h a l f b a k e r yWarm and Fussy
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The idea of a maglev train inside an evacuated tunnel is regarded as a gold standard of transportation: with no aerodynamic drag, you can get from point A to point B at fantastic speed with only the energy required to change altitude; in fact, if the destination is at a lower point, you might at the
end have a net *gain* in energy.
Realistically though, it's hardly going to happen: for starters the tunnel has to be able to stand up to a constant outside pressure of (at ground level) 14psi, and be constantly kept at a vacuum. The train has to *contain* a constant pressure against a vacuum. This means that if things go wrong, then things *really* go wrong.
So...
The idea is pretty simple: fill the tunnel with H2 and make the train carriages airtight (perhaps a slight overpressure).
H2 is cheap, easy to produce and is 1/15th the weight of air which means 1/15 the effort to push the atmosphere aside at the front and pull it back together at the rear.
Inspired by <link>
edit: and shot down as conceptually baked by <other link>... darn.
and previously hypothesized in a link description by [NotationToby]
I don't believe I'm saying this, but
Sky_20Tube
Inspired by "Jim" [FlyingToaster, Apr 15 2011]
darn
http://www.superson...oinhydrogen_STV.pdf
describes a plan for a fuel-celled turbofan vehicle [FlyingToaster, Apr 15 2011]
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nooooooooooooooooooo - oooooooooooooooo - oooooooooooooo |
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Surely it's not supersonic; rather it is partly able to travel faster because the speed of sound is greater in hydrogen. Travelling at speeds that would be supersonic somewhere else is not travelling at supersonic speeds. |
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[NT]'s link doesn't work for me. |
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I was confident that this would be baked, at least in concept. People have messed around with filling train tunnels with steam, partial vacuums, etc. for long enough for someone to have thought of hydrogen ages ago. |
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he musta screwed it up somehow, but the link from his other link works to get the pdf. |
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And he said the thing about being subsonic inside the tube there. I swear I didn't cheat! |
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... re: the STV, it seems a bit counter-intuitive to me that a turbofan operated by an onboard fuel-cell would or could be more efficient than a maglev. |
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I think a turbofan can be rather efficient - 0.95 or so. The efficiency of a good propeller should approach the glide ratio of a good wing, and if the blades' tips are close to the tube, tip losses are reduced. |
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I realise this would be very fast. But if it's claimed to be supersonic, then I should be able to carry around a vial of cold uranium hexafluoride and claim that my bike is supersonic. |
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There's the Jimi Hendrix song in which he says "no throwing cigarettes butts out the window." which would be good advice for this idea. |
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Damn, I'm doing a roaring trade in time hops so people can get their idea in first.. |
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Great for transporting anaerobic lifeforms, or "cargo only" transport. |
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Aerobic lifeforms get their say too: two thousand miles an hour is umm two thousand miles in an hour: a bit of compressed air and some duct tape around the edges of the carriages.... |
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Leave the air out of it.... |
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in fact there are quite a few things to leave out of it, protons being what they are... |
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I thought we were leaving the neutrons out of it, hydrogen being what it is. |
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I work with hydrogen on a weekly, if not daily, basis, and I have only one thing to say about it: |
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Fast-moving objects of sufficient size to transport cargo or passengers must either be constructed using prohibitively expensive hi-tech materials, or with a far more affordable and readily available conventional material called steel. If a piece of steel strikes any reasonably hard object it produces sparks. Unless this train and the hydrogen-filled tunnel are both made out of an ingeniously-engineered composite of carbon nanotubes and vat-grown gummy bears, I'm not riding it. |
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Of course, this critique is fairly hypocritical, given some of the outlandish ideas I come up with. |
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As with most ideas involving hydrogen as anything other than a fuel, I offer my standard alternative: helium. Almost as light, and no go boom. |
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In this case, the difference in density is rather important. Twice the density means half the speed. |
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Of course, the hydrogen in the tube can't burn, as it's too pure. If the train or the tube leaked a lot, then combustion could be supported. |
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//I work with hydrogen on a weekly, if not daily, basis, and I have only one thing to say about it:// |
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Ahh, so you are weekly interacting... |
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It was noble of you to say so. |
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hydrogen will not ignite unless mixed with an oxidizer. |
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That's what I said up there. |
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//the hydrogen in the tube can't burn// |
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True; I suppose I was thinking about a scenario involving the rupture of either the tube or the pressurized cabin, but I completely failed to illustrate that crucial detail of my argument. Also, my aforementioned weekly interaction with hydrogen is in the form of plasma, so whenever somebody mentions it, I automatically think of fire. |
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Re: the drawbacks of helium; sure, it would limit your train's top speed to only about 1,000 mph (a figure %100 pulled out of my hat), making it just too slow to be any good to anyone. Also, before anyone jumps on me, I know that helium is much more expensive than hydrogen (I have a couple of bottles of that kicking around, too). I just think your added expense could be re-couped on the other end, i.e. when something inevitably goes wrong and your 1,000+ mile long tunnel _does not_ spectacularly erupt in a ribbon of flame visible from high orbit. |
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Actually, it's even worse - half the speed, with the same power consumption, resulting in twice the energy use per unit distance. (Wait, that's not quite right.) The other drawback of helium you've already mentioned at the end of your last annotation. All round, less bang for your buck. |
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Half the speed, with the same power consumption, yes. But still a considerable improvement over a tunnel filled with plain old air, which I believe was the whole point in the first place. |
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And it's less bang for _more_ buck. A lot less bang. (took me a second to get that; lol) |
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Sorry, I goofed. Half the speed, same thrust, therefore half the power, but the same energy per unit distance. |
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Another benefit of hydrogen, mentioned in the second link, is that you only have to carry the oxygen in the train; the hydrogen in the tube is the fuel. |
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Speaking of the second link: |
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Oldtimer #1: That's torn it!
