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Rob Dekker

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Re: The Hyperloop
« Reply #250 on: January 31, 2018, 05:44:26 AM »
But beyond air drag is the force required to move the weight of the train, and overcome the friction of the wheels.  Long freight trains require several locomotives — much more power — to move all that weight.  The benefits of the hyperloop pod (if handled right ;) ) is the physics of running in a near vacuum, rather than through air, and a suspension system that becomes almost frictionless at speed, allowing the pod to coast long distances at high velocity with only an occasional “push” — meaning much less energy is required.

I agree with most of what you are saying, but with a couple of side notes :

Air friction (and thus energy usage) increases with the square of speed of the train.
So if you want to go twice the speed as HSR, you increase energy use due to air friction by a factor of 4. Hyperloop wants to mitigate that by reducing the air density, but this goes only linearly. So if you want to go twice the speed, and keep air friction the same, you have to reduce air density by a factor of 4 (go to 250 mb).

The problem is that creating lower pressure also demands energy. Especially when compensating for leaks. The NASA paper that Sleepy posted previously :
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170001624.pdf

gives a good overview of when it makes sense to put the train in a tube, and which pressure it should operate on to be more energy efficient than simply building a HSR system. See fig. 12.
This suggests that yes, it makes sense to lower the pressure in the tube, but it may not be energy efficient to operate it at the near-vacuum conditions, and ultra-high speeds, that Elon originally anticipated.

And then there is the issue of the pod cross area versus the tube cross area, which puts essentially an upper limit on the speed you can run the pod at (at least without resorting to exotic new technologies like putting a large vacuum pump in front of the pod running at 10X the RPMs of a jet engine).

My guess as an engineer is that the first real commercial Hyperloop (if it would be built at all) would be a traditional MagLev running at speeds less than 300 mph inside a tube at reduced pressure but not near vacuum.
« Last Edit: January 31, 2018, 05:49:54 AM by Rob Dekker »
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Sleepy

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Re: The Hyperloop
« Reply #251 on: January 31, 2018, 06:17:30 AM »
A friend had brought back video she'd taken of a HSR in China, and one of the things that
impressed me was the length of the train. It appeared to be at least a mile long, possibly some multiple of that.
Sleepy's video mentions that the advantage of the Hyperloop system is that it lowers air resistance.


Doesn't each car in a HSR train, with the exception of the lead car, share the air resistance of that lead car?
Wouldn't a 3 car train have ~ the same air resistance as a single car or a 10 car train?
No, it won't. Here are percentages from a Swedish train buff for the air resistance of a typical (rather long...) high-speed train with 14 cars:

4.5% Front
3.5% aft
45% bogie and wheels
8% power take-off
4% air intake
7.5% Bottom
27% Surface friction from sides and ceilings

That is also one of the main reasons for Rodolphe Nieth to design a maglev train in underground vacuum tunnels, back in 1974. And the end of that saga came in 2009. One of his early supporters, Marcel Jufer, wrote this paper in 2008:
https://www.researchgate.net/publication/37449974_Global_modelisation_of_the_Swissmetro_maglev_using_a_numerical_platform
And this in 2010:
https://www.researchgate.net/publication/255720968_REPORT_Collaboration_with_the_Korean_Railroad_Research_Institute_KRRI_Status_Synthesis_of_the_Swissmetro_Project_Swissmetro_Maglev_and_KRRI_Tube_Train_Developments

In the last one you will also find the history for this. It started when Elon was three years old... Without flame throwers and boring company.  ;)
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Rob Dekker

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Re: The Hyperloop
« Reply #252 on: January 31, 2018, 08:17:22 AM »
One more thought :
The only advantage of putting a train in a (vacuum) tube is reduced air friction, and thus reduced cost.
However, building a vacuum tube is crazy expensive.
I'm not just taking about the cost of the steel, or the cost of pumping a vacuum, but there are many other issues :
- Thermal expansion (day/night) of the tube is something like 150 meter over a 600 km track. Even if you somehow manage to let the whole tube 'role' linearly, that expansion would cause the entire start and end stations to move back and forth 150 meters every day and night. How is that going to work ?
- Airlocks would be needed at start and stop stations. Airlocks that can draw a vacuum in minutes. Has that been tested yet ?
- How do you turn the pods around at start and stop stations ?
- How do you 'split' a MagLev track, so trains can go to more than one end station ?
- How do you prevent catastrophic failure of the entire system (killing everyone in any pod in the tube) in the case one pod comes of its tracks and penetrates the tube ?
- What to do if a pod gets stranded off the track in a 600 km long tube, with no way out for the passengers ?
- and so on..

It's so much easier to build a HSR system, and that is hard enough.

