With vactrains, we could theoretically ride from Europe to North America in just one hour. One would think they’re well on their way revolutionizing transportation, right?
Well, it’s easier said than done. There are numerous problems with actually building (and funding) the construction of a vactrain, but what exactly is a vactrain?
Vactrains are magnetically levitated (maglev) trains travelling in a vacuum tube. Vactrains are built off of the principle of defeating motion’s worst enemy: air resistance.
Motion’s worst enemy
One of the main reasons regular trains do not go faster is because as they move, they are pushing a thick wall of air in front of them. Since they are pushing air from in front of them, they leave a region of low pressure behind them which pulls them backwards (suction). Air resistance is proportional to the square of the train’s speed. For example, the force on a train going 60 mph is four times the force on it if it were going 30 mph.
Imagine if that air wasn’t there. There would be nothing pushing and pulling the train backwards. There would be little to hold it back from very high speeds. Since a pure vacuum cannot be achieved, a partial vacuum (at least 7% of atmospheric pressure) could be used with effective results.
Professor Emeritus of mechanical engineering and ocean engineering from the Massachusetts Institute of Technology spoke to the BBC about an experiment he conducted, saying “We built a half mile long tube at the playing fields of MIT, evacuated it, and then shot things through it in order to measure what sort of velocities we could obtain. We started with ping pong balls, and then went to mechanical models.” He and his team achieved speeds of up to 580 mph (930 km/h). If the tube were filled with air, it would only go half this fast.
Air puts a heavy limit on max speed. So put a train in a vacuum – brilliant! Unfortunately, ideas in theory never work out as well in the real world. Here are some of the major challenges faced by those attempting to build a vactrain.
Since a perfect vacuum cannot be achieved, the train is still pushing air out of the way. In Elon Musk’s design for the hyperloop (similar to a vactrain), he proposes an air compressor in the front of the train to push that air to the back. This would get rid of the pressure in front while increasing pressure in the back. Increasing pressure behind the train would reduce the suction, thus solving two problems at once.
Landscape and turning
With something moving at such a high speed, sharp turns just cannot happen. If anything, gradual, wide turns would need to be used. Ideally, a vactrain would be built for straightforward trips with no turns or curves whatsoever. Also, terrain would need to stay level or nearly level for the entire track. Rises in elevation would cause issues with pressure regulation (the higher you go, the lower the atmospheric pressure is). General changes in elevation could yield design difficulties, such as vibration.
Changes in elevation, along with rough terrain would introduce vibration. Vibration causes fatigue in materials, with fatigue accounting for 50-90% of mechanical failures. More significantly, the motion of the trains at such a high speed may make more noise than they can contain. This applies to both vactrains and hyperloops.
For a partial vacuum to be maintained, there cannot be much space between the trains and the tubes containing them. In the case of the hyperloop, a type of vactrain that is suspended on an air cushion instead of maglev, Musk and his team suggest a tolerance of only 0.5-1.3 mm. With such a small space between a tube and train, one would better hope that nothing significant hits the tube. That, or the tube has little to no variation in width, elevation, etc. Imagine a train going 760 mph and it gently touches a wall. We’ve likely seen a scene from a NASCAR race in which a car gently touches the wall. If those accidents aren’t deadly enough, imagine a train full of people moving three times as quickly. It would be utter catastrophe.
Evacuating the tubes
Ironically enough, this is the easy part (relatively speaking). The train would actually have to pass through multiple airlocks, each with a reduction of pressure. After full evacuation, the train would reach top speed. Also, air would leak at the stations, so there would need to be seals.
Looking at the model, once could see a method of letting people out of the train (pressurized on the inside). The cone opens, leaving a cut of the inside open for pods to be released. The tube itself is sealed and kept at a constant low pressure. Though it’s called a hyperloop, it doesn’t loop at all. It’s a straight line stopping at the ends.
Transportation of the future?
Is it possible? Certainly. Is it likely? Doubtful. With such close tolerances needed for vactrains to properly function, the cost would be too high for it to take off. It’s an interesting technology, granted, however its high cost and instability make it more of a liability than a next step in the evolution of fast public transportation. Maglev (magnetically levitating) trains are already in use, and they are fast and reliable. Like all technologies, it is only a matter of time when maglev takes off even more than it already has.