Space Station Economical Resupplies

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Offline Freeholder

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Space Station Economical Resupplies
« on: 10/12/2007 22:02:44 »
Space Station in orbit resupplies.  That is my topic.  I wonder if a hydrogen balloon would lift itself into the stratosphere?  And if it were weightbearing with,say, water to be given to the space station, could a snatching device be made?  I have something in m,ind something like how the fast mail strains operated by the post office in years gone by snatched from the station a sack of mail.

Now, since the orbits the space vehicles are moving in require a speed to stay up, That is the vehicles need a at around 24,000 mph (escape velocity I think) to stay in orbit, could a hooking device that would grab the supply without tearing it apart be imagined.

Or can another way be found that would provide supplies less expensively that the millions $
the supply socket costs now.  Arthur C Clarke proposed that a bucket convey system made up of the carbon fullerines could reach a stationary station.

And then, whatever is happening with the rail guns that can shove a vehicle to the escape velocity? Anybody hear anything? 

What do you think?

 
Perfesser

My Army colonel nicknamed me The Perfesser in Taif, Saudi Arabia, and it stuck  also for the Army Evacuation Hospital in Fort Lee, Virginia until I retired in 1991.  So I guess it will do.

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lyner

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« Reply #1 on: 10/12/2007 23:14:38 »
http://science.nasa.gov/headlines/y2000/ast07sep_1.htm
The Space Elevator.
I fancy this system most of all. Once it has built ("once"!) it is the cheapest way to get up into Low Earth Orbit.

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another_someone

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« Reply #2 on: 11/12/2007 01:04:21 »
Aside from cost, I really am not comfortable with the safety aspects of a space elevator.

The problems with using a pickup from a balloon (although future technologies may overcome this) is that the present altitude record for a balloon is just short of 52Km.  In general, there is an inevitable conflict of requirements, in that a balloon can only where there is an atmosphere to give it lift, yet a space vehicle pickup, if it is to limit the amount of heat it has to tollerate, as well as the inevitable loss of energy associated with that, wishes to stay above the atmosphere altogether.

What is certainly very feasible is that some means is got to take a supply ship into low space orbit (no more than 100Km - the altitude achieved by Burt Runtan's SpaceShipOne), and then get picked up from there and pulled to a higher orbit by some sort of space tug of the kind you are talking about.

My own preferred fanciful way of getting a vehicle into low orbit is on a focused microwave beam.  The big problem with getting spacecraft into orbit is not the energy required to get them there (that is an unavoidable cost, whatever technology you use), but the cost of carrying the extra fuel to carry the fuel to get you there (this is a problem with any vehicle that must carry its own fuel with it as it rises through the atmosphere).  By using a microwave beam on which the space vehicle can be lifted up, the fuel for the lift remains on the ground, so the only actual fuel that is used is that required to actually lift the vehicle itself.

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lyner

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« Reply #3 on: 11/12/2007 18:14:23 »
Quote
Aside from cost, I really am not comfortable with the safety aspects of a space elevator.
Why? Do you think people will run into it?
If it is ever made, there will be no other craft ,at height, likely to hit it (everyone would go by elevator) and aircraft could easily miss it - the same as they miss existing obstacles. We would, of course, need a huge exclusion zone.

A high altitude balloon would have a tiny payload and would have to dump its gas - or have to support a tether with which to haul it down. Helium is costly in large quantities (and getting costlier, I believe).

The microwave beam is discussed, on occasions and is, I agree, attractive but what are the sums involved? How big an array is needed to produce a sufficiently narrow beam which wouldn't waste transmitted energy? Many wavelengths, to produce a small spot beam, for sure. The receive antenna would need to be correspondingly large to catch the beam and could produce a lot of drag - energy which would need to be supplied. Some tradeoff would need to be reached.
What actual propulsion system would be used? A reaction system would still need to have something to force out of the back - the atmosphere would run out at altitude, and a source of propellant would have to be carried - extra weight to carry.
I did once read of a proposal for a net, kept aloft by microwave radiation pressure, which could be used as an alternative to satellites for transmitting signals. This would have been kept up by radiation pressure - a very weak force.
The system does have a strange fascination about it, though.

