so THAT's what a space elevator is?

I'd seen a lot of headers about this thing, but was never sure what it
was until I saw The Universe on Teusday. An orbiting satellite that
lowers a platform on a 60,000 mile long cable? Is it even possible or
just a far off dream?

How much could it lift before you pullled it out of orbit?

wrote:
I'd seen a lot of headers about this thing, but was never sure what it
was until I saw The Universe on Teusday. An orbiting satellite that
lowers a platform on a 60,000 mile long cable? Is it even possible or
just a far off dream?

Depends on how difficult it is to solve the materials problem. There are
currently no practical engineering materials strong enough to build a
space elevator. There is a consensus that carbon nanotubes hold promise
for this application but they are currently a lab curiosity; a practical
engineering material utilizing them is years away, if not decades.

How much could it lift before you pullled it out of orbit?

Depends on the distance between geostationary altitude and the c.g. of
the elevator. The higher the c.g., the more tension in the tether, and
therefore the more mass the tether can lift. This can be accomplished by
either extending the tether well past twice geostationary altitude
(which would optimize the elevator for, say, launching craft on escape
trajectories) or by putting a countermass on the upper end of the tether
(which would optimize the elevator for Earth-GSO and back).

Depends on how difficult it is to solve the materials problem. There are
currently no practical engineering materials strong enough to build a
space elevator. There is a consensus that carbon nanotubes hold promise
for this application but they are currently a lab curiosity; a practical
engineering material utilizing them is years away, if not decades.

I thought Kevlar et al. would do if you build it with a taper, with even
a "reasonable" taper ratio?

Jan

Jan Vorbrüggen wrote:


I thought Kevlar et al. would do if you build it with a taper, with even
a "reasonable" taper ratio?

Nope. You need specific strength (i.e. strength/mass) 15-20 times that
of the best existing fibers (Dynewema, Kevlar, Spectra etc) before the
total mass comes down to a feasible level (i.e. not requiring hundreds
of heavy launches). And even that assumes a "bootsrap" construction
with a minimal ribbon at first, which is then built up over a year or
two by crawlers adding additional material .

The strength-mass relationship for a space elevator material embodies
the same ugly logarithm as the rocket equation, and for exactly the
same reason. If your rocket Isp isn't high enough, the rocket becomes
impossibly large because too much of the propellant is accelerating
propellant rather than payload. If your elevator ribbon material isn't
strong enough, it becomes impossibly large because too much of its
strength is supporting its own weight rather than useful loads
climbing it. Same gravity well, same exponents.

Monte Davis
http://montedavis.livejournal.com/

Another issue would be LEO satellite and space junk that would
eventually find the elevator. And this stuff would hit with enough
kinetic energy to snap the cable. Then you'd have about 150 mile long
cable falling down, which cannot be a good thing... Not good for a
continent, or an ocean (tsummini).

robert casey wrote:
Another issue would be LEO satellite and space junk that would
eventually find the elevator. And this stuff would hit with enough
kinetic energy to snap the cable.

Not necessarily. There are fault-tolerant tether designs that can handle
a break in one strand. Google "Hoytether" for one.

Then you'd have about 150 mile long
cable falling down, which cannot be a good thing... Not good for a
continent, or an ocean (tsummini).

Don't be silly. A tether will not have nearly enough mass to cause a
tsunami. If you believe otherwise, show your work (equations).

robert casey wrote:

Then you'd have about 150 mile long
cable falling down, which cannot be a good thing... Not good for a
continent, or an ocean (tsummini).

Nope. The same very high strength/weight required to make it possible
at all means that the ribbon doesn't -- CAN'T -- have much mass. Its
density would be somewhere between that of cellophane and newsprint.
What doesn't fly off into space, or acquire enough velocity in falling
to burn up when it hits stmosphere, would flutter down harmlessly.

Monte Davis
http://montedavis.livejournal.com/