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Here is an exercise for you.

Go to http://pandodaily.com/2012/07/12/pandomonthly-presents-a-fir.... Jump to 43 minutes in. Listen for two minutes. Come back and explain how to explain that interchange WITHOUT having a tunnel. If you listen for slightly longer when he talks about storing energy in the system so that it can run 24x7, it seems pretty obvious that whatever the system is, it has a lot of weight moving very fast.

We also know, because Elon Musk has said so, that, It is not an evacuated tunnel.

If it is going to be reasonably efficient, it can't have a lot of air in it.

So those are the design constraints for what Elon is thinking of. What can you propose?

Now why would you not want an evacuated tunnel? Well the common complaint about an evacuated tunnel is how hard it is to maintain the vacuum, and what to do if containment is breached. My proposal gives a way to maintain the vacuum (the plungers). Minor breaches of containment are not that big of a problem (the air that gets in is blown out again).




> Come back and explain how to explain that interchange WITHOUT having a tunnel. If you listen for slightly longer when he talks about storing energy in the system so that it can run 24x7, it seems pretty obvious that whatever the system is, it has a lot of weight moving very fast.

Explain that exchange without it being a giant pneumatic tube like the Jetsons. If we take it at face value, he basically agreed that it's a pneumatic tube, but I'm pretty sure that's not really what he's thinking of, and you seem to agree, given that your proposal is absolutely not a "Jetsons tube".

I agree that storing energy without batteries likely means a lot of moving mass. That doesn't mean it's a big weight flying through a tunnel. It could also be a flywheel or something else. Or it could be that the "moving mass" is an incorrect guess and it's something chemical, but not in a traditional "battery" (e.g. a breakdown of water into component gases).

> We also know, because Elon Musk has said so, that, It is not an evacuated tunnel.

We also know that Elon Musk tends to propose (and build) practical solutions. It seems unlikely that he would be proposing what amounts to an evacuated tunnel with unnecessary complexity tacked on. If he's stating that it's not an evacuated tunnel, then it's presumably not just a semantic game.

> If it is going to be reasonably efficient, it can't have a lot of air in it.

So he's designed pointless complexity into the system just for the sake of saying it's not an evacuated tunnel. "It's not evacuated. It just doesn't have a lot of air in it."

It's be a lot simpler if he just proposed a traditional partially evacuated tunnel if that's what he wants.

> So those are the design constraints for what Elon is thinking of. What can you propose?

My lack of a proposal does not make yours feasible or sensible.

> Now why would you not want an evacuated tunnel? Well the common complaint about an evacuated tunnel is how hard it is to maintain the vacuum, and what to do if containment is breached. My proposal gives a way to maintain the vacuum (the plungers). Minor breaches of containment are not that big of a problem (the air that gets in is blown out again).

Your tunnel has all the same problems as an evacuated tunnel and more. A standard evacuated tunnel also has a way to maintain the vacuum. It's called a pump. This doesn't stop a breach from being problematic. You're ignoring the fact that pushing all that air out of the way again is hard, whether it's being done with a pump or a piston. If there's a breach, your "plungers" will run into huge pockets of air that have to be evacuated. This will slow them down, potentially causing collisions depending on how large the breach is. It will actually slow them much more than an evacuated tunnel, because they form a plug in the tunnel, meaning they need to displace most of the air instead of some of it.

Your plungers also have to form a tight seal in order for them to be effective. I'm not even sure that's possible. 70 tons of mass flying through a tunnel with a tiny gap all around. What happens when we hit that breach and this mass wobbles a bit? How big does the gap have to be to ensure safety? How big before the plunger isn't actually expelling gas anymore? And you admit that the friction the plungers encounter will make them hot enough to be deadly to humans. This is all wasted energy.

P.S. You also said this thing is going to be underwater. How on earth do you imagine than an underwater breech would be minor? And how is this air being expelled underwater anyway? Are you planning on building an exterior tunnel big enough to hold an air gap plus the transit tunnel (plus the return tunnel) all underwater? For a tenth of the cost of a bullet train?


Why are you inventing constraints that don't exist?

Elon's pauses make it clear to me, and presumably to you, that he was checking what she said versus what he was thinking, and finding that, to a layman, the two would seem similar. Even if underlying technologies are different.

His wording suggested strongly to me that his solution was a tube. Did it not suggest that to you?

No matter how many times you say "pointless complexity", that doesn't mean that there really is pointless complexity. (And we're disagreed on the complexity.)

I'm quite aware of how hard it is to push air out of the way. A quick comparison of the mass of the plunger and the mass of the air says that a bubble of air will not actually slow the plunger down much. Being in air constantly would, but a bubble of fixed size wouldn't. (You'd have to think about what effects it might have on passengers inside.)

There is absolutely no need for a tight seal, as long as the flaps open and shut at the right time. You can stand a long ways away from a moving truck and still feel the wind pushing away. And the closer that it gets to Mach 1, the farther away you can feel that gust.

The friction losses are hard for me to estimate. The upper limit on the temperature of the plungers is caused by the heating of air that is suddenly compressed to 1 atmosphere, and the friction from air moving past at hundreds of miles per hour. However if only a little bit of air is involved, the heating might happen at reasonable rates. Without parameters that I don't have and a fluid dynamics simulation that I am not prepared to do, I can't tell. That said, I can tell you that the rate of heating per volume (and therefore the ease of dealing with it) goes down as you make the plungers larger, up as you speed up, and down as you reduce air pressure. So if you have little enough air and large enough objects moving through it, the heat generation gets more and more reasonable.

And about your PS, I'm trying to guess at how Elon gets around right of ways. Underwater is not part of my basic idea at all.


What constraints do you imagine I'm inventing? That it's practical?

Musk's pauses indicate to me that he was internally asking himself whether his proposal is somehow like the Jetsons tubes. That doesn't necessarily mean he was looking for literal parallels. And if he were, I don't see why "pneumatic tube that sucks a single person up" would turn into just "tube" to a layman, either. He's also mentioned the Concorde and a railgun. How do those fit into your vision? Will a layperson see those in your car/tunnel system?

I'm not just saying "pointless complexity". I'm telling you why it's pointless. Your system is certainly more complex than an evacuated tube, and without a compelling reason to embrace such complexity, then it is indeed pointless.

Let's step back a bit, though. You're saying that your system is easier to implement than an evacuated tube. Let's assume that's true (though I don't think it is). Is it also easier to implement than a partially evacuated tube? Those are expected to travel at or past Mach I, and don't need the car to move nearly 100% of the air in the tunnel. How is your proposal superior? You talk about using the plungers to evacuate air, but the energy cost to remove the air must still be paid. Your 70 ton slugs might carry more momentum, but a car in a partially evacuated tube wouldn't need to displace all the air it encounters, and so wouldn't need as much momentum. You're constantly paying the cost to push around a bunch of air. That cost is largely avoided with a more traditional design.

I'm not certain about your idea that you don't need a tight seal. Yes, at Mach I a gust would blow pretty hard. But in a near-vacuum you'll not be blowing much air. Remember that you're going to have to blow with >1 atmosphere of pressure to push anything out and past the flaps. Also remember that this "gust" is the result of resistance caused by the giant plug you're pushing down the tube, resistance that another design might just avoid or at least minimize. And any air not evacuated by the "gust" will contribute to further drag as the plunger pushes past it.




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