unloaded while it is still moving, and the empty car then come down the other side.

You cannot simply leave an empty car to travel up, since when at the bottom, it will present too light a load to the cars above, and cause them to accelerate to too high a speed, and when at the top, it will present too little pull on the cars below.  If you want to shut the system down, it will have to be a gradual process, with cars becoming gradually lighter, and then maybe even replacing cars with dummy lightweight cars (without load) until you can finally remove all the cars from the system (which may be necessary for maintenance, or in times of threat to the system - e.g. bad weather, or incoming meteor that may pose a risk).

The journey time will inevitably be quite considerable, since the system cannot run very fats, not least because it needs to be loaded and unloaded while still in motion; but it will still have to travel 100,000Km to reach the end station.  Even if one allow for 100Km/hr (which will be quite a high speed to try loading and unloading with), that is still a 1000hr journey.

Having reached the space terminus, you are then well above any Earth orbit you will be wanting to use (OK if you are building an interplanetary craft, but too high for satellites, etc.).  You will also be travelling too fast to be able to slot into the very high orbit (since it is this excess speed that is allowing you to create your lift).

I would still love to know what the fail-safe systems are in the event of a cable failure?  Even if you can arrange that the cable fails when there are no cars on it (but failure does not always happen according to when you wish it to happen), that cable is still holding the very large terminal satellite in position, and the cable snapping will cause that satellite to go flying off into space.
 

lyner

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Space Station Economical Resupplies
« Reply #49 on: 19/12/2007 12:01:07 »
Materials: this is a very long term project, of course; carbon nanotubes would have been well proven - or some alternative - by the time this thing was built. The whole thing depends upon this and we can only discuss the rest of the problems if we assume someone has found a strong enough material. Every fifty years, something significant  will come along; watch this space - if you have the time.

Energy and Force: These are different - as we must remember. O Level Mechanics rules here.
The structure itself will just provide a force - the tension in the tether is just there to allow the vehicles /  lifting cables or whatever to 'push down'. The work done (force times distance)  is achieved by energy input from the Earth's resources.  This is just the same as when you lift a brick with your arm - you don't talk about the Earth's surface contributing to the energy.
The thing that makes this system attractive is the fact that the tether moves down very little, compared with the upwards movement of the load.  The proportion of energy  lost in moving the massive satellite down is tiny (could be zero, if you choose), compared with what is used in moving the the small vehicle up. Compare this with the  rocket engine alternative which uses vast amounts of energy to push out its propellant at high speed.
A little energy will be needed to adjust the tether length, now and again, but the force times distance is very little, as the distance is a few km not 40Mm.

Centrifugal / centripetal; back to O level again.
We were hit (literally) if we used the word 'centrifugal' when at School. Yes, of course, when a string is whirling round with a conker on the end, there is tension and a force inwards and outwards. BUT the 'centrifugal' force is only there because the centripetal force is constraining the conker to move in a circle and providing a radial acceleration.  When you are in orbit, there is only one force acting on you. That force is gravity and it is inwards - keeping you accelerating in a circle.  - it is a centripetal force. If you switch off gravity or cut the conker string, there is no motion outwards; in both cases, the motion is tangential (Newton I).
If you cut the tether just below the space station, it will fall towards the Earth because of the weight of the (mainly lower) parts.

Long or short cables: Having a cable loop, as in a conventional lift, is very attractive. It balances out the vehicle weights. You would only need to pay for the payload lift. One long cable would certainly be very hard to control; I could imagine all  sorts of problems with longitudinal standing waves being set up which would produce extra stresses.  A series of short loops sounds much better; transferring between loops would not be a major problem but you migh need to synchronise transfers from one cable to another so that each loop was balanced. It would depend on the nett and tare weights. Only the cars with life support facilities would need to have much mass.
There is an alternative to cables and that would be to use electric motors with regenerative braking; all the descending cars would be generating power towards the lifting motors.  This would make it much easier to stabilise the loading of the tether because you could control the speed / acceleration of each car, actively damping out longitudinal waves .

Safety:  A major factor, of course. If you are thinking in terms of collision with debris then you could deal with a 'direct hit' on the cable by a large object by having a number of cables, spread out. They could be linked, at intervals, by horizontal ties. If one cable is severed then the others could take up the tension and the structure would survive. A major repair job, not a rebuild, and much less risk of any loss of life.
The space station crew would be quite safe, even if a single tether were used. They would remain in orbit but would need a bit of rocket power to adjust their position.
The elevator would be less hazardous than the present shuttle system - that's brown trousers every time it lands!
If a car became detached or the cable was broken there could be a serious problem when near the Earth but, for the majority of the journey, it would end up in some an of obit (elliptical) and would have a chance of being rescued. The falling bit of tether would present a bit of a hazard, I admit, but it would 'fall' to Earth in a region near its base - Newton I rules, again. It would not wrap itself around the Earth.

 

The Naked Scientists Forum

Space Station Economical Resupplies
« Reply #49 on: 19/12/2007 12:01:07 »

 

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