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[Sheb]

Well, I guess any real application would have 3 parallels tubes, a la Eurotunnel. Two train tubes (one by direction) and one for maintenance between the two. You can use that one for evac.

[10ebbor10]

Point is, I'm not sure if the design would allow to leave enough place to easily evacuate. The tubes need to fit pretty good, in order to minimize air resistance. (Otherwise you could just go with an open air tube.)

[MonkeyHead]

6 to 10 billion is waaaaay off what it would cost to buld this. Waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay off.

[10ebbor10]

6 to 10 billion is waaaaay off what it would cost to buld this. Waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay off.

Yup, I also find it strange that a complete metal tube would weight less, and be cheaper to build than 2 simple rails.

[MonkeyHead]

*does some sums*...

The tube would have to be a couple of mm thick for that to hold true. Probably not the strongest tubes in the world then.

[andrea]

I think that in the comments of the article somebody mentioned a backup wheel system for when the air cushion fails. If the fall is accounted for, it should be enough to keep the train going until the next emergency exit. Now, removing the train and restoring the railroad would take quite a bit of time and money. But it should happen rarely enough not to matter much.
The price tag is probably going to increase a lot, if it is ever developed. A good part, as many people suggested, would be land costs. But land costs aside, there doesn't seem to be anything too expensive. That bit about being lighter than tracks seems a bit odd, but he is also considering the weight of foundations and similar stuff in that weight. At least when it comes to bridges, I guess it would be cheaper because you don't need to keep an heavy base below tracks.

edit: are the other projects supposed to be elevated too? because if so, I guess it really is lighter.you need more metal to carry the weight of a train if that metal is shaped like as a rail rather than as a tube

[MonkeyHead]

The tube will need to be strong enough to take the weight of a hyperfast train impacting on it when the air cushion fails, or its going to punch through it and plummet into the ground. Thats gonna need to be one hell of a tube.

[10ebbor10]

The distribution of the weight doesn't matter for the strength of the single pillar that has to hold up the entire construction.

And well, you can assume that failures are going to happen quite often. At least as often as they happen with other trains. Probably more. Also, the train pushes quite a bit of air in front of it, even with decent aerodynamics, metal fatigue is going to be a problem.

[andrea]

Weight distribution doesn't matter for the pillar, sure. but it matters for material resistance. A tube performs much better than a beam for both for bending and shear stresses( the most dangerous ones in such a project), therefore you can use less materials and make it lighter. the only problem might be instability of the "skin", but we have plenty of experience avoiding that thanks to aircrafts. In fact, it would even be easier, since loads always act in the same way. external pressure, plus compression in the top when the train is on the track.

( Note once again that this is a valid point only if competitors are elevated too. a ground track doesn't need to concern itself with carrying the train.)

As far as the train punching through the tube when air cushion fails...  I don't see that happening. it isn't going to levitate that high, and speed doesn't really matter for fall damage. There are fairly large safety limits in civil engineering. if it can stand the train moving, it can stand it falling a few centimeters.
Now, friction when the cushion fails would be a much bigger problem. But I guess some wheels might allow a safe, if not entirely comfortable, stop.
I would be more concerned about it derailing and punching through the sides. But that stuff doesn't tend to happen, as long as drivers don't step out of speed limits. ( which sadly they sometimes do. See for example the recent derailing in spain.)

Aerodynamic pressure on the front might pose some problems, but depending on how it is designed, it may not be significantly worse than those of aircrafts. But on this point I really can't say anything. Besides, it is the  kind of thing that can't be properly estimated before building a prototype.

[10ebbor10]

Wouldn't using wheels as back up negate the lightweight benefit? After all, you have to place a reinforced rail into the tube to handle the load when the cushion fails.

IIRC, the train's engine only maintains speed, and is not powerful enough to accelerate. Might not need a driver at all, actually. Problems with the decelerators at the station might be problematic though.

[andrea]

If you want to gain any sort of speed yes, you would need tracks. but if it merely needs to avoid excessive friction , they don't need tracks. Part of the weight of elevated traditional railroads is that they must not cause excessive vibrations. either levitation or low speeds can counter that. In our case either we are flying  on an air cushion, we are crawling slowly on wheels, or we are decelerating and we would rather be all shaked up than killed in a fire or in the fall that might follow.
The weight of the train would be supported by the tube, no matter if we are making contact with air cushion or something else.
note that the wheel thing is just something I read in comments and seemed a decent enough idea.
( also, I am not an expert in the field, so take what I say with a grain of salt. I only have a basic understanding of what such advanced trains need to work)
( I would also like to issue a correction on my previous post. The structural benefit is mostly regarding bending, but that is the one that causes problems anyway)

edit: well, I'll give a good read to the 60 pages PDF attached to the article, so I can make somments that are actually useful, rather than speculation.

[Another]

I think it is also entirely possible to have some compressed air tanks be mechanically automatically used in case of power failure to maintain air cushions for a few minutes it takes to emergency brake such train.

[10ebbor10]

Compressed air tanks are a bad idea, especially near humans. There's quite a lot of force after such an explosion, they weight a lot, and you need a lot of air to maintain the cushion. Besides, the design doesn't have brakes. (And it'd be a bit pointless to keep the cushions online while you're going for friction slowdown anyway.)

[Another]

The same point as having emergency wheels but without the need to add rails for them. If you are spending a lot of air to maintain the cushions you are already doing it wrong because it would waste a lot of energy to maintain normal operation.

[alway]

The Interstellar Starship Congress starts today, with livestream & schedule here: http://www.icarusinterstellar.org/congress-announcement/
Tune in over the next 4 days to learn about all sorts of tech and such related to interstellar travel; from near term stuff like solar sails to terraforming and current investigations of warp drives.