Navigant Research Blog

Hyperloop Faces Technical Hurdles

Sam Jaffe — August 12, 2013

If a prototype of the Hyperloop – Elon Musk’s futuristic superfast train – ever gets built, it will have to overcome some very significant engineering challenges that may or may not be solvable.  After reading through the published document that explains how the concept works (it reads something like a cross between a patent application and a Popular Science article), I can’t say that it will or it won’t work.  But I do have opinions on the plan’s strengths and weaknesses.

I should point out that, while I’m not an engineer, I have helped design systems that use compressed air and deal with the aerodynamics of large structures.  I’ll start with my opinion of what the plan has going for it, followed by important challenges that still require a solution.

The most important element of the concept paper is that it doesn’t really invent anything.  Everything described in the plan has been built for other applications and proven to work.  The electric engine and battery pack would be variations of what Tesla has built in the Model S.  The steel tubes, through which the passenger pods would travel, would be carefully aligned versions of pipeline tubing.  The compressor on the front of the pod would be similar to any industrial compressor.

Not So Outrageous

Every entrepreneur claims that his or her idea will be cheaper than the currently available technology, and just about every entrepreneur turns out to wildly overestimate how cheap his or her system will be.  Musk’s paper does an excellent job of detailing exact pricing of each element of the project so that there’s no magical thinking involved in the pricing expectations.

Musk began this project when he realized how awful high speed rail is as a transportation solution.  Although we’re comfortable with trains, few of us really appreciate how much it costs to build a brand new rail line.  The $68 billion price tag for California’s rail project seems outrageous, but it’s actually not too expensive in the world of high-speed rail.  It’s worth considering that perhaps the craziest option is to spend $68 billion on a more conventional alternative to the Hyperloop.

The biggest concern with this plan has to do with temperature.  The pod will be compressing air and expelling it downwards and backwards.  All that air compression creates an enormous amount of heat, which can damage the pod and its machinery.  Musk’s solution is to add to each pod a water tank that will capture that heat and turn it into steam to be collected at the next station.  Although the thermodynamic calculations are correct, a small pod with only a few cubic feet of room for a heat exchanger leaves little space for an efficient exchange of heat.  That means that the flow of water must be increased, requiring a lot more water on board.  There may be an elegant solution for this challenge, but it’s not in Musk’s current paper.

Wind stress is another challenge.  Any structure elevated 100 feet off the ground is going to be under a lot of wind pressure, which will act on it in weird and sometimes multiple directions.  If that structure is a heavy tube stretching hundreds of miles in either direction, you effectively have a big sail.  Will the concrete pylons be powerful enough to resist that pressure?

The Hyperloop may or may not ever get built, but there are few examples of a billionaire spending his own time and money on giving the world a unique idea that’s been well thought-out and clearly worth investigating further.  Musk’s vision and audacity should be applauded.

31 Responses to “Hyperloop Faces Technical Hurdles”

  1. yah says:

    FYI, USA Today just quoted you on the Hyperloop as an expert source critical of the subject. Hopefully you are aware of this, and are prepared for the obvious onslaught of negative responses flooding your blog (i.e. if a tube whose support is designed to withstand earthquakes is affected by wind, wouldn’t an elevated freeway system suffer the same fate too?… i.e. engineers did do a rough solve using math and simulations of the problem of heat generated by the system… where’s your calculation to show that it’s not sufficient?)

    Not to say that the Hyperloop doesn’t have potential hurdles that are not discussed in the alpha version (i.e. how would accumulated debris in the tube be removed? and what size of debris strike could the intake fan handle? Cars (gasoline cars) could not be transported in a cargo pod, because they can be considered a hazard (because of the gasoline in the tanks).

    All I’m saying is that good debate = solid retorts that are based on empirical evidence. Not the baseless debate that states “I don’t think this is a good idea because I say so”.

  2. aus tin says:

    If you’re not an engineer, why are you commenting on an engineering problem? There are very few problems in this world that don’t have an engineering solution.

