Skytrain: would it work?

I’m reading a David Brin book called Existence. In it he has a transportation system called skytrain*. Basically a skytrain is an engineless rigid airship tethered to and pulled by a locomotive. The zeppelin floats about 300 meters above the engine and carries all the cargo/passengers. Would such a system work? I can think of some technical issues, but none of them seem to be dealbreakers. Would it be a significant savings of energy over ordinary trains?

  • Which has nothing to do with Vancouver BC’s rapid transit system except for a common name.

I’m struggling to see any advantages. It sounds like the worst of both worlds. It flies but it sucks at clearing obstacles because of its tether. It still needs all of the rail infrastructure because it has a locomotive on the ground, and the locomotive has to tow not only the cargo and whatever structure it’s contained in (as it does on a conventional train) but also a bunch of gas and probably also ballast since the airship will be tugging upwards on the locomotive and reducing its traction. And the faster you tow a balloon, the lower it will get, so the speed would be limited unless you added a lot of extra gas which would mean using more ballast and fuel.

Yeah, you’re right, Lord Feldon. I thought about it some more after posting and see the problem. They’d have to clear all obstacles near the tracks such as buildings, light poles, trees, and bits of geology. And then hope the gondola doesn’t scrape the ground. Increasing the lift to avoid that would raise the locomotive from the tracks and reduce traction (i.e. efficiency).

However, Skytrain is also the name given to elevated light rail running above existing roads in various places around the world, such as Bangkok. And that does work pretty well.

It’s not impossible, but impractical.

I suppose the idea is that you eliminate the rolling resistance from the rail. But rolling resistance of steel wheels on steel rails is tiny to begin with. And you are replacing that with the air resistance of a balloon, which will be enormous.

What about wind? A balloon wouldn’t follow directly behind a locomotive. It will be downwind of the locomotive. The locomotive needs to be heavy enough to anchor the balloon even in strong crosswind. The area around the track needs to be clear of obstructions.

And how do you board it? When the locomotive is stopped at a station, the balloon will be downwind of the locomotive, which can be any direction. You’d need a boarding platform on a rotating mechanism.

Size vs. carrying capacity will be another problem. The Zeppelin-NT is 75 m (246 ft) long and 14 m (46 ft) in diameter, and carries 12 passengers and 2 crew. If you remove the 3 engines (about 120kg each), propellers, fuel tank, control systems, etc, it can probably carry 25 or 30 people. What use is that? You could just carry them on the roof of the locomotive rather than on a towed balloon. Even a tiny railbus would have much more capability, and be significantly cheaper to operate, than this “skytrain.”

How much of the energy used to move trains is used on rolling friction? Reducing that seems to me to be the only possible benefit of a skytrain (except for the view).

Presumably to even consider this, you have to think about really large amounts of cargo. The equivalent of at least 200 trains cars. A single blimp won’t work. You’ll need a lot of big fat blimps.

Trains are already very, very efficient. The air resistance of the blimps, OTOH, seriously hurts efficiency. Without doing the Math, it’s going to be a clear loss.

And how long is the train still going to be in order to tow a bunch of blimps?

Note that in shipping, a major part of the cost is the transfer of cargo*. The easier to make this the better. How are you going to quickly put containers on/off this contraption?

  • E.g., some coal plants have rail systems that turn the coal cars upside down to dump their loads. The cars run continuously thru without stopping. They do this because it’s fast and efficient. How would these blimps quickly load or dump coal?

In the book, how was the loading and off-loading of passengers and cargo handled? When it comes to trains, you just pull up to the station, hop on, train takes off a few minutes later. you arrive, hop off, new passengers hop on, and train takes off again a few minutes later. This “Skytrain” dealie has got to be just a bit more complicated, right?

The obvious way to embark and disembark passengers would be to winch the blimp down to the train.

But this scheme includes the serious drawback that’s inevitable with airships: their vulnerability to bad weather (wind, thunderstorms, etc.). Add to this high construction and maintenance costs for the airship, the need for a track with no overhead obstructions (underpasses, wires, etc.), and a net decrease in fuel efficiency compared to a normal train. Offsetting all this is the small advantage of a nice view for passengers.

Overall, not an attractive scheme.

Can anyone who has read the book tell us what the supposed advantage of this system was?

The Zeppelin-NT I linked to above has a range of 560 miles, and has a 306-gallon fuel tank. Even if we assume 50% fuel reserve for the stated range, that’s 3.7 miles per gallon. That’s comparable to a Greyhound bus that can carry 50 people. So, fuel consumption per passenger per mile is a factor of 4 worse than a bus.

A towed balloon may be slightly more efficient than an airship (propeller vs. rail traction), and carry more passengers. But even if it were twice as efficient, and can carry twice as many passengers, it’ll still only be comparable to a bus, and much worse than a train.

Even then, it would weathervane around the locomotive. Your “station” is a circular field twice as large as the blimp.

I suppose you could anchor it with 2 locomotives, front and back, to force it to align with the rail. But if you do that, the wind forces would be far greater.

p.p.s. If the only advantage is a better view for the passengers, it would be cheaper to build very tall trains. It might look like the Brighton and Rottingdean Seashore Electric Railway. (Though 300m height is probably a stretch…)

ETA: a better solution would be gondola lift (cable car) system with 300-meter towers.

That is…fuckin’ cool.

Yeah, I’ll second this. Brin is a good enough writer, and technically-minded enough, that he wouldn’t come up with something like this without some reason.

Tunnels would suck.

Not just the tunnels. My town happens to sit on one of the Union Pacific’s main lines. We get literally dozens of freight trains through here each day. There are three viaducts over the tracks in a space of 2.37 miles (not to mention going east or west from that point.) What do you with the cable when it keeps hitting the bridges? Sure, you might be able to make it work in less dense areas, but that’s not where the freight is.

Well, the only instance so far (I’m about halfway through the book) was a passenger vehicle, although it may have also had some cargo. We didn’t actually see them embarking. We’re told about disembarking from that particular trip but

everyone has to take the emergency exits. From something like 100 meters altitude, I think.

In fact, the locomotive has a winch on it just for that reason.

As far as cargo, it did say it only took intermediate value cargo. So coal and other bulk cargo is out, as is anything that has to get there soon. So all trips may be mixed cargo/passengers. We just didn’t happen to see the cargo compartment.

He does give the size of the zeppelin, although I can’t remember exactly what it is. Larger than the Hindenberg, I remember that, although it weighs only half as much.

I seem to recall that Sir Joseph Banks imagined that hydrogen gas balloons would be most useful as a way of lightening the load of horse-drawn wagons. At first he didn’t seem to consider the advantages of free flight.

And maybe he was right in some ways; cargo balloons have never really caught on, at least partly because dirigibles get blown about a lot when they try to land. If the dirigible were already tethered to a locomotive on the ground this task might be just a little easier.

Sounds like (and is as practical as) some sort of steampunk idea - especially if the locomotive is steam powered…