Oldtimer #2: Darn it! |
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That's one of mine. Ithankyou. |
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Dangerous, ill considered, likely to kill thousands if
put into practice, involving three or more of: high
speed, trains, space, advanced materials,
submarines. On the whole a big, big bun. |
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//drawbacks of helium//
But if everyone starts speaking in a squeaky voice then at least you know there's a cabin leak. |
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In fact that might be fun, just when the executive in front of you is just getting to the really serious part of the huge merger negotiations on their mobile phone... |
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...which clearly creates the opportunity for an app to correct the speakers voice in the event of helium leakage. |
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A better use for a tube filled with hydrogen is in a space cannon, for lofting projectiles into space a la Jules Verne. |
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Hydrogen embrittlement and high speed passenger vehicles are probably not a good mix. |
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Boom --- helium is better... |
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Now Jim reckons there is a competing idea --- but you are going to need a weather balloon to play... |
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There's no "boom": the oxygen has to get to the hydrogen first before there's any chance of combustion. Even worst-case scenario, a lightweight tunnel made of carbon-fibre, or even a simple envelope, compromised and set aflame, it would be mildly impressive for a few seconds but there'd be much more damage from a moving train suddenly hitting heavy heavy air than there would be from the H2 going up in flames. |
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//embrittlement//
My new mot du jour. |
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Air can be introduced anywhere in the case of
emergencies. It could be used to slow the train down as
well. However the a/c needs to be a little different. |
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PS When Concorde used to fly (made partly by the French),
the cruising altitude was getting on for 60,000ft. The
external air pressure at that time was around 1psi. And it
had windows, although they were small to slow down any
explosive decompression which might have caused the
Champers to froth up a bit too much. |
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1. Moving people is passe. People are fine right where they are. But cargo gots to move. |
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2. Small and unsafe pipe would be fine for cargo. |
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3. Consider: hydrogen filled pipe. Cargo train has onboard oxidizer - maybe H2o2 or compressed O2. Hydrogen encountered in front of train enters combustion chamber ramjet style, oxidized and is blasted out the back as superheated steam. |
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Not only do you get the low resistance of hydrogen, the train itself decreases resistance directly in front of it by consuming local hydrogen, sucking itself forward in addition to ramjet in the back. It needs to carry only half the fuel. |
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Also, wiggly little hydrogens leak out and would need to be replenished anyway. When steam condenses on cold tunnel walls, pressure falls and facilitates replenishment with new hydrogen for next train. |
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All right Voice, you can ride in it. Bring your scuba gear. Don't stomp the bananas, now; we have to sell them. |
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// It could be used to slow the train down as well//
That's a really good point - you could hardly expect vacuum brakes to work. |
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Where do the exhaust gases go? I assume you just
wait until the water condenses and trickles down a
drain. |
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//Half the speed, same thrust, therefore half the
power, but the same energy per unit distance.// No,
same energy per unit time, but for longer due to
lower speed, so more energy. |
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No, first the shot, then the powder ... |
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It's actually a bit arbitrary. If you kept the power the same, then the speed would be between 1/2 and 1, and the energy per distance would be greater, as you say. If you kept the thrust the same, then the energy per unit distance would be the same (by definition), and the speed would be 1/2. I assumed the latter, by assuming in turn that the mach number was the same (slightly less than 1) in each medium. |
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Okay, if we're now talking about propelling the train using hydrogen combustion, why not introduce it in intervals; something like a V3-ish 'pump-gun?' I'd think it would save on the amount of gas consumed, and if timed correctly, wouldn't create the lurching effect familiar to all those who have ridden the Boston T. It might even be used to slow the train as well, by introducing heavier inert gasses, although I've only given that second concept about 3.5 seconds of thought. |
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[spidermother] Thrust is power. I'm not sure what
you think it is. |
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edit: actually thrust is force, knowledge is power. |
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Thrust is force. It's measured in Newtons or equivalent. Thrust times speed is power. |
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Obviously, some power is consumed in generating the thrust in addition to the thrust-times-speed power, but that's simply inefficiency. |
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