Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Freeholder on 10/12/2007 22:02:44
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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?
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http://science.nasa.gov/headlines/y2000/ast07sep_1.htm (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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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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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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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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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.
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---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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---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.
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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.
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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.
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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.
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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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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.
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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.
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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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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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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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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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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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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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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.
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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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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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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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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.
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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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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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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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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).
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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°
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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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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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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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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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Yes, you are right!
Owch. Back to the drawing board.
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Space elevator:
(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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Space elevator:
(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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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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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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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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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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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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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.
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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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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.
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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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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
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 sp² 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/cm³,[17] its specific strength of up to 48,000 kN·m/kg is the best of known materials, compared to high-carbon steel's 154 kN·m/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
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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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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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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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.
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you wouldnt have to travel up it at 100km/hr the whole time. You can dock at a different speed than you travel up. The pulleys at the bottom loading area would move very slow compared to the pulleys at the top of the elevator. I dont see any reason why you couldnt get it to move 1000km/hr.
I'd also like to see what the military could do about stopping space debris.
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How big would the coils of wire need to be to generate power in space and transfer it to the earth using cordless electromagnetic induction systems . The same type of system used to charge tooth brushes and very soon mobile phones ,powertools etc.
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How big would the coils of wire need to be to generate power in space. . . . . .
You can't use the same method for power transfer at such a distance. The toothbrush charging system is, essentially, a transformer. The system used for charging toothbrushes is a particularly inefficient form of transformer because you need a complete path of iron core to contain all the magnetic flux for good efficiency. (Useful - but inefficient).
I still reckon that linear motors would be a much better propulsion system - virtually no limit on top speed!
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The tooth brush system maybe but they will soon be bringing out a new and much more efficient system with one central unit attached to the fusebox of your house and car with a range that allows you to charge all rechargeable apparatus in your home. You will no longer need to plug things in. As soon as you enter your house they will start charging provided they have a small electronic component installed. I heard about the new system on the beeb the other day ,I dont know exactly how it all works but they gave a demo using a working system.
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It is rubbush that. As another poster has pointed out...any such system will be inefficient and in an age where we are supposed to conserving energy it makes no sense.
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Yes that has to be either rubbish or you heard wrong. If you don't have a good magnetic circuit, you lose most of your energy. You either need a good magnetic contact or a good electrical contact for any significant amount of power transfer. End of.
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There is a company who make a charging pad where the item (modified or made compatible in the first place) to be charged is just plonked on it but I have been unable to get an efficiency figure which I would think is not very good.
I would have though that a 'docking station' where spring loaded contacts are used is just as convenient and does not require a pick up coil in the item and will be lossless.
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It's probably easier to get a good electrical contact - involving very little pressure - than it is to get a good low reluctance contact in a magnetic circuit. It may be less of a problem if you use a high frequency for your induction loop system but it sounds a bit of a gimmick rather than anything worth having..
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yeah your right the misus said it was a station that you had to place the item on and have an adapter fitted to the device.
She even rememebered its name. Splashpower
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They dress it up as if it was new technology that they have invented.. Really dates back to Faraday's age.
http://www.splashpower.com/
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"Up to a centimeter away from the SplashPad the energy transfer drops very slowly." The inductive coupling operates at a frequency of, "a few kilohertz," Halfpenny said.
This is crazy.. The conversion AC-DC-RF...coupling...RF-DC will lose power along the chain. The overall efficiency will be poor.
Better to go AC - DC in one simple efficient step.
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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).
For a stationary object (i.e. one that feels no force), the forces acting upon it must be balanced. For a satellite in orbit, it feels no force, and therefore the forces acting upon it must be balanced.
You could take the relativistic approach, and suggest there is no force acting upon the satellite at all, because gravity is not a force but a distortion of space, and so the satellite is merely in freefall. Alternatively, you could say there is a balance between centrifugal and centripetal forces. It makes no sense to me to say that gravity is the only force acting upon it, since those within the satellite cannot detect an imbalance in forces.
As for cutting the conker string, ofcourse there is a motion outwards. Ofcourse it is also true that the conker keeps going in a straight line. You are looking at the same thing from at once a polar coordinate system and a rectilinear coordinate system - and each statement is true with reference to their own coordinate system.
From the point of view of someone who is above the Earth, and does not want to get further away from the Earth, then the polar coordinate system may be a more natural way for him to look at his predicament than a rectilinear coordinate system.
