Comments and alternatives, by Jim Cline, regarding the anchored
tether Space Elevator project proposed by Bradley Edwards, developed
under grant from NIAC between 2000-2002, and presented at ASCE's
Space 2002 conference:
- Effect on the KESTS to GEO potential project: Although the original reason for creating the KESTS to GEO form
of space elevator access to space from the ground, was to bypass
the extraordinary strength to weight limitations of potential
tether material when the earth tether space elevator concept was
originally formed long ago, the nature of KESTS to GEO now go
far beyond that of mere bypassing tether limitations. Nonetheless,
since the tether technology now seems nearly ready for the "bending
metal" stage, I recommend full steam ahead on Brad Edward's Space
Elevator concept project. The tether concept was given a grant
by NASA 2 years ago to proceed, while KESTS was not given a grant,
so the tether is ready to go! Let's go for it ASAP!
- Brad Edward's tether project uses a newly-developed carbon nanotube matrix ribbon for the
tether material. It is to be placed in GEO using 4 shuttle launches, enabling
a start for the tether cable to be emplsced to the earth surface
from GEO while the tether counterweight extends beyond GEO. Once
this is emplaced and anchored to surface platform, crawlers will
climb up the tether adding material to the tether, tacking it
as it goes along. The resulting completed tether would have a
taper in its crossectional area, 2.4 times as wide at GEO as it
is at the surface platform's site. A new type of high efficiency
laser on the ground will beam energy up to the crawlers to power
them, and then to power payload carrying vehicles as they climb
the tether to space during routine transportation. The new carbon
nanotube matrix material that makes this possible has a density
of 1300 kg/m3 and a working tensile strength of 100 GPa.
- Since I have been mulling over space elevator concepts for over
3 decades, some comments come to mind now, which possibly might enable
the project to have new features beyond Edward's original proposal.
Some of these thoughts have already been communicated to Brad
Edwards by e-mail, but he is very busy, lacking the hired help
he needs with his project. So the following is my effort to provide
such thoughts.
- Although the Mooncable project I proposed in 1971-72 to NASA would
have been built up from a small "seed" tether, much like Edward's
proposal uses "crawlers", this interesting new tether material
properties suggests a possible easier construction technique.
If this tether material can support its own weight using a constant-area
crossection cable from ground to GEO during the primary construction
phase, ever-widening tether ribbon can be pulled up from the surface
platform, as the tether counterweight beyond GEO spools up the
tether ribbon material (powered by solar panels on the counterweight,
servopositioning its position to maintain a constant upward tether
pull at the earth surface platform assembly site. When the tether
has reached its full operational width at the surface site, then
the stored tether material is de-spooled guided by crawlers as
they lead it downward alongside the initial tether, and laterally
attach the added ribbon material to build up the desired widening
taper. Being made wider by facilities on the earth surface as
the material is lifted up by the upward pull on the tether from
the counterweight's centrepetal force, instead of innumerable
tack operation in space, the resulting cable could be of greater
strength and reliability.
- The key question is can the tether material support its own weight,
with a little extra weight added, during conditions of no live
loads on the cable during construction. To determine this,
Thus there is at least a 20% extra load the tether can lift in
addition to its own weight. Its useable tensile strength under
these conditiions could easily be more, say a 50% load it can
carry during construction. This means for each ground-to-counterweight
length of tether ribbon material raised, the tether widens by
50 %, an exponential increase in constant girth until it reaches
final operational girth at the earth surface. Then the spooled
material stored at the counterweight is used to provide a taper
for the cable, with the crawlers guiding de-spoolinng counterweight
material down instead of having to lift it up. Note that the counterweight,
as before, continues to adjust its length to provide the desired
upward pull at the earth surfaced platform, as it probably would
continue to do throughout the service life of the tether elevator.
- Another thought that comes to mind would provide non-equatorial
ground terminal sites, much as the "Mirrored KESTS" concept described
on this site. This would require some means for coping with variable
high winds at times on the part of the pair of tethers along the
part that is within the atmsphere. As in my paper, "Kinetically
Strengthened Lifteing Bridges to GEO" also to be published in
ASCE's "Space 2002", similarly the tether pairs could be anchored
at, for example, Japan and Australia, or Maine and Comorodo Argentina.
- Continuing on the concept of adding cable taper only in the final
stage of construction, using a linear or slightly inverted taper
during construction:
- Spooling up of the tether ribbon by the counterweight beyond GEO,
would require solar power panels to provide the energy to do the
spooling, and the electric motors which spool are driven in a
servoposition function to provide the required tension at any
given moment. This signal might be updated frequently from the
tension system on the earth surface site.
- The use of 1300 kg/m3 density, 100 GPa tetjer material, per Brad
Edward's info, allows a no-taper cable to be emplaced with 80
GPa at GEO. If the ultimate strength of the material is 200 GPa,
if the initial constant-area cable were to start pulling up a
cable 20 % wider ribbon than itself, it doesn't exceed the 2:1
safety ratio. Since the cable is widened at the earth surface
site, its lateral stresses requiring shear coupling will be vastly
less than that done if crawlers climb up tacking the new ribbon
as they go, tacking low tensioned tether material onto to tether
that has already has much tensile stress applied; this enables
less taper in the final cable since the stresses are even. And
without up-crawlers with their load of ribbon, the cable under
construction does not need to handle such live loads, further
enabling linear taper sections during construction.
- Since the initial emplacement probably will have no personnel
in space during construction, the danger of being holed by a meteor
is slight, and presumably little time for etching by atomic oxygen
then too. Personnel on the earth surface platform would be at
risk if cable were severed during construction, of course.
- The spooling up of the length of cable 40 km long for each step
increase, spools up a lot of tether material, so the maximum rate
of increase of ribbon width would minimize this. If the cost of
the tether material is not extreme, then perhaps the spooled tether
material could be used for other construction purposes later.
Otherwise, it is valuable for the previously-mentioned addition
of material for the final operational tether taper, and for lowering
from GEO for new seed tethers, such as for two-way tether pairs.
Conceivable additional income could be received from sale of these
"seed tethers" to other corporations and countries around the
world... Japan would no doubt be an early customer of such a "seed"
tether, with its keen awareness for the need for high quality
long term energy supply to maintain a high tech civilization level,
and for its concurrent need for recycling of toxic industrial
process materials, and awareness of value of high real estate.
Page updated 2002 04 17 by James E D Cline
Copyright © 2002 J E D Cline