So, I went looking for the International Academy of Astronautics report
mentioned in Edwards' talk, to see what they wanted for it. Did a search
on the full title, "Space Elevators: An Assessment of the Technological
Feasibility and the Way Forward" and a link came up to this site
http://www.virginiaedition.com/sciencedeck/spaceelevators.shtmlbut also another link almost the same as above, to a pdf. I don't know
whether it is an error, but it seems the report is downloadable.
So I read through it, and I am rather baffled. There is a lot of good
solid science and engineering displayed, many issues thoroughly addressed,
but a few really glaring omissions, pretty much completely overlooked.
One is that they admit up front that fibres have not yet been demonstrated
long and strong enough to be adequate, though they anticipate that will
be solved soon - the report is published in 2013, with data up to late 2012,
but in his talk this summer, Edwards claims that work this year has yielded
the necessary performance. However, while discussing this, the report casually
mentions that while individual fibres have the needed strength, all attempts
to wind them into a yarn have shown they slide past each other, resulting in
less than a tenth the required strength for the composite. This seems like
a pretty big obstacle.
Closely related to that, the model proposed employs a broad, low density
cable, with many discrete strands, under the assumption that damage from
space debris will be inevitable, and so the cable is spread out like a
ribbon to reduce the damage to a small fraction of the total cross section.
The damage is then expected to be repaired by (most likely robotic) climbers
which monitor the health of the cable regularly. However, there is no
discussion whatsoever of how these repairs can be effected. Considering
they still don't know how to make the component strands of the main cable
bond to each other to make a usable rope, The much more difficult problem
of adding in a splice to a broken rope, while hanging in situ, seems
like it would be a huge problem to solve. Note that for a repair to be
effective, the resulting cable must be as smooth and flexible as it was
before, so that extra wear is not inflicted on the repaired section by
passing climbers, and crucially, the tension of the repaired strand must
match its initial value, so the load is still shared equally among the
strands; otherwise the repair is of dubious value.
Then there is the issue of atmospheric challenges to the cable, such as
the jetstream, which make the notion of solar powered climbers with
huge low mass panel arrays too vulnerable to deploy. They have a few
optional solutions, one of which is to have the elevator proper sit
entirely above 40km altitude. How they deal with the bottom 40km is
enough to make one doubt the soundness of the entire exercise. I could
spend a day listing all the profound problems with it, but you'll just
have to read it to believe it.
