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.
You quoted:
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.