https://www.tesla.com/sites/default/files/blog_images/hyperl...
> Another extreme is the approach, advocated by Rand and ET3, of drawing a hard or near hard vacuum in the tube and then using an electromagnetic suspension. The problem with this approach is that it is incredibly hard to maintain a near vacuum in a room, let alone 700 miles (round trip) of large tube with dozens of station gateways and thousands of pods entering and exiting every day.
> All it takes is one leaky seal or a small crack somewhere in the hundreds of miles of tube and the whole system stops working. However, a low pressure (vs. almost no pressure) system set to a level where standard commercial pumps could easily overcome an air leak and the transport pods could handle variable air density would be inherently robust. Unfortunately, this means that there is a non-trivial amount of air in the tube and leads us straight into another problem.
Basically, the only energy efficiency can come from the seals, since those essentially allow you to store the energy spent to move the air out of the way of the train ahead of time. Otherwise, whether you move the air by pushing the train through it, or move the air by pumping it out in front of the train, the total energy expenditure will be similar.
And again, the problem of maintaining even a somewhat low pressure in a hundreds of kilometers long above ground tube with no airlocks is well outside our current engineering capacity.
You speak pretty confidently about that, can you actually show any of that?
It also depends on how high the utilization of the tunnel is.
Who says there are no airlocks?
The proposal has been simulated by both SpaceX and Tesla, I don't think they made some basic mistakes about it being impossible to sustain a low pressure tube.
