MagLev vs. Hyperloop — Technical Discussion
Oct 09, 2013
In which it may be possible to, in fact, blend the two technologies together, creating terrestrial 'wormholes,' not in space, but certainly in time.
Last week, I introduced you to my top ten reasons, in reverse order of importance. Now, I wish to present the technical bases for my arguments.
The top reasons are (from last week):
10) Hyperloop is inflexible.
9) Maintaining even a partial vacuum at this scale is difficult and expensive.
8) Hyperloop uses large, noisy fans in front of each pod.
7) There is too little clearance between the pods and the tube's inner walls.
6) Switching between tubes is impractical.
5) Hyperloop pods tend to roll, which might also damage the motors.
4) Hyperloop pods are cramped.
3) Hyperloop cannot carry some of the larger standard freight containers.
2) Escaping from a transportation tube can be a dicey experience.
1) Tesla is the best company to manufacture LeviCars.
The above describes the problems with Hyperloop in terms of their effects. Below are sections that will suggest these same problems in terms of their causes, and how these causes may be remedied. Some of these suggestions are quite speculative.
There are problems with the current version of Hyperloop, many of which are related to the use of long uninterrupted transportation tubes [see items (5) and (2), above]. Basically, this section discusses how to add a MagLev rail to a transportation tube, or else to incorporate a MagLev trough into such a tube.
But first, we need a thorough technical description of everything that goes into this:
A brief description of Magnetic Levitation (MagLev)
I will use the term “D-P MagLev” to mean any sort of Danby and Powell MagLev system. Like all practical superconducting MagLev systems, D-P Maglev is characterized by having the superconducting magnetic coils on the vehicle, not the transportation conduit (“rail”), as many people suppose. The conduit contains two sets of inexpensive aluminum loops. The unpowered loops in the first set interact, by means of induced current, with the moving vehicle’s superconducting magnets to provide not only levitation, but also a great deal of stability. The second set consists of energized loops providing propulsion and braking, using a Linear Synchronous Motor (LSM).
The conduit could be a full monorail, a pair of virtual rails, or a trough (the Japanese MagLev, based on Danby and Powell’s work, is a rectangular trough), and the trough could conceivably be circular. Since these two sets of aluminum loops overlap each other without touching, they could be flush with the bottom of a circular trough, equivalent to the bottom part of a cylindrical tube. A circular-trough MagLev has not yet been designed, but, in consideration of the stability afforded by D-P MagLev, it should work well as the lower part of a transportation tube. Thus, it possible to combine the advantages of D-P MagLev with those of transportation tubes.
I believe that Mr. Musk is overestimating the cost of MagLev, while underestimating the cost of Hyperloop. (Perhaps he also assumes that the rails must contain expensive superconducting magnets, rather than cheap aluminum coils.)
Technical Discussion of the Problems with Hyperloop and other Transportation Tubes
This section describes the problems in terms of their causes, and later sections suggest how these causes may be remedied. Some of these suggestions are quite speculative.
A transportation tube is any enclosed large tube for conveying vehicles long distances. Let us consider three exemplary kinds of transportation tubes, the first two being “low-pressure” and last being “high-pressure”:
• Hyperloop: Very low pressure with a suction turbine in front of each vehicle;
• Pneumatic: Tubes in which high-pressure air pushes the vehicle.
First, let us consider some important disadvantages of all three kinds:
(1) These are long, solid tubes. If a vehicle gets stuck in one, the tube is closed down completely, and the people inside all the vehicles will have to be rescued. This can be particularly dicey for underground tubes. The tubes can be equipped with escape hatches, but resetting and reasealing the escape hatches can be a major operation, and may shut down the system for days. The hatches will also cause irregularities in the inner walls of the tube, which, in the case of Hyperloop, might cause extra friction at best, or collisions between the pods (or their skis) and the inner walls at worst.
(2) The tubes can leak due to many causes, ranging from stress and torsion due to seismic activity, to deliberate sabotage. For low-pressure tubes (Hyperloop and Vactain), an air leak into the tube can pretty much shut it down by increasing the air resistance. The air from the leak has to diffuse towards the vacuum pump, and that takes time, and would undoubtedly cause air currents within the tube. There would have to be multiple pumps, and lots of them, nearby each together. For higher-pressure pneumatic tubes, leaking is less of a problem.
(3) Because these are solid tubes, you cannot see out of them. Both Hyperloop and ET3 suggest using display screen showing virtual scenery. It is long established that people like to see where they’re going.
In addition, Hyperloop has the following problems:
(4) Hyperloop’s air-bearing skis run very close to the walls of the tube — 0.020" to 0.050" (about 0.5 mm to 1.3 mm). [In contrast, ET3 uses a different kind of MagLev, and has a 4" (10 cm) clearance, about the same as D-P MagLev.] At the high vehicular speeds, any irregularities or vibrations in the wall can lead to a collision between the wall and the pod (or its skis). Both overground and underground tubes are subject to seismic activity. Overground tubes could sag or be affected by collisions with other objects. If the tubes are mounted on pylons near highways, a wayward truck could disrupt the tube.
A recent news article desribes a simulation that indicated two previously-unknown problems:
(5) The pods get very hot, due to the high speed and confined space, even though the air is very thin. It seems to me that the partially-evacuated tube has none of the advantages of a fully-evacuated tube, but also none of the benefits.
(6) Hyperloop pods tend to roll, that is, rotate within the tube, with a chance that the pod could travel upside down.