    This one, in particular, deals with extremely low pressures, which means the generated heat will be minimal. The hyperloop whitepaper details how much energy is lost to air friction and the exact mass/heat flow rates through the system. It’s not rocket science (that’s Musk’s other company, SpaceX).

  3. Dan says:

    Here we go again, a blogger that thinks he/she is an expert. You stick to your cars, slow trains and overpriced air fares, we will meet the future…something that most bloggers are afraid of!

  4. Spencer Nyelian says:

    About the author: Jaffe, who holds a BA in English literature from Wake Forest University and an MA in Near East studies from New York University, is the author of two books, one on business and religion and another on the Korean economy.

    *****************************

    Dude is not an engineer or a physicist. I see no justification for quoting his opinion in USA Today.

  5. CB says:

    Hyperloop is doable. All component technology has been proven. The working prototype will be up and running within 2 years. Problems will emerge as the project advances, just like with far more complex projects Like Apollo or Manhattan. The American empire must regularly do something really big that captures human imagination in order to remain relevant. The human genome project, the internet, cell phones etc. So hyperloop appears next in line to run the gauntlet of small-minded American naysayers. The problem is that human mass consciousness, especially “specialist” consciousness, is often less malleable that the physical world it allegedly seeks to harness. In a way though, this reactionary consciousness acts as a filter allowing only the most worthy and fortunate ideas to break through. The trouble is that it also strangles many tremendous ideas in the cradle. If America is the cradle of world innovation, it had better start acting like it.

  6. Justin says:

    Engineer here. The concerns detailed above about rejecting heat from the proposed 325 kW compressor are legitimate. There are serious challenges dealing with that much heat rejection inside a vacuum system and the proposed steam system is underdeveloped. The wind stress considerations above are junk. We know how to build strong and dependable structures.

    I don’t fault Sam Jaffe for the USA today article. It reads to me that Chris Woodyard set out to write a sensational article saying “it won’t work” without spending enough effort.

    Think of unbelievable things we have accomplished: 22 nm transistors in your computer, adding to strands of DNA in living organisms, supersonic flight, the moon landing. Nothing is possible until someone sits down and figures out.

    The real question in this (and any engineering project) is not if it can be done (the answer is yes). The question is if it can be done and make money at the same time.

  7. Michael W says:

    As an experienced engineer (and in high pressure compression systems), I can say there are zero important problems with only an engineering solution. Sam is right to point out the compression, heat, and cooling challenge.
    Thermal systems are challenging to properly design in normal situations and we are talking about a cooling a pound per second from 1000F to 80F (in the main system). That is not a simple task and especially so in a light small package.
    I offered my constructive criticism to eliminate the secondary compression and cooling system, utilize on board high pressure air storage at 1000 bar (620kg/m^3) that would be refilled at stops. The compression and vessels already exist today for hydrogen. It would take less than 1 m^3 to supply the air bearings, breathing, air, additional propulsion, and redundant safety systems (brakes, hatches, controls, etc). The cream on top would be that letting high pressure let down could provide significant cooling potential through Joule-Thomson effect.

  8. GoodArticle says:

    This was a good critique, it’s what Elon asked for. I came here from the USA Today article, and think they should be embarrassed for quoting this. Not because it’s a bad article, but because it is just someone’s opinion.

    I wanted to hear some critique of the hyperloop, and this was what I was looking for. Just because the author doesn’t have engineering credentials doesn’t mean what he’s saying is wrong. I’d like to see his points furthered/countered with someone more technical but I still think it’s good that he wrote this.

    I think a lot of the dissonance in the comments section will come from people expecting to see a really good technical critique of hyperloop only to come read someone’s opinion. This isn’t the author’s fault.

  9. DS says:

    Did folks here not read his last sentence? “Musk’s vision and audacity should be applauded.”

    And then this: “I’ll start with my opinion of what the plan has going for it, followed by important challenges that still require a solution.”

