3 ms·

"A large network of road tunnels many levels deep" It's not just one tunnel...

by zild3d 9y ago

ktta 9y ago
See I don't know what specifics they're talking about when they say 'a large network'. It'll obviously be more efficient to dig a wider tunnel than multiple tunnel on a single route. So, unless I'm wrong, to me it looks like it'll only be one tunnel, with one track to and one track back. How many people do you think can be transported in a direction in a 'sled' in one lane? I'm talking about one point to another point throughput not the complete network. I'm betting the numbers will be very underwhelming.
- njarboe 9y ago
  The car in the video looks like Model S size. About 16 feet long. Add 2 feet on each end for an even 20 feet for the sled. At 125mph one gets 660,000 feet per hour. So if you can pack the cars next to each other, the maximum is 33,000 cars per hour. So 33,000 people per hour with each person getting a space that would rent in SF for $1000/month. Pack that sled space with people at the same density as a train at rush hour (50?) you get 1.65 million per hour. Mix and match your people density and sled packing to determine your own rate. Seems like in reasonable scenarios you could get train like rates with one tunnel. Bart says at peak commute it does 60,00 per hour. Maybe other systems do better, but those BART trains are full.
  - so33 9y ago
    >each other, the maximum is 33,000 cars per hour. Yeah, but that's 33,000 cars per hour without any of the cars stopping to exit the tunnels. The trains used in BART and other public transit systems can very easily exceed 60+mph already, but they don't because they have to stop, bringing down the average speed of the system. BART, by the way, does 28000 passengers per hour per direction in the Transbay Tube (https://en.wikipedia.org/wiki/Transbay_Tube https://en.wikipedia.org/wiki/Transbay_Tube). You can probably fit over 200 people into a BART train car at absolute crush capacity.
    - njarboe 9y ago
      By the way, the maximum rate of 60,000/hour in each direction for BART I quoted was from BART itself[1]. I should have provided a link. Primary sources are good. Of course the sleds in an operating system would not be absolutely packed together. This was just a Fermi estimation to get some idea of the flow rate the parent comment was wondering about. Looks like max rates would vary from somewhat worse than trains to much better. The sleds would be automated so that when one wants to exit the others would give it enough space to do so without problems. Another Fermi estimation could be done to get an idea of how long the exit tunnel would need to be for a sled to stop given a range of deceleration rates. I think Musk is on the right track but that his biggest hurdles may be social and political in the USA. Maybe China or India will want to try it out (or the Swiss). [1] https://www.bart.gov/sites/default/files/docs/2016Factsheet_v12.pdf https://www.bart.gov/sites/default/files/docs/2016Factsheet_...
  - ktta 9y ago
    It is very easy to come up with theoretical numbers and look nice. We already know know the subway infrastructure works. Forget what exists currently, and try to calculate the theoretical throughput of the subway system. I bet you'll come up with much more that what it is currently. On first look at your calculations the flaws I find are these: 1. 125mph is a very, very optimistic number in my opinion. In reality I think anywhere near 100mph is what I would expect. 2. You can't fit 50(!) people in a 20ft x 6.5ft 'sled' for hours. If something goes wrong, they'll trapped in a tunnel with no escape to surface for hours!!! 3. The numbers might make financial sense if you cram them with theoretical people at peak hours. But the thing is, most of the day (and night) those sled will mostly be empty. So you can only count the sleds to be used close to their limit for a couple hours a day morning and night. 4. You can't just daisy chain 33k sleds along the track. As I've said before, one malfunction would possibly wreck havoc in the system. Which can actually lead to people's death. Think of them as you would an airplane, but you can't land in an emergency like you can in an airplane 5. This is close to what @so33 said. The last couple miles and the first couple miles will be very congested because people slow down, and getting off them would take time and many stations. The only way to remove bottlenecks would be to make lots of places to easily disembark, which would increase the costs.
    - njarboe 9y ago
      There is a lot details to work out of course but Musk surely has thought about this more than you or me, with a brain that is optimized for this sort of thing. He has the clout to talk to the best people in the field, people to do computer simulations, and a record of success in very hard businesses. He is serious about this. Look at the open jobs list. More people trying and doing and less people just criticizing the better. My biggest concern would be Musk spreading too much of his focus away from SpaceX and having that venture flounder.

