The current state-of-the-art in high-tensile-strength steel has a strength of about 2GPa. Carbon fibre reinforced plastics are lighter than steel, but no stronger. (CFRP is popular for more subtle engineering reasons) More exotic materials like boron fibre composites might be able to do a bit better, but not much (and they cost a FORTUNE)
Whoa, whoa, whoa! NASA has misled you with its talk of a minimum tensile strength. Because all that strength is required for the structure to support *itself*, lightness counts. The correct figure of merit is not tensile strength, but tensile strength divided by density.
Taking your figure of 2GPa for both materials, and taking the density of steel as 8000kg m-3, the relevant figure is 250 kilonewton metres per kilogram. For CFRP, assuming a density of 2000kg m-3, then the figure is 1 meganewton metre per kilogram, four times higher than steel.
(note to readers: a gigapascal is just another way of saying 10^9 newtons per square meter)
So where weight is a paramount consideration, CFRP is not preferred to steel for subtle engineering reasons at all, but for the very unsubtle reason that steel could not support its own weight, and CFRP can.
Another common figure of merit, used to give a rough intuitive idea of how much weight a material can bear, is to divide the previous figure by the Earth's gravity at sea level, 10 newtons per kilogram, to arrive by how much of its own length a cable could support before breaking. For steel, using your numbers above, that's 25km, and for CFRP it's 100km, again four times as much.
Even though the gravity against which an orbital cable would have to support itself is less than sea level, decreasing rapidly with height, it's clear that neither of these materials is quite up to it, but it's equally clear that CFRP is closer to it than steel.
In short, I'm a LOOOOOOOOOONG way from being convinced on the materials front.
I'm with you there.
(Materials Science and Technology, Brunel University 1995)
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branchandroot
no subject
Date: 2003-09-14 11:04 (UTC)Whoa, whoa, whoa! NASA has misled you with its talk of a minimum tensile strength. Because all that strength is required for the structure to support *itself*, lightness counts. The correct figure of merit is not tensile strength, but tensile strength divided by density.
Taking your figure of 2GPa for both materials, and taking the density of steel as 8000kg m-3, the relevant figure is 250 kilonewton metres per kilogram. For CFRP, assuming a density of 2000kg m-3, then the figure is 1 meganewton metre per kilogram, four times higher than steel.
(note to readers: a gigapascal is just another way of saying 10^9 newtons per square meter)
So where weight is a paramount consideration, CFRP is not preferred to steel for subtle engineering reasons at all, but for the very unsubtle reason that steel could not support its own weight, and CFRP can.
Another common figure of merit, used to give a rough intuitive idea of how much weight a material can bear, is to divide the previous figure by the Earth's gravity at sea level, 10 newtons per kilogram, to arrive by how much of its own length a cable could support before breaking. For steel, using your numbers above, that's 25km, and for CFRP it's 100km, again four times as much.
Even though the gravity against which an orbital cable would have to support itself is less than sea level, decreasing rapidly with height, it's clear that neither of these materials is quite up to it, but it's equally clear that CFRP is closer to it than steel.
In short, I'm a LOOOOOOOOOONG way from being convinced on the materials front.
I'm with you there.
(Materials Science and Technology, Brunel University 1995)