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.

[Image Source:Hyperloop]

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.

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.

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.

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.

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 the railroad tank car vacuum implosion displayed below - only many times more violent.

Spontaneous Decompression

If the tube was punctured for any reason, outside air would violently enter the tube as it attempts to equalize the pressure gradient.

At typical room temperatures, air molecules travel around at about 2000 km/h. In a room filled with air, the molecules bounce around in random directions, striking other molecules as they move. As individuals, the particles do not carry a significant amount of momentum.

However, inside the Hyperloop chamber, there are few molecules for the air to bounce off of. The atmosphere would violently force the air inside where the molecules would continue to travel with the force equivalent to an elephant traveling nearly 2000 km an hour for every square meter. Given the two meter diameter, the cross section of the tube would measure about three square meters. If a breach ever occurred, the air would rush in supersonic speeds with the force of 30,000 kilograms over the entire cross section.

The air would continue to race down the track with explosive force until the pressure equalizes or until it slams into an object - most likely, into the train capsules.

At just 3 PSI (pounds of pressure per square inch), air can cause significant damage to a human body with the potential to result in the loss of human life. At 5 PSI, buildings would begin to collapse and fatalities would be widespread. With 10 PSI, reinforced concrete buildings become severely damaged or can collapse entirely. Most people would be expected to die.

In the case of the Hyperloop, air would enter the tube at 15 PSI (!) equivalent to one atmosphere or 10,000 kg per square meter. As it enters any perforation, the atmospheric pressure would tear open the tube like a tin can. Any and all capsules that stand in the way would be instantly shredded apart. The results would almost certainly be deadly.

Deadly Collisions

Engineering a capsule that could withstand the force during a spontaneous decompression disaster would be immensely difficult given the nature of the design. The capsule must be strong enough to support the atmospheric pressure inside the cabin, yet must remain light enough as to not destroy or compromise the tube as it travels down the track. The capsule will operate at 1/1000th of an atmosphere, making it rather improbable for it to sustain the impact of the incoming 1 atmosphere. Implementing security features that could withstand the force of 30,000 kg of pressure traveling at the speed of sound would be difficult.

Of course, one thing going for the capsule is the compressibility of air. Perhaps the air would compress, slightly dampening the initial blast - although it is rather unlikely it will reduce the destructive force acting upon it.

Assuming a capsule could somehow survive the initial blast of air, more problems would still be yet to come. Once the air hits a capsule, it would be forced to quickly accelerate down the track as the air rushes in.

The air would maintain the force of 10,000 kg per meter squared, or 10,000 newtons per square meter - all of which would be exerted on the face of the capsules. Assuming it does not instantly shred to pieces, the capsules would accelerate down the track until is smashes into one another with deadly force.

One capsule weighs about 2,800 kg, according to the alpha documents. Assuming the cart is fully loaded with humans, it would weigh in at about 4,000 kg.

As previously discussed, a cross section measures approximately three square meters, which would result in almost 30,000 kg, or 30,000 newtons of force to be exerted on the capsule during spontaneous decompression.

Now, using some simple physics, the acceleration the capsule would experience under spontaneous decompression can be approximated (Force=Mass x Acceleration → A=F/M).

In seconds, the capsule would accelerate to over 100 km/h

Almost instantaneously, the cart would accelerate at 7.5 meters per second squared in the best of conditions. If the capsule was not fully loaded, the acceleration would be even more dramatic. If it was nearly empty, the acceleration would be over 10 meters per second squared - faster than what would be experienced during free-fall with no air resistance(9.8 m/s^2).

In a little under four seconds, a fully loaded capsule with 14 passengers weighing 100 kg each would reach speeds upwards of 100 km/h. On a track shared by many capsules, fatal collisions would be imminent in the event of spontaneous decompression. In the best of conditions, decompression would be devastating.

The acceleration that an object experiences after decompressing from a vacuum to atmospheric pressures is astounding.

A professor at Purdue University used the effect of decompression of a vacuum tube to turn a ping pong ball into a deadly projectile.

In the Video, Purdue University mechanical engineering technology professor, Mark French, demonstrates an air-powered bazooka capable of shooting pin pong balls faster than an F-16 fighter jet.

