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What happens when the Rolling Roads get to the rollers? 
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Joined: Mon Jan 12, 2009 8:27 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
I've just re-read it, which was what prompted the question, and the one that was the primary focus of the story was a 20-strip road: 5-100mph in 5mph increments.

Which means that, topologically, there is no rational way to design it so that you can build things on the strips that I can see: those strips have to get to the end, and either turn 180 degrees in the pitch axis, under a roller and go back, or in some fashion turn 180 degrees in the yaw axis, and go back on the other side -- which would most easily be done by using 2 rollers at 45 degrees to the direction of travel and 90 degrees to one another.

And the reference Ed is looking for is to Methuselah's Children, which I chanced to read immediately afterwards.

And, finally: "Look! Topic drift!"


Fri Feb 27, 2009 6:00 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
I contend that the design issues are soluble using a flat-plate model that pivots around, say, a one mile radius at each end. I can sketch a workable model easily.

Whether or not the various issues of practicality are soluble... well, let's chalk it up as a story facilitating MacGuffin. :)

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Fri Feb 27, 2009 7:22 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
Is there a mathematician in the house? :-)

I know that the question is "what's the centripetal force generated, measured in G's of lateral acceleration, by an object describing a 1 mi radius curve at 100 mph" (or possibly more; it depends on the design of the plates), but I don't have the math to figure it out off hand; I suspect strongly that it's a pretty substantial fraction of a full G, unbanked; that seems pretty high to me.

Beamjockey: you're a rocket scientist; what's the answer here? :-)


Fri Feb 27, 2009 9:47 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
The Rolling Roads certainly would be an engineering wonder. What this reminds me of is London Bridge of centuries ago. How often today do we see any structure on a bridge, except the occasional toll booth right at the end? Yet London Bridge was a busy and important city street, with houses and businesses over the River Thames. If anyone were to develop an equivalent structure today, it certainly would be a financial and bureaucratic nightmare, as well as an engineering wonder ....

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Sat Feb 28, 2009 10:41 am
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Post Re: What happens when the Rolling Roads get to the rollers?
Baylink wrote:
I know that the question is "what's the centripetal force generated, measured in G's of lateral acceleration, by an object describing a 1 mi radius curve at 100 mph" (or possibly more; it depends on the design of the plates), but I don't have the math to figure it out off hand; I suspect strongly that it's a pretty substantial fraction of a full G, unbanked; that seems pretty high to me.

A fast horseback calculation of 100 mph going around a 1-mile radius is about 0.13g. Very mild banking - perhaps 2 degrees - or a doubling of the radius (or even a much larger extension; there's no reason for these roads to be confined to a narrow strip at their ends) would reduce this to a negligible factor.

Next objection?


Sat Feb 28, 2009 12:25 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
James Gifford wrote:
Next objection?


Lubrication.

Heat dissipation.

:geek:


Sat Feb 28, 2009 12:44 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
Peter Scott wrote:
Lubrication.

Heat dissipation.

:geek:


I envision a vast corps of attendants who are responsible for lubricating the system and jacking down and replacing any roller found running hot. :roll:

Certainly, there are quite a few technological hurdles. But I don't see any fundamental reason why we couldn't build such a monstrosity. Certainly nothing contravening either the laws of physics or known limits of engineering.


Sat Feb 28, 2009 1:16 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
Baylink wrote:
Is there a mathematician in the house? :-)

I know that the question is "what's the centripetal force generated, measured in G's of lateral acceleration, by an object describing a 1 mi radius curve at 100 mph" (or possibly more; it depends on the design of the plates), but I don't have the math to figure it out off hand; I suspect strongly that it's a pretty substantial fraction of a full G, unbanked; that seems pretty high to me.

Beamjockey: you're a rocket scientist; what's the answer here? :-)


I get the same answer Jim does: about 13% of a G, so your intuition is correct.

If there are turnaround zones at the ends for flat-plate sliding roads, they could easily have curves with larger radii than 1 mile (and hence lower accelerations) without adding much cost to the project.

I must admit that I've read the story many times without getting a clear understanding of how the roads were supposed to work...

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Sun Mar 01, 2009 12:56 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
James Gifford wrote:
Certainly, there are quite a few technological hurdles. But I don't see any fundamental reason why we couldn't build such a monstrosity. Certainly nothing contravening either the laws of physics or known limits of engineering.


Really. A machine two hundred miles long moving at a hundred miles an hour. No unobtanium required, eh? Massing, let's be conservative and say 500 lb/linear yard or 100,000 tons. Okay, so roughly an aircraft carrier; we'll assume that all the motion generating machinery is static and does not have to be accelerated. Let's see, energy required to get it moving (assuming whole thing moving at average of 100kph) = 0.5 * 1E5 * 2000 / 2.2 * (1E5 / 3600) ** 2 = 4 E10 Joules or roughly .01 kiloton. Seems too easy. But we can assume negligible air resistance.

Damn. I started out expecting to shoot a bullet in the head of this thing and now I'm starting to think it's possible. Darn you, sir.


Sun Mar 01, 2009 4:03 pm
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Post Re: What happens when the Rolling Roads get to the rollers?
I am darned. :lol:

Remember that the drive comes from many, many points along the road, not from some single source like a propeller or push link. Heinlein implies that there are drive rollers every, what, few hundred yards? By distributing the drive force and the power requirements (including heat dissipation) it becomes much more practical. Don't try to make the calculations as if it's a single-point drive or concentrated force.

RAH also noted that it can take hours to restart a strip, so I think he'd thought it through that the 100mph strip would need to be spun up very slowly, and perhaps even "train" style where some parts start ahead of others and then pull the slower sections along.

It's still a complete ridiculous monstrosity. For one thing, it requires this massive installation to run at full speed even when traffic is very light, the complete opposite of our individual-vehicle model.


Sun Mar 01, 2009 4:11 pm
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