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Offline Pumblechook

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« Reply #4 on: 11/12/2007 20:15:10 »
Proposals I have seen for a space power station beaming power to Earth users a 2 km diameter parabolic dish in space and 4 sq km of rectennas panels on Earth working at 2.45 GHz..rather large!   Working the other way..2 km would be 20 times the diameter of the biggest dish we have on Earth... 400 times the area.   The dish would have be an accurate curve within 12 mm (1/2 inch).

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another_someone

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« Reply #5 on: 11/12/2007 20:17:21 »
Quote
Aside from cost, I really am not comfortable with the safety aspects of a space elevator.
Why? Do you think people will run into it?
If it is ever made, there will be no other craft ,at height, likely to hit it (everyone would go by elevator) and aircraft could easily miss it - the same as they miss existing obstacles. We would, of course, need a huge exclusion zone.

Aircraft could easily miss, they also could easily miss the WTC buildings.

Given that this structure reaches out into space, you will also have to take account of collision with high velocity space objects (e.g. a meteor).  Even low altitude anthropogenic space debris becomes an issue.

There is also simply the risk of catastrophic failure.  This is a huge building, more like a bridge than a skyscraper, and could easily be subject to failure through error of design, poor maintenance, or endless other causes.  You will ofcourse have to factor in the effects of vibrational resonance, etc.

If it does fail, you will not only have risk to people on the elevator, but to the population subjected to falling debris beneath the elevator.

How to you undertake the continual inspection and maintenance of the structure, and have you factored in the costs for this?

A high altitude balloon would have a tiny payload and would have to dump its gas - or have to support a tether with which to haul it down. Helium is costly in large quantities (and getting costlier, I believe).

No, I don't believe helium is practical, but hydrogen is.  Since the major disasters involving hydrogen balloons before WWII, our understanding of managing the safety of hydrogen has improved significantly (after all, the space shuttle launch system contains enough of the stuff), and I don't see it as any less safe than methane, or numerous other materials we feel perfectly comfortable handling these days.  Hydrogen is lighter than helium, and so would actually be a preferable gas to use in that respect, and far more available.

I don't thing the balloon idea will work (as I said, the problem as I see it is that it could not get high enough to allow a space tug to pick it up - and as you say, how you bring the balloon down for reuse is another matter), but I think the issue of availability of helium is a red herring.

The microwave beam is discussed, on occasions and is, I agree, attractive but what are the sums involved? How big an array is needed to produce a sufficiently narrow beam which wouldn't waste transmitted energy? Many wavelengths, to produce a small spot beam, for sure. The receive antenna would need to be correspondingly large to catch the beam and could produce a lot of drag - energy which would need to be supplied. Some tradeoff would need to be reached.
What actual propulsion system would be used? A reaction system would still need to have something to force out of the back - the atmosphere would run out at altitude, and a source of propellant would have to be carried - extra weight to carry.
I did once read of a proposal for a net, kept aloft by microwave radiation pressure, which could be used as an alternative to satellites for transmitting signals. This would have been kept up by radiation pressure - a very weak force.
The system does have a strange fascination about it, though.

My own idea (probably has been thought of elsewhere) would not be a mass reaction propulsion system at all, but an electromagnetic reaction system.  The idea is that the base of the vehicle would have an array of antennae that receive the incoming microwaves, and would use the power to generate an outgoing microwave that would interact with the incoming microwave to create a repulsive force (essentially magnetic levitation at a distance, and at high frequency).

Yes, it would need a large cross sectional area (not that the cross sectional area of the space shuttle's fuel tank is exactly small), but the amount of drag created is highly dependent of speed, and if one is not looking at a mach 3 flight (at least not while in the denser part of the atmosphere), then the drag could be kept to manageable levels.

The ground array may need to be large, but it need not be a single antenna, but divided amongst lots of small, widely separated, antenna, creating a synthetic antenna of large size.  The ground antenna itself could be synchronised by using a low power locator transmission generated in the vehicle onto which the desperate ground signals are locked.  This would even allow the transmitting antennae to be located aboard ships deployed in the middle of the ocean, with the ships subjected to the motions of the sea, but remaining locked onto the locator signal.  It would also compensate to some extent  for any refraction caused by atmospheric effects, since the locator signal would be subject to the same refection.
« Last Edit: 11/12/2007 20:24:41 by another_someone »

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Offline Pumblechook

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« Reply #6 on: 11/12/2007 20:26:22 »
---The ground array may need to be large, but it need not be a single antenna, but divided amongst lots of small, widely separated, antenna, creating a synthetic antenna of large size. ---

No..  That technique can be used in radio astronomy to increase resolution but this does not increase the overall gain.    You get lots of wasteful sidelobes. Getting the phasing right with widely space antennas is very difficult. 

