I've been away from the discussion for a long time (travel). Perhaps some of this has been covered but I didn't see it in a quick scan.
I guess they are going to "muscle" their way through the Kantrowitz limit, by mounting more linear motors in the track.
I don't fully understand this. It seems like there is an assumption that the pod will have to force its way through a standing mass of (low density), compressing in as it moves.
Will it not be the case that the air inside the tube will also be moving in the direction of the pods in the case of a true loop? Each pod will push some air forward as the pod ahead leaves a small vacuum in its wake. After awhile the air in the loop should be moving at pod speed.
Everything in the tube should be traveling at the same speed. Energy inputs should be needed only to overcome moving air/tunnel wall friction.
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There will need to be sidings. They haven't been designed yet (at least publicly) mainly because they are not yet needed. The first task is to get the basic part of the system working. A pod should be able to leave the main tube and move onto a siding at full speed. A door can close at each end of the siding, slower at the rear to create some forward resistance to aid braking.
When stopped doors can close close to both ends of the pod, the pod moves to the passenger platform area and another pod take its place for entering the main loop. Very little vacuum to break or recreate.
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By going into deep tunnels the problem of heat expansion disappears. Land costs drop to about nothing, just the cost of surface area entrances.
As long as explosives can be kept out of the pods then the probability of a terrorist attack drop close to zero. There is currently technology that uses fiber optic lines to detect any activity in the vicinity. The systems have been built for perimeter detection.
The fiber optic cable is buried, a regular stream of light bursts is sent down the cable. Any seismic activity within range causes a readable change in signal arrival time. The system can detect approaching footsteps and can determine human or animal. It can detect shoveling meters away. It can tell if activity is getting closer or moving away.
A similar system should make it pretty much impossible for someone to tunnel down to attack the loop tunnel.
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If there's a need to suddenly kill the vacuum perhaps the solution would be to have an additional tunnel between the outgoing and incoming tunnel. Leave it at outside air pressure or even somewhat pressurize it. (Rent the space for utility cables.)
Put emergency air exchange as determined distances. If the system needs to be quickly stopped then input air between pods all around the loop. That would help rapid braking. It would eliminate the "wall of rushing air" problem. And it would provide breathing air for pod passengers.
I think the solution for an above surface tube was to create venting doors along the loop which would kill the vacuum and provide air for passengers.
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Aisles and toilets. Needed for multiple hour journeys. We don't put toilets on our subway systems because people are not on for extended times. We might end up with smaller diameter tunnels for NYC to DC type travel and larger tunnel/pods for LA to Dallas/Fort Worth.
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While some people believe we should not travel far and/or fast they will not be able to sell that belief to the general public. People do not want to give up something they already have. If people were willing to sacrifice we would have solved the climate change problem long ago.
We have to find acceptable alternatives. For long distance travel we need an alternative that is close to being as fast as passenger jet travel and about the same cost. If what we come up with is only almost as fast and only almost as inexpensive then it will take governmental action to force the change. What we really, really need is faster and cheaper. At least as fast and cheaper or faster for at the same cost.
The 'loop looks like it could be faster. Not only traveling somewhat faster but cutting 'travel to the airport' and waiting for takeoff/taxi time. Cheaper? Way too early to know.
A lot of smart people think there's a good chance they can make it work. I think the smart thing would be to hold back on the condemnation and see what tricks they have up their sleeves.