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.
I doubt it will be that simple.
It is true that the tether will start to fall to Earth, but as its fall accelerates, it will start to burn up in the atmosphere (when that happens will depend on many factors, and it will certainly not happen in the initial stages of the fall), and when the lower portions of it have burnt off, depending on how much tether is left, it may continue to fall (and burn up as it hits the atmosphere), or may fly off into space (most likely its momentum by then would be sufficient to continue sending the remainder of the tether Earthwards).
It is very unlikely to fall straight down, if for no other reason than the stiffness of the tether will make this unlikely, as would any winds it encounters. Once it starts to topple in one direction or the other, it will probably continue further in that direction - but I agree that it wont as such wrap itself around the Earth.
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.
I am sure there must be ways of applying damping to the cable loop to make that less likely.
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.
That would add considerable weight to the whole mechanism, add vibration at the transfer points, and each of the transfer points will inevitably involve energy loss. The increased complexity will also lead to far higher risk of failure.
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.
That was my original notion of what this was about, before the idea of the cable loop was explained; but that now means you need massive conductors to take the current (even for fairly low currents, because of the length of the system, you need to keep resistance to an absolute minimum - you may even want to look at superconductors).
You also have to think about how the electrical energy is converted to mechanical energy. You could have a linear induction motor built with coils build into the tether, but that now adds more weight to the tether. Alternatively, you transfer the power to the cars, and have motor drives in the cars, but now you need a way to transfer that energy across (a 36,000Km live rail system?).
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.
Could you not damp longitudinal waves simply by building discontinuities in the materials used to construct the tether, and let those longitudinal waves be dissipated as heat at the discontinuities (it would require that the tether then be efficient at carrying away that heat, since there will be no atmosphere for most of the distance into which you can dump the heat generated). Ofcourse, those same discontinuities will also represent potential weakpoints in the structure, and these have to be accounted for in the load distribution management of the design.
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.
How large an object are we talking about?
If we are to protect ourselves against an object that is 100m wide, we need a cable separation that is significantly wider that 100m, we need to make sure that there are sufficient numbers of cable to be able to survive the loss of one cable (i.e. 2 cables will not be sufficient, since the loss of one cable would represent an instantaneous increase of 100% load on the other cable, plus the shock). Also, one has to look not only at the loss of one cable, but the loss of all cables in any given line that an object may travel along through the cables.
Since the idea is that the load of the cars is spread across all of the cables, if one takes the simplest example (I am not sure if the simplest example is sufficient in terms of load sharing) of 9 cables distributed as if in a 3 x 3 matrix, but with the middle cable removed, and each cable being 110m from its neighbour (thus allowing a 100m object to hit 3 cables in a row, and the remaining 6 cable distributing the weight between them), that would require the car to be at least 220m x 220m in size. If you are looking to survive a hit by a largest object, or a hit by multiple objects in a cluster, then the size has to increase accordingly. It is possible that you could improve survivability a bit better by not distributing the cables in a regular grid, and so reduce the number of cables in a direct line with each other. On the other hand, you may also want to think about ricochets.
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.
It depends on where they are.
If you are talking about a 36,000Km orbit, then I agree, the risks are slight. If you are talking about a 100,000Km orbit, then in fact they are on a hyperbolic trajectory, and if the tether is cut, they will quickly go out of orbit unless massive amounts of rocket power (needed to shift a massive object) were used to decelerate them and/or reduce their orbital height by a massive amount.
Also, the loss of a shuttle, as tragic as it is, cost the lives of 7 people, and the loss of a single vehicle. A major failure of a large integrated system of this nature could lose a lot more than that (it is as much about distributing risk as eliminating it).
The elevator would be less hazardous than the present shuttle system - that's brown trousers every time it lands!
The present system is totally inadequate, but then it is also something that is only launched a few times a years, with massive checks and rechecks (which, as has been seen, if not done, will lead to disasters) every time it goes up. On the other hand, the system we are proposing will have to run continuously, without the ability to spend several months with the system grounded for checks; so the level of innate reliability must be orders of magnitude better because our ability to cope with problems during operation is less.
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.
Depends where?
Again, if we limit ourselves to a geostationary orbit, then all orbits below that are indeed elliptical, but if we extend ourselves above that (and given the angular speed we will be travelling at in order to remain geostationary) then we will be on a hyperbolic trajectory, and will not be going into any kind of orbit.
