0 Members and 1 Guest are viewing this topic.
Aside from cost, I really am not comfortable with the safety aspects of a space elevator.
QuoteAside 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.
---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.
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.
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.
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.
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?
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.
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...
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.
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.
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.
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.
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.
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.
(and they would all have to be situated exactly on the equator).
Space elevator:Quote(and they would all have to be situated exactly on the equator). Why? The tether would just sway around a bit (very slowly) would that matter? You could adjust the length, continuously, to get the period right.
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.
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.
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.
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, 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.
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.
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.