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Okay, but if you're floating under water, you've added a ton of new concerns. You're looking at a lot more than 1 atmosphere of pressure if you're under water, and a containment failure goes from stopping the transit to drowning everyone. Also you now don't have the solar panels Musk mentioned (they don't do so well underwater), and the oceans are not magically immune from the effects of earthquakes. Tsunamis can most definitely cause plenty of damage.

LA is also 17 miles from the coast, so if you're getting from downtown SF to downtown LA in 30 minutes, you've got less than 15 minutes to travel nearly 400 miles underwater in order to leave enough time to even attempt to get to LA under the deadline. You're now travelling at Mach II, almost twice as fast as Musk proposed.

If he really meant the actual downtown of Los Angeles, then there is definitely a problem getting out of there in time to make anything feasible. And all of the ground, air and underground around there is going to be an issue. Heck, http://en.wikipedia.org/wiki/Westside_Subway_Extension shows 4-8 billion just to extend a subway through part of it.

As for oceans, I think the risks are less than you think. A tsunami does great damage when it hits land, but in the open ocean you just bob a few inches. The mechanics behind that are simple - a wave coming in to shore starts to get bigger once the depth of the land is the same as the depth of the wave. Tsunamis are very deep waves.

I don't pretend to know what Musk has been hinting at. I don't see why you're clinging desperately to the "not an evacuated tunnel" constraint while dismissing other constraints you don't like, though. I assume Musk said "downtown" intentionally, since he said it twice (once for each city). Maybe his idea simply isn't feasible, given the constraints he's mentioned, but if we're going to dismiss inconvenient constraints, "Mach I without a vacuum" seems like a good place to start.

As for oceans, a Tsunami is still going to put a lot of stress on a 400-mile underwater tunnel. And at the points you get in and out of the tunnel, you're going to be at the shore anyway, where the Tsunami is its strongest. Also boats.

...but if we're going to dismiss inconvenient constraints, "Mach I without a vacuum" seems like a good place to start.

OK, let's discuss Mach 1 without a vacuum. A condition that I had not through, but which actually makes surprising sense as a design limit with the system that I am suggesting.

The Chapman–Jouguet condition says that a supersonic shock wave travels at the speed of sound in the air behind it. The ideal gas law says that this speed is almost entirely determined by temperature, not pressure. If you were to go anywhere above Mach 1, the temperature behind the shock wave gets hotter than room temperature very, very quickly. At Mach 1 exactly you should be able to maintain a thin (~0.2 micrometers is quoted by Wikipedia as the measured value) bow shock.

That shock can trap a huge pressure differential behind it, which could allow an open container to maintain normal air pressure while moving through a much lower density of air. If you are moving below that speed, the pressure differential that you maintain is much less than if you speed it up. Therefore for efficiency you'd want the vehicles to be moving at Mach 1 relative to the gas. Assuming that the gas is dragged along, this would let you actually go slightly faster than Mach 1. But if we assume that we have a lot of tunnel and occasional objects, the air would move much closer to the speed of the tunnel than the objects moving through, and therefore the top speed would be approximately Mach 1.

The exact relationship between the air pressure of the tube and the air pressure behind that flap is very complex. But with the flap design that I suggested, at pressures above 1 atmosphere there would be leakage and an opportunity to remove air. If the pressure fell below 1 atmosphere, that opportunity would disappear. Therefore the flap system that I suggested would do a good job of letting air leak out of the tunnel at about the necessary rate to maintain 1 atmosphere inside of the open containers as the speed of the whole system increases and air leaves the tunnel. (It should not be hard to design ways to bring air into the tunnel...)

This reinforces the point that the tunnel need not be evacuated, and explains why Elon said it would run at the speed that he predicted.

To mince words, Musk used the word "tunnels" (and not "tubes"), perhaps intentionally?? Tunnels are know for leaking water (and air), while tubes (or pipelines) are implied to not leak.