    The author isn’t a naysayer, he’s only providing his opinions and continuing the dialogue about this new/old idea that is garnering some positive attention on a topic (high-speed mass transit system) that really needs it considering our current — and failing — attempts with high-speed rail.

    It’s obviously fine if you disagree with him, but let’s not take his stance/argument out of context. I think he’s hoping for an innovative idea like this to take root as much as the next person.

  10. Rush Strong says:

    “Any structure elevated 100 feet off the ground is going to be under a lot of wind pressure”

    You’re probably correct – which is probably why the proposed structure is on twenty foot tall pylons (spaced 100 feet apart).

    Mat we see a retraction?

  11. Sully says:

    I’m not an engineer either, but I did stay in a Holiday Inn once.

    The thing I most like about the Musk proposal is that he has the nerve to state a measurable accomplishment goal that is less than the lifespan of the universe. As to the technical details that is for the engineers to decide.

  12. Jumbybird says:

    You obviously didn’t read the proposal, where it says the system will be depressurized.

  13. Daniel Pineda says:

    Something tells me the commenters on this page should do some math themselves before making judgments about the ability of other people to do so.

  14. LarryT says:

    Jaffe is correct on most points, this will never get off the ground. Musk might be good at making money, but this is not an e-commerce website. And has anyone been to a Wal-Mart lately? Take a real good look at what Americans have devolved into. We’re not going to be doing anything anymore other than invent new ways to market Cheetos, tattoos, and flipflops.

  15. Sam Jaffe says:

    Thanks for all the comments, even the negative ones. Musk’s goal was to inspire debate and on this forum that’s what we have. Sorry for the mass reply, but I’m travelling and have to get this out quick, here are my responses to some of the points raised in comments:

    LarryT: I 100% disagree with you. First of all I never said it wouldn’t get off the ground. Secondly, America is the best place to launch a new technology like this.

    Jumbybird: I understand that the system is depressurized. That helps with the heat dissipation, but doesn’t solve it. Molecules being pushed aside still causes heat, even if there are less of them.

    Rush Strong: The pylons will vary in height from 20 feet to 100 feet, depending on elevation. Many of my concerns regarding wind stress are still mostly valid at 20 feet. They just become bigger problems at 100 feet.

    Michael W: Isn’t pressurized air part of Musk’s solution already? My initial thought is to dispense with onboard cooling and somehow get your heat transfer from the pipe itself. In fact, why not cool the pipe itself (and the air inside) down to near zero Celsius in order to help with the heat problem. Doing so would make the tubes condense an enormous amount of water on the outside, which could be collected and used for Central Valley agriculture. Crazy?

    Spencer: If you filter out all non-engineers and non-physicists from technical discussions, then I guess you won’t be able to read Musk’s paper either. He is neither an engineer nor a physicist.

    Austin: I disagree with you that this isn’t rocket science. Determining how to propel a pod inside a closed tube is actually harder to do than how to make a rocket go into space. Imagine if the rocket you were designing had a safety threshold of 1 millimeter. Veer off by more than that in any direction and you have a catastrophic failure. However, as I said in the blog post, I do think this is doable, regardless of how USA Today chose to interpret my words.

  16. Danny says:

    Why USA Today lead me to Sam, who is not even qualified for a peer review. Could be outright rejected by the open source community. Where exactly is wrong with Elon’s calculation, and where is your calculation, Sam?

  17. Danny says:

    I actually think spinning might be a big risk when things are out of control.

  18. Derek says:

    You are not a credible expert for a technical critique of this system. The technology is sound.
    The most difficult part of the Hyperloop project is political. Getting a corridor through central California, is an exercise in Eminent Domain and coordination of multiple cities, counties, and land owners – none of which is straightforward – and a hurdle for any ground based transportation project. Hyperloop is better than the high speed rail line currently proposed in just about every way.

  19. Dave says:

    Mr Jaffe, it sounds like you slept at a Holiday Inn Express last night.

