Low density tunnels for trains

[yael perkal]

What do you think about building sealed long tunnels for trains, that after they are being built, there are pumps vacuuming it out of air, at high percentages of the atmospheric density. Suppose the train is sealed too and compressed like airplanes, and there are sealed passages in the stations connecting the train to to open world, to let people get in and out of the train. What do you think about reducing drag and fuel consumption in this model?

 

[Tres Juicy]

What do you think about building sealed long tunnels for trains, that after they are being built, there are pumps vacuuming it out of air, at high percentages of the atmospheric density. Suppose the train is sealed too and compressed like airplanes, and there are sealed passages in the stations connecting the train to to open world, to let people get in and out of the train. What do you think about reducing drag and fuel consumption in this model?

 

 

 

The benefit of reduced fuel consumption would very likely be heavily out weighed by the cost of maintaining an adequate vacuum.

[CaptainPanic]

The benefit of reduced fuel consumption would very likely be heavily out weighed by the cost of maintaining an adequate vacuum.

 

We can calculate this with some rough back of the envelope calculations.

 

A modern train uses something like 5 MW of power (ballpark figure). By going to a vacuum, this power consumption should become less, so you can use a certain percentage of that power for creating a vacuum.

 

Now it's just a matter is calculating what a vacuum pump requires (and I'm too busy today to do that), and looking into leakage into tunnels. Btw, I think you're likely pumping water vapor, not air. At some point all the air has been pumped out, and only water leaks into the (underground) tunnel. You might therefore achieve your vacuum by cryogenic cooling too. I'm not really sure which is more efficient.

 

Would there be any data on how much water seeps into the Channel tunnel? Then you have a flow of water (mass/time), and you can calculate how much power (depending on the level of vacuum) you need to have to get it out.

[Greg H.]

We can calculate this with some rough back of the envelope calculations.

 

A modern train uses something like 5 MW of power (ballpark figure). By going to a vacuum, this power consumption should become less, so you can use a certain percentage of that power for creating a vacuum.

 

Now it's just a matter is calculating what a vacuum pump requires (and I'm too busy today to do that), and looking into leakage into tunnels. Btw, I think you're likely pumping water vapor, not air. At some point all the air has been pumped out, and only water leaks into the (underground) tunnel. You might therefore achieve your vacuum by cryogenic cooling too. I'm not really sure which is more efficient.

 

Would there be any data on how much water seeps into the Channel tunnel? Then you have a flow of water (mass/time), and you can calculate how much power (depending on the level of vacuum) you need to have to get it out.

 

If you're planning to do this in the US, Canada, and/or Mexico, remember to factor in the cost of switching nearly the entire locomotive fleet off of diesel fuel to pure electric (they should have done this years ago anyway). You would also have to design specialized cars for those cargoes that cannot be shipped in a vacuum (livestock, for instance).

 

The next big question is - would the efficiency gains outweigh a (relatively) simpler conversion, such as changing the trains over to a maglev configuration? (By simpler I mean you could continue to use existing rail infrastructure, with the appropriate updates, and existing rolling stock - again, with the appropriate updates, as well as existing capital facilities such as stations and servicing facilities).

[CaptainPanic]

If you're planning to do this in the US, Canada, and/or Mexico, remember to factor in the cost of switching nearly the entire locomotive fleet off of diesel fuel to pure electric (they should have done this years ago anyway). You would also have to design specialized cars for those cargoes that cannot be shipped in a vacuum (livestock, for instance).

Regarding the livestock, the bottleneck is not the maximum velocity, but loading times and waiting times at depots. There is no point in using a high speed rail system if the cargo is gonna spend a couple of hours standing at a depot afterwards.

 

I would only connect main cities with high speed trains. And you're gonna have to buy new trains anyway. Take the European high speed network as an example, and put it in a tube, and add some airplane technology to allow passengers to stay conscious.

 

The next big question is - would the efficiency gains outweigh a (relatively) simpler conversion, such as changing the trains over to a maglev configuration? (By simpler I mean you could continue to use existing rail infrastructure, with the appropriate updates, and existing rolling stock - again, with the appropriate updates, as well as existing capital facilities such as stations and servicing facilities).

