New planes, not super trains.

I’ve come to conclude that airplane travel makes a lot more sense than high-speed trains over any reasonably long distance. Consider the marginal energy cost of transporting a 90kg (200 lb) person on a 737–800, the most common commercial jet in US service. For this plane, the ratio of lift/drag at cruise speed is 19. The average lift/drag over a short trip will be somewhat lower, perhaps 15 because take-off and landing are less-efficient than cruise flight, but even at this rate the energy cost per meter of transporting the 90 kg person will be low: 90 x 9.8/15 = 58.8 J.

We can compare this to the energy content of jet fuel, 44 MJ/kg. One kg of jet fuel could transport this person some 750 km if airplane engines were 100% efficient. As it happens airplane engines are only 21% efficient suggesting a mere 157 km/kg. This is still not bad given the jet fuel costs only about $1/kg, about $3/gallon. Since 1 mile = 1609 m, we find the marginal cost of transport is 1¢ per mile, virtually nothing.

Two generations of Wright flier showing a dramatic improvement in lift/drag. High lift per drag, as in the plane at right, is the key to low-cost air travel.

The above is the marginal cost — the cost of adding an additional 200 lb person on a short flight, e.g. from Detroit to NY (500 miles). The flight is about 500 miles, and at 1.0¢/mile, the cost to the airline is $5. No wonder airplanes offer crazy-low fares to fill seats.

The above is just the marginal cost, but the average energy cost isn’t that much higher in absolute terms. On a reasonably full 737 flight, the passengers and luggage weigh about 1/4 as much as the plane and its fuel. Effectively, each passenger weighs 800 lbs, suggesting a 4¢/mile energy cost, or $20 of energy per passenger for the flight from Detroit to NY. This is far cheaper per passenger than driving, mostly becasuse most drivers are not as good filling the car as airlines are at filling a plane. Though the fuel rate of burn is high in a plane, about 5000 lbs/hr, the high speed and the high number of passengers, 80–100 mean that 737 gets, on average, more than 80 passenger miles per gallon, even for short trips.

Passengers must pay more than $20; wages, capital, interest, profit, taxes, and landing fees add to the cost. Still, one can see how discount airlines could make money if they arrange a good deal with a hub airport, one that allows them low landing fees and allows them to buy fuel at near cost.

Compare this to any proposed super-fast or mag-lev train. Current US passenger trains, when fairly full, boast a fuel economy of 200 passenger miles per gallon, but they are rarely full, and the mpg is likely to be lower when the speeds are higher. With the current, 70–80 mph trains, it takes some 20 hours to go Detroit to NY, in part because they stop often, and in part because they go via longer routes, going around lakes and visiting major cities along the way. With a 750 mile route, even if the train got 150 passenger mpg, the 750 mile trip would use 5 gallons per passenger, $15, barely less than the $20 for the flight above. This is a savings of $5, but it comes at a cost of 20 hours or so of a passenger’s life. Even speeds were doubled, the trip would take 10 hours including stops. For most people, that’s not worth the $5 savings. And it’s unlikely there would be savings since for rail traffic, you need to include the cost of track and track upkeep, and the cost of land. These will cost more than $5/passenger.

Futurists like to imagine there will be societal benefits to promoting this slow form of transport, but I don’t see the benefits. While I’d be happy to see better freight trains, and better signaling for short, conventional passenger rail that would use freight lines, I don’t see any benefit to government-funded mag-lev lines for 150+ mph trains that are likely to be half full — as the Japanese bullet train is.

A better place to put government money is in short -take-off and landing plane, and in a new generation of comfortable buses. Some years ago, I noted that Detroit’s Coleman Young airport no longer had commercial traffic since its runway was too short at 1051m. There are military planes that could use this runway, and I’m happy to report that Bombardier’s new CS100s should work there too. A CS100 requires only 1463m of runway, carries 120 passengers, about the same as a Boeing 737, and is quiet enough for city use. So far Boeing has managed to keep the planes out of the US, but I trust that this is just a delay. Even if the cost per mile is higher than for a 737, 5–6¢/mile, this is far lower than the cost of any likely mag-lev train, and quite a lot easier to get going.

For shorter trips between cities, I’m inclined to think money is best spent on modern busses, instead of on rail. Busses are as fast and energy efficient as trains, but cheaper since they share the road costs with cars and trucks. I would not mind seeing higher bus speeds, and higher car speeds too, but even this does not have to be super high-tech. The German Autobahn allows 100+ mph travel, and I would not mind seeing something similar in the US. If you want low pollution and high efficiency, how about hydrogen hybrid buses?

Robert Buxbaum, December 3, 2017. I taught engineering for 10 years at Michigan State, and my company, REB Research, makes hydrogen generators and hydrogen purifiers.

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