The device works by removing all the air in a vacuum chamber. Then, by instantaneously re-pressurizing the chamber, the ball is ejected at supersonic speeds.

The video gives clear and severe warning of the dangers of such a device thousands of times smaller than the Hyperloop that exists under similar circumstances. Although the ball has a high drag coefficient and a mass of 2.3 grams, French says "There is not enough money you could give me to get me to step in front of that gun."

Clearly, the device is incredibly dangerous.

Decompression is a dire problem

The effects of the vacuum gun experiment would be similar to what would happen in the Hyperloop system given a spontaneous decompression event. Just like the ping pong ball, the train would quickly accelerate as the air continuously rushes in. With no drag, the capsule would accelerate to supersonic speeds.

Decompression is a dire problem that could and likely would be fatal in a vacuum train system. Thus far, no breaking systems have been proposed to prevent the capsules from accidentally accelerating due to spontaneous decompression - more on that below.

Decompression would not only ruin the system, but it would likely be fatal to all those unfortunate enough to be riding in the tube at the time of the accident. Unfortunately, a wide range of events could cause a perforation in the tube.

What could cause decompression

Just about any small defect in the tubing could cause disastrous decompression. The tubes exist under such extreme environments, that even small defects could cause the atmosphere to crush the tube like an aluminum can. However, even assuming the tubular system was engineered with absolute precision and perfection, many more dangers threaten to destroy the Hyperloop.

Propelling the Hyperloop capsules along is a massive turbine that Hyperloop claims will propel the vehicle down the track at nearly supersonic speeds.

The turbine functions in much the same way as a regular turbine engine on an aircraft, only the Hyperloop's engine will spin much, much faster.

Airliners fly high in the atmosphere to reduce drag and increase efficiency. Although, the engines still require oxygen to create combustion. Aircraft fly at a specific height to maximize efficiency yet at a level with enough oxygen to maintain enough thrust to stay aloft.

Airliners use turbine engines that spin in excess of 3000 RPM. At those speeds, each turbine blade carries the centrifugal force of 110 tons, equivalent to the weight of a full-size locomotive.

Containing turbine blades during catastrophic failure

Of course, turbine engines are designed to contain the blades within the engine in the event of a catastrophic failure. If they did not, the blades would quickly become dangerous projectiles that travel over a thousand kilometers an hour. The rogue projectiles could easily slice through the thin aluminum lining of any aircraft.

Below is an example of what happens in the event of such a failure (0:15 seconds in)

The engine can contain the projectile blades, however, aircraft have more room for error than a vacuum train.

In the video, after the failure, the engine vibrates dramatically. In the air, a plane's wing retains some flexibility that enables the engine to continue to vibrate without structurally compromising the entire aircraft. The aircraft can also maneuver in the air with backup engines to compensate in the event of a loss of one engine.

Alternatively, inside of a vacuum tube, the vibrations would rattle the tube apart, causing a catastrophic and fatal failure. The intense vibrations would likely structurally compromise the tubes, either causing an implosion or even worse yet, spontaneous decompression. The train has merely inches of maneuverability, making a collision with the vacuum tube practically imminent. Unfortunately, it is not the only issue with the turbines.

Less atmosphere requires faster turbines

As previously mentioned, airliners operate in an atmosphere 200 times denser. A traditional turbine engine could not generate enough compression in the vacuum tube to propel the capsule down the track.

According to Phil Mason, a chemist, and YouTuber, the only foreseeable solution to generate nearly enough thrust is by implementing a turbomolecular pump.

Unfortunately, for the pumps to operate, they must spin at speeds exceeding 20,000 rates per minute. The speeds at which they operate are nearly 10 times more than a turbine engine. At those speeds, instead of building an engine case capable of retaining a rogue turbine blade spinning with a centrifugal force equivalent of a 100-ton locomotive at the end of each blade, the forces generated would be in excess of 1000 tons per blade.

As of today, there are no turbomolecular pumps big enough to propel a full-scale vacuum train at supersonic speeds. However, it is with good reason. Engineering a case that can withstand the force of a blade traveling at hypersonic speeds with the force of 10 full-size locomotives is preposterous.

