Humanity today possesses such knowledge and technologies that it is possible to achieve even the craziest, sometimes unimaginable ideas. One of the favorite ideas in the entire history of mankind is in general the journey and the speed at which it is moving. Futuristic Hyperloop is the best example of a journey of the future.
The Hyperloop is a proposed pod transport system that would operate inside a near-vacuum tube and travel at high speed. This high-speed mode of transportation would have the following characteristics: immunity to weather, collision free, twice the speed of a plane, low power consumption, and energy storage for 24-hour operations
Is the Hyperloop a reality or is it an idea that will not happen with current technology?
The Hyperloop was designed around the idea that a pod could carry passengers at extraordinary speeds if it were in an environment where it wouldn’t need to contend with much friction.
The essential part of the whole scheme is the vacuum within which the passenger capsules travel.. Within such a closed system, any level of propulsion will send the capsule shooting “bullet-like” along the “rifle barrel” of the tube at ultra-high speeds, with the absolute minimum of effort.
In this way, the Hyperloop transport technology, could propel passengers at 1,200 km/h along a 560 km route in only 35 minutes, which is considerably faster than trains and less damaging to the environment than aircraft.
There are still many engineering hurdles to overcome if the Hyperloop scheme is to become a reality, like building the tubes strong enough to deal with the stresses of carrying the high-speed pods, finding energy and cost efficient ways to keep them operating at low pressure, speed reduction upon reaching the destination, thermal expansion of the rail, safety…
Creating a vacuum of 1 mbar is not rocket science, but when you “scale-up the model” along, for example 200 km of a 4 m-diameter tube (i.e. over 2.5million m3 of space), a proper vacuum pumping system requires a lot of expertise and understanding of vacuum physics, material knowledge as well as vacuum simulation skills. The pump system itself needs to handle both a fast pump down of the system as well as holding the pressure (which represents 99% of the operation time).
The tube the pods travel through is also a point of contention.. However, a tube that is going to carry passengers as speeds of over 1000km/h cannot afford to have bumps, dents, and sharp bends. This is one of the reasons why high-speed trains lines are incredibly straight and, when bends are needed, they are incredibly gradual.
The second issue with the tube is the expansion and contraction of steel (or any material for that matter). In one simple experiment that demonstrates expansion very well, a metal ball that just fits through a ring will no longer fit if the ball is heated with a blow torch. Steel (according to hyperphysics) will expand 13 parts per million when heated by 1ºC. So, for perspective, a 1-meter length of steel will expand by 13μm for a temperature rise of 1ºC.
This may not seem like much but, when considering a tube run of 600km in a typically mild environment (such as the UK) where the temperature varies about 20°C or 68ºF, the overall length change equates to 156 meters. This does not include hotter climates where temperature changes throughout the day are more drastic.
Expansion is easily solved in most designs with the use of flexible joints such as those found in bridges and railway lines.The Hyperloop would also have been an easy fix if it were not for the vacuum requirement. Seals do exist for such setups (such as moving vacuum joints), but how many would the Hyperloop need? This depends on the length of tubing that is manufactured but if the tubes are too long and/or welded together then distortion in the structure will quickly surface when under thermal stresses. The tube which contains a near-vacuum atmosphere is already under stress so any dents or warping in the cylindrical structure could result in structural failure.
But the expansion issue does not end there. The top side of the tube will be at a higher temperature than the bottom, which will not only cause buckling but also cause expansion problems for the entire length of tubing. If the topside is 10 degrees hotter than the bottom side then the expansion difference between the top and bottom side will be approximately 78 meters.
Even if all the technical problems with the tube itself are solved, the safety issues the Hyperloop faces are unprecedented.
The first problem comes from the vacuum inside the tube. If a person is exposed to an atmosphere that is not breathable, then a respirator is usually sufficient to keep someone alive. However, vacuums have a nasty habit of killing living organisms really fast (unless you are a tardigrade, in which case you’re fine). So, in the event of an emergency, passengers cannot leave the pod until the tube itself is re-pressurized.
If a failure occurs on the Hyperloop and a section of the tube becomes exposed to the atmosphere, then air from the outside will rush into the tube until the pressure is equalized. However, the air will not move slowly into the system but instead create a powerful air front whose pressure is equal to one atmosphere traveling close to the speed of sound (considering that the average speed of molecules in the air travel at 500m/s). Such a front could devastate any pods in the entire length of tube with each pod potentially creating more damage to the tube in the form of debris (in a similar fashion to a cascade failure of satellites).
Cylinders are used to contain vacuums and high-pressure gasses/liquids because a cylinder is one of the strongest structures known (next to the sphere). Corners and dents in a shape make it impossible to uniformly distribute internal stresses and pressures. This is why submarines, particle accelerators, and even spacecraft all use cylinder-like structures.
If the Hyperloop were to be dented (or, say, shot with a bullet), that cylindrical structure would suddenly have new weak points. An event where the tube, itself, is not breached may result in the tube collapsing under atmospheric pressure. An event resulting in a breach could cause a tear and (as stated before), a pressure wave.
Where Is It Now?
The concept has been around for a long time but until now the technology has been lacking. However, the technology may have just caught up with the concept now. There are well-funded companies racing to be the first to deliver a working service but despite the optimistic timescales these projects are still very much in the pilot and experimental stages.