Enclosed tube maglev system tested in China | Hacker News Reader

Enclosed tube maglev system tested in China

phys.org

49 pointsjyrki19 comments

While this is cool, it's probably decades away. People should note that low-speed maglevs have arrived.

http://www.globaltimes.cn/content/860672.shtml

http://english.cri.cn/6909/2014/05/16/2702s826842.htm

A few of these would be nice in NY or LA. JFK to Manhattan in 20 minutes. Cross the width of Manhattan in 4 minutes.

I would be curious to know what is the advantage of using maglev technology if you only reach speeds of 120 km/h. "Firstly, there is no friction, and little noise or vibration; secondly, it eliminates the risk of derailment; and lastly, it is an eco-friendly method of transportation without emissions" looks like nothing special to me.

I'm not really sure how to answer this. You're kind of asking me to explain the value of safety and noise. I guess most people really put a high price on safety. Accidents do happen on subways:

http://www.newsday.com/news/new-york/f-train-derails-in-nyc-...

Subways, and rail in general, can be quite noisy. Paris uses rubber tires on their subway, for example.

http://en.wikipedia.org/wiki/Paris_M%C3%A9tro

In Silicon Valley, one of the reasons people don't want HSR near their homes is because of the noise:

http://www.businessinsider.com/silicon-valley-high-speed-rai...

Ever hear the above ground train pass by in DUMBO in Brooklyn, NY?

Anyway, a super-smooth quite ride at a mile a minute certainly does seem like an appealing way to get around a big city.

Honestly, the safety argument looks pretty moot to me - monorails don't derail either. For the noise, that's cool, but I'm under the impression that a high speed maglev will be quite noisy too.

Monorails seem to run pretty slow. A little more than half the speed?

http://en.wikipedia.org/wiki/Tokyo_Monorail

As for the "high speed maglevs" making noise, you should read my comment again. I was clearly talking about low-speed maglevs. Yes, at 300 mph you are going to make some noise.

high speed maglev will be quite noisy too.

At <100mph, the TR08 is fairly quiet [0]. When I was en Elmsland for the noise testing, it was common for the vehicle to sneak up on us when it was running at lower speeds. At higher speeds, especially over 150 mph when aerodynamic noise kicks in, it is very loud.

[0] http://ntl.bts.gov/lib/22000/22500/22570/fra0213.pdf

Aren't noise barriers more cost effective solutions?

http://en.wikipedia.org/wiki/Noise_barrier

Depends on how you quantify "cost." For a lot of people aesthetics and livable appeal factor into that.

Can any physicists compare this briefly to hyperloop? The prototype tube has curves but would this have to go in a very straight line similar to hyperloop? Does it have the same safety issues in terms of being a closed pipe? - obviously that prototype with open air seats isn't going to carry anyone 1000mph+.

It's cool, they'll just add a windscreen.

I don't think open air seats are good when you've got a low atmospheric pressure, either. It looks like the prototype is a mock-down at a lower scale. The principal advantage of hyperloop is that it takes the atmosphere (which is a liability for maglevs) and turns it into an asset,- by collecting it at the front and redirecting it below as a mode of levitation. This means the tracks can be unpowered. I'm not sure if the tracks are powered in the chinese maglev, but the article said "superconducting" so that's energetically costly either on the pod or on the tracks.

An interesting maglev alternative to superconduction is inductrack, which, however, cannot be combined with magnetic propulsion - https://en.wikipedia.org/wiki/Inductrack

The atmosphere isn't a liability for Maglevs, you could make hyperloop with maglev tech and it'd still work fine.

The tracks don't need to be powered for most Maglev concepts; superconducting maglev doesn't use powered track. Superconducting maglev is also energetically extremely cheap from an operating perspective - it's arguably energetically expensive during construction because you need a shitload of permanent magnet material (the entire track surface is coated with it). The superconductors are the cheap part from a system perspective. Electromagnetic suspension - like with shanghai maglev - also doesn't need a powered track, the relatively small amount of power required can be supplied by onboard batteries and periodically refreshed. Inductrack can be combined with magnetic propulsion, it's just not a form of magnetic propulsion in itself. There's no obstacle to integrating a separate propulsion stage into the track (as is proposed in the inductrack based container movement system proposed for... LA?)