I don't think we are going to see a Hyperloop any time soon.
« Last Edit: January 31, 2018, 08:40:23 AM by Rob Dekker »
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Sleepy

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Re: The Hyperloop
« Reply #253 on: January 31, 2018, 09:43:03 AM »
Quote
I don't think we are going to see a Hyperloop any time soon.
I concur Rob, the devil is in the details and this idea is 44 years old, and counting.

Interestingly the Swiss also concluded (like that NASA study) in their final study from 1999 that the the tube will need to be larger. Adding pic 13. The difference is that they never read Elons paper from the future.

A few short quotes:
They recommended a sandwich-type cladding made of one
impermeable layer between two concrete rings.

They were counting on that the rock would cool the tube, but still stated that local thermal
conditions of the rock and/or an increase in the operating frequency, could lead to thermal problems and construction of a tunnel cooling system was required.

They concluded that the vacuum level and the tunnel diameter is a compromise between investment costs and operating costs, to fulfill criterias related to safety, thermal issues etc.

All with speeds of just 500km/h with a vacuum of only 100 mb, or 0.01 atm...

There's a lot more, unfortunately all in German:
https://web.archive.org/web/20130927194601/http://www.swissmetro.ch/sites/default/files/downloads/studien_schlussbericht_de.pdf
But still, no real world tests and Swissmetro went into liquidation in 2009 due to lack of support.

No wonder. Then came Elon in 2013 and hyped almost everything.
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Sigmetnow

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Re: The Hyperloop
« Reply #254 on: January 31, 2018, 02:06:08 PM »
Without Hype'loop it's like the Jetsons without the jet.  ;D

Compared to spending hours stuck in stop-and-go traffic…
speeding along care-free at 124 mph / 200 kph would be like heaven on earth — or, heaven under earth! ;D
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Sleepy

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Re: The Hyperloop
« Reply #255 on: January 31, 2018, 02:39:30 PM »
Without Hype'loop it's like the Jetsons without the jet.  ;D

Compared to spending hours stuck in stop-and-go traffic…
speeding along care-free at 124 mph / 200 kph would be like heaven on earth — or, heaven under earth! ;D
Again, you started the comparison with the hyperloop...
and subway tunnels are not that exiting or even new. :)


No wonder:
Humanity's Longest-Lasting Legacy: Miles of Holes
https://www.livescience.com/47530-human-activity-changing-geological-timeline.html
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Sigmetnow

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Re: The Hyperloop
« Reply #256 on: January 31, 2018, 03:20:11 PM »
Without Hype'loop it's like the Jetsons without the jet.  ;D

Compared to spending hours stuck in stop-and-go traffic…
speeding along care-free at 124 mph / 200 kph would be like heaven on earth — or, heaven under earth! ;D
Again, you started the comparison with the hyperloop...
and subway tunnels are not that exiting or even new. :)

...

? ? ?

I did not mention the hyperloop here, you did.  “124 mph / 200 kph“ is the speed of the skate/sled concept, which my previous comments described.  Travelling in your own car, or in a mini-pod, directly through a Boring Company tunnel to your destination, is a far cry from any subway train.

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Sigmetnow

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Re: The Hyperloop
« Reply #257 on: January 31, 2018, 03:26:30 PM »
...
- Thermal expansion
...

Thermal expansion is probably of greater concern in the desert than anywhere else.  Yet Dubai is planning a hyperloop. 

http://gulfnews.com/news/uae/transport/dubai-to-have-hyperloop-prototype-by-2020-1.1926075

http://www.dubai-hyperloop.com
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Sleepy

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Re: The Hyperloop
« Reply #258 on: January 31, 2018, 04:52:39 PM »
? ? ?

I did not mention the hyperloop here, you did.
Let's try this then.
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Sigmetnow

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Re: The Hyperloop
« Reply #259 on: January 31, 2018, 06:15:15 PM »
? ? ?

I did not mention the hyperloop here, you did.
Let's try this then.

**sigh**

Not in a mood to argue semantics.  Let’s just drop this.
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Sleepy

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Re: The Hyperloop
« Reply #260 on: January 31, 2018, 06:50:13 PM »
Fine, but semantics? Definitely not.
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Rob Dekker

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Re: The Hyperloop
« Reply #261 on: February 01, 2018, 05:15:38 AM »
...
- Thermal expansion
...

Thermal expansion is probably of greater concern in the desert than anywhere else.  Yet Dubai is planning a hyperloop. 

http://gulfnews.com/news/uae/transport/dubai-to-have-hyperloop-prototype-by-2020-1.1926075

http://www.dubai-hyperloop.com

From your link :

Quote
Dubai is all set to have a 20km prototype of hyperloop by 2020.