  20. Joshua says:

    “The pod will be compressing air and expelling it downwards and backwards. All that air compression creates an enormous amount of heat, which can damage the pod and its machinery. Musk’s solution is to add to each pod a water tank that will capture that heat and turn it into steam to be collected at the next station. Although the thermodynamic calculations are correct, a small pod with only a few cubic feet of room for a heat exchanger leaves little space for an efficient exchange of heat. That means that the flow of water must be increased, requiring a lot more water on board.”

    Damn. There goes the jet propulsion engine.

  21. Michael W says:

    The current proposal calls for lugging compression, cooling, and power systems required for additional compression for the the air skids at low speeds. It is equivalent to the top power output of Smart Car (gas or electric) to produce an air pressure 1/10th of atmospheric pressure for likely a greater weight. Bringing very high pressure air on board would be like replacing the secondary compression system with a ‘battery’ that can help drive the primary compressor, cool the compressed gas, etc. Admittedly, you are introducing air into system that would have to be pulled out, but that is a relatively trivial feat compared to the rest of the system.
    Having a cooled tube would provide very little benefit at absurd energy costs. Cooling the tube by X degrees would result in a compressed air temp on the order of X degrees. At about 1700lbs per linear foot of a tube pair, the energy and time required to cool the tube boggles the mind. Additionally, every bit of water condensed would be an additional thermal load. Finally, colder air is denser air so there will be increased drag, compressor power, etc.

  22. Michael W says:

    Since I can’t edit:
    Cooling the tube…air temp *reduction* on the order of X degrees.

    Also, I have to disagree with your response regarding Musk’s credentials. He has a degree in physics and is leading a private company that has completely several first in space transport and will likely continue to do so. That should count for at least partial credit.

  23. rogerio says:

    How are they going to clean out all of the metal and body parts when (not if) this thing goes horribly wrong? An 800mph scraper/plunger?

  24. Derek says:

    I think we are also missing a big part. No windows. Imagine being locked in a steel tube with all the same folks you fly with now. Sorry, but a 5″ TV screen with an Adam Sandler movie just doesn’t make up for missing out on a view of flying 100′ over the ground at 600+ mph. Yea, a steel tube is cheap. But, there are many more human factors to evaluate here.

  25. audiresys says:

    The attempts to silence the author or other observers because they are not engineers are appalling—the flip side of this country’s disdain for intellect, or perhaps its extension by valuing formal credentials over ability to think. I have seen too many formally qualified but factually idiotic practitioners in numerous fields. The proof of competence to comment is in the quality of the comments. Let us skip the “Are you an engineer?” rubbish.

    Even if there are no showstoppers, it may be that a hyperloop is not cost-effective in comparison to a maglev, which in turn might not be more cost-effective than scheduled air transport.

    Constructed Transrapid 09 maglev track, exposed to full aerodynamic resistance, for the 560km SF-LA route is likely to range from US$0.6bln (Transrapid proposal in Australia, 2008) to $22bln (the Shanghai Pudong Airport line, built in difficult geological conditions in 2001–2003). The relative simplicity and economies of scale over the 19 times longer SF-LA route suggests that costs could be much reduced from the Shanghai levels. The 500km/h cruising speed, achievable over almost the entire route except the vicinity of stations, would yield approximately the same block time as that of the airlines, 1:15h, but downtown to downtown (probably more conveniently to most) and without the security, the associated lines, the requirement to be present long in advance of boarding, and the long walking distances at the airports, all resulting in much faster travel times than by scheduled airline for most passengers and much more pleasant journeys. Unlike on the hyperloop, passengers would also enjoy much roomier accommodations (being able to stand up, walk, have a drink, get a meal, use the bathroom) and natural light. Maglev has a history of being routinely disfavored for conventional high-speed rail, which historically has been much less expensive. However, this apparently has the potential to change, and the unusually expensive and not very air-competitive California High Speed Rail project may be one that maglev can beat—while being twice as fast.