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another_someone

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« Reply #7 on: 11/12/2007 20:34:09 »
---The ground array may need to be large, but it need not be a single antenna, but divided amongst lots of small, widely separated, antenna, creating a synthetic antenna of large size. ---

No..  That technique can be used in radio astronomy to increase resolution but this does not increase the overall gain.    You get lots of wasteful sidelobes. Getting the phasing right with widely space antennas is very difficult. 

The phasing should not be an issue - that is the purpose of the locator transmission from the vehicle itself - if the individual transmitters lock their phases on to the same incoming transmission from the locator signal, then their phases should be correctly maintained.

Sidelobes may be wasteful, but exactly how wasteful?  Even if you lose 50% of the power that way, that is substantially better than the current fuel usage for space launch.

I was at least as concerned about losses due to atmospheric heating caused by the microwaves, but I would still expect that to be highly preferable to anything we have now.
« Last Edit: 11/12/2007 20:38:12 by another_someone »

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Offline Pumblechook

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« Reply #8 on: 11/12/2007 20:52:18 »
A phasing system could be made to work I suppose.  Sideloads would be considerable I would have thought.   I am fimiliar with stacking yagi antennas and you have to have them within 1 to 2 wavelenths of one another...very close.,,to avoid excessive sidelobes.     Think about just 2 antennas widely space..  Say each with a one degree beam.  The two signals combine at a point where the path length is the same  but also at thousands of other points where the path difference is N Wavelengths within half a degree of the bearing of main beam.  It is very similar to the interference patterns caused by letting light pass through two slits.  I suspect also that  individual paths would not be stable enough either in terms of not travelling in straight lines and also arriving in phase due to slight changes in the speed of transmission as the atmosphere varies in pressure and humidity from point to point.  The inonosphere can cause probs ...may cause variable phase shifts ...and polarisation changes...Faraday rotation.
« Last Edit: 11/12/2007 20:59:30 by Pumblechook »

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Offline Pumblechook

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« Reply #9 on: 11/12/2007 21:08:22 »
As ragards atmospheric attenuation.  This is normally negligible below 30 GHz but heavy rain can be a problem above 10 GHz.   
Maybe once or twice a year our satlellite TV signals disappear for a few minutes during very wet weather.  They need to fade by about 6dB (1/4 of normal signal) for the receiver to fail.  As part of an another thread on microwave ovens I did an experiment.  3 sheets of wet paper (total thickness 0.3mm when dry) was enough to make the receiver fail when put over the horn of the Low Noise Block..frequency was 12 GHz. 
« Last Edit: 11/12/2007 21:12:55 by Pumblechook »

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another_someone

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« Reply #10 on: 11/12/2007 21:37:23 »
A phasing system could be made to work I suppose.  Sideloads would be considerable I would have thought.   I am fimiliar with stacking yagi antennas and you have to have them within 1 to 2 wavelenths of one another...very close.,,to avoid excessive sidelobes.     Think about just 2 antennas widely space..  Say each with a one degree beam.  The two signals combine at a point where the path length is the same  but also at thousands of other points where the path difference is N Wavelengths within half a degree of the bearing of main beam.  It is very similar to the interference patterns caused by letting light pass through two slits.

This is true with two antennae, but you could easily have 30 antennae spread around.  Yes, there is a cost involved, but we are talking about totally reusable components in an industry that regards millions of dollars as small change.

  I suspect also that  individual paths would not be stable enough either in terms of not travelling in straight lines and also arriving in phase due to slight changes in the speed of transmission as the atmosphere varies in pressure and humidity from point to point.  The inonosphere can cause probs ...may cause variable phase shifts ...and polarisation changes...Faraday rotation.

All this is true, but the whole point is that (assuming the locator transmission used a frequency not far off the upbeam transmission frequency), then this should effect equally the location signal, so the upbeam would be automatically adjusted for any phase shifts caused by atmospheric conditions (you could even, with a bit more work, even adjust for for different attenuations along different signal paths by measuring the attenuation of the locator signal coming back down the same signal path).