At 20km, thermal expansion is manageable.
At 600km, there is a real problem.
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Rob Dekker

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Re: The Hyperloop
« Reply #262 on: February 01, 2018, 05:37:06 AM »
I did not mention the hyperloop here, you did.  “124 mph / 200 kph“ is the speed of the skate/sled concept, which my previous comments described.  Travelling in your own car, or in a mini-pod, directly through a Boring Company tunnel to your destination, is a far cry from any subway train.

I kind of like that animation of a skate/sled concept.
But it would leave huge holes in the surface when the 'sled' is in use.
Would it not be more practical to simply drive the cars through that tunnel ?
To prevent air toxification, you could restrict it to EVs, which would be a great incentive to drive EVs instead of ICE vehicles.
Also surely a Tesla Model S can reach 200 km/hr, so with a bit of electronic guidance the 'sleds' would be obsolete.
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Sleepy

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Re: The Hyperloop
« Reply #263 on: February 01, 2018, 06:00:34 AM »
I haven't pondered too much over that pimped subway, but it sure is a weird way to solve public transportation by moving 17.5 times more weight than needed.

And there are numerous differences between the loop and the hyperloop, at least 450+.
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Bob Wallace

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Re: The Hyperloop
« Reply #264 on: February 01, 2018, 06:53:52 AM »
Quote
At 20km, thermal expansion is manageable.
At 600km, there is a real problem.

Please explain.  Are you assuming there are no expansion joints along the path?

DrTskoul

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Re: The Hyperloop
« Reply #265 on: February 01, 2018, 07:31:06 AM »
Quote
At 20km, thermal expansion is manageable.
At 600km, there is a real problem.

Please explain.  Are you assuming there are no expansion joints along the path?

https://www.kompaflex.com/en/products/large-diameters/

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Rob Dekker

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Re: The Hyperloop
« Reply #266 on: February 01, 2018, 09:00:51 AM »
Quote
At 20km, thermal expansion is manageable.
At 600km, there is a real problem.

Please explain.  Are you assuming there are no expansion joints along the path?

https://www.kompaflex.com/en/products/large-diameters/

How much expansion can one of these expansion joints absorb under a vacuum ?
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oren

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Re: The Hyperloop
« Reply #267 on: February 01, 2018, 09:05:10 AM »
I haven't pondered too much over that pimped subway, but it sure is a weird way to solve public transportation by moving 17.5 times more weight than needed.

And there are numerous differences between the loop and the hyperloop, at least 450+.
I think so too about the LA tunnel but it kind of belongs to the Boring Elon thread, not here...

Sleepy

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Re: The Hyperloop
« Reply #268 on: February 01, 2018, 09:30:05 AM »
Agree oren, but it was Sig who brought the loop here, which also was the reason for my earlier discussion with him above. And there seems to be some confusion of which is what in some other comments.

Continuing with the hyperloop.
You would need a lot of expansion joints, and therefore also get thousands of potential points of failure. This has been mentioned many times before by a lot of engineers. Also upthread.

Remember, we are not talking about the loop = 150 mph on rails here.
We are talking about hypersonic promises at 0.001 atm / 0.015 psi / 750 microns.
Compare to the Swissmetro study above, 310 mph at 0.01 atm / 0.15 psi / 7600 microns.

I'll repeat my personal point of view from upthread:
The main reason for using a vacuum pump on a heat pump installation, is to evaporate and withdraw water from the system, you need at least 500 microns of vacuum. 100Pa is ~750 microns. Lower micron numbers mean a stronger vacuum. 0 microns is a perfect vacuum while 760,000 microns is the measurement of a space with no vacuum at sea level. The main reason for leakage in HVACs is often the copper flare flanges. Stressed installers often fail on those and eventually the refrigerant will leak out. And that is a brass nut tightening on the copper flange itself. That copper flange needs to be perfectly made and the flare fitting must be perfect. Still they can leak. The larger pipes on domestic heat pumps are often 3/8-1/2 inch. Rather thin and they dont have pods at supersonic speeds travelling inside them. Only refrigerant at high pressure.

I would really like to see a real world installation of the Hyperloop that can maintain 750 microns with a pod travelling though at mach 0.8. This would then have to be done many, many times over an extended period without any failiures.

And that with thousands and thousands of welded steel or rubber joints.
750 microns, 100Pa or 0.015 psi equals 99.9% vacuum. Adding an image with the expanded air ratio for temperatures at 60° F or 15.5° C.

Edit; adding the reason for choosing mach 0.8 above, it's from the study posted here:
https://forum.arctic-sea-ice.net/index.php/topic,1094.msg139894.html#msg139894
And this section:
https://forum.arctic-sea-ice.net/index.php/topic,1094.msg140010.html#msg140010
Those are pretty good reasons to assume that speeds higher than that, are not feasible.
« Last Edit: February 01, 2018, 10:43:13 AM by Sleepy »
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Bob Wallace

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Re: The Hyperloop
« Reply #269 on: February 01, 2018, 05:41:27 PM »
Quote
You would need a lot of expansion joints, and therefore also get thousands of potential points of failure.

