Can a 600km-long vacuum tube send people from LA to San Francisco within 30 minutes? Technological wunderkind Elon Musk is absolutely certain it can.
The Hyperloop is a proposed pod transport system that would operate inside a near-vacuum tube and travel at high speed (faster than cruising planes). The goal is to enable passengers to travel great distances in a short amount of time. Musk first released a whitepaper on the subject, "Hyperloop Alpha" (PDF) in 2013.
It has been estimated by Elon Musk that the system, which would initially link Los Angeles and San Francisco, would cost $6 billion and provide passengers with tickets for as low as $20 a trip. Part of its appeal is that the Hyperloop could feasibly allow for safer travel than cars provide as there would be no opportunity for collisions and far less vulnerability to inclement weather.
A section of the hyperloop. Image courtesy of Kevin Krejci [CC-BY 2.0]
To help with the development of the transport system, Musk went open-source. The Hyperloop Pod Competition was held which had students and companies alike competing with pod designs. The first phase of the competition involved design concepts and ideas with the second phase involving an actual scale down Hyperloop testing various pod designs. SpaceX now plans to hold a second competition in 2017 to help further develop the Hyperloop concept and make it a reality.
However, there are many challenges facing the promise of the Hyperloop, including challenges to the physics of the basic design and questions of efficiency.
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 initial concept has been to reduce friction by sending pods along an air cushion created by air compressors and linear induction motors.
The original conception of a Hyperloop pod. Image used courtesy of SpaceX.
For this system to be efficient, it would need to operate in a near-vacuum environment to escape the Kantrowitz Limit. Hence, Musk has envisioned a track built inside a long vacuum tube.
Power for the system could be provided by solar panels attached to the top of the transport system which could generate extra power to feed into the grid (as the number of panels used would exceed the number needed to power the system).
Work on the Hyperloop is plentiful for electrical engineers. (Check out this job listing for an electrical engineer at Hyperloop One). Beyond just the design stage, maintenance is a major issue when you're talking about 600km of tubing. The solar system, alone, would require a great deal of wiring and maintenance.
The tube the pods travel through is also a point of contention. The initial Hyperloop plan is for a 600km length connection between LA and San Francisco. 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.
Between the LA and SF line lies the famous San Andreas fault which is known to cause major earthquakes. For a structure that must be incredibly smooth with no kinks, placing it across a fault line is not the best idea. But this goes for placing a tube structure anywhere on the planet. Subsidence, landslides, ground shifts, and pot holes all pose problems to such a structure.
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 68ºF, the overall length change equates to 156 meters. This does not include hotter climates where temperature changes throughout the day are more drastic. To make matters worse, these temperatures are taken in the shade as opposed to the temperature of an object that is in direct sunlight.
A prototype tunnel at the Hyperloop One test site in California. Image used courtesy of Hyperloop One.
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.
Safety is paramount in modern society and the Hyperloop (for it to become a “5th mode of transport”) must achieve high levels of safety to stand a chance with the American people. 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.
Musk has not yet ventured to say how 600km of tube could be adequately secured from either naturally-caused or human-caused dents.
Where Is It Now?
Currently, the Hyperloop is still going strong in its development.
One test performed shows a maglev system reaching 116mph in two seconds. However, while this may seem like progression, the reality is that the test was effectively a glorified roller coaster (which can reach speeds of up to 150mph).
Many will quickly point out the almost mile-long vacuum tunnel that tested various designs. The top speed for all the test pods was near 60mph with pods shortly coming to a stop once the electric car used to push them disengaged.
Based on such tests and the many inherent problems with the basic concept of the system, the outlook for the Hyperloop isn't exactly rosy. At very least, there's much more research to be done. Such a system may be more suited for a place on the surface of the moon or a body whose atmospheric pressure is next to 0 which removes the requirement for moving seals, vacuum chambers, temperature control, and mobile stations.
In the meantime, keep an eye on the coverage of the Hyperloop and see if you can differentiate what's technological progress and what's just good marketing.