If you wanted to go further, each upbeam from each ground transmitter could itself send a unique identification signal either encoded in the power beam, or alongside it; and the locator signal could also encode the health (relative attenuation, etc.) of each identification signal, so giving the ground stations even more information to allow them to adjust their power signals.

And ofcourse, the more transmitters you use, the less significant is an adverse signal path from a transmitter.
« Last Edit: 11/12/2007 21:48:38 by another_someone »

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Offline Pumblechook

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« Reply #11 on: 11/12/2007 22:03:48 »
Keeping 30 transmitters locked in frequency and phase (with path compensation) and (less critical) amplitude is not  going to easy.  On the other hand constructing huge steerable parabolas would not be easy either.   

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Offline syhprum

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« Reply #12 on: 11/12/2007 22:20:26 »
Should we perhaps reconsider whether we need a space station ?, does it serve any scientific or political purpose ?.
I understand about 100 billion pounds has been spent on it so far and many worthy scientific projects sacrificed to build and maintain it.
syhprum

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« Reply #13 on: 11/12/2007 22:41:08 »
Keeping 30 transmitters locked in frequency and phase (with path compensation) and (less critical) amplitude is not  going to easy.  On the other hand constructing huge steerable parabolas would not be easy either.   

You are not keeping 30 transmitters locked in frequency and phase.  You are keeping one transmitter locked in frequency and phase, and reproducing that 30 times.

Each transmitter is locked onto the locator signal, and is totally ignorant of the other 29 transmitters.

The only case where there may need to be some recognition that there are 30 transmitters is in the computer within the vehicle itself that is analysing the health signal sent alongside the power beam, and so sending information back to each ground transmitter to say that station X is 5% attenuated, while station Y is 7% attenuated, so station Y should boost its power a bit to compensate.  Not rocket science (well, it is, but it is not [:)]).  Similarly, the internal computer within the vehicle can asses the transit time of each incoming signal, and tell a ground station to alter its phase, which it will do in relation to the common clock signal that the ground station receives from the locator beacon.

The whole thing merely depends on time signals encoded in each transmission, and maintaining time accurate to within about 1/8th of a cycle of the power transmission carrier signal (which should be possible with atomic clocks).

The biggest headache is making sure you carefully calculate the signal delays in the transmitters and receivers of the control signals to ensure that you can carefully calculate the actual time time in transit of the signal (although, in reality, the actual time in transit does not matter so much as relative time in transit, so as long as you can make sure that all the signal delays are the same, it does not matter exactly how long they are).

Given the size of the target we are looking at, I would guess we could use a wavelength of anything around 2cm to 10cm; which would correspond to a frequency just above one hundred megahertz.  We could, I would imagine, comfortably go to longer wavelengths yet (the base of the vehicle could be enlarged to 10 metres or more - the space shuttle external tank is over 27m is cross section).

This would allow an atomic clock, with an accuracy of better than 5 nanoseconds per day, to remain within 1/8th of a cycle (within 1.25 nanoseconds) for about 6 hours.
« Last Edit: 11/12/2007 23:40:14 by another_someone »

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another_someone

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« Reply #14 on: 11/12/2007 22:46:54 »
Should we perhaps reconsider whether we need a space station ?, does it serve any scientific or political purpose ?.
I understand about 100 billion pounds has been spent on it so far and many worthy scientific projects sacrificed to build and maintain it.

Not really the question that is relevant.


The relevant question is whether the dozen or so space launches we have each year is sufficient for our future space needs (whether that be to supply a space station, whether it be for interplanetary exploration, whether it be to launch communication satellites, whether it be to develop zero gravity industrial processes - whatever opportunities space may bring).

Whether specifically a space station is something we want or not is one question, but the real question is whether we wish any significant long term presence in space at all.  Even maintaining our present level of space launches, although feasible with current technology, is expensive and not particularly environmentally friendly (let alone, not particularly safe).  I don't see what we are doing at present (let alone any ramping up of capacity, in whatever way) to be sustainable in the long term; so we either stop almost all extraterrestrial activity, or we have to think of better ways of going about it.

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lyner

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« Reply #15 on: 11/12/2007 23:40:47 »
Have you actually done the sums to calculate the strength of what is surely no more than light pressure?  Or, it could be magnetic force involving the Earths magnetic field?

