Arg, my (in)famed ability to skip around a subject and assume everyone either knows what I do, or doesn't care, strikes again. I should have been clearer.
Density (and hence related stuff like bending/buckling resistance) is the "subtle engineering reason" (subtle, that is, compared to pure tensile strength) I was referring to, and I meant the context of general engineering. As you correctly point out, in this particular context, specific tensile strength becomes even more important.
In fact, IIRC, the 62GPa figure I'm throwing around above is for a material with a density broadly similar to CFRP, or maybe even lower, and assumes that the cable is tapered (thin at the bottom, thick in the middle (GEO), thin at the far end) to minimise loads. For something as heavy as steel, the number is really stupidbig.
With a space elevator, it all gets horribly complex, thanks to the interaction with centripetal acceleration and the counterweight cable (or mass). The gravity drop-off is, I think, only relevant in the sense that it defines how high the mid-point (where the cable is apparently "weightless") is - and even there, centripetal acceleration dominates once you get more than a few thousand km up.
For the purposes of argument, I picked the lowest possible number to illustrate how far we currently fall short, even against THAT target.
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branchandroot
no subject
Date: 2003-09-14 11:33 (UTC)Density (and hence related stuff like bending/buckling resistance) is the "subtle engineering reason" (subtle, that is, compared to pure tensile strength) I was referring to, and I meant the context of general engineering. As you correctly point out, in this particular context, specific tensile strength becomes even more important.
In fact, IIRC, the 62GPa figure I'm throwing around above is for a material with a density broadly similar to CFRP, or maybe even lower, and assumes that the cable is tapered (thin at the bottom, thick in the middle (GEO), thin at the far end) to minimise loads. For something as heavy as steel, the number is really stupidbig.
With a space elevator, it all gets horribly complex, thanks to the interaction with centripetal acceleration and the counterweight cable (or mass). The gravity drop-off is, I think, only relevant in the sense that it defines how high the mid-point (where the cable is apparently "weightless") is - and even there, centripetal acceleration dominates once you get more than a few thousand km up.
For the purposes of argument, I picked the lowest possible number to illustrate how far we currently fall short, even against THAT target.
(Aeronautical Engineering, Manchester 1994)