For it to work, the Hyperloop would require an absolutely perfect turbomolecular pump

Any engineer learns early on in their university career that all components are designed with a certain degree of error. Though it may seem shocking to some, even NASA's most high-tech rockets are designed with a degree of error in mind. It is the reason parts fail, which is okay, as long as it is caught in time.

One of the greatest challenges engineers face is vibrations. Vibrations can rattle bolts loose, cause micro-fractures, or create a catastrophic failure. In the event of a turbomolecular pump spinning at tens of thousands of RPM, even the most minute of failures could result in catastrophe.

If the engine began to vibrate, it would quickly disintegrate, turning the turbine blades into mini projectiles.

If the tip of a blade came lose, it could easily perforate the Hyperloop tube. Then, all the air would rush in, destroying the system and killing all that are inside.

The multi-ton capsules intended to carry passengers also pose as liabilities themselves.

Weighing in at nearly 3,00o kg a piece, the Hyperloop tube would have to withstand the constant force and vibrations as each capsule travels through the pipes at hundreds of kilometers an hour. The capsules would wear down the structural integrity of the tubes. With regular maintenance and properly functioning tubes, it would not be an issue. However, if engineers did not catch a faulty tube (and there will be thousands of tubes), it could fail and result in spontaneous decompression once again.

Too much air creates significant problems

Backing up the problem with pressures, the Hyperloop could also fail if a pocket of air somehow enters the system.

As the capsule travels at hundreds of kilometers an hour with a turbine rotating tens of times faster than that, a pocket of air would act more like a wall. If a capsule encountered an air pocket, the pressure difference would create such a violent impact that the turbine blades would become instantaneously damaged. Even the smallest of fragments could severely damage the turbine blades, causing untold damage. The turbine would become unbalanced, yet would continue to spin at astronomical speeds.

Even a small variation in the turbine blades could create enough vibration that could cause the engine to tear itself apart, once again ejecting parts into the tube, creating catastrophic decompression.

Naturally, one solution is to add vents that could re-pressurize the tube before it cascades into a total system failure. However, it would require having thousands of extra parts that exponentially increase the risk of failure.

Of course, engineers would compensate for the pressure and other dangers to the best of their ability. Constructing such a tube would require thick steel. However, steel comes with its own set of problems.

In the heat of the Sun, that problem comes in the form of thermal expansion.

Thermal Expansion

Steel is sufficiently strong enough to sustain a near perfect vacuum in ideal conditions. However, another problem arises due to a property of steel itself.

Throughout the year, the temperature changes a substantial amount across most of the world. The change of heat would cause the Hyperloop tube to physically change its size.

The thermal expansion of steel is rather minute. However, it is enough to be considered during the construction of bridges that regularly expand and contract. Engineers introduce thermal expansion joints that allow a certain degree of expansion, allowing the bridge to expand and shrink without compromising the structural integrity.

A thermal expansion joint on a bridge enables the bridge to expand and contract. [Image Source: Wikipedia]

Although the expansion is minimal for structures less than a kilometer, for structures extending hundreds of kilometers (like the Hyperloop) the effects can be quite dramatic.

Steel maintains a thermal expansion rate of about 13 parts per million per degree Celsius.

A reasonable assumption of the range of temperatures expected in the United States varies from 0 degrees Celsius to about 40 degrees. Given a temperature variance of 40 degrees, the thermal expansion would result in a variance of nearly 300 meters.

The Hyperloop will require thermal expansion joints to function. Installing the joints on bridges is easy enough, however, they do not need to maintain a seal holding back billions of kilograms of force.

Phil Mason predicts the Hyperloop will require a joint every 100 meters. Over the entire distance, it would accumulate 6000 moving vacuum seals - all of which are a significant point of failure.

"A failure on any one of them would be disastrous to everyone inside" Mason comments on one of his videos deducing the Hyperloop.

Steel tubing does not heat evenly

In the US heat, the Hyperloop would be subject to temperatures exceeding 40 degrees on an annual basis. The thermal expansion experienced would create a problem in more ways than one.

The top of the pipe will be subject to more sunlight, and consequently, more heat. A temperature difference of just three degrees on the pipe would cause the top portion to expand nearly 25 meters more than the bottom.

