Saturday, September 25, 2021

Md. & Japan in maglev train partnership


The government of Japan has pledged $2 million to help fund studies on the feasibility of a magnetic levitation train that could take passengers from Baltimore to Washington in less than 15 minutes, the Baltimore Sun reports.

Governor Larry Hogan signed a trade agreement yesterday with Ambassador Kenichiro Sasae of Japan, making Maryland the third state to sign such an agreement to try to make a go of the fast trains that ride on air, about 1 centimeter from the guide rail.

Two kinds of maglev train

The key to maglev is that no metal wheels are in contact with the tracks, which means there’s no friction between the train and the track to slow the train down. The train glides on air, about a centimeter off the guide rail, because magnetic forces, generated by electromagnets in the guide rail and magnets in the train itself, make the train levitate.

One type of levitation is accomplished by lining up like poles on the two magnets, which repel each other. Think of this magnet as being below the train and pushing it up off the rail continuously.

The other type of levitation is accomplished by lining up unlike poles and turning the magnets off and on in rapid succession. Think of this magnet as being above the train and pulling it up continuously. Now, it can’t pull it all the way up into contact with the magnet, so either you have to turn the magnet off before the train touches it or you have to alternate unlike and like poles in the rail or train so as the train travels along, it experiences push-pull alternation that keeps it levitating nicely. Either way, the train drops and is pulled back up several times every second.

But whichever type of levitation is used, the only thing slowing down a maglev train is the air resistance or drag, which, I’m sorry to say, increases as the square of the velocity. When you double the velocity, you generally quadruple the drag and, therefore, the amount of energy needed to maintain the speed, although the precise relationship gets complicated due to the mass of air molecules.

From Newton’s second law of motion, the aerodynamic forces on the body (lift and drag) are directly related to the change in momentum of the fluid with time. The fluid momentum is equal to the mass times the velocity of the fluid. Since the air moves, defining the mass gets a little tricky and aerodynamicists usually relate the effect of mass on lift and drag to the air density. The mathematical derivation for this conversion is given on another slide dealing with momentum effects on lift. As a result of this derivation, we find that lift and drag depend on the square of the velocity. NASA

And the velocity of these trains can get pretty high. In at least one case, a vacuum tube in which the trains can travel has been proposed. This solution would negate the drag, as menionted in the video above. But this solution hasn’t been put into practice, again, most likely, due to a prohibitive cost associated with building the vacuum tube.

Mr Hogan visited Japan in 2015 and was impressed with a maglev train operating in that country, the Sun reported. The trains are sometimes called bullet trains, since they travel at speeds higher than 350 mph. You can also ride on operating maglev trains in China and South Korea.

In addition to the increased air drag at the higher velocities, the cost of building and maintaining the system can be prohibitive, making cross-country travel by bullet train a less desirable option than air travel. But for the short trip between downtown Baltimore and Washington Union Station, well, that’s what this feasibility study will determine.

Plus, while airplanes can theoretically travel between any two points using any path, commercial air routes are in practice about as inflexible as a guide rail would be for a maglev train. And airplanes benefit from greatly reduced drag at higher altitudes, where the air is thinner and the planes don’t hit very many molecules that create drag.

The cost, just for building a maglev train between Baltimore and Washington, is estimated to be in the neighborhood of $10 billion, or about $261 million per mile.

Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.

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