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Space Elevator An Impossible Dream?
bj8rn writes "Three months ago, the dreams of a space elevator finally seemed to be coming true after a successful test. An article in Nature, however, suggests that there's reason to be pessimistic. Ever since carbon nanotubes were discovered, many have been hoping that this discovery would turn the dream into reality. Pugno, however, argues that inevitable defects in the nanotubes mean that such a cable simply wouldn't be strong enough. Even if flawless nanotubes could be made for the space elevator, damage from micrometeorites and even erosion by oxygen atoms would render them weak. It would seem that sci-fi will never be anything other than what it is: a fiction."
Sorry for being slightly off topic, but as a non physicist, I've always wondered why the other seemingly obvious problems with such a device are never really considered problems. I am thinking of storm type winds blowing it off balance or making it resonate, the danger to aeroplanes, the disastrous consequences of breakage, etc. Why aren't these problems?
There is a slight difference with that: the acceleration takes place over the entire flight to orbit, not entirely on the ground. Instead of having a steady acceleration of at most a few G's, a station based on the ground "throwing" an object strait up would have all the acceleration before it left the structure, probably killing everything alive on board. Coming back down isn't so bad as you have a terminal velocity, and acceleration is limited to gravity, only 1 G. The only way a ground based structure could do a "throw" would be if the "throw" were not strait up, but rather at an angle very shallow to the earth's surface, giving the acceleration on a track over miles. The problem with that of course is that the total air resistance while leaving earth is far greater, meaning even more, instead of less, energy. No, overall if you want a steady non-rocket based acceleration into space, the space elevator is *still* more viable.
Does a line appended to your comment give your post meaning in and of itself, or only in relation to those without?
Imagine a spider on it's web; the web is torn at one point. The spider fixes it by producing more silk.
Imagine a small robot, even a nanobot, space elevator cable, made of many strands of carbon nanotubes. Imagine a way to pull carbon out of air and repair the cable.
A spider produces silk from the food and air it consumes; a nanobot could repair nanotubes in much the same way, by "breathing" carbon dioxide or pure carbon and doing repairs. Hell, it doesn't even need to MAKE carbon nanotubes, it could pick them up at "ground floor" and deliver them up the cable.
A self-repair system. No need to invoke convoluted biology and DNA.
Just because we can make fibres with enough tensile strength to withstand loads for this thing doesn't address the entire construction question. My understanding is that we only have FIBRES. They have to be joined in a matrix of something, and that matrix will not have enough shear to support the tensile strength of the fibres. In carbon fibre materials, for example, an epoxy resin is cured around the carbon fibre. The fibres, because of their orientation and strength make a very strong structure; materials such as unidirectional CF can be used to strengthen the structure along load paths. But if the shear limits of the epoxy are exceeded, the thing will break. A great example of this is a Formula One chassis. Enormously strong becasue of their carbon fibre/epoxy structure, but when the break they shatter. So how will the fibres in the cable be formed into a structure? Seems to me the limit will be the glue, not the carbon.
Eh, you're both right. Either we become extinct on this one lousy rock in the next century or two, or we expand out into space and basically become unkillable. After that all we need is time.
One line from Vernor Vinge's _A Deepness in the Sky_ that caught my attention was the almost casual mention that when the human race had expanded to a volume many hundreds of light years wide, "Earth had had to be recolonized from scratch 4 times" since the civilizations would last for a few thousand years and then self-destruct to totality.
Which is what really pisses me off about NASA. All we need for them to do, all we've _ever_ needed from them, was cheap and reliable access to LEO. Probes, stations, zero-g experiments, even the moon missions, it's all really super cool but we've got universities and companies and throngs of avid would-be astronauts who would do it if they only could afford to.
Dyolf Knip