This is a linkpost for https://www.bhauth.com/blog/industrial%20design/hydrogen%20tubes.html
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In Germany, we are talking about building pipelines for hydrogen transport. Would those hydrogen tubes dual use so that you can transport a lot of hydrogen with them as well?
Tubes for this would be more expensive but could be used for piping hydrogen. Pipes for moving hydrogen would be much smaller and at high pressure.
Gas pipelines have lower losses at high pressure, which is why natural gas pipelines are typically >40 bar.
Possibly of interest: the fastest rocket sled track uses a similar idea, they put a helium filled tube over the final section of the track:
Just as meteors are burned up by friction in the upper atmosphere, air friction can cause a high-speed sled to burn up, even if made of the toughest steel alloys. An engineering sleight-of-hand is used to increase those "burn-up" limits by reducing the density of the atmosphere around the track. To do this, one needs a safe, non-toxic, low-density gas such as helium. Helium is only one seventh the density of air, significantly reducing friction between the high speed sled and the atmosphere. Enter the "helium bag" concept.
No one person takes ownership of the idea, so it was probably a combination of brainstorming and inspiration. But like any elegant engineering solution, simplicity is at its heart. It involves enclosing a portion the track with a plastic sheet, not unlike the plastic drop cloth found at the hardware store. This tube is sealed off and pumped full of helium to force out the air. A helium-filled tube that can stretch for more than a mile then covers the track.
Shows that the idea can basically work without any advanced technology. I think this is a video of it in action, where the white thing above the track is the polyethylene tunnel which is then destroyed as the sled goes through it:
A vehicle in a hydrogen-filled tube can't use air around it for engines
Why not? Your "fuel" tanks could simply carry oxygen to burn the surrounding hydrogen "air" with.
and shouldn't emit exhaust.
Exhaust would be water vapor, easily removed even passively via condensation and drains. Hydrogen will (of course) have to be replaced to maintain pressure.
It can't use "air" around it for engines because what's around it isn't "air".
Oxygen is much heavier than the fuel it's used with, and you'd either need liquid oxygen (which increases costs) or pressurized tanks (which would perhaps double that mass). That's still lighter than batteries, yes, but engines are also needed. Piston engines are inefficient and/or heavy, and gas turbines are somewhat expensive.
It's not that difficult to separate water and hydrogen, that's true, but processing that much gas is still rather impractical when batteries have enough specific energy. Simply condensing it in the tube is...possible, but would increase drag, especially considering density variation issues, and you'd have to deal with getting it out of a long sealed tube without leaking hydrogen.
Also, if batteries are good enough, the cost of replacing the hydrogen alone probably makes batteries better than burning the hydrogen.
Condensation is not just possible but would happen by default. You described the tubes as steel lined with aluminum in contact with the ground, if not buried. That's going to be consistently cool enough for passive condensation.
Getting water out of a long tube shouldn't be hard with multiple drains, and if there's any incline, you just need them at the bottom. You can just dump it in the ground. Use a plumbing trap to keep the gasses separated. They're at equal pressure, so this should work, and the pressure can also be maintained mostly passively with hydrogen bladders exposed to the atmosphere on the outside, although the burned hydrogen will have to be regenerated before they empty completely, but this can be done anywhere on the pipe. Hydrogen can be easily regenerated by electrolysis of water, which doesn't seem any more expensive than charging the batteries. It might be even cheaper to crack if off of natural gas or to use white hydrogen when available.
Are turbines more expensive than electric motors for similar power? It's true that conventional piston engines are heavy, but batteries are also heavy, especially the cheaper chemistries.
Alternatively, run electricity through the pipe to power the vehicles so they don't have to carry any extra weight for power. It's coated with conductive aluminum already. If half-pipes could be welded with a dielectric material and not cost any more that would work. Or use an internal monorail, but maybe only if you were going to do that already. Or you could suspend a wire. That's got to be pretty cheap compared to the pipe itself.
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1…run electricity through the pipe…
Simpler to do what some existing electric trains do: use the rails as ground, and have a charged third rail for power. We don’t like this system much for new trains, because the third rail is deadly to touch. It’s a bad thing to leave lying on the ground where people can reach it. But in this system, it’s in a tube full of unbreathable hydrogen, so no one is going to casually come across it.
Using sliding electrical contacts for power is fine for current high-speed trains, but it doesn't work as well above 200 m/s.
You quoted:
the vehicle can cruise at Mach 2.8 while consuming less than half the energy per passenger of a Boeing 747 at a cruise speed of Mach 0.81
This is not how Mach works. You are subsonic iff your Mach number is smaller than one. The fact that you would be supersonic if you were flying in a different medium has no bearing on your Mach number.
I would also like to point out that while hydrogen on its own is rather inert and harmless, its reputation in transportation as a gas which stays inert under all practical conditions is not entirely unblemished.
The beings travelling in the carriages are likely descendants of survivors of the Oxygen Catastrophe and will require an oxygen-containing atmosphere to survive.
Neglecting nitrogen, you have oxygen surrounded by hydrogen surrounded by oxygen. If you need to escape, you will need to pass through that atmosphere of one bar H2. There is no great way to do that, too little O2 means too little oxidation and suffocation, more O2 means that the your atmosphere is explosive. (The trick with hydrox does not work at ambient pressure.)
Contrast with a vacuum-filled tunnel. If anything goes badly wrong, you can always flood the tunnel with air over a minute, going to conditions which are as safe as a regular tunnel during an accident which is still not all that great. But being 10km up in the air is also not great if something goes wrong.
Barlow's formula means that the material required for a vacuum tunnel scales with the diameter squared. For transporting humans, a diameter of 1m might be sufficient. At least, I would not pay 42 times as much for the privilege of travelling in a 6.5m outer diameter (i.e. 747 sized) cabin instead. Just lie there and sleep or watch TV on the overhead screen.
Those Mach numbers are for the relevant speed in air. I would have written that differently, but that's how the cited paper worded things.
Mostly-sealing against part of the tube before cutting it is less problematic than dealing with a large pressure difference.
Aerodynamic support and propulsion in hydrogen is less expensive than magnetic propulsion and support in a vacuum-filled tube. Building an unpressurized tube is cheaper than a tube that doesn't buckle under compressive forces. And so on.
Maybe vehicles would need to carry some shaped charges to cut a hole in the tube in case of emergency.
That would likely create sparks, and provided the tube has been cut the hydrogen is going to explode.
Hydrogen can only burn in the presence of oxygen. The pipe does not contain any, and combustion isn't possible until after they have had time to mix. It's also not going to explode from the pressure, because it's the same as the atmosphere. The shaped charge is obviously going to explode, that's the point, but it will be more directional. That still doesn't sound safe in an enclosed space. Maybe the vehicle could deploy a gasket seal with airbags or something to reduce the leakage of expensive hydrogen.
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