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« Reply #16 on: 12/12/2007 01:17:10 »
Have you actually done the sums to calculate the strength of what is surely no more than light pressure?  Or, it could be magnetic force involving the Earths magnetic field?

Very difficult to accurately calculate without knowing lots of issues concerning how focussed the beam is, what the conversion efficiencies are, etc.

Looking at commercial electromagnets, they seem t be able to light about 2.2 tonnes per kilowatt.  So, for a 100 tonne vehicle (including payload), it would require about 50 kilowatts of power to lift.

Clearly, we are dealing with oscillating electromagnetic fields rather than stationary magnetic fields, and we are dealing with fields that are focused over a great distance rather than locally generated.  We are also dealing with fields that are trying to react against induced fields rather than fields relying on paramagnetism.

One linear motor that I can see in use for maglev train applications claims about 5000lbs of thrust (453Kg) for 1.125MW of power.  Since linear motors used induced fields for their reaction, they would be closer to ideas here than DC electromagnets.  This would imply that you would require 248MW of power to lift 100 metric tonnes.

Ofcourse, any power over and above that required to life the vehicle amounts to upward acceleration.  A 10% increase over that required to lift the vehicle would amount to an upward acceleration of about 0.9ms-2, which would amount to about 2mph upward acceleration per second.

I cannot say that I would have the least idea where to start calculating all the parameters without a lot of experimentation.

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Offline daveshorts

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« Reply #17 on: 12/12/2007 10:11:14 »
Going back to the original question, one reason that this is not done is that lifting you up even to 200km doesn't get you into orbit, you then have to accelerate up to about 24 000km/hr or you just fall back down again.

This requires significantly more energy than lifting you up there. I am not saying that you wouldn't need a much smaller rocket as it would probably save most of the first stage on the rocket which is usually the biggest, but it doesn't save as much energy as you would think.

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another_someone

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« Reply #18 on: 12/12/2007 15:26:24 »
Going back to the original question, one reason that this is not done is that lifting you up even to 200km doesn't get you into orbit, you then have to accelerate up to about 24 000km/hr or you just fall back down again.

This requires significantly more energy than lifting you up there. I am not saying that you wouldn't need a much smaller rocket as it would probably save most of the first stage on the rocket which is usually the biggest, but it doesn't save as much energy as you would think.

Yes, it requires energy, but my understanding is the manner in which the energy is supplied.

My understanding is that if the tug that pulls the supply ship is massive enough (and it would have to be very massive to be meaningful), then most of the extra energy (maybe even all of that energy) could be supplied simply from the kinetic energy of the tug.  Then, the tug will lose some kinetic energy, and will need to recover that energy before it picks up the next supply ship, but with enough tugs available, each tug could pick up only one supply ship a year, and spend the rest of the year slowly picking up its lost kinetic energy (e.g. through some slow solar powered mechanism - e.g. solar sails).

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Offline daveshorts

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« Reply #19 on: 12/12/2007 17:15:42 »
Although can you imagine the actual manoeuvre, you would have to go from stationary to 24000kph in the stretch of your tether.  Even if you have a long tether, you then have a huge amount of elastic energy to dissipate somehow without causing the payload to collide with the tug...

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« Reply #20 on: 12/12/2007 18:47:37 »
Although can you imagine the actual manoeuvre, you would have to go from stationary to 24000kph in the stretch of your tether.  Even if you have a long tether, you then have a huge amount of elastic energy to dissipate somehow without causing the payload to collide with the tug...

Although I can imagine various ways to mitigate that, I do agree it is a non-trivial concept at many levels.  All I was suggesting is that one can imagine a theoretically possible way of achieving it, not that it would be easy.

I would imagine that much of the 'elastic energy' would best be taken up by a winch (much like the slack taken up on a fishing line when a large fish first bites, where the line is first allowed to spool out, before being reeled in). Ofcourse, managing the friction on the winch could be a bit challenging.
« Last Edit: 12/12/2007 18:51:55 by another_someone »

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Offline BolianAdmiral

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« Reply #21 on: 12/12/2007 21:14:55 »
The space elevator is perhaps the most ridiculous idea yet. Not only would it look just silly as all hell, I don't care how super-strong they make that thing... if you have ANY construct that thin, in relation to the size of the Earth and the Moon, it'll just snap like a twig, end of discussion. I really don't see why anyone at all supports the idea of a space elevator... it's just so incredibly silly.