    As the history of supersonic air transport shows, even a doubling of speed over a nontrivial (e.g. transatlantic) distance may not be a product that a sufficient number of customers wish to buy at its higher cost. The same problem has haunted maglev: its higher costs were deemed not worth the 40% drop in trip times. If a hyperloop can achieve the speed Musk envisions in commercial service, it will have 55% faster trip times than maglev. But over SF-LA, that’s only 0:40h one-way. Is this worth the cost, which is likely to be much greater, especially considering the costs of initial development of the technology and the risks? I seriously doubt it. If SST and maglev technology has not generally been able to jump over that gap, why would hyperloop, especially with its much lower comfort level?

    A successful technology is not necessarily the fastest (especially in the U.S., where passengers are much less concentrated near downtowns than in Europe or Japan, and so are saddled with much longer average trip times to and from the long-distance stations or airports) but the one that hits the right price/performance point. For example, much of Musk’s extraordinary Mach 0.91 cruising speed (faster than the maximum speed of any in-service transport aircraft) is due to the lower air pressure, and thus resistance, in the tube; take that away, and his acceleration technology is the same as a maglev’s—a linear motor. It should be relatively easy to increase speed substantially by doing the same with a maglev (encasing it in a tube and pumping most of the air out), and it would be much cheaper: the vehicle would be steadied by the magnetic suspension in two dimensions, as any maglev is; the tube could be much wider relative to the vehicle, avoiding the need to rebalance pressure; the tube would not have to be perfectly straight and kept that way; there would be no need for emergency repressurization (a breach would simply reduce speeds); and the tube could be made of a transparent material to allow light and sight. But there would be an extra cost for this saving of perhaps 0:20h. Would people pay this cost for this additional saved time, especially if it is imposed on everyone (because once constructed, the tube is not optional)?

    Musk mentions magnetic suspension as an adequate and existing solution to the need to steady the vehicle laterally in the tube, and dismisses it with a single sentence because of its cost. This is probably the single greatest and most obvious error in the entire proposal. Maglev is indeed costly, but probably much less costly than his complex and undeveloped stabilization approach, and is already in use. I am far from a maglev fan because I recognize the need for any system to be economically justifiable, but hyperloop is far more extreme still, and thus should be evaluated against maglev, with almost certainly much lower costs, no substantial technological risks, operational certainty, and passenger experience superior in every way other than a trip duration shorter by a margin that many people might deem trivial.

    Other than this use of a straw-man comparator, the two aspects of the proposal most jarring to me are both related to cost:

    - Musk expects to use existing right-of-way (along a highway) without sharing the cost of acquiring it. Even a state not on the verge of bankruptcy and again paying employees in IOUs would never have this. Any highway corridor is (1) limited to the length of the roadway, in which case a hyperloop would have to stand on newly acquired adjacent land (not cheap even with eminent domain), or (2) includes lateral scope for additional lanes, but it would have to be replaced and the state would want the cost of its replacement, or (3) contains no room for lateral expansion at all, forcing the condemnation of existing structures and the construction of new bridges at great cost in addition to that of seizing the land.

    - He grotesquely underestimates the vehicle cost, to the point where it is lower than a used trolley car of no historical significance, whereas his vehicle is actually an extremely complex assembly of multiple critical systems in a very small space. The small size of the vehicle suggests that the vehicle cost per seat is likely to be far higher than on a full-length maglev train, in which most of the technology is built into the track rather than carried on the vehicle.

    Then there are the potential showstoppers:

    - At the projected Mach 0.91 cruising speed, even with air pressure of only 100 pascal (the equivalent of 48km above sea level), there might still be some transonic buffeting, for which there is little tolerance where the gaps between the vehicle and the tube walls are so narrow. This could cause friction drag and, if sufficient, damage, as well as discomfort to passengers. A computational fluid dynamics study would be needed to determine whether this problem exists and to correct it, if possible. Again, a problem already solved by maglev technology.