Simpler = high speed trains like in Europe, Japan, China.

If you're gonna go Maglev anyway, you might as well take it a step further. To be honest, I don't really believe in large Maglev networks. Too complicated. Changing tracks is as complicated as moving a house.

 

But I think you were questioning the investment costs of these tunnels... and I agree. It's likely to be far too expensive to contemplate.

[Enthalpy]

http://www.swissmetro.ch/en

http://de.wikipedia.org/wiki/Swissmetro

[Greg H.]

 

But I think you were questioning the investment costs of these tunnels... and I agree. It's likely to be far too expensive to contemplate.

 

That is the central point I was trying to make - you may see some efficiency improvements, but the cost may be so overwhelming that the rail industries will simply say "No." and walk out of the proposal.

 

That aside, I agree that a simple(!) alternative is to simply move to higher speed rails like they have in Europe. I lived in Germany for a couple of years and I was impressed by two things - travelling by train didn't take all day, and the trains actually ran on time. In comparison, Amtrak is, frankly, embarrassing.

 

Granted, Germany is much smaller than the US - but I think the basic ideas are there. Start with regional high speed rails, and gradually expand them to connect the regions together. But when it takes longer to ride a train to Chicago from St. Louis than it does to drive, and it costs as much as flying, why should I bother with the train at all?

[CaptainPanic]

[...]the rail industries will simply say "No."[...]

Now, there's your problem.

Rail networks should be state owned, just like road networks.

[yael perkal]

Thank you, I assumed somebody have already thought about it before, as it very simple concept.

http://www.swissmetro.ch/en

http://de.wikipedia....wiki/Swissmetro

 

Thank you all, I really like this forum

[InigoMontoya]

Another issue I've not seen addressed.... Heat.

 

Even electric motors generate heat. Where do you plan on dumping the heat? In the cold vacuum of space, spacecraft can have problems keeping cool. Now you ditch the cold expanse of space as a radiative heat sink (you lose the Sun as a source too so maybe it's not THAT bad) and you have to keep some seriously powerful motors rather than simple electronics cool.

[yael perkal]

Another issue I've not seen addressed.... Heat.

 

Even electric motors generate heat. Where do you plan on dumping the heat? In the cold vacuum of space, spacecraft can have problems keeping cool. Now you ditch the cold expanse of space as a radiative heat sink (you lose the Sun as a source too so maybe it's not THAT bad) and you have to keep some seriously powerful motors rather than simple electronics cool.

 

This is interesting.

[yael perkal]

This is interesting.

 

But, if there is less air then there is less friction and less heat. I think that the tunnels should have heat absorbent, natural source is better like flow of water.

[Enthalpy]

Let's imagine it takes 1MW mechanical power to move the complete train, of which 100kW are dissipated at the train and the rest is gained back when braking.

(By the way, it depends on where the engines are: on the train or at the track, and we didn't tell how power goes to the train).

 

Put infrared radiators at the railway engine: 2m high, 15m long, at each side, with good emissivity.

If the tunnel's walls are at 300K the radiators will reach 440K or +167°C. Inconvenient as is, but the design can improve.

 

The tunnel itself gets heat over a huge length and can conduct it away.

Take 100kW at 10m/s during acelerations, one train every 5min: 33W/m.

0.4W/m/K in rocks, tunnel r=3m, conduct heat to R=100m: 46K warmer under these heavy conditions. Circulate water in the tunnel to ease the portions of acceleration and braking.

[vampares]

The maintance of a vaccuumed commuter transport system would be way too expensive. It is also dangerous. And there no air to provide cooling to the brakes and engine ect.

 

I think the best way to deal with the forward pressure in a tunnel is have channels that run the length of the train for the air to travel. In this way it would be similar to the aerodynamics of a car. Air goes up over and the behind the vehicle to replace the vacuum that is left behind. Provided there is smooth continous flow there should be a minimal loss of efficiency. The front end of the train could be designed like a scoop to push the air into the channels. I think the addition of gates to prevent air loss at the back of the train might help as well.