The best way to go about resupplying the ISS would be to have a set of unmanned and computer-controlled "tanker" orbiter shuttles, that could be launched into space and fuel or resupply the station, via docking, and then return to Earth to be prepped for the next run.

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lyner

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« Reply #22 on: 12/12/2007 21:45:46 »
You can't use the same principles used for maglev (for distances of a few cm) to a vehicle at a distance of many miles. What sort of field pattern would you need and what size of electromagnet would you need?
You would have to use em waves, if you want to concentrate the field and that leaves you with a minuscule force, explainable in terms of momentum change of photons (fractions of a Newton for kW/sq metre power density). Where would the extra factor of a million come from and what would hou do with all the wasted energy (reflected or dissipated)?
This problem is, actually, a simple application of Maxwell's equations, which also predict light pressure and I don't think there's a valid way round it.

The space elevator is perhaps the most ridiculous idea yet. Not only would it look just silly as all hell, I don't care how super-strong they make that thing... if you have ANY construct that thin, in relation to the size of the Earth and the Moon, it'll just snap like a twig, end of discussion. I really don't see why anyone at all supports the idea of a space elevator... it's just so incredibly silly.

The best way to go about resupplying the ISS would be to have a set of unmanned and computer-controlled "tanker" orbiter shuttles, that could be launched into space and fuel or resupply the station, via docking, and then return to Earth to be prepped for the next run.
Well, that's me well and truly dumped on!
However - why should anything be snapped like a twig? It would bend - by many km, of course. The size, related to the earth is irrelevant - the earth is not going to be jerking about in any surprise fashion; it will be going round steadily, just as it is now. It only has to be strong enough to handle manmade impacts. The Twin Towers were built on the cheap and only fell down because they caught fire - not due to the initial impact.
The elevator system has been considered seriously but, of course, it awaits some suitable material for the tether.

Where do the 'tankers' save you any energy? You need just the same amount of fuel, however you care to lift your payload of fuel for the other rockets to use. The re-usable stages on the shuttle already do the same job as your tankers but could, of course, do a better job if they could be flown back to land safely rather than needing to be fished out of the sea and reconditioned.

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Offline BolianAdmiral

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« Reply #23 on: 13/12/2007 00:45:26 »
The Earth rotates at about 1,000 MPH... there will come a point in altitude, where the spped will have an effect on even the strongest structure that is that thin, compared with the Earth's surface... it will bend to a degree, and then just be sheared or snapped or pulled off.

Tankers or other unmanned vehicles would be better, because they'd be cheaper, resusable, won't look uber-silly, and are just plain more practical than building an elevator to space. The very mental visual of a space elevator being built is so comical to me, because of its impracticality. I mean, just the cost alone would be as sky-high as the elevator.

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« Reply #24 on: 13/12/2007 01:09:38 »
You can't use the same principles used for maglev (for distances of a few cm) to a vehicle at a distance of many miles. What sort of field pattern would you need and what size of electromagnet would you need?
You would have to use em waves, if you want to concentrate the field and that leaves you with a minuscule force, explainable in terms of momentum change of photons (fractions of a Newton for kW/sq metre power density). Where would the extra factor of a million come from and what would hou do with all the wasted energy (reflected or dissipated)?
This problem is, actually, a simple application of Maxwell's equations, which also predict light pressure and I don't think there's a valid way round it.

OK, I shall go through some of the issues as I see them (some ideas have come in and gone out of my concept at various times, but discussing them does make clearer what needs to be in, and what can remain out - not that it proves it will work, only that if it is to work, what are the ways it needs to address its problems).

It may be that a simple beam will provide no more pressure than you suggest, with most of the radiation remaining within the reflected beam - which is why (that and to minimise problems associated with high power radio beams bouncing all over the place, into people's homes, etc.) from the beginning the requirement was that whatever solution was to be achieved had to bring uncontrolled reflections down to negligible levels (can never be zero, but there has to be very strict limits placed upon the power dissipated in that way).