Atmospheric drag absolutely is a liability... That's the whole point of an evacuated maglev.. To make an artificial atmosphere with lower drag. Note how the mockdown chassis cross section is small relative to the tube... That's also a drag limitation; and note how hyper loop has a nearly full tube cross section.

Sorry, I didn't word that very clearly - yes it's a liability, but it's a liability that's fairly independent of the levitation mechanism. I think it's more clear to say "That's the whole point of evacuated tube transport, to make an artificial...". Whether you use wheels or maglev or air bearings is a separate issue. Hyperloop doesn't turn the atmospheric drag into an asset - it just uses a particular method of overcoming the large liability. Because of how it overcomes it, it might be able to get away with a higher blockage ratio, but to say that with any real confidence you need to specify the vacuum level in the high-vac example you're comparing it to. And then do a bunch of compressible flow math that I've not yet seen for a high-vac ETT. This paper (http://jmt.swjtu.edu.cn/EN/abstract/abstract8587.shtml) looks at the incompressible flow case... but that's kind of missing the point I think.

The story also says "Zigang has a tube that lowers air pressure in the running environment to 10 times less than normal." (This probably makes little sense to someone who's good at physics of gases--reducing by 100% or 1x is the lowest you can go with that.)

In general, I guess this story has about the level of quality that one should expect of a science & engineering story sourced from the _Daily Mail_.

Hmm I didn't read it quite like that - I read it as reducing it to 10%, a reduction of 10 times from normal 100%.

There are a couple of differences between this and Hyperloop.

1. The hyperloop white paper proposed air bearings as the levitation mechanism, rather than maglev. This could be swapped out without much change to the overall concept (and, in my opinion, should be if it's ever built - maglev gives much more generous tolerances and ease of control).

2. The hyperloop anticipated a low vacuum; around 1/1000 atmospheres. While that sounds like not much air, it's still quite a lot when you want to go around the speed of sound, especially if you want to do so in a tube; pressure builds up in front of the vehicle and causes huge drag. The hyperloop addressed this with an integrated compressor stage in the front of the vehicle; basically sucking in air to avoid the drag. This used quite a bit of power, required cooling, etc... and the resulting drag was still significant to the point that regular medium power 'boosters' would be required along the track to keep the train at a reasonable speed. It also meant the speed was still limited, despite the partial vacuum, to around 1000kph (aircraft speeds). If you want to get away without the compressor you need a much higher vacuum - I don't know exactly where the equation changes but the number usually thrown around is about 1/100,000 atmospheres to 1/1,000,000 atmospheres. This is basically hard vacuum. If you can achieve it then it means no compressor, no significant onboard power source, and no meaningful limit to the top speed (you could go 2000 - 3000 km/h easy if you could make the track straight enough). The bad part is that, all else being equal, it's roughly 100 times harder to make a vacuum of 1/100,000 atm than a vacuum of 1/1000 atm. So... if you're really confident you can build extremely well sealed tubes then you should go for the hard vacuum approach. If you're confident you can't, you should go for the low vacuum compressor approach. If you're not sure, you should build a prototype :-)

The limitations regarding straight lines and the safety issues are roughly identical for the two concepts. The hyperloop is arguably less safe due to having a big spinning compressor right in front of you and a large onboard power source, high pressure steam storage etc.

How does one pressurize the cabin in a serious vacuum tube? How long will it take and will there be special requirements to adjust pressures for travelers in such devices?

Is this not essentially equivalent to going for a ride in the space station for us out of shape schlubs?

From the article, it doesn't appear to be a full vacuum tube, just much lower pressure, which airplanes have been protecting their passengers against for years.

Also, the reason astronauts need to be so physically fit is not because the environment outside their ships is a vacuum, its because they have to be able to handle the immense g-forces of launch and reentry, as well as be able to operate in micro-gravity, which wouldnt apply here.

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