Elon Musk's Hyperloop proposal had substantial public interest. With various initial Hyperloop projects now having failed, I thought some people might be interested in a high-speed transportation system that's...perhaps not "practical" per se, but at least more-practical than the Hyperloop approach.

aerodynamic drag in hydrogen

Hydrogen has a lower molecular mass than air, so it has a higher speed of sound and lower density. The higher speed of sound means a vehicle in hydrogen can travel at 2300 mph while remaining subsonic, and the lower density reduces drag. This paper evaluated the concept and concluded that:

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

In a tube, at subsonic speeds, the gas must move backwards around the vehicle as the vehicle moves forwards. This increases drag, but

Gap flow increases the required power by at most 36% for any vehicle and tube system for which the ratio of tube-to-vehicle diameter is 2.38.

Larger tubes give lower drag.

Compared to a tube filled with vacuum, there are multiple advantages:

• The vehicle can be supported aerodynamically.
• The tube doesn't have large compressive forces.
• Leaks would not cause rapid gas flow.
• Propellers are an option for propulsion.
• Air brakes could be used.
• Airlocks are easier to implement.

airlocks

A hydrogen/air airlock could be implemented as follows:

Imagine 2 vertical tubes, connected at the ends to form a loop. The top of the loop is filled with hydrogen, and the bottom is filled with air.

In 1 tube, there's a piston with some (low-volatility) organic liquid sealing around it. The other tube is designated the airlock tube, and has an airlock chamber in its middle with doors.

When a vehicle arrives, the piston is raised to push hydrogen down into the airlock chamber. A door opens, and the vehicle enters the airlock chamber. The piston is then lowered to push air up into the airlock chamber, and a door is opened to outside.

high-speed train problems

The faster a train goes, the straighter its tracks need to be. They must be straight on a small scale to avoid vibration; this is an engineering problem. They also must be straight on a larger scale to avoid high accelerations; this makes buying land for them more difficult, and often requires digging or elevated tracks or tunnels. The speeds that would justify a hydrogen tube would require very large turn radii, perhaps 80 km.

Costs of elevated track ("viaducts") typically range from $50 million to $80 million per mile, which is normally too expensive to use them for most of a route.

A train supported aerodynamically in a tube can have good cushioning, so vibration is less of a problem than with wheels. The other problem depends on the speed rather than the train technology, but trains with steel wheels have another problem: they can only handle small slopes, which can make routes longer or require tunneling.

A hydrogen atmosphere is only important at high speeds, and accelerating to high speeds takes some time, so that's only worthwhile for reasonably long routes. The longer a route is, the harder it is to make it very straight. Also, competing with aircraft is harder for longer routes.

Supporting trains on air has been proposed; you can see Wikipedia on hovertrains and ground-effect trains. It's feasible, but more expensive than wheels on steel rails.

tube transport problems

The biggest problem with trains that run in a tube is probably that a tube is more expensive than a track. Leaked 2016 documents from Virgin Hyperloop One estimated the cost of a 107 mile Bay Area project to be between $9 billion and $13 billion, which is $84M to $121M per mile.

I guess that's only about as expensive as California rail projects, but money was getting siphoned off from those, and projects with higher base costs would probably be even more expensive. Also, it's expensive enough that, considering all the costs involved, short flights would be cheaper.

When something breaks, vehicles can get stuck. If they're in a tube, it's much harder for passengers to exit or get fresh air. Maybe vehicles would need to carry some shaped charges to cut a hole in the tube in case of emergency.

Passengers on trains often like to look out the windows. That's more difficult when the train is in a metal tube.

tube construction

Hydrogen leaks through and embrittles steel, so aluminum is needed to contain it, but a thin layer of aluminum inside a steel or concrete tube is adequate. Pressure on the tube would probably be comparable to wing loading of an aircraft, so perhaps 1 psi.

Pneumatic tires would have higher pressures, which could be anywhere along the tube length if emergency braking was needed, but that pressure would only ever be on the bottom center of the tube, and is still much lower than what train wheels produce.

Because of the low average pressure, even dirt would provide enough support to the tube, and digging trenches in dirt is cheap; the problem with that approach is having enough stability over time to maintain good tube alignment. I think either deep piles or active control with hydraulic supports would be needed.

Gas pipelines use bends to handle thermal expansion, but that's not an option here. Short sections of corrugated metal pipe would be needed.

How cost scales with tube diameter is a good question. Supposing vehicles about as wide as a 737 fuselage, and a 2.5x diameter ratio, tube diameter would be ~9m.

power

A vehicle in a hydrogen-filled tube can't use air around it for engines, and shouldn't emit exhaust. A lot of proposals for vehicles in tubes specify linear motors, but very long linear motors are expensive. However, because the efficiency of a vehicle in a hydrogen-filled tube is so high, power isn't a big problem; a vehicle with Li-ion batteries should be able to go 3000 km at 2x the speed of a 737.

I think a reasonable approach is to use pneumatic tires with electric motors for support and propulsion up to perhaps 200 mph, then rely entirely on aerodynamic lift and propellers at the rear at higher speeds. (Preferably counter-rotating propellers with variable pitch.) If tires are filled with hydrogen, leakage through rubber is ~6x as fast as nitrogen, but that's not a major problem.

wings

The vehicles in hydrogen-filled tubes would be a type of ground-effect aircraft. They'd have propellers, very short wings with very long chord, and probably fins on the top and bottom.

At low speed, the wings would need to be close to the tube edges, while at higher speeds, more clearance would be desirable. Perhaps the large wings on the sides could have retractable wingtips - multiple smaller wings inside the big wings that can be extended out to the tube edges at lower speeds.

The low density of hydrogen reduces drag, but it also reduces lift. Most aircraft take off in higher-density air than they cruise at, but the hydrogen density would be constant and low, meaning takeoff speeds would be higher. That's compensated for by ground effect in a smooth tube and a long takeoff distance, only really limited by the maximum practical speeds of tires - which have been used at over 400 mph, but 250 mph would be a more reasonable limit for tires that need to last a while.

conclusion

I think a 9m diameter hydrogen tube for high-speed vehicles could be made for between $50M and $100M a mile, given decent construction management. That's a crude but still complicated extrapolation from pipelines and trains, and does not include land costs.

That's comparable to the Shanghai maglev train, but that only goes 186 mph, and a much longer route would be needed to get up to higher speeds. Like the Shanghai maglev, that would only make sense as a national prestige project, and it would be a much more expensive one. On the other hand, it could be cheaper than the Chuuou Shinkansen project.

Is $25 billion for a single 300-mile link that takes 30 minutes to travel a good investment? (More than that, actually, considering the land costs, development costs of vehicles, and the need for stations. And you might want 2 tubes.) In financial terms, I'd say it isn't. What makes more sense to me as a practical transportation system is large double-decker high-speed buses on dedicated roads, perhaps with overhead electric lines. But in terms of public interest and national prestige, more speed is more better, and supersonic public transport at ground level is unprecedented.