=transport =mechanical engineering =science fiction =design
Back in 2007, I got paid a few
dollars for a SF story that mentioned a train traveling in a tube filled
with hydrogen.
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.