    - The entire cycle (sensors > computer > mechanics) controlling variable valves and pressurized air injectors would have to occur quickly enough to keep the vehicle from touching the tube walls or buffeting too much. I’m not sure there is a system that carries out similar tasks with such rapid response. The mechanical actuators, specifically, may be incapable of responding timely.

    - We know how to build long structures, such as tunnels, reasonably straight by using laser beams to guide the construction. But a SF-LA pipeline would be one to two orders of magnitude longer and with far smaller tolerances than anything built using such guidance so far (typically underground railway tunnels).

    - Furthermore, the antiseismic/antiwind resilience measures would have to contain any forces outside the tube, without permitting it to be deformed.

    - Finally, the car transport proposal looks extreme by any cost-efficiency measure. It would greatly complicate loading and unloading, as anyone who ever used a car ferry knows (and here there would be much less space). It would add substantial trip time at the terminal stations. It would not be practical at intermediate stations (terminating vehicles there would mean reducing the turn-up-and-go frequency of the network). It would require a large amount of space at each downtown station, at great expense. And it is unnecessary. Most hyperloop passengers would be those attracted away from the airlines. They are used to renting cars away from home. All they would need is a competitive car rental offer at their outstation.

    In sum, hyperloop, as some Californians used to say, is far out. There are closer technologies: air transport (already there), 360km/h (done) and possibly 400km/h (forthcoming) conventional rail, and maglev. Musk says hyperloop is cheaper. I find that assertion noncredible, at least on the currently available evidence. Musk says hyperloop is faster. True, but probably not sufficiently to justify the cost and risks. Musk acknowledges that hyperloop has not been developed beyond a rough estimate (which is obviously lacking in many respects). The risk of this complex idea that exists only on paper compares unfavorably to the other technologies, which are all ready except 400km/h conventional rail, whose risks are much lower. Again, I am not a fan of any of these specific technologies; I simply submit that before we seriously consider a paper technology that’s far out, let us consider ones closer in that are already available.

    At that, I would love to see work on hyperloop experimentation and scale proof-of-concept, as it would, at a minimum, advance science even if not transport.

  26. Philip Coates says:

    > “Any structure elevated 100 feet off the ground is going to be under a lot of wind pressure, which will act on it in weird and sometimes multiple directions. ”

    Well, isn’t this the same problem that giant bridges experience? And especially ones spanning a bay, areas which are often windier than inland areas where the Hyperloop would operate?

  27. Philip Coates says:

    Audiresys says: “Any highway corridor is (1) limited to the length of the roadway, in which case a hyperloop would have to stand on newly acquired adjacent land (not cheap even with eminent domain), or (2) includes lateral scope for additional lanes… or (3) contains no room for lateral expansion at all”.

    He doesn’t seem to include the possibility of erecting the pylons in and running the structure down the highway median where such exists. That portion of the highway system would seem to require ~no~ additional land.

  28. Philip Coates says:

    Audires: “Unlike on the hyperloop, passengers would also enjoy much roomier accommodations (being able to stand up, walk, have a drink, get a meal, use the bathroom) and natural light. ”

    Remember that you are talking about a trip not of 4 or 5 hours, but about a half hour. People take bus rides or subway rides of that length without needing to eat or use the bathroom.

  29. Philip Coates says:

    Audires: “Musk says hyperloop is cheaper. I find that assertion noncredible, at least on the currently available evidence.”

    Why don’t we just not jump the gun on this until we have some actual evidence? As provided by a short-distance demo project.

  30. Philip Coates says:

    Audires:

    “The entire cycle (sensors > computer > mechanics) controlling variable valves and pressurized air injectors would have to occur quickly enough to keep the vehicle from touching the tube walls or buffeting too much. I’m not sure there is a system that carries out similar tasks with such rapid response.”

    What about existing maglev systems? Do they deal with such fine tolerances?

  31. Philip Coates says:

    Derek: “I think we are also missing a big part. No windows. Imagine being locked in a steel tube”

    Subways. Millions ride them everyday.

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