[tsimionescu]

It can't be made feasible from an engineering POV, not just economically. It's virtually impossible to maintain a relevant vacuum in hundreds of kilometers of tunnel. The temperature stresses alone would cause leaks unless using some very special materials (if any exist).

[panick21_]

The original Hyperloop system is designed to not require a vacuum as strong as other vacuum systems of the past.

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.

[tsimionescu]

Well, the best way to get very high speed rail is to abandon the pumps and tubes entirely and do it in open Air, e.g. Maglev. You won't see any significant energy efficiency gains until you get to pretty low pressures, and even then that will be entirely compensated by the effort of pumping out air if you can't maintain an excellent seal.

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.

[panick21_]

I have not done the math on the energy required to create the original low pressure and maintaining it compare to how much you save.

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.

[Ekaros]

I suppose relatively bad vacuum is possible with enough energy expenditure that is pumps. What in my mind always made it stupid is throughput. First overall and then getting stuff in and out. Oh and safety... Minor thing being stuck in vacuum capsule middle of some hundred of kilometres long tube. I sure hope there is plenty of spare air...

[bluGill]

We can do it in the ISS. Not an easy problem, but it is a solvable problem. The problem is all the solutions will cost a ton of money.

[avianlyric]

The ISS isn’t kept at a vacuum and isn’t hundreds of kilometres long, and space needs no assistance from us to maintain a vacuum, gravity’s got that covered.

[bluGill]

The outside is a vacuum, the inside is at pressure. While the forces are reversed, the problem is otherwise the same.

[tsimionescu]

The ISS is not a tube hundreds of kilometers long, so I have no idea how this compares. Also, the ISS doesn't require pipes, and can easily have many segments which are airlocked from each other, which a high-speed train would not do well with.