The Hyperloop would bend, and likely, would buckle under the hot summer heat.

Oil pipes often face similar thermal expansion issues that are expected on the Hyperloop. Of course, it is rare to hear of a pipe bursting from thermal expansion.

The reason being of the absence of reports is a result of the clever engineering that allows the pipes to contract and expand willingly. Thermal expansion loops can often be seen along oil pipes. The loops come in various shapes, however, one of the most recognizable can be seen below.

Thermal expansion loop. [Image Source: Wikimedia Commons]

The bend prevents the pipes from buckling and cracking as the pipe expands and contracts. Unfortunately, implementing such a dramatic bend in a vacuum train system would cause too much strain on the tube.

Trains speeding through the tunnel would experience massive g-forces that would stress the pipes and the passengers on board. The expansion loops would also be prone to structural deterioration, making them a weak point along the track.

No foreseeable solution - Yet

The only comparable vacuum tube anywhere near the magnitude of the proposed Hyperloop is the CERN Large Hadron Collider. The LHC features nearly 50 km of vacuum tubing. However, it does not face thermal expansion problems since it is placed deep within the ground where temperatures remain relatively constant.

The engineers behind the Hyperloop have somewhat addressed the issue, although it is rather vague. They explain,

"A telescoping tube, similar to the boxy ones used to access airplanes at airports would be needed at the end stations to address the cumulative length change of the tube."

It appears as though there are no intentions to introduce moving thermal expansion joints along the track. Instead, the tubes will be welded together and a "telescoping tube" will accommodate for the movement at each end of the Hyperloop. Unfortunately, that means each station at both ends will have to accommodate for a minimum of a 150-meter movement in either direction.

Moreover, it means there will be no points of access along the tube. If for any reason the track spontaneously decompresses, the capsules would be trapped somewhere along the 600 km track. Without the vacuum, the train could not travel fast, or perhaps at all.

The stranded passengers would be left with no escape. Without a means to quickly evacuate and rescue people, it is entirely probable that everyone inside the Hyperloop would die due to asphyxiation or sheer panic.

Once again, vents could be introduced to provide emergency re-pressurization and escape routes, however, they will always add more points of potential failures, increasing costs, and risks.

An easy terrorist target

Unfortunately, in this day and age, people are more concerned than ever about the looming threat of a terrorist attack. Designing a tube hundreds of kilometers long that transports hundreds of people at a time gives rise to the very real possibility of a terrorist attack.

Once again, a single puncture would prove catastrophic to all those inside in an above ground system. Agencies could employ security measures, although it would dramatically increase the running cost, likely to the point where there could not be a reasonable return of investment.

Burying it underground

The Hyperloop could technically be buried underground, which would eliminate both the threat of a terrorist attack and would alleviate the stresses encountered due to thermal expansion. Unfortunately, it would also restrict the ability to install emergency vents and would also exponentially increase the costs.

Currently, the longest tunnel ever made for transportation spans a mere 60 km through a mountain in Switzerland. The tunnel also accumulated a staggering cost of US $12.3 billion.

The cost averages out to a total of a little over US $216 million per kilometer. Using the same system to build the Hyperloop would drive the cost up to 130 billion dollars. Significantly higher than the proposed total cost of just US $1.5 billion.

Will the Hyperloop ever exist?

The answer remains uncertain. However, from an engineering perspective, it is rather unlikely.

The Hyperloop is a fantastic idea, however, the practicalities of real-world implementation cannot be ignored.

The Hyperloop is absurdly expensive, and moreover, insanely dangerous. The entire system is prone to a single point of failure that would be catastrophic to the entire structure. A simple breach and all passengers inside would perish almost instantaneously.

The Hyperloop is not impossible, however, it is entirely impracticable, expensive, and insanely dangerous. Right now, the Hyperloop won't work.

Engineers have been trying to perfect the systems for nearly a century, and the technologies are still not advanced enough for real world implementation. That being said, the idea should not be abandoned. The idea needs significantly more refining before it reaches a level deemed to be safe for public transportation. It will require time. However, that time is not now.

Till then, stick to planes, trains, cars, or better yet, your bike.

Sources: Hyperloop, Phil Mason

Written by Maverick Baker