From Interesting Engineering via Hyperloop One
On paper, the Hyperloop is an engineering marvel that promises to set supersonic travel underground. The system is proposed to carry people around the world at speeds nearing, and eventually exceeding, the speed of sound. The idea is to carry people inside a vacuum tube at supersonic speeds. Although it looks great on paper, in the real world, a full-scale Hyperloop may not be realized for many more years to come.
Currently, there are many problems plaguing the Hyperloop – begging the question, is it practical?
Small scale preliminary experiments reveal the Hyperloop is entirely feasible and more so, it functions extraordinarily well. However, constructing a perfect tube hundreds of kilometers long capable of sustaining a near perfect vacuum will undoubtedly be one of the greatest engineering challenges in the 21st century.
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Vacuum Trains: How they work
The Hyperloop is a theoretical transportation system currently undergoing prototype testing from various companies, perhaps most famously, by Elon Musk.
The idea is to reduce the pressure in a tube and then place a sort of train within the system. Reducing the pressure results in a few benefits; One, air resistance is removed, and two, the pressure gradient can be used to propel the trains at great speeds.
Reintroducing atmospheric pressure behind the capsule forces the air to propel the train down the pipe as air rushes back in to equalize the pressure gradient. The method is sufficient enough to propel the capsule at speeds nearing that of sound. However, Elon Musk envisions a variant of the idea where a special turbine engine will propel the capsule down the track.
Although many people attribute the invention of the vacuum train to Musk, the idea has existed for almost 100 years. However, larger scale vacuum trains were never constructed – and with good reason. The trains are prohibitively expensive and there are unavoidable dangers brought on by the extreme environments required to devise a functional system.
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Important things to note
The proposed system of the Hyperloop will technically not be operating under a perfect vacuum. Rather, the alpha documents reveal it will remain at a pressure of about 100 Pascals – equivalent to about 1/1000th of an atmosphere (1/1000th of the pressure experienced from the weight of the atmosphere at about sea level).
However, at those pressures, the difference between a perfect vacuum and the proposed pressures the Hyperloop will operate at are practically negligible.
Comparatively, large airliners fly at altitudes with more than 200 times more air than what the proposed Hyperloop capsules will travel through. Airliners fly at an altitude of about 10 km up whereas the Hyperloop tube would have the same internal pressure level that is experienced 50 km up in the atmosphere – essentially near-space conditions.
A Boeing 747 operates at about 10 km up and experiences 200 times more pressure than the internal pressures of the Hyperloop. The Hyperloop operates at about 100 Pa, or about 1 mb (millibar). From the origin on the chart, the Hyperloop will operate at just one unit (mb) to the right – an equivalent pressure experienced at an altitude of 50 km – approaching the equivalence of space itself. [Image Source: ManashKundu]
The pressure exerted on the inside of the tube will remain at around 0.015 Psi (0.000977 of an atmosphere) – whereas the atmospheric pressure on the outside of the tube approaches 15 Psi (nearly one atmosphere). Therefore, for all intents and purposes, the Hyperloop can be assumed to be operating at a near perfect vacuum.
Now, Musk and other companies believe the technology is ready to support the weight of the entire atmosphere over hundreds of kilometers.
However, the problems still remain. It is not an impossible task, although with current technologies, it will likely remain unfeasible to develop a full-scale vacuum train for many more years to come – here is why.
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The Problems Plaguing the Hyperloop
Constructing a tube hundreds of kilometers long would be an engineering marvel in of itself. However, introducing a tube hundreds of kilometers long that operates at a near perfect vacuum which can support the force of capsule weighing thousands of kilograms as it travels hundreds of kilometers an hour is nothing short of sci-fi fantasy.
Small scale experiments reveal the fundamentals of the idea are sound. Although, in the real world, there are too many factors that cannot be accounted for with a small scale design.
In the real world, there are tens of thousands of kilograms of atmospheric pressure which threatens to crush any vacuum chamber. There is also the problem with thermal expansion which threatens to buckle any large structure without proper thermal expansion capabilities. The Hyperloop would also be stupendously expensive. There are many unavoidable problems facing the Hyperloop that threaten the structural integrity, and every human life on board. The problems can be addressed, but at a great cost.
Below are the most compelling problems engineers must still address before any full-scale vacuum train system will carry a human life.
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Pressure
Continuously lingering above the near perfect vacuum tubes of the proposed Hyperloop is thousands of kilograms of atmosphere.
Before the Hyperloop becomes operational, the transportation tubes that will stretch hundreds of kilometers across the US will have to support the entire weight of the atmosphere above it. Essentially, the weight will accumulate about 10,000 kg per meter squared. That is, for every square meter of tube, there will be over 10,000 kg crushing down on it.
Since the proposed Hyperloop will extend 600 km with a diameter of about two meters, it will maintain a surface area of about four million meters squared. Given one square meter will experience 10,000 kg of force, the Hyperloop will have to endure nearly 40 billion kilograms of force over its entire surface.
A small compromise in the structure of the tube would result in a catastrophic implosion. If the tube became punctured, external air would tear into the tube, shredding it apart as it violently rushes in to fill the void. The effects would be similar to a railroad tank car vacuum implosion – only many times more violent.
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PenneyVanderbilt 
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