As you say, close to Earth (not only within the atmosphere), even if you end up in an elliptical orbit, if that orbit impinges upon the upper atmosphere then you will rapidly lose angular velocity and will fall to Earth (actually, more likely, burn up in the upper atmosphere).
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.
Why?
Why is the tether any different to a car - all matter, no matter what its mass, at a given altitude and given velocity, in the absence of atmosphere, will follow the same path.
It is true that the tether may cover a range of altitudes, rather than a (almost) single level of altitude that the car would occupy, so that would mean that different parts of the tether would be pulled into different orbits, causing to to rotate - but each section of the tether will be effected by exactly the same forces as any of the cars would be.
Also, what happens if the tether snaps with the cars attached? In this case you cannot say that what happens to the cars is different from what happens to the tether, because they are still locked together.
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1. When you cut the string there is no FORCE outwards. If there were a force, the conker would accelerate away. It doesn't; it moves away, of course, but at its original tangential speed. There is no point introducing Einsteinian ideas to a simple bit of classical Newtonian mechanics. Einstein, himself wouldn't have because he understood both the old and the new. What 'outwards' force is there when in orbit? Nobody detects one when on a satellite - everything 'floats' or falls towards the Earth. The satellite (and contents) is ACCELERATING towards Earth, constantly. If it were 'stationary' it would fall (accelerate) right towards the centre of Earth. It is not in equilibrium; forces are not balanced so Newton 1 doesn't apply. This scenario is precisely the one which Newton was thinking about when he came up with his laws of motion and gravitation. Oh yes, and the 'reaction force', which has to be there (Newton 3) is pulling the Earth towards the Satellite and the acceleration is g (Newton 2). Get your basics right or your conclusions can be wrong.
2. A series of short loops would allow different speeds - fastest when the car is outside the atmosphere and slowing down as it gets to its destination. Clutches / springs would allow for the speed changes. The result of a breakage or a fault would be much worse for one long cable rather than many short ones.
3. You mention resistive losses for an electricity supply. What sort of friction forces do you think would operate on a 7000km cable loop with the necessary guides and pulleys? Electrical power distribution tends to be less lossy than mechanical forms and it doesn't wear out. The motors for the cars would not need a lot of power - a few kW would produce a small, constant, acceleration which would soon have a car traveling at very high speed. You wouldn't need railway traction powers for this job. At a few 100kV, the current could be less than a hundred A - a trivial resistance problem for aluminium cabling. How many surface transports use cable rather than electric traction? (Situation is different, I admit,)
4. The probability of meeting a 100m object is small and the collision cross section of the tether is vastly less than that of the Earth - which would pose a major problem! Why do you need 200m wide cars? Small cars would go up each line. And about multiple cables being hit. Objects of 10m (still much bigger than we see very often) would be very unlikely to hit more than two lines if they are arranged in a large enough circle. It would be mostly space between the cables (think spider's web). As we don't know the actual statistical size and speed distribution of debris, we can't come to any real conclusions but 'large' objects do not land on Earth often. Some data might be useful.
5. As you say, the shuttle system is very unsafe. You want the tether system to be totally safe. It is an unfair comparison. The equivalent scenario to the whole tether being destroyed might be the same as that of a major crash of a fuel transporter rocket falling on a major city. Both are unthinkable but impossible to rule out completely. Enough redundancy, built into the tether structure, could reduce the probability of major catastrophic failure to near zero - just as with rocket system design. The tether would obviously be sited in mid Pacific / Atlantic so damage to the population would be low - a fuel tanker could, in principle, come down anywhere. I think the risks would need serious analysis rather than gut reaction.
6. For a broken tether, the space station would, by design, be very near the geostationary position, assuming it is more massive than the tether, (the whole basis of the system design) so there can be no doubt that it would remain near there, with or without the tether. In any case, control ballast which was further out or inside could be jettisoned in case of a disaster and things would happen very slowly to the flying portion of the structure; plenty of time to take action. The cars on that section would be relatively safe, also.
The lower part of the tether is very different from a car, once detached, in that it would be pulled down by the weight of the bottom portion - very quickly; at something like 10m/s^2 . The car, once detached, would be in an elliptical orbit, as stated before, and this orbit would only involve hitting the Earth for cars nearer the Earth. For most of the journey, the cars would be further away than this and their orbits would make them recoverable if they were detached. Most cars would be unmanned so they could would not matter.
