The Speculist

Stephen,

This reference (page 18, WARNING lots of Scary Mathematical Symbols in the rest of the document) indicates that the net payload of any non-equitorial space elevator falls off non-linearly (as COS of the angle between local zenith and the cable) with increasing latitude [absolute value], reaching zero (or no payload) at approximately 48 degrees North or South.

This would mean that (given an elevator designed to lift 2x a certain minimum cargo at the equator) any location in (or above) the continental U.S. or Hawaiian Islands could be the down station for an elevator.

Non-equitorial elevator anchor points must be designed to counter forces on the cable in the direction of the equator either passively through the foundation of the anchor (like the anchor points of a suspension bridge) or actively (some kind of continuous thrust supplied by an airborne terminal).

EJG

Also,

Anything south of the northern boarder of Louisiana (33 degrees North) would have greater than 50% of the lifting capacity of an equitorially-stationed anchor point.

EJG

I still think that costs of having a midway point to the 'bottom floor' of the space elevator is needless complexity. If the ribbon is going to be hundreds of miles long, what's another couple to extend it right to terra firma? Then you don't have to worry about the expenses of getting materials to an obscene height, and you don't have to worry as much about the complicated balance to keep the space elevator and the orbiting counter weight from falling to the earth or flinging off into space.

Finally,

Of particular interest, at least from the geopolitical perspective, is the fact that, while only Europe south of the Alps can be directly served by elevator, almost all of China and all of Japan, India, Australia, the continent of Africa, and all of South America except Patagonia and the extreme southern Andes, CAN be directly served.

I disagree with ChefQuix that eliminating 100,000 feet of atmosphere, weather, gravity well, and traffic is of negligable benefit. Also, some of the engineering, at least as I understand it, indicates that non-equitorial elevators might have to be dropped "straight down" to the Equator and then moved to their chosen destination. This is tough to do with a fixed architecture.

EJG

Yet another benefit of the dark station concept. Because you aren't anchored, you do not need to be in geosyncronious orbit. This means that you can have a much lower orbit, if you have you platform moving at some speed east to west. A lower orbit means less length for the nanotube tethers and less tension. Less tension means less strength required.
This would actually make the anchor platform easier for aircraft to land on. If the platform if travelling at 350mph, an aircraft flying at the same approximate speed could land on a ridiculously tiny platform (also good for keeping the platform weight to a minimum). The moving platform would also circumnavigate the planet for full global access.
The downside to this is that if the tethers are electrically conductive (which nanotubes are), then passing them through the Earth's magnetic field will generate a current and kill the station's momentum if the current is grounded. On the plus side, pumping current through the tethers would provide momentum for orbit correction.

Michael:

That reference to nonequatorial space elevators doesn't seem to contemplate two elevators linked by an equatorial counterweight.

I wonder if the triangle approach would have the same lift capacity fall-off problem that the reference was contemplating.

It does seem that with any space elevator system, it pays to be as close to the equator as possible.

South Florida may continue to be important to the U.S. space program even after we've retired rockets.

ELG:

Take a look at this map:

http://www.maps-continents.com/maps/north-america-globe.gif

If the U.S. was going to put up a space elevator in South Florida counterbalanced in the southern hemisphere, it would be best to put the south elevator in...Chile. Maybe off-coast? In international water.

Without the triangle we might consider cooperating with Mexico. A South Mexican elevator would probably be close enough to the equator that the lift fall-off would be manageable. Also, the slope of the elevator would be over water. That would be better in the case of a disaster (objects would fall into the ocean).

Panama would be even better because it's closer to the equator. And since it has the canal, material to be shipped into space could be brought in from the Pacific and the Atlantic.

I'm envisioning a space elevator port positioned immediately adjacent to the canal (on the Pacific side - keeping the slope of the elevator over water) with ships unloading people and cargo going straight into orbit.

Sure would be nice if the Canal Zone was still American soil. Thank you Mr. Carter.

Stephen:

Well, at least we have a lesson learned, there. Wherever we put the elevator, the Elevator Zone will need to be US soil, agressively defended and managed by us.

Stephen,

Although the cited reference does not specifically address the 'sky-bridge' case (as far as I know that's an original coinage and I think I'll use it until I find out otherwise), among the advantages to be captured by placing the anchor points out of the equatorial plane in the original concept were: access from a greater fraction of the planet's surface and removing most, if not all, of the length of the elevator from both the Van Allen Belts and the debris-strewn areas between LEO and GEO that have been used for 50+ years of rocket-based astronautics. That said, (if I'm reading the mathematics right, the correlation of net lifting capacity and anchor latitude would not be offset by a symmetrical anchor on the other side of the equator, just as the lateral forces on the suspension cables of a bridge are not offset by an anchor on the far shore. Please note, I'm getting rather out of my depth here. Perhaps we could recruit Engineer Poet and Prof. Hall into this discussion for additional learned counsel?

Acrinoe,

Although I think your "wandering platform" terminus concept is a creative application of forces that might otherwise be considered disadvantages to be overcome, I don't think that it would shorten the overall length of the elevator cable by an appreciable amount. Again, while Professor Hall could give us exact figures as to the difference in counterweight altitude a delta-v of less than Mach 1 would allow, my gut says that its going to be much less than 1% of the total length (and therefore self-supporting mass) of the elevator.

Phil,

It seems a bit colonialist to make the assumption that, simply because an elevator located on 'Merican soil wouldn't be the most powerful elevator in existance, we should require secured access to equitorial territory. I rather envision multiple non-equitorial elevators with down-terminals at high altitude served from the ground by heavy-lift airships. (Alas, ATG, the company that we cited last year as the builder of the N.A. Semper Melia is in bankruptcy, but other airship companies will, forgive me, 'rise to the challenge') At first there might only be a handful of these "down ports", even in a nation as wealthy and economically powerful as ours, but, eventually, they would likely be as common as international airports, perhaps as common as railyards.

EJG -

Dude, we're going to have to limit you to accusing one individual or group of being "colonialist" per week. :-)

I'm all for putting the elevator on existing US soil. But if we put it someplace else (out to sea, for example, as Rand Simberg has suggested) we just have to make sure that wherever it is, it's ours.

I've exceeded my limit already?

One of the advantages of having a bandito / revolutionary alter-ego is the opportunity to sling slogans.

¡Luche a los contra-liberales, a los colonialistas y a imperialistas!

[Trans. "Fight the anti-liberals, the colonialists, and the imperialists!]

Mark that I / we are against colonialism, not colonization.