Going back to the "20 km no problem"  from coast to coast it's about 4,500 km from LA to NYC.  So, 225 expansion joints on a coast to coast run. 

And we might see a hub and spoke system with a two hour run east from LA and then passengers would switch to a route headed to various locations.  The number of potential leak points along a single stretch might be closer to 100.

Then there's leakage and failure.  With a refrigeration system losing the coolant results in system failure.  With a small leakage into the tube it might mean using more energy to keep the partial vacuum in place while the leak repaired.  Or perhaps there would be constant leakage that would have to be factored into the energy budget.

Quote
I would really like to see a real world installation of the Hyperloop that can maintain 750 microns with a pod travelling though at mach 0.8. This would then have to be done many, many times over an extended period without any failiures.

Apparently work will start on the first long distance tube next year (IIRC).  It shouldn't be too long before we see a real world test.

Sleepy

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Re: The Hyperloop
« Reply #270 on: February 01, 2018, 08:41:13 PM »
Quote
You would need a lot of expansion joints, and therefore also get thousands of potential points of failure.

Going back to the "20 km no problem"  from coast to coast it's about 4,500 km from LA to NYC.  So, 225 expansion joints on a coast to coast run. 
That would mean that every expansion joint would have to be able to move 10 metres, if you calculate on a maximum temperature difference of just 40K and only 0,5mm/m for steel pipes, and also zero thermal effects from the pod at supersonic speeds inside. That Swiss study above recommended a cooling system dispite beeing built into rock and concrete.

Quote
With a small leakage into the tube it might mean using more energy to keep the partial vacuum in place while the leak repaired.  Or perhaps there would be constant leakage that would have to be factored into the energy budget.
99,9% vacuum is indeed a partial vacuum, technically speaking. But also like 50km up, in the stratopause. There was a section regarding energy consumption and cost vs tube pressure in the first link in my post above to that NASA study.
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Bob Wallace

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Re: The Hyperloop
« Reply #271 on: February 01, 2018, 08:53:33 PM »
I realize that I'm nitpicking, but I am interested in figuring stuff out....

How would heat be generated by the pod if it's traveling through a near vacuum and emits no heat other than what might leak out from the compartment?  There would be no road friction heat.

Sigmetnow

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Re: The Hyperloop
« Reply #272 on: February 01, 2018, 08:57:13 PM »
I did not mention the hyperloop here, you did.  “124 mph / 200 kph“ is the speed of the skate/sled concept, which my previous comments described.  Travelling in your own car, or in a mini-pod, directly through a Boring Company tunnel to your destination, is a far cry from any subway train.

I kind of like that animation of a skate/sled concept.
But it would leave huge holes in the surface when the 'sled' is in use.
Would it not be more practical to simply drive the cars through that tunnel ?
To prevent air toxification, you could restrict it to EVs, which would be a great incentive to drive EVs instead of ICE vehicles.
Also surely a Tesla Model S can reach 200 km/hr, so with a bit of electronic guidance the 'sleds' would be obsolete.

Briefly in response:
- Could move the next skate/sked into place as the current one departs. (Details of entrance functions haven’t been shown yet. ;) )  Horizontal elevator doors?
-“Elevator” entry means entrances take up no more room than a big parking space, meaning no big ‘station’ or tunnel entrance needs to be built, so lots of entrances can fit within existing city blocks.
- ICE cars will outnumber EVs for some years yet; turning them off and whisking them away reduces pollution as well as traffic.
- A fleet of computer-controlled skates/sleds all with the same specifications are more likely to be able to maintain the close, one-car-a-second flow needed for speed and efficiency.  The speed and exacting performance needed is beyond the ability of most cars — what if they enter without enough charge to complete the trip?  Few cars will have anything like the computer control required for such travel, for years yet. 
- Sleds/skates allow easy inclusion of “people pods” and cargo pods with equivalent performance, and programming, into the system.
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numerobis

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Re: The Hyperloop
« Reply #273 on: February 01, 2018, 09:10:30 PM »
I realize that I'm nitpicking, but I am interested in figuring stuff out....

How would heat be generated by the pod if it's traveling through a near vacuum and emits no heat other than what might leak out from the compartment?  There would be no road friction heat.

There's still air friction, whatever waste heat the propulsion produces (that depends on the propulsion system), and heat radiating out of the pod (I assume it'll be at room temperature, whereas the tunnel would start out rather cooler).