Launching from an 'extended' section of tether, beyond the geosynchronous distance, doe not automatically put you into an escape condition. You would just be in a bigger ellipse, unless the tether was very much extended. That would not be part of my idea . Detached cars near the Earth would not have the same trouble as spacecraft re-entering; they would start off with no effective KE and their PE would gradually transfer to KE. This could be dissipated with a system of parachutes rather than needing the dreaded heat resistant tiles.
The falling tether would, of course have a lot of GPE (the top bits, at least) and this would end up as 'disaster energy' once it hit the ground. It wouldn't be hard to relate this to asteroid impact, actually; perhaps someone would like to do this for us? A nice after-Christmas Dinner activity. But,certainly, the smaller the tether, the less of a disaster.
George, this makes a change from usual. I am nearly always the one pouring cold water on your ideas. This time it's the other way round.
I'm up for buying a few shares in this one, perhaps.
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2. A series of short loops would allow different speeds - fastest when the car is outside the atmosphere and slowing down as it gets to its destination. Clutches / springs would allow for the speed changes. The result of a breakage or a fault would be much worse for one long cable rather than many short ones.
3. You mention resistive losses for an electricity supply. What sort of friction forces do you think would operate on a 7000km cable loop with the necessary guides and pulleys? Electrical power distribution tends to be less lossy than mechanical forms and it doesn't wear out. The motors for the cars would not need a lot of power - a few kW would produce a small, constant, acceleration which would soon have a car traveling at very high speed. You wouldn't need railway traction powers for this job. At a few 100kV, the current could be less than a hundred A - a trivial resistance problem for aluminium cabling. How many surface transports use cable rather than electric traction? (Situation is different, I admit,)
Do you need guides and pulleys?
You need guides and pulleys on Earth because of all the environmental forces (e.g. winds) that would effect the car, but they do not exist in space, so if the cable is just pulling the car upwards, what is going to deflect its path? Why the guides?
You need pulleys at the end points, and maybe need pulleys or guides in the first 100Km, within the atmosphere – but do you really need them beyond that?
Bear in mind that if you do need guides and pulleys along the way, these are going to wear out, and you will have an impossible maintenance job constantly replacing them (assume a life expectancy of 10 years constant use, and 1 guide per 100m, so you will need 360,000 guides, and an average of 36,000 guides replaced per year, which is 100 guides per day replaced – and this for each cable).
I am not saying that 100A is a lot, but pass 100A down 36,000Km of aluminium (you could power it from both ends, and so bring it down to 18,000Km), it had better be very thick aluminium if you are not going to lose a fair amount of voltage on that trip. Then you also have to worry about what you do with the heat associated with that resistance – you have no atmosphere to dissipate that heat, so you have to conduct the heat away, the full 18,000Km distance.
On the other hand, if you do use superconductors, the lack of atmosphere to carry heat becomes an asset rather than a problem. Nonetheless, it will be a lot more weight than the light weight nanotube structures people were talking about.
4. The probability of meeting a 100m object is small and the collision cross section of the tether is vastly less than that of the Earth - which would pose a major problem! Why do you need 200m wide cars? Small cars would go up each line. And about multiple cables being hit. Objects of 10m (still much bigger than we see very often) would be very unlikely to hit more than two lines if they are arranged in a large enough circle. It would be mostly space between the cables (think spider's web). As we don't know the actual statistical size and speed distribution of debris, we can't come to any real conclusions but 'large' objects do not land on Earth often. Some data might be useful.
The largest meteor to land in recent years (1920) was about 3m across when it hit Earth, but a lot of that ablated before reaching the ground – but I agree, probably I have about a factor of 10 overestimate – 10m objects might be sufficient.
Yes, arranging them is a circle is fine, but a circle is a hypothetical shape, and in reality you would be arranging them in a polygon. But, if you are going to have 9 cables, are they really going to be more compact arranged in a 9 sided polygon than arranged as a 3x3 matrix (slightly offset to minimise alignment)?
The reason why one would wish to have a single car linked across all the cables, rather than one car per cable, is because if you did have a single cable strike, if you had one car per cable, you would be guaranteeing setting that car adrift, whereas spanning the cars across the cables would allow the car to survive the cable strike.
