4 ms·

“Cheaper than high speed rail” - why is high speed rail expensive? I believe because precision contact between rail and wheel is required. How would this be d

by graeham 14y ago

“Cheaper than high speed rail” - why is high speed rail expensive? I believe because precision contact between rail and wheel is required. How would this be done cheaper in a tunnel? One issue with a tunnel is that even if the air and carriages are moving at the same speed, there is still drag or friction between the air and the wall of the tunnel by Poissiulle's law. At the proposed speeds (~300km/h) and distances (600km), this becomes a lot, and higher pressures (if my quick calculations are right) lead to higher energy requirements through higher air density. In a vacuum, this friction would not exist. I can post my calcs if there is interest - I used a Moody chart and Darcy's friction equation, and ended up with an energy requirement that there would have to be ~80 million carriages going each way to be as efficient as a Telsa roadster, and neglecting any other losses.

jacquesm 14y ago
600 kilometers / 80 million carriages gives me 7.5 mm per carriage length so I think there is a bit of a problem with that calculation. The limiting case is when the whole tube is full of carriages, say they're 4 meters long that would mean there are a maximum of 150,000 carriages in that 600 Km long tube at absolute maximum capacity, something that you probably should not want to get close to. More realistically, a regular roadway has a spacing of about 30 meters per vehicle at 60 Mph, you might be able to pack them in that tight in a tube like this but that's still pretty tight and leaves very little room for error when shifting carriages in and out of the loop. At that spacing you could stuff (600,000 / 34) = 17650 carriages in (none of those assembled into impromptu trains, which of course would increase the density). If we assume train like assemblies of 10 cars with 30 meter spacing you'd be looking at a 40 meter train + spacing is 70 meters, or 8500 trains, so 85000 carriages. Those would then be pushing a column of of air 30 meters long ahead of them.
- graeham 14y ago
  I was trying to compare the energy required to overcome friction of the air against the tunnel to driving the route conventionally. 80 million is an excessive number to expect want to travel at one time, which implies that driving is much more efficient from an energy required perspective.
  - jacquesm 14y ago
    Yes, I got that, but the fact that there isn't room in there for that many carriages means that the friction can't be that high. Unless there is a way in which the friction in a tube is so high that the little bit of air surrounding the vehicle would exert that much drag on the tunnel wall (so one vehicle length + say a few mm distance to the tunnel wall). I'm not sure what the figure should be but this seems very high.
    - graeham 14y ago
      When fluids flow, there is a `no slip` condition at surfaces - that is the velocity of fluid at a surface is the same as the surface`s velocity. So in a stationary tunnel, the fluid may be moving at an average of 300 meters per second, but will be stationary at the wall. This makes a shear stress in the fluid and is what causes drag. In fact, making the air gap between the carraige and wall smaller will increase this drag since there is less distance for this transition to happen between stopped and fully moving air. Consider if each carriage is pulling 30m of air a long distance at great speed. Consider if instead of the air moving against a stationary tunnel, a 30 meter section of tunnel was moving at the same distance and speed through stationary air. The drag on this would be high, much higher than a car at highway speeds.
      - lazyjones 14y ago
        What if it isn't air, but an actual fluid, e.g. water? It cannot be compressed, so it's "crash proof" (carriages cannot hit other carriages with water in between). It also has plenty of mass, so once it has started moving, it will push carriages around easily. A torus with permanently moving water (probably a more high-tech fluid), where carriages are injected / removed after acceleration / before deceleration?
        eru 14y ago
        You'd still have the friction between carriage and wall.
        14y ago
lnanek2 14y ago
Rail is also expensive because it needs track the entire length of the trip. Huge problems on the west coast right now running rail where it is wanted because you need the land...
Kibber 14y ago
What if we fill the whole tube with carriages, without any space between them and very little space between the carriage wall and the tunnel wall? That would leave much less room for turbulence and perhaps reduce friction. If that works, you could then have "station" sections where you could place/remove "inner" carriages to/from the carriages running inside the loop and thus you'd never have to disconnect the main carriage loop.

by graeham 14y ago

jacquesm 14y ago

600 kilometers / 80 million carriages gives me 7.5 mm per carriage length so I think there is a bit of a problem with that calculation. The limiting case is when the whole tube is full of carriages, say they're 4 meters long that would mean there are a maximum of 150,000 carriages in that 600 Km long tube at absolute maximum capacity, something that you probably should not want to get close to. More realistically, a regular roadway has a spacing of about 30 meters per vehicle at 60 Mph, you might be able to pack them in that tight in a tube like this but that's still pretty tight and leaves very little room for error when shifting carriages in and out of the loop. At that spacing you could stuff (600,000 / 34) = 17650 carriages in (none of those assembled into impromptu trains, which of course would increase the density). If we assume train like assemblies of 10 cars with 30 meter spacing you'd be looking at a 40 meter train + spacing is 70 meters, or 8500 trains, so 85000 carriages. Those would then be pushing a column of of air 30 meters long ahead of them.

graeham 14y ago
I was trying to compare the energy required to overcome friction of the air against the tunnel to driving the route conventionally. 80 million is an excessive number to expect want to travel at one time, which implies that driving is much more efficient from an energy required perspective.
- jacquesm 14y ago
  Yes, I got that, but the fact that there isn't room in there for that many carriages means that the friction can't be that high. Unless there is a way in which the friction in a tube is so high that the little bit of air surrounding the vehicle would exert that much drag on the tunnel wall (so one vehicle length + say a few mm distance to the tunnel wall). I'm not sure what the figure should be but this seems very high.
  - graeham 14y ago
    When fluids flow, there is a `no slip` condition at surfaces - that is the velocity of fluid at a surface is the same as the surface`s velocity. So in a stationary tunnel, the fluid may be moving at an average of 300 meters per second, but will be stationary at the wall. This makes a shear stress in the fluid and is what causes drag. In fact, making the air gap between the carraige and wall smaller will increase this drag since there is less distance for this transition to happen between stopped and fully moving air. Consider if each carriage is pulling 30m of air a long distance at great speed. Consider if instead of the air moving against a stationary tunnel, a 30 meter section of tunnel was moving at the same distance and speed through stationary air. The drag on this would be high, much higher than a car at highway speeds.
    - lazyjones 14y ago
      What if it isn't air, but an actual fluid, e.g. water? It cannot be compressed, so it's "crash proof" (carriages cannot hit other carriages with water in between). It also has plenty of mass, so once it has started moving, it will push carriages around easily. A torus with permanently moving water (probably a more high-tech fluid), where carriages are injected / removed after acceleration / before deceleration?
      - eru 14y ago
        You'd still have the friction between carriage and wall.
      - 14y ago

lnanek2 14y ago

Rail is also expensive because it needs track the entire length of the trip. Huge problems on the west coast right now running rail where it is wanted because you need the land...

Kibber 14y ago

What if we fill the whole tube with carriages, without any space between them and very little space between the carriage wall and the tunnel wall? That would leave much less room for turbulence and perhaps reduce friction. If that works, you could then have "station" sections where you could place/remove "inner" carriages to/from the carriages running inside the loop and thus you'd never have to disconnect the main carriage loop.