Generally speaking, how do you make an evacuated train tunnel? What do you use to suck the air out and keep it out? Does it have to run constantly?
Thanks,
Rob
How do you, present tense? You don’t. Nobody does this right now.
How could you? That’s a question that has many answers, and nobody is yet sure what the most practical one is.
There are many types of vacuum pump, so it is hard to say what type would be used. Apparently, the “hyperloop” concept envisions a vacuum of about 1 torr ~= 1 millibar. This is a very soft vacuum. Laboratory pumps for this range are called “roughing pumps,” and are typically single stage positive displacement pumps, of which there is a huge variety available, both dry and oil-based. They operate by repeatedly expanding a small volume to a large volume. One of the simplest is just a piston with check valves to suck air into the piston on each half-stroke, and expel it on the other half stroke (like a bicycle pump run backwards). More interesting designs include things like a scroll pump or a Roots blower, described in this Wikipedia article:
It would most likely cycle on and off, or run continuously with a variable pump rate, in order to maintain the desired pressure. It would likely be most efficient if each pump is run close to its optimal pumping speed.
You start by being a billionairethan you scale up an old pneumatic tube system and put a maglev train in it.
Simple
Well, as Chronos says, there isn’t a full scale model of this working anywhere yet. Supposedly construction will start on Musk’s version in a few more years, but I’ll believe it when I see it done. Here is a cite with some of the principals. Here’s a brief YouTube video on it if you feel up to watching.
To answer your specific questions though, I think Musk’s method would be low pressure, not full vacuum (from memory it’s the equivalent pressure you’d get at 100k feet, but it’s been a while since I looked at this thing so don’t take that with more than a grain or two of salt). It uses a ground effect system (I think…again, from memory) as well as maglev, so I guess it’s sort of a hybrid system…and the trains also use the same compressor to provide some of the thrust. It’s a pretty elegant design, with solar panels on the tops of the track to provide some of the energy needed, and going along existing routes. I have no idea if this thing will ever get built, but it COULD work, at least that seemed to be the consensus from my other engineering buddies over beers a year ago.
it contains some information
http://boards.straightdope.com/sdmb/showthread.php?t=699329&highlight=hyperloop
You need the stuff dreams are made of.
Well, they’ve already got the maglev train going in Japan, so all they need to do now is build a vacuum tube around it and they’re good.
Best guess at how many pumps you need? Does the tube need to be made of something exotic?
Thanks,
Rob
i think it can be done with current materials. It won’t be successful because of cost versus benefit.
I’m not an engineer, but I wouldn’t imagine that you need anything all that exotic. To be certain, there’s a lot of volume that’s at a lower pressure than the outside air, but it’s spread over a very long distance. I would expect that if you took a one inch square on the surface of the tube and drew a pie slice (with that square as the rounded side of the crust) to the center of the tube, that’s the only volume of vacuum that that inch of tubing needs to be proof against.
That said, if a section failed catastrophically, you’d have air rushing into the tube at many hundreds of miles per hour (based on the ping pong cannon that the Mythbusters constructed). Assuming that doesn’t damage the train, it would at least rocket the thing backwards suddenly, very rapidly, which could send the people and possessions inside to rocket towards one end of the train at (literally) break-neck speed.
Such an eventuality could be mitigated by flooding the whole tube with air, the moment a failure was detected. But if the failure happened right next to a train, I think it would be bad, no matter what.
Wait, we are the stuff that dreams are made on…
It’s people! The hyperloop is made with people!
Or you could invent inertial dampers. 
Additionally this train will be going from subsonic speeds to supersonic speeds as the air pressure increases.