5. As you say, the shuttle system is very unsafe. You want the tether system to be totally safe. It is an unfair comparison. The equivalent scenario to the whole tether being destroyed might be the same as that of a major crash of a fuel transporter rocket falling on a major city. Both are unthinkable but impossible to rule out completely. Enough redundancy, built into the tether structure, could reduce the probability of major catastrophic failure to near zero - just as with rocket system design. The tether would obviously be sited in mid Pacific / Atlantic so damage to the population would be low - a fuel tanker could, in principle, come down anywhere. I think the risks would need serious analysis rather than gut reaction.
Given the takeoff path of the shuttle, over the sea, the tank cannot fall onto land (which is exactly what happened when the challenger suffered a catastrophic failure on launch).
But my concern about the tether system, where is seriously differs from the shuttle, is the duty cycle. The shuttle spends most of its life in maintenance, and very little of its life actually in service (this is expensive, but increases safety significantly – and despite this, disasters happen). The system we are talking about, once constructed, will not be taken down, and will be almost 100% duty cycle, and what little maintenance it will have will be in situ rather than in a workshop.
6. For a broken tether, the space station would, by design, be very near the geostationary position, assuming it is more massive than the tether, (the whole basis of the system design) so there can be no doubt that it would remain near there, with or without the tether. In any case, control ballast which was further out or inside could be jettisoned in case of a disaster and things would happen very slowly to the flying portion of the structure; plenty of time to take action.
The problem was trying to address two different concepts – thebran13's concept of a terminal satellite at 100,000Km, and the more conventional concept of a terminal satellite at around 36,000Km.
With a terminal satellite just inside geostationary orbit, and a dummy mass about 100Km above it, so the the initial break would allow the terminal satellite to be pulled up to geostationary orbit by the dummy mass, before releasing the dummy mass and parking in geostationary orbit.
The cars on that section would be relatively safe, also.
The lower part of the tether is very different from a car, once detached, in that it would be pulled down by the weight of the bottom portion - very quickly; at something like 10m/s^2 . The car, once detached, would be in an elliptical orbit, as stated before, and this orbit would only involve hitting the Earth for cars nearer the Earth. For most of the journey, the cars would be further away than this and their orbits would make them recoverable if they were detached. Most cars would be unmanned so they could would not matter.
I think you are missing a key point.
While you are right to be concerned about the differential gravitational effects, but remember that the different portions of the system are travelling at very different velocities (same angular velocity, but very different linear velocities). Thus, the lower portions of the system (this could be true even of portions a mere 1,000Km below the terminal satellite) will have a backward drag on the upper portions, both pulling it down, and reducing its angular velocity, and also causing a rotation of the system.
Launching from an 'extended' section of tether, beyond the geosynchronous distance, doe not automatically put you into an escape condition. You would just be in a bigger ellipse, unless the tether was very much extended.
Not my understanding.
It is not where you are that matters, it is where you are and what your angular velocity is.
Yes, being outside geostationary orbit, if your angular velocity was less than geostationary, could then put you in either a circular or elliptical orbit. The trouble is that you are beyond geostationary orbit, but still carry the angular momentum for the geostationary orbit, so you are carrying too much angular momentum to retain either a circular or elliptical orbit, so you will be taking a hyperbolic trajectory.
That would not be part of my idea . Detached cars near the Earth would not have the same trouble as spacecraft re-entering; they would start off with no effective KE and their PE would gradually transfer to KE. This could be dissipated with a system of parachutes rather than needing the dreaded heat resistant tiles.
Sorry, I don't understand this.
Ofcourse they have KE. You may be saying they have no KE in the vertical direction, but they certainly have KE in the horizontal direction, and this has to be accounted for in your calculations.
I assumed you had taken account of that KE when you spoke of them going into an elliptical orbit (because their KE is too small to take them into a circular orbit).
The falling tether would, of course have a lot of GPE (the top bits, at least) and this would end up as 'disaster energy' once it hit the ground. It wouldn't be hard to relate this to asteroid impact, actually; perhaps someone would like to do this for us?
But the tether would still have angular momentum, just as the cars would.
George, this makes a change from usual. I am nearly always the one pouring cold water on your ideas. This time it's the other way round.
Is that not the whole point – and idea is only worth while after you throw every problem you can think of at it, and it still survives.
I'm up for buying a few shares in this one, perhaps.
Not unless you really want to lose the money.
Even if the idea works, there are very few major infrastructure projects, even if they have proved a long term success, that have actually made money for the original speculators (just look at the channel tunnel – it would be a great success, if only it did not have to repay the the cost of construction).
