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Drang seems to think that Musk wants dampers (inside pillars) to handle large thermal expansion:

> Eventually, the dampers in the pillars won’t be able to handle the movement because it’ll be greater than the pillar width. Eventually, the movement will be greater even than the distance between the pillars.

But the tube is "not rigidly fixed" to pylons. It slips past pylons during expansion or contraction:

> By building a system on pylons, where the tube is not rigidly fixed at any point, you can dramatically mitigate Earthquake risk and avoid the need for expansion joints.

In fact, the author quotes the above in the beginning of his article, but then assumes exactly the opposite.




I believe fixed and rigidly fixed have very different meanings in this context. Your suggesting that the tube floats on the pylon(e.g on rollers). The hyperloop paper suggests that it's attached with dampers that are, for lack of a better term, elastic.


I took it to mean that the tube was rigidly (or near rigidly) fixed in the vertical direction, more generously dampened in the transverse direction, and completely free to slide over the pylons in the longitudinal direction.


I disagree. I think "fixed" means the tube attached to the pillar to prevent it from jumping out of it. See rings in figure 3 (page 11):

http://www.spacex.com/sites/spacex/files/hyperloop_alpha-201...


Given that the proposal describes dampers used to absorb change, I think this is wrong.

    By building a system on pylons, where the tube is not 
    rigidly fixed at any    point, you can dramatically    
    mitigateEarthquake risk and avoid the need for expansion  
    joints. Tucked away inside each pylon, you could place    
    two adjustable lateral (XY) dampers and one vertical (Z) 
    damper.
    
    These would absorb the small length changes between
    pylons due to thermal changes, as well as long form
    subtle height changes
(see page 11)


See section 4.2.3.

The tube will be supported by pillars which constrain the tube in the vertical direction but allow longitudinal slip for thermal expansion as well as dampened lateral slip to reduce the risk posed by earthquake.


It says dampers absorb change. But it doesn't mean only dampers do that. Of course they couldn't handle hundred meters expansion (near ends). I don't believe Musk would make such an easy mistake and force dampers handle all of thermal expansion.

That said, the quoted text is somewhat confusing. I had to read it a few times before I got the idea.


Given that the proposal mention slip joints at the stations that needs to be long enough to handle the aggregate expansion, it is pretty clear that the tube "floats" at least to a certain degree.


'slipping past pylons' during expansion would be a pretty terrible solution though. Can you imagine how hard it would be to keep it straight and level, to the tolerances required, while not even being able to fix it to the pylons? To say nothing of the fact that some of those precast tubes are going to be precast straight, and others precast curved. They're not interchangeable.


> To say nothing of the fact that some of those precast tubes are going to be precast straight, and others precast curved. They're not interchangeable.

I don't know enough about steel and construction, but I think the tubes could all be made straight. Hyperloop curves have very large turn radius. Steel tubes are not completely rigid, they can curve a little by themselves.


Fair point - maybe that would allow the necessary flex.

The minimum bend radius they allow for the 300mph segment is 3.67km. If you made a complete circle it would have a circumference of 23km, and comprise 769 30m segments. Each segment would need to contribute 0.46deg of the 360deg curve.

To achieve this over 30m you need to deviate the end of the pipe by 24cm from where it should be... which actually sounds like a hell of a lot to me, especially if it's flexed through this range regularly (as it would be if constantly switching places with a truly straight segment).


Thank you for calculating this! It's now a question of how much a steel pipe can bend naturally. I tried to find this information, but I couldn't.


> while not even being able to fix it to the pylons

Tubes are fixed to the pylons, just not rigidly (in longitudinal direction).


True, but that means you're going to have some absolutely awesome bearing system that gives almost zero friction along the major axis of the pipe while also giving almost zero slop in the other axis. Which is damned hard. Adaptive maglev bearings maybe? ;-)


Why do you want zero friction along the major axis? Linear speed of pipe movement through the bearing would be very low: not more than couple hundred feet per day. And what do you mean by zero slop?

By the way, if you want to continue speculating on Hyperloop thermal expansion issue, you can join my speculations - I wrote a few comments under this blog post:

http://blog.rongarret.info/2013/08/everything-you-need-to-kn...




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