Sleepy

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Re: The Hyperloop
« Reply #274 on: February 01, 2018, 09:27:20 PM »
I realize that I'm nitpicking, but I am interested in figuring stuff out....

How would heat be generated by the pod if it's traveling through a near vacuum and emits no heat other than what might leak out from the compartment?  There would be no road friction heat.
So am I Bob, that's why I'm still here. The idea is still fascinating and has fascinated people for more than a hundred years.

At transonic speeds it will still get hot, 750 microns is not enough to evaporate water at lower temperatures, you need at least 500 microns for that. And then if the 99,9% vacuum fails... 
Aircrafts never cruise near transonic speeds for a very good reason. Either they go Mach 0.8 or they will have to go a lot faster than Mach1.

Edit; they=hyperloop
« Last Edit: February 01, 2018, 10:42:11 PM by Sleepy »
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Sigmetnow

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Re: The Hyperloop
« Reply #275 on: February 01, 2018, 09:31:49 PM »
I realize that I'm nitpicking, but I am interested in figuring stuff out....

How would heat be generated by the pod if it's traveling through a near vacuum and emits no heat other than what might leak out from the compartment?  There would be no road friction heat.

What air remains in the tube is compressed in front of the pod at speed (plus there is a compressor, in Musk’s design), which generates heat, necessitating an ‘intercooler’....

“The Concorde could fly at about Mach 2. The ambient temperature is much lower than room temperature, but the heatup compared to ambient was about 182K for the skin and 153K for the nose.”

Wikipedia:  “Owing to air compression in front of the plane as it travelled at supersonic speed, the fuselage heated up and expanded by as much as 300 mm (almost 1 ft).”
https://en.m.wikipedia.org/wiki/Concorde
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Sleepy

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Re: The Hyperloop
« Reply #276 on: February 01, 2018, 10:33:38 PM »

https://www.kompaflex.com/en/products/large-diameters/

How much expansion can one of these expansion joints absorb under a vacuum ?
Had some time over and sat down to find out. I found very few numbers, maybe placing an order will provide better results? :)

But those larger might not be suitable for high vacuum since they offered high vacuum smaller ones and also high pressure, also smaller. But there were some numbers at least:
https://www.kompaflex.com/en/references/high-pressure/
The largest expansion there: +/- 20mm. So 40mm on that one. But only 1500 cycles...

If that were to be true for the real hyperloop joints, a 600km tube would require more than 7500 of these joints! That's using a 40K maximum temp difference and 0,5mm/m which is slightly lower than for steel pipes. And you will have to replace all of them after a while.  ;D

Adding an image from that project pdf with the larger ones in DrTskolus link above.
That was a staggering 16 joints which according to them, never has been done on site by another manufacturer.
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Sigmetnow

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Re: The Hyperloop
« Reply #277 on: February 01, 2018, 11:13:35 PM »

At 20km, thermal expansion is manageable.
At 600km, there is a real problem.

It wouldn’t be one uninterrupted tube.  There will be stations along the way — a good place for slip joints.

“Specially designed slip joints at stations will be able to take any tube length variance due to thermal expansion. This is an ideal location for the thermal expansion joints as the speed is much lower nearby the stations. It thus allows the tube to be smooth and welded along the high speed gliding middle section.”
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Re: The Hyperloop
« Reply #278 on: February 02, 2018, 12:19:45 AM »
From the 'white paper'...

The paper conceived of a hyperloop system ... at an average speed of around 600 mph (970 km/h), with a top speed of 760 mph (1,200 km/h).

Passenger jets travel 878 – 926 km/h; 546 – 575 mph.

The speed of sound is 1,192 km/h; 741 mph.

Elon envisioned the pod traveling at about the speed of passenger jets, not the speed of sound.

Sigmetnow

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Re: The Hyperloop
« Reply #279 on: February 02, 2018, 01:10:39 AM »
From the 'white paper'...

The paper conceived of a hyperloop system ... at an average speed of around 600 mph (970 km/h), with a top speed of 760 mph (1,200 km/h).

Passenger jets travel 878 – 926 km/h; 546 – 575 mph.

The speed of sound is 1,192 km/h; 741 mph.

Elon envisioned the pod traveling at about the speed of passenger jets, not the speed of sound.

You are correct.

“The capsules travel at 760 mph (1,220 kph, Mach 0.99 at 68 oF or 20 oC).”

I guess my point was that even in low atmospheric pressures such as the Concorde at high altitude, or the hyperloop pod in a low-pressure tube, there would be frictional heating from air molecules.  (Enough to require a hyperloop “intercooler.”)
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oren

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Re: The Hyperloop
« Reply #280 on: February 02, 2018, 02:03:47 AM »
Is it reasonable to assume a 40k range for thermal expansion purposes in a concrete-lined tunnel underground? I know nothing on the subject but it sounds a lot, especially when the assumption is for the whole length of the tunnel, rather than some local heat source like the pods etc.
Probably in a desert climate the effect will be magnified, but in a California climate? Any data anyone?

