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Old 08-16-2009, 12:21 PM   #11
Langy
 
Join Date: May 2008
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Default Re: Orbital elevators and long, long days

And on the original topic:

An orbital ring is a type of space elevator that doesn't need to go out to Geosynchronous orbit (or even be on the plane of the equator). I think this is the 'pinwheel' concept mentioned earlier, though that isn't a term that's usually used as far as I know (or could find out with a quick Google search).

With an active off-planet industry already in place, an orbital ring isn't too expensive either, at least according to the original designer's calculations - only a few billion dollars, though the Wiki article doesn't mention how much of a payload they can carry.

When compared to real-life orbital elevator designs, the ones in GURPS Ultra-Tech are extremely expensive - an order of magnitude or more off from the calculations done by experts in the field, if I remember right. Mind, said experts might be biased, but I'm fine with optimistic projections in my sci-fi.


EDIT: Specifically, the orbital elevators in Ultra-Tech are $40 billion per ton per day. In the design that I've got here sitting in my lap, the projected budget was $6 billion dollars for a 4.45 ton per day payload cable (it's a 6.85 ton per day total cable, but the climber masses 7 tons - there are roughly one climber sent up per 3 days, though an actual trip takes 7.5 days total). Construction time was 30 months, total capacity 20 tons with a 13 ton payload, and 125 trips per year. This comes down to $1.35 billion per ton per day for the first cable. Cables after that were priced at ever-decreasing rates, with the second at $0.5 billion per ton and built in 6 months. The third cable was priced at $5 billion for a 140 ton payload at 125 trips per year - or a price per ton of payload per day of $100 million. The fourth was the same size but cost $80 million per ton per day.

The lower prices are due to bootstrapping - using the earlier built elevators to lift the new cable up into orbit.

Last edited by Langy; 08-16-2009 at 01:03 PM.
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Old 08-17-2009, 07:41 AM   #12
mlangsdorf
 
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Default Re: Orbital elevators and long, long days

Pinwheel is another name for skyhook:
http://en.wikipedia.org/wiki/Skyhook_%28structure%29

Basically, instead of tethering the elevator to the ground, you leave it free-floating in orbit, and then make it spin. You still need some kind of flight system to catch the skyhook, but then it lifts the package into orbit.

Skyhooks have a disadvantage in that they require energy to maintain. You periodically have to add momentum to keep them spinning. But you can add that momentum from outside the gravity well, where the momentum is relatively cheap.
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Old 08-17-2009, 09:06 AM   #13
Anders
 
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Default Re: Orbital elevators and long, long days

Quote:
Originally Posted by Fred Brackin View Post
You've got to have photosynthesis or there will be no atmospheric oxygen. That's what I was getting at.
Not necessarily. Most of the oxygen will be bound up with hydrogen - i.e., water. However, hydrogen is a very light gas and will escape, leaving an excess of oxygen. There was a recent article in Scientific American about how photosynthesis may not have had such an enormous influence on atmospheric chemistry.
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Old 08-17-2009, 09:42 AM   #14
Anaraxes
 
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Default Re: Orbital elevators and long, long days

Quote:
allowing somehow for variations in its length and orientation...
or to build an elevator to L1, L3, L4, or L5
Wouldn't there be a similar issue with an elevator to the Lagrange points? The points themselves aren't dynamically stable, as I understand it. They're the tops of a hill (or saddle point), not the bottom. Stable positions are in orbit around the point proper. So there's at least some small variation in the position of the elevator's endpoint. (And you'll have to account for the tug of the elevator cable on the Lagrange orbit.)

I have no idea from an engineering point of view if this is a problem or not. Everything has some elasticity. Maybe the difference isn't significant compared to the length and strength of the elevator cable.

I kind of like the sound of an elevator to the moon. Has quite a fantastic ring to it.
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Old 08-19-2009, 05:28 PM   #15
Agemegos
 
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Default Re: Orbital elevators and long, long days

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Originally Posted by Asta Kask View Post
Not necessarily. Most of the oxygen will be bound up with hydrogen - i.e., water. However, hydrogen is a very light gas and will escape, leaving an excess of oxygen. There was a recent article in Scientific American about how photosynthesis may not have had such an enormous influence on atmospheric chemistry.
In theory you can have water dissociated into oxygen and hydrogen by UV light, and that hydrogen can then escape to space by the Jeans Escape process.

But last I heard:
  • water tends to freeze out in the atmospheric "cold trap", so the upper atmosphere is rather dry;
  • free hydrogen and oxygen tend to recombine more quickly than hydrogen escapes;
  • as the process establishes some concentration of oxygen in the upper atmosphere, that tends to form a UV-opaque layer which shuts off photodissociation (unless the ozone layer is below the cold trap).

And therefore the conventional wisdom was that the process was an order of magnitude or two too slow to have been the origin of most of Earth's atmospheric oxygen.

What's changed? I stopped subscribing to Scientific American when it started being written by journalists and that smirking ignoramus Steve Mirsky.
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