So I have a question to anyone who might know.
Is there any reason to go with low-pressure tunnels at all? For example, having a plexiglass tunnel at .5 atm doesn’t sound that dangerous to me, and it should be easier to build and maintain, but does it actually provide any irl benefit? Like what’s the production costs/train speed/energy savings relation here? What’s the highest low pressure that starts to make sense? Like, do you have to go down to .01 atm, or can .1 or .5 provide enough of a benefit? If not now, what kind of material advances might help?
Just curious about long-term feasibility of that whole thing.
I think it will depend on future technology and that it is not feasible nor cost effective with current technology (or it would be built already).
Having a consistent pressure (fully tunnel or fully open air) might help, as the shinkansen needs the very long-nosed trains to help with the ‘pop’ entering or leaving a tunnel.
However that could also be alleviated with the concrete tubes at the end of each tunnel that gradually get more enclosed
@perviouslyiner @fearout The *ORIGINAL* concept was a low pressure tube with a train in. If this could be done it could reach aviation speeds. I make no comment on how low the pressure would need to be for say 700mph, but this is *hard*.
However, as a comment early on in the link explains, nobody has built one because it’s hard. Billionnaires who get interested in this end up building very short train lines. Elon Musk’s Hyperloop is a system of trucks that carry cars, which is just a ridiculous idea.
Having said that, we need more trains.
@perviouslyiner @fearout IMHO there is no good reason for people to travel at more than 300mph or so. There are no sustainable ways to do so with current technology, and the fastest way to cut aviation emissions is to stop flying. Flying is much too cheap given the harm it causes, and public transport is too expensive given how efficient ecologically it is.
That does mean much less travel between America and Europe. But so what? You can holiday closer to home. If your relatives moved several thousand miles away, they probably don’t want to see you anyway. 😀
Hi, aerospace engineer here. As far as benefits go it depends.
If we assume the tube is constant volume and constant temperature. The ideal gas law says that in this case, the pressure would change proportionally with density. So if you lower the pressure by 50% the density should lower by about 50%.
Drag force is also proportional to density. So a 50% decrease in density will result in a 50% decrease in drag. This is true for subsonic speeds. The speed of sound is 343 m/s or 770 mph.
Drag also has a square relationship with velocity. So drag gets extremely high when there is an increase in velocity.
If we take the speed of the shinkansen(90 m/s or 200mph) as a baseline and lower the pressure by half. The new speed the Hyperloop would be able to travel with the new speed is 127m/s or 284 mph. That is faster 40% for the same amount the trains will have to work, but to build all of that infrastructure, spend all the money creating a lower pressure environment and maintain that pressure for thousands of miles is just not worth it. The vacuum tube is just not practical to make.
Edit: If you maintain a reduced pressure and increase speeds about 30% of the speed of sound, the subsonic equations I used start to be less accurate. But in that case drag increases dramatically in transonic and supersonic regimes.
This is the kind of actual discussion that I hope for in these discussions. While many people focus on the dangers of the vacuum tube proposed for the Hyperloop infrastructure, I always wondered about the benefits. It’s not like putting a train in a vacuum will suddenly make it go infinitely fast.
So, the question is how much faster would it go? Once you have that number, you can adjust the car vs plane vs train chart that CityNerd showed off. All it would do is deepen and lengthen the railed transit curve some amount. It would potentially increase the distance two cities could be and still provide a benefit over airplane travel. It’s just a question of how many city pairs it would help to include as a rail option.
Going from 200 mph to 284 mph won’t make that much of a difference. Yes, it’ll open up more city pairs for high speed rail, but when comparing those benefits against the cost of the massive tube construction it’s not going to seriously pencil out as a net benefit.
Here’s the video where CityNerd lays out their reasoning and charts a rough model of where high speed rail is going to be a more reasonable choice for travel based on the distance needed to go: https://youtu.be/pwgZfZxzuQU?t=477
@azimir @Erismi14 I’d be interested in seeing “the cost of building a massive tube” compared to “the cost of building a massive highway”.
DOTs across the country have been using phony math to justify ludicrously expensive highway projects for decades – given a train in a tube would be higher speed and higher throughout, I feel like using their same logic we’d see huge “economic benefits” from connecting two new business centers with a transport mode that allows workers to work in-transit.
I think if they are doing a vacuum tube, they should get as close to a vacuum as possible.
I think if the USA is going to spend trillions on rail infrastructure, I think we should start with doubling or tripling the amount of trains on Amtrak first. It’s not as sexy as the Hyperloop, but it would get people riding trains more often