Bob Wallace

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Re: The Hyperloop
« Reply #281 on: February 02, 2018, 02:21:29 AM »
There's a risk of mixing up a 'loop built in steel tubes above ground and and a 'loop built many feet below ground in tunnels.

Just pointing that out so that people can be alerted to be clear in their comments.

Rob Dekker

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Re: The Hyperloop
« Reply #282 on: February 02, 2018, 05:11:48 AM »
https://www.kompaflex.com/en/references/high-pressure/
The largest expansion there: +/- 20mm. So 40mm on that one. But only 1500 cycles...

If that were to be true for the real hyperloop joints, a 600km tube would require more than 7500 of these joints! That's using a 40K maximum temp difference and 0,5mm/m which is slightly lower than for steel pipes. And you will have to replace all of them after a while.  ;D

Thank you Sleepy. That is good data to start understanding the problems with thermal expansion on Hyperloop design.

And to address the comments from oren and Bob, I think it is fair to assume that thermal expansion is much less of a problem for tubes that run underground.

But then, a tube that runs underground will be even more expensive (and has more issues) than tubes running above ground, which is more expensive (and has more issues) than simply running a MagLev without a tube, which is more expensive (and has more issues) than simply running HSR.

We'll see where this goes, but based on that reasoning, I don't have much confidence that Hyperloop will ever make it beyond prototype stage.
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Rob Dekker

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Re: The Hyperloop
« Reply #283 on: February 02, 2018, 05:20:13 AM »
It wouldn’t be one uninterrupted tube.  There will be stations along the way — a good place for slip joints.

I don't think that really works.
For starters, sealing vacuum chamber slip joints don't exist yet.

So the only 'slip joints' that you could put in are at the start and end stations of the longest non-stop route, regardless of any split-offs to intermediate stations.
At 600 km the thermal expansion is something like 150 - 300 meters, depending on the temperature range you need to handle.
That means you would need a design that allows for entire stations to move back and forth 150 - 300 meters. And that assuming that the tube itself moves almost frictionless over its support system for the entire 600 km route.
« Last Edit: February 02, 2018, 05:37:12 AM by Rob Dekker »
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Sleepy

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Re: The Hyperloop
« Reply #284 on: February 02, 2018, 07:58:20 AM »
https://www.kompaflex.com/en/references/high-pressure/
The largest expansion there: +/- 20mm. So 40mm on that one. But only 1500 cycles...

If that were to be true for the real hyperloop joints, a 600km tube would require more than 7500 of these joints! That's using a 40K maximum temp difference and 0,5mm/m which is slightly lower than for steel pipes. And you will have to replace all of them after a while.  ;D

Thank you Sleepy. That is good data to start understanding the problems with thermal expansion on Hyperloop design.

And to address the comments from oren and Bob, I think it is fair to assume that thermal expansion is much less of a problem for tubes that run underground.

But then, a tube that runs underground will be even more expensive (and has more issues) than tubes running above ground, which is more expensive (and has more issues) than simply running a MagLev without a tube, which is more expensive (and has more issues) than simply running HSR.

We'll see where this goes, but based on that reasoning, I don't have much confidence that Hyperloop will ever make it beyond prototype stage.
Yes, we need more data. A lot more.
Reiterating Elon's self-contradictory idea paper from 2013 won't help much.

Dispite much lower specs and built into the rock with concrete, the Swiss study still recommended a cooling system:
https://forum.arctic-sea-ice.net/index.php/topic,1094.msg140614.html#msg140614
Concrete has a temperature expansion coefficient that is similar to steel.

I used 40K above, building the hyperloop above ground would require a lot more. Tin roofs here are specified for thermal expansions from -35°C up to +75°C, according to Swedish professionals.

Adding an easy graph to eyeball.
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Rob Dekker

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Re: The Hyperloop
« Reply #285 on: February 02, 2018, 08:09:53 AM »
Another issue with Hyperloop is the pod design.
Here is an image from Elon's original white paper :



If this pod is supposed to operate at near vacuum conditions, there is no way that you can mount one of these Tesla pop-up doors in it. You need to have a spacecraft-style sealing lock instead.

Which makes me wonder how much Elon thought his idea through.
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Sleepy

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Re: The Hyperloop
« Reply #286 on: February 02, 2018, 08:23:14 AM »
Probably not enough. A genius provides new ideas, like Tesla. Most of us normal humans are very good at improving other peoples inventions. Like Otto's engine from 1876. Which is as old as our tunneling dreams.
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Archimid

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Re: The Hyperloop
« Reply #287 on: February 02, 2018, 02:54:27 PM »
Sleepy, I believe genius to be the exact opposite of what you describe. Ideas are a dime a dozen. Succesful execution of ideas is the hard part. People that can successfully execute ideas in extraordinary ways are genius. Under this definition Musk is a genius not for his crazy ideas, but for the succesful execution of them.

