The trouble with trying to make trains go faster (BBC)

[eppur_se_muova]

By Katia Moskvitch

Since George Stephenson’s Rocket, designers have been trying to make trains go faster and faster. Despite all the innovations, particularly in the last 50 years it’s still a dream that all cities around the world could be connected by high-speed trains that complete journeys in a flash, allowing you to arrive at your destination relaxed and untroubled. Why is this the case?

Going fast on rails brings its own special set of problems. Human bodies are simply not built for rapid acceleration, we experience certain low frequency motions that create discomfort – a feeling of “motion sickness". We also experience rapid acceleration, for example, each time we take off and land in a plane.

Then there is the logistics of trying to send a train faster along a track. Going fast means pushing air out of the way, which also requires a lot of power. A train travelling at 300mph (480km/h) uses roughly 27 times more power than one travelling at 100mph (160km/h). And at ground level the air is a lot denser than it is at 35,000ft (10,600m) where airliners regularly cruise. That means more resistance, and therefore more vibrations.

But train operators and companies are pushing for ever-greater speeds, and they have been trying out a range of innovative designs that they hope will make trains go super-fast and be ultra-comfortable at the same time.

If trains could travel just in straight lines and without any dips, then high speeds would not be a big issue. It’s the bends and the ups and downs that create a problem, especially in Europe, with its many rivers and mountains and old train lines following long-travelled routes.
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more: http://www.bbc.com/future/story/20140813-the-challenge-to-make-trains-fast




No breakthroughs here, just a surprisingly detailed background read.

[NYC_SKP]

1. K/R but 27 times more power?

I beg to differ, I believe that resistance (and thus required power) increases exponentially, by the square of the speed; 3x the speed = 3x3=9 times the resistance.

[eppur_se_muova]

2. Well, according to Wikipedia ...

The power required to overcome the aerodynamic drag is given by:

P_d = F_d·v_b = (1/2)ρv³AC_d

Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

http://en.wikipedia.org/wiki/Drag_%28physics%29#Power

Alternatively worded, force is integrated over distance, and at three times the speed, three times the distance is covered, so the work per unit time also triples. The work per unit distance would only increase ninefold -- important if you're calculating mileage. But work per unit time tells you how powerful the engine needs to be.

[NickB79]

3. Which is also the reason we should have kept speed limits at 55 mph. NT

[NYC_SKP]

4. Makes sense, to calculate mileage, it increases with the square.

But over a given time period, with the cube.

Don't you think that in comparing efficiencies of travel, that fuel economy would be the better comparator?

Or is it fair to say that, as reaching 300 mph would require a bigger power plant to put out that additional power, that power AND mileage are important considerations?

[intaglio]

5. In the UK the really big problem is not the "Go" but the "Stop"

At 60 mph a train takes ³/₄ of a mile to stop; at 120 mph, 3 miles and there are very few places where maximum speed can be built up and maintained.

Check this link for more detail Trains and Train Safety - Making Trains Safer

[Kaleva]

6. The rabid accleralation issue maybe why human cannonballs can't be used for mass transit

Picture a line of human firing cannons and nets across the US. When a passenger is fired from the first cannon, he, hopefully, lands in a net where he clambers off and gets into the nearby cannon to repeat the process till he arrives at his destination.

Atomic Annie had an effective range of 20 miles so we'd need about 150 of them in order to provide cross country coverage.