At any rate, the subject of Evacuated Tube Transport Technologies (ET3) tm was disclosed in US patent 5,595,543 in 1999. For the record, Coanda's system was pneumatic (and took alot of power to move the air). Search for "evacuated tube transport" on youtube for an overview. I have not noted any claims for 'hyperloop', that are not exceeded by ET3 (tm). see www.et3.com and www.et3.net

"You're looking at a lot more than 1 atmosphere of pressure if you're under water"

There are already some very long vehicular tunnels at depth (the Chunnel, Seikan Tunnel, etc.). I don't think one atmosphere more or less would be a showstopper.

The point is that if you can deal with many atmospheres of pressure, you would just do a proper evacuated tunnel and get the benefits of lower friction, etc., instead of trying to find a way to make it work with air still in the tunnel.

The Chunnel also cost about $17 billion in today's dollars and is only 31 miles long (including the above-ground parts). I don't think this is a feasible model for long-distance transit, especially when Musk claimed a cost of $6 billion for SF to LA.

The Chunnel was also built by governments with contractors, subcontractors, sub-sub contractors and so on.

With SpaceX Elon has demonstrated an ability to reduce costs by a factor of 10 in large part by eliminating the use of contractors. If you look on the PandoDaily interview, just before the Hyperloop is discussed he was railing on about how inefficient the 405 highway expansion is. I wouldn't be surprised if he thinks that he could save an even bigger factor in that kind of public work than he did with SpaceX.

I'm not saying that he's right. I'm saying that his cost projections are extremely unlikely to start with existing construction projects as a base line.

I dont know about 'hyperloop' but ET3 (tm) tubes (if placed underground) require less than 1/15th as much earth removal than a train tunnel or freeway tunnel. Our patented (US 5,950,543 and others) ET3 system optimizes tube size to strike a balance between low cost and high utility (as cars do), we use a freeway network design philosophy (not like a train track) see www.et3.com and www.et3.net to learn more.

Far easier than an evacuated tunnel, though, is a tunnel built on standard-pressure hydrogen/helium. Light gasses have a much higher speed of sound, and their lower density means lower drag. A system of hydrogen-filled maglev tubes that can max out at 2500mph with freight-weight loads over long hauls would be more than a little bit valuable.

A tunnel filled with hydrogen seems to be a very bad idea.


A tunnel filled with hydrogen has a large surface area for potential leaks.

I don't think I need to debunk what happened to the Hindenburg yet again (and, incidentally, what didn't happen with it, its sister-ship the Graf Zeppelin II or its ancestor the Graf Zeppelin I in 682 other flights).

Suffice it to say, the challenges of an electrically grounded hydrogen tunnel lined with heavyweight materials using modern technologies & sensors are a hell of a lot different than the challenges of an ultra-lightweight, ultra-thin flammable gas bag before the advent of plastics, in an electrical storm.

A tunnel filled with vacuum has it a hundred times worse, because you're dealing with 15PSI trying to get in rather than 0PSI pressure difference diffusing out. We already regularly use larger-molecule methane at thousands of PSI.

I'm not sure what exactly you mean when you say "debunk what happened to the Hindenburg". The thing became engulfed in flames and crashed. Not much debate on that. Or are you talking about debunking the claim that hydrogen airships are fundamentally unsafe, a claim I didn't actually make? (Incidentally, the fact that other hydrogen airships didn't suffer the same fate doesn't mean they were actually safe. Lots of homes have aluminum wiring but have managed to avoid burning down. You'd still be wise to replace any aluminum wiring you find in your home.)

Obviously a modern hydrogen tunnel would be much different than a zeppelin, and have different challenges. Still, 400 miles of 10-foot diameter tunnel would have plenty of opportunities for problems (whether it's evacuated or filled with hydrogen or something else). I'm not sure an evacuated tunnel is necessarily worse from a safety standpoint, though. It might be harder to engineer, but the failure mode seems safer. A hydrogen tunnel means that a leak will mix hydrogen with an oxidizer. If it's at 0 PSI, this could mean a leak lets oxygen into the tunnel (either due to high-pressure weather or due to hydrogen rising from the leak, leaving a low-pressure environment in the tunnel). If you wanted to fill a tunnel with hydrogen, I think it would be wise to keep at least a small positive pressure to ensure than any leaks are outward only.

I'm totally unclear why methane stored at thousands of PSI is relevant. Is there some 400-mile long, 10-foot diameter, above-ground methane tunnel I'm not aware of?

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