One of the differences between a simple beam bouncing off a mirrored surface and a maglev system is that, however leaky, the maglev system nonetheless forms part of a magnetic circuit, where most of the energy remains within the circuit.  Thus, it does not matter how much of the magnetic field is converted into acceleration since, so long as the field remains contained within the circuit, ultimately 100% much be converted into acceleration, because there is nowhere else for it to go.  Ofcourse, this is an ideal position, and in the real world there will always be losses in the system, so some proportion of the magnetic lines will be wasted, and not converted into acceleration at all.  Nonetheless, it does demonstrate that the only way to go is to create a circuit from the EM waves that mimics the magnetic circuit of the maglev system.  If we achieve this, then we solve the problem of unwanted reflections, and the problem of most of the wasted energy.

So, the question is how do we lock the EM wave into a circuit?

One way in which this is done in laboratory conditions is, if one has to mirrored surfaces that are exactly parallel to each other, and introduce an EM wave between them, then you can create a standing wave that simply bounces betwixt one mirror and its opposite number.

OK, lets step out of the laboratory and back to the launch site.  There is no way that the mirrors on the underside of the launch vehicle are ever going to be, let alone remain, parallel to even one, let alone all, of the 30 ground stations transmitting the lift beam.  So we need to modify the solution somewhat.

The problem we have is that a simple mirror will only reflect an EM wave in the plane of the mirror itself, and will not reflect it along the incoming path unless the plane of the mirror happens to be perpendicular to the incoming path.  I believe that there are, and has been since the 1960s, meta-materials that will allow an incoming EM wave (only over a narrow bandwidth, but that is not a significant limitation for us) to reflect and incoming EM wave precisely along the path it came in, no matter in what direction the incoming EM wave is coming from.  Since this is happening in the bulk of the meta-material, it is not actually happening across a plane at all, so the orientation of the beam with respect to a plane is of no consequence.

If one excludes thermal losses and wave scattering for the moment, then in theory, if one surrounds the lift beam transmitting aerial with this meta-material, and covers the base of the launch vehicle with this meta-material, then one can generate a perpetual standing wave (irrespective of location and orientation of the reflectors involved) that will last until all the energy contained within the EM wave has been converted into upthrust.

Ofcourse, one aspect of this that I have presently ignored, is that a standing wave, in order to be maintained, must not only be accurately reflected the incoming path, but the points of reflection must be an integral multiple of the half wavelength of the EM wave.

The problem is that, even if one could maintain such an accurate distance between one ground station and the launch vehicle, as soon as the launch vehicle begins to move, that distance will have changed, and the standing wave will collapse.  To do that with 30 ground stations and the launch vehicle would be inconceivable, even in a transient manner.

That means either that the point of reflection within the meta-material has to be constantly changing, in order to maintain a standing wave, or we need another solution to maintaining a circuit.  It may be possible to design a meta-material that somehow maintains this standing wave by changing the virtual point of reflection within its body as the distances change, but I do not know enough (or virtually anything) about the meta-materials and their design to make any assumption that this may be possible.

So the other alternative is to maintain a feedback of the EM wave without resorting to standing waves.  This would still require the use of the same meta-meterial on the base of the launch vehicle to reflect back the incoming EM wave, but the ground station would use a very different philosophy.

The ground stations transmitters, rather than being surrounded by this meta-material, would be surrounded by receiving dipoles, that will receive the reflected wave, rectify the wave, and then send that energy back into the newly synthesised upbeam that is phase controlled as I described in the earlier posts.

I believe that in a lossless system (an ideal world), this would convert 100% of the transmitted EM beam into uplift of the launch vehicle, because there is simply nowhere else for the energy to go.

Ofcourse, the real world is not lossless.  I have taken no account of thermal losses that may occur in the meta-material when it reflects the wave.  I have taken no account of thermal losses, or secondary emissions, that may occur in the ground station as it receives in reflected wave and turns that energy around into the upbeam.  I have taken no account of scattering of the wave when in transit between the ground station and the launch vehicle.  No doubt there are other losses also that I have not taken account of.

The real question, that I have no idea of, is whether these losses can be kept to a low enough level to ensure that most of the energy of the wave (or, at least, an adequate percentage of the energy of the wave) is converted into uplift rather than lost in these various real world inefficiencies.
« Last Edit: 13/12/2007 02:17:42 by another_someone »

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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.

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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).

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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!!

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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 »

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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 »

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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'?

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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.

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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.

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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.

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

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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.

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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)?

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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.

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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.


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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.

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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 »

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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 »

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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
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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
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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.

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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.

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another_someone

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« 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.

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lyner

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« 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.