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A number of issues - thank you.
Quotes take up too much room - I assume you are across this so you will remember what has been said.
1. I really wouldn't fancy a 70,000km loop of wire, flailing around with nothing to guide it. AS I, actually don't fancy the idea of doing it mechanically, in any case, then I don't want to go into too much detail regarding lots of small loops. I can see many advantages - not least , when some replacement sections are needed.
2. Electrical Power vs Mechanical: As soon as you get above the atmosphere, there is plenty of sunlight to provide electrical power at stations all the way up, so you may not even need long supply cables. The effective night is not that long, at distances of 10km+ and you could suspend operations, if necessary- maintenance time, perhaps. Once above 100km, or so, the cars could even be solar self-powered - present satellite panels provide several kW and that could be sufficient, as I have already said - low acceleration would be no embarrassment.
3. I don't understand the idea behind the very long tether system of thebrain - it wouldn't be in a naturally geostationary orbit so what would it do? I suspect it would just trail behind the geostationary main station - or even be attracted to it. It's not part of my system and needs some clarification.
4. I obviously meant that the cables would be arranged around a circle - what else could I have meant? Such an arrangement would have all the advantages of a spider's web (redundancy). Have one car in the middle, if you like but that would pull the lines together if there were much driving force. Un-tensioned cross links would provide the same safety function and cars could pass each other safely as they would be separated by tens of metres on separate cables.
5. You are right about the tanker crash scenario as long as launch methods are kept the same. Will it always be the case that orbital height is reached within a few hundred km?
6. In the event of the tether being actually severed, the 'drag, to which you refer, would have been there all the time. There would now be less and the outer portion would go outwards rather than inwards.
7. The trouble is that you are beyond geostationary orbit, but still carry the angular momentum for the geostationary orbit, so you are carrying too much angular momentum to retain either a circular or elliptical orbit, so you will be taking a hyperbolic trajectory.
No; forgetting the tether, if you are going a bit faster (which is all that would happen) your orbit would take you a bit farther out by the time you were 90o further around on the orbit - an ellipse. It's not angular momentum but KE that counts for orbits; KE + PE is a constant. Adding KE (i.e. going a bit faster) at the low point gives extra PE (height) at the high point. You need a lot of KE to make your escape.
8. When you are low down on the tether - near LEO position- your KE is much less than the KE of an normally orbiting object - that's what I mean by negligible effective KE. Instead of an orbit time of 90minutes it is 24 hours - that implies a tiny fraction of the normal KE (about 1/120 ) This is very relevant to the amount of energy which the vehicle would have to shed.
9. I am still tempted with it as an investment!
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the point of the longer tether, is it would allow you to ramp up the upwards force for an object that was moving up it, if it was shorter it would spend too much time where gravity is the stronger force. if you designed a pulley system, the higher the tether the more energy you could liberate, that would allow you to pull objects up faster, and harder, and give it a jump start to wherever you wanted to go.
just because the tether is outside of geostationary orbit, doesnt mean that it wouldnt rotate around the earth, it would still stick straight up. The geostationary satelite is not the guiding force. what determines if it stands or not is the overall force upwards verses downwards. (as long as friction is somewhat negligilbe)
Also the large mass or counterweight would be located at 36000 km up, not at the end of the longer cable. That would generate too much strain on the cable. However when you initialy built the structure the counterweight has to be outside of geostationary orbit, since most of the mass of the tether is located where gravity is the prominent force. In that scenario (a shorter tether) the purpose of the counterweight is to hold the cable up.
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just because the tether is outside of geostationary orbit, doesnt mean that it wouldnt rotate around the earth, it would still stick straight up. The geostationary satelite is not the guiding force. what determines if it stands or not is the overall force upwards verses downwards. (as long as friction is somewhat negligilbe)
You will, of course, have some calculations to back this up. It certainly isn't what conventional ideas of the energy and forces involved in orbits under and inverse square law would predict. Are you proposing a rigid tether, 70,000km long? If not, how can you prove that your system will work with a flexible line? Unless the velocity of the 'remote' end is high enough for circular motion (which it isn't), the mass will follow an elliptical orbit.
What does 'ramp up the upwards force' mean? Please use terms that actually mean something in the classical mechanics / dynamics sense.
I sense that you are drawing a parallel with throwing a stone out of the end of a pipe - or, possibly, the Aboriginal 'Woomera' principle. This is no good without a rigid system.