Bob Wallace I love the discussion so far and find the counterpoints compelling but you are right about the distinction between underground tunnels and surface tubes. The distinctions between the two get muddled in the discussion.
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crandles

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Re: The Hyperloop
« Reply #288 on: February 02, 2018, 03:31:58 PM »
Sleepy, I believe genius to be the exact opposite of what you describe. Ideas are a dime a dozen. Succesful execution of ideas is the hard part. People that can successfully execute ideas in extraordinary ways are genius. Under this definition Musk is a genius not for his crazy ideas, but for the succesful execution of them.

Liked this fun list of honorifics:
Quote
All hail Elon Musk, First of his Name, King of the Martians and the First Molemen, Protector of Tubes, the Unbalded, Breaker of Industries, Father of Dragons, uniter of the seven continents
https://www.wired.com/story/elon-musk-spacex-rocket-travel-plan/
(very GoT ish)

Sleepy

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Re: The Hyperloop
« Reply #289 on: February 02, 2018, 03:58:35 PM »
Sleepy, I believe genius to be the exact opposite of what you describe. Ideas are a dime a dozen. Succesful execution of ideas is the hard part. People that can successfully execute ideas in extraordinary ways are genius. Under this definition Musk is a genius not for his crazy ideas, but for the succesful execution of them.
Elon is an entrepreneur.

Swedens most famous entrepreneur died recently, Ingvar Kamprad. Care to calculate his footprint?
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Archimid

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Re: The Hyperloop
« Reply #290 on: February 02, 2018, 04:05:28 PM »
Ha. Nice article, but I was thinking more along the lines of his accomplishments like accelerating the advent of electric cars, reusing rockets profitably and arguably creating the first succesful internet bank.

I have always believed that Elon's priority is to colonize Mars. Space X will get him there, the Boring company will build the habitat and Tesla will help save the world from self destructing before he can get to Mars. So the GoT messianic references are right on point.
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Bob Wallace

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Re: The Hyperloop
« Reply #291 on: February 02, 2018, 05:22:22 PM »
Quote
Which makes me wonder how much Elon thought his idea (Hyperloop) through.

Obviously not all the way through.  Elon presented what he thought was an interesting idea but stated that he did not have time to fully develop it.  He gave the idea to whomever wanted to take it and run.

Elon may have made a bad assumption that there would be a way to allow for steel tube expansion while maintaining a near vacuum.  It's left up to others to find a solution, if there is a solution.

Elon assumed 'sports car' seating in low diameter pods.  Perhaps much of the market wouldn't accept that.  We see current designs using pods more the size of regional passenger jets with narrow aisles and toilets.  Perhaps the larger diameter would make the costs too high.  Perhaps there will be a return to small diameter pods.

It's impossible to see all the answers when you're standing at the starting line.  I'm looking forward to see if other smart people who have taken on this project will be able to find the needed answers. 

Sigmetnow

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Re: The Hyperloop
« Reply #292 on: February 02, 2018, 09:31:21 PM »
It wouldn’t be one uninterrupted tube.  There will be stations along the way — a good place for slip joints.

I don't think that really works.
For starters, sealing vacuum chamber slip joints don't exist yet.

So the only 'slip joints' that you could put in are at the start and end stations of the longest non-stop route, regardless of any split-offs to intermediate stations.
At 600 km the thermal expansion is something like 150 - 300 meters, depending on the temperature range you need to handle.
That means you would need a design that allows for entire stations to move back and forth 150 - 300 meters. And that assuming that the tube itself moves almost frictionless over its support system for the entire 600 km route.

If you have 10 stations along the 600 km path, and two slip joints per station, 20 joints would only need to deal with 7.5 - 15 meters each.  The stations wouldn’t move.
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Sigmetnow

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Re: The Hyperloop
« Reply #293 on: February 02, 2018, 09:47:01 PM »
...
Liked this fun list of honorifics:
Quote
All hail Elon Musk, First of his Name, King of the Martians and the First Molemen, Protector of Tubes, the Unbalded, Breaker of Industries, Father of Dragons, uniter of the seven continents
https://www.wired.com/story/elon-musk-spacex-rocket-travel-plan/
(very GoT ish)

They missed “Zombie Defender,” Musk’s self-ascribed Twitter profile during the recent toy flame-thrower funding days.  ;)

“The rumor that I’m secretly creating a zombie apocalypse to generate demand for flamethrowers is completely false.  You’d need millions of zombies for a so-called “apocalypse” anyway. Where would I even get a factory big enough to make so many!?“
https://twitter.com/elonmusk/status/957494364028071936

The man works insanely hard.  But he also knows how to have fun, and isn’t afraid to share it. :)
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Sigmetnow

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Re: The Hyperloop
« Reply #294 on: February 02, 2018, 10:59:45 PM »
...

