Re: New (?) take on orbital elevators (perhaps dumb!)

Richard Burke <info@xxxxxxxxxxxxxxxxxx> wrote:
This almost certainly a really dumb idea, but I don't have the maths or
the engineering knowledge to know for sure, so here goes... (Also, if
you have dealt with the idea before, then I apologise.)

As I understand it, there are four main technical obstacles to
constructing an orbital elevator out to geosynch:

- developing materials with sufficient tensile strength to 'hang' 22,000
miles, and also to deal with the tensile pull of the necessary
counterweight.

Note that dealing with the pull of the counterweight is the same as
dealing with the weight of payload. This is obviously still something that
has to be considered, but you can decrease your payload weight to suit
your materials once you have something that will actually work with an
elevator at all.

- safety. What happens if the structure snaps - or if someone plants a
bomb? Presumably the Earth-tethered end would smash down into the
atmosphere and wrap itself around the equator. (Might fly outwards
first, I guess.)

You've been reading too much Kim Stanley Robinson. :)

A realistic cable is going to weigh just a few pounds per *mile* and is
going to be operating very close to its maximum strength. Most of it will
burn up in the atmosphere, the parts that don't will probably
disintegrate, and the parts that don't disintegrate will flutter gently to
the ground. The only real danger is to people on the elevator at the time
of the break. Escape systems will add cost and weight, and as we discussed
in another thread recently, will need to be extremely fancy in order to
save people from a couple of "dead zones" where passive systems are unable
to allow for survival.

- how to construct it. Build it in orbit and drop an end down? Feed it
up from Earth with some bizarre rocket system to keep the end vertical?

I believe the current idea is to build the elevator on the ground and
launch it into orbit on one or more rockets. Once it's up and deployed,
you can add on to that cable or use it to lift new ones. The initial
launch would be expensive, but possible.

- money.

Always.

So, I got to thinking, why are these things problems? And the answer is
that it has to be 22,000 miles long and tethered to a point on the
equator. If it was shorter and *not* on the equator, could it perhaps be
less strong, safer, easier to build and cheaper...?

So here's the probably-dumb bit:

How about building a structure at one of the poles with two opposite
lines that rotate to provide centrifugal 'force'? (Think of a helicopter
rotor with 1,000 mile blades and you get the idea.) Would this work? I
can see all sorts of potential problems:

My main question is, why have two?

I'm not sure if 1000 miles enough. Your limit is having the cable be
perfectly straight, which means that it's horizontal at the anchor and
proceeds in a straight line from there. At 1000 miles long in this limit
case, the ends are only 125 miles above the surface. They'll have to be
going at least orbital velocity at the ends to stay up (and much more to
be near this limit case) which means that the bits of the cable inside the
atmosphere would have to be going extremely fast as well. The rotation of
the Earth is almost ignorable here; it rotates once every 24 hours,
whereas this cable is going to be completing one spin every few minutes.
You'll have problems with cable strength and also with just keeping it
from burning up.

All of these problems get better as the cable gets longer, I believe. I'm
not sure if they ever become practical.

- the mechanism to keep the rotation going (it would have to be powered
or the ends would just fall out of the sky)
- the tensile strength required (though that's probably lower than for a
standard orbital elevator design)
- managing the movement of the tethers through the atmosphere
- designing the pivot
- orbital and gravitational effects at the tethers' outer ends
- yada yada yada

BUT...

Might it not actually be easier than a 'standard' orbital elevator
design? You could fling mass out for very little energy by putting
similar masses out on both arms simultaneously. The stresses at the
pivot-point would presumably balance to zero. Power for the drive that
keeps rotation going could be pumped up the arms.

Stresses at the pivot probably aren't a problem. For structures on Earth,
you can just keep piling on more concrete until you have enough. The main
energy cost is going to be accelerating your masses to orbital velocity,
and that isn't helped at all by launching two at a time.

You'll also need some system to keep the thing spinning around, but what?
Doing it at the base probably won't work because it has no good way to
accelerate the end. Doing it at the end means rockets which means
inefficiency. Maybe there's some way to push off the Earth's magnetic
field?

You might even be able
to use the part of the arms that are in the atmosphere to generate lift.

Lift at hypersonic velocities brings with it a lot of drag. You'd probably
just want to concentrate on minimizing drag and having the "lift" be
generated by the circular movement.

If there's damage to the system, the bits fly off into space. Hey, it
might even be cheaper!

I'm sure it's a dumb idea, but is it dumb as a matter of *principle* -
or is it dumb the same way that the idea of a standard orbital elevator
is dumb - i.e., theoretically possible, just not possible for us right
now, too many technical obstructions? And if it's only dumb the same way
as the standard model is dumb, then is it a better idea?

Don't be hard on me. I'm good at ideas, but I'm not claiming to have any
hard expertise here.

I get the impression that it's even crazier than a space elevator based on
the above, but I could be wrong.

--
Michael Ash
Rogue Amoeba Software
.

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