"A large network of road tunnels many levels deep" It's not just one tunnel...

by zild3d 9y ago

"A large network of road tunnels many levels deep" It's not just one tunnel...

ktta 9y ago

See I don't know what specifics they're talking about when they say 'a large network'. It'll obviously be more efficient to dig a wider tunnel than multiple tunnel on a single route. So, unless I'm wrong, to me it looks like it'll only be one tunnel, with one track to and one track back. How many people do you think can be transported in a direction in a 'sled' in one lane? I'm talking about one point to another point throughput not the complete network. I'm betting the numbers will be very underwhelming.

njarboe 9y ago
The car in the video looks like Model S size. About 16 feet long. Add 2 feet on each end for an even 20 feet for the sled. At 125mph one gets 660,000 feet per hour. So if you can pack the cars next to each other, the maximum is 33,000 cars per hour. So 33,000 people per hour with each person getting a space that would rent in SF for $1000/month. Pack that sled space with people at the same density as a train at rush hour (50?) you get 1.65 million per hour. Mix and match your people density and sled packing to determine your own rate. Seems like in reasonable scenarios you could get train like rates with one tunnel. Bart says at peak commute it does 60,00 per hour. Maybe other systems do better, but those BART trains are full.
- so33 9y ago
  >each other, the maximum is 33,000 cars per hour. Yeah, but that's 33,000 cars per hour without any of the cars stopping to exit the tunnels. The trains used in BART and other public transit systems can very easily exceed 60+mph already, but they don't because they have to stop, bringing down the average speed of the system. BART, by the way, does 28000 passengers per hour per direction in the Transbay Tube (https://en.wikipedia.org/wiki/Transbay_Tube https://en.wikipedia.org/wiki/Transbay_Tube). You can probably fit over 200 people into a BART train car at absolute crush capacity.
  - njarboe 9y ago
    By the way, the maximum rate of 60,000/hour in each direction for BART I quoted was from BART itself[1]. I should have provided a link. Primary sources are good. Of course the sleds in an operating system would not be absolutely packed together. This was just a Fermi estimation to get some idea of the flow rate the parent comment was wondering about. Looks like max rates would vary from somewhat worse than trains to much better. The sleds would be automated so that when one wants to exit the others would give it enough space to do so without problems. Another Fermi estimation could be done to get an idea of how long the exit tunnel would need to be for a sled to stop given a range of deceleration rates. I think Musk is on the right track but that his biggest hurdles may be social and political in the USA. Maybe China or India will want to try it out (or the Swiss). [1] https://www.bart.gov/sites/default/files/docs/2016Factsheet_v12.pdf https://www.bart.gov/sites/default/files/docs/2016Factsheet_...
- ktta 9y ago
  It is very easy to come up with theoretical numbers and look nice. We already know know the subway infrastructure works. Forget what exists currently, and try to calculate the theoretical throughput of the subway system. I bet you'll come up with much more that what it is currently. On first look at your calculations the flaws I find are these: 1. 125mph is a very, very optimistic number in my opinion. In reality I think anywhere near 100mph is what I would expect. 2. You can't fit 50(!) people in a 20ft x 6.5ft 'sled' for hours. If something goes wrong, they'll trapped in a tunnel with no escape to surface for hours!!! 3. The numbers might make financial sense if you cram them with theoretical people at peak hours. But the thing is, most of the day (and night) those sled will mostly be empty. So you can only count the sleds to be used close to their limit for a couple hours a day morning and night. 4. You can't just daisy chain 33k sleds along the track. As I've said before, one malfunction would possibly wreck havoc in the system. Which can actually lead to people's death. Think of them as you would an airplane, but you can't land in an emergency like you can in an airplane 5. This is close to what @so33 said. The last couple miles and the first couple miles will be very congested because people slow down, and getting off them would take time and many stations. The only way to remove bottlenecks would be to make lots of places to easily disembark, which would increase the costs.
  - njarboe 9y ago
    There is a lot details to work out of course but Musk surely has thought about this more than you or me, with a brain that is optimized for this sort of thing. He has the clout to talk to the best people in the field, people to do computer simulations, and a record of success in very hard businesses. He is serious about this. Look at the open jobs list. More people trying and doing and less people just criticizing the better. My biggest concern would be Musk spreading too much of his focus away from SpaceX and having that venture flounder.