If this pod is supposed to operate at near vacuum conditions, there is no way that you can mount one of these Tesla pop-up doors in it. You need to have a spacecraft-style sealing lock instead.

Which makes me wonder how much Elon thought his idea through.

Musk’s SpaceX Dragon capsule has a passenger and cargo door that maintains a breathable atmosphere inside while withstanding direct contact with launch forces to space, the hard vacuum in space while attached to the International Space Station for weeks or months, and then survives a flaming reentry and splash down in the ocean. It’s a good bet he knows what’s required for a hyperloop door.
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Rob Dekker

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Re: The Hyperloop
« Reply #295 on: February 03, 2018, 04:03:13 AM »
...

If this pod is supposed to operate at near vacuum conditions, there is no way that you can mount one of these Tesla pop-up doors in it. You need to have a spacecraft-style sealing lock instead.

Which makes me wonder how much Elon thought his idea through.

Musk’s SpaceX Dragon capsule has a passenger and cargo door that maintains a breathable atmosphere inside while withstanding direct contact with launch forces to space, the hard vacuum in space while attached to the International Space Station for weeks or months, and then survives a flaming reentry and splash down in the ocean. It’s a good bet he knows what’s required for a hyperloop door.

Sure. But to make my point : SpaceX Dragon has a little hatch like this :



Try to get grandma through that one  ::)

Jokes aside, a practical Hyperloop pod would need an improved (due to larger pressure differential) airplane plug door, and it cannot be anything larger than that. And certainly not the Tesla style flip-up doors that Elon showed in his whitepaper.

But here we go : If the door cannot be larger than an airplane door, the pod needs an 'isle' to walk through. And you need to be able to walk through that isle. So the pod would need to be much larger than Elon's original idea, and that also means the tube will need to be larger diameter.
« Last Edit: February 03, 2018, 04:14:25 AM by Rob Dekker »
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Rob Dekker

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Re: The Hyperloop
« Reply #296 on: February 03, 2018, 04:25:48 AM »
If you have 10 stations along the 600 km path, and two slip joints per station, 20 joints would only need to deal with 7.5 - 15 meters each.  The stations wouldn’t move.

You misunderstand the issue.
Slip joints for vacuum chambers don't exist yet.

You can only have a slip joint when there is no vacuum.
So you either have to stop at everyone of the 10 stations (and go through two air locks at each station), which make fast long distance travel impossible, or you need to come up with a different solution for the thermal expansion problem.
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Rob Dekker

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Re: The Hyperloop
« Reply #297 on: February 03, 2018, 04:48:01 AM »
Let me add that instead of that, you can also invent a perfectly sealing slip joint for vacuum chambers. If you do that, and it is low cost (like a rubber ring) you can put such a slip joint in every (100 meter or so) tube section, and the issue is solved.

It is going to be interesting to see how they deal with this thermal expansion problem in the Dubai prototype : They are going to have to deal with about 10 m of thermal expansion.
« Last Edit: February 03, 2018, 05:00:38 AM by Rob Dekker »
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Rob Dekker

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Re: The Hyperloop
« Reply #298 on: February 03, 2018, 05:05:30 AM »
Let me note that thermal expansion and size of the door are just TWO of the many issues with the very concept of Hyperloop design.

Other issues include :

- Airlocks would be needed at start and stop stations. Airlocks that can draw a vacuum in minutes. Has that been tested yet ?
- How do you turn the pods around at start and stop stations ?
- How do you 'split' a Hyperloop track, so trains can go to more than one end station ?
- How do you prevent catastrophic failure of the entire system (killing everyone in any pod in the tube) in the case one pod comes of its tracks and penetrates the tube ?
- What to do if a pod gets stranded off the track in a 600 km long tube, with no way out for the passengers ?
- and so on..

Not even talking about keeping this system all under near vacuum.
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Tor Bejnar

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Re: The Hyperloop
« Reply #299 on: February 03, 2018, 06:17:04 AM »
Quote
Airlocks that can draw a vacuum in minutes.
I imagine the station has a short "jet bridge" that connects to the pod after the pod stops.  Unlike a jet bridge used in today's airports, it will form a pressure-tight fitting immediately around the door opening, so that virtually none of the vacuum is affected.  Once tightly connected, any sort of door (that can be airtight) can open in any sort of way, if space is available.
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