Specifically, Hyperloop’s Alpha Specification, on the top of page 25 (section 4.2), states:
The Hyperloop travel journey will feel very smooth since the capsule will be guided directly on the inner surface of the tube via the use of air bearings and suspension; this also prevents the need for costly tracks. The capsule will bank off the walls and include a control system for smooth returns to nominal capsule location from banking as well. Some specific sections of the tube will incorporate the stationary motor element (stator) which will locally guide and accelerate (or decelerate) the capsule. More details are available for the propulsion system in section 4.3. Between linear motor stations, the capsule will glide with little drag via air bearings.
Other problems inherent in the above quote, but not mentioned in the news article are:
(7) Even if the pod does not travel upside-down, lack of roll control might make for a nauseating ride.
(8) Unless the pod’s rotor, and the stators (which protude from the tube’s inner wall) are perfectly aligned when they meet, there will be a collision.
(9) Periodic acceleration can make for a bumpy ride.
Since D-P MagLev provides inherent stability and can be built into the transportation tube as a circular trough, it can solve all these problems without requiring the addition of tracks.
In Defense of Pneumatic Tubes
In general, the use of Pneumatic tubes in which high-pressure air pushes the vehicle, is rejected, by Hyperloop and others, mainly because it would require a high-speed or supersonic air stream to rub against the inside wall of the transportation tube. Pneumatic tubes are, however, more tolerant of leaks than low-pressure transportation tubes.
My own feeling is that there might be a solution out there, that would permit air to go at very high subsonic speed, or at supersonic speed, without causing a lot of friction. The history of aerodynamics is riddled with “impossible” things being true. After all, golfballs counterintuitively travel twice as far with dimples than without, and, when the curve ball was first demonstrated in baseball, some scientists even pronounced it “impossible”. The following is almost-pure speculation: Perhaps a shark-skin pattern or some sort of plasma device could be used.
Again, speculating: One possibility is to use a D-P MagLev system with a Linear Synchronous Motor (LSM) in a monorail within the transportation tube. The monorail would also contain both high- and low- pressure pneumatic pipes, with computer-controlled electrically-activated valves that can suck air from the transportation tube ahead of the vehicle, and vent air into the tube behind the vehicle.
There would not have to be a perfect seal around the vehicle — a small amount of air could leak forward or backward with minimal effect on the functioning of the system. Perhaps some sort of huge valve, resembling the tricuspid valve of the heart, made of actively-controlled elastic sheets, could prevent backward flow of air in the tube. It would be retracted every time a vehicle comes through.
In this brief section, I have not expounded any solution, but rather called for more research to be done. Of course, all this is sheer speculation — I haven’t solved any problems, just posed them.
How to Combine the Best Features of Hyperloop and MagLev in a Super-High-Speed System
The higher-speed transportation conduits would supplement the standard-speed Maglev. In all likelihood, these higher-speed conduits would be more expensive than the standard-speed rails, and will also take some time for research and refinement before they are built. Therefore, long-term plans should be for the standard-speed network to be developed first, and the higher-speed systems added later.
These higher-speed segments would be for passenger service only. Freight can move fast enough at 300 mph, but passengers would want to move yet even faster.
Ordinarily, without the higher-speed segment, a vehicle would typically be driven from its original source to a MagLev depot (no more than ten miles in any built-up, urban or suburban area) where it would be placed on the hexOgrid, and travel at 300 mph (500 kph) to another depot, from whence it will be driven to its final destination.
With the high-speed system, the vehicle would travel from the first depot, at 300 mph, to a transfer point where it would accelerate and be inserted into a higher-speed conduit, likely a transportation tube. It would then travel hundreds or even thousands of miles to another transfer point, where it would decelerate and be placed on the standard-speed MagLev network. It would travel on this network to a final depot, and then be driven the few miles to its final destination. It is also possible that the trip might be in five (or seven or nine) alternating segments of standard-speed and higher-speed conduits.
When traveling on a MagLev rail, the vehicle is mounted on a bogie, an undercarriage containing the superconducting coils, power supply, and any other devices needed to interface with the MagLev rail, plus other systems that may be needed or personal comfort and safety. Often, when traveling in a transportation tube, the vehicle must be fitted with a dome or fairing, to manage air resistance at the higher speed, or to keep the air in while traveling in an evacuated tube. It would be better to use these on the entire trip, both in the standard-speed and higher-speed modes. The MagLev undercarriage and dome or fairing can be applied at the depot where the vehicle enters the standard-speed network for the first time, and removed at the depot where it leaves it for the last time.
As mentioned above, we could put a Linear Synchronous Motor in a monorail within the transportation tube. The monorail would also contain low-pressure pneumatic pipes, with computer-controlled electrically-activated valves that can suck air from the transportation tube ahead of the vehicle. For Hyperloop, this would reduce air friction and help counteract the Kantrowitz Limit. The suction turbine would still be needed, but it could be smaller. For Hyperloop and Vactrain, these pipes could be used to maintain a vacuum.
All we need is to have shortcuts between one part of the standard-speed network, and another part, far away. These could be straight transportation tubes, with no switching points. I propose calling such a shortcut tube a “wormhole”, after the popular name for an Einstein–Rosen bridge, as mentioned in Star Trek, The Next Generation.
Combining Hyperloop’s Compressor into MagLev Wormholes
This is a very speculative idea: When a MagLev vehicle is on a standard-speed rail and is about to enter a wormhole, it is mated with a Hyperloop-like front compressor. The compressor’s housing has a fairing in back, that matches the front of the vehicle’s fairing. The compressed air from the compressor is fed into a front-to-back pipe in the MagLev bogie, and leaves the rear of the bogie. There is no need for air-bearing skis. This truly combines D-P MagLev, LeviCar, and Hyperloop!
It might also be possible to have one compressor in front of several MagLev vehicles in tandem, but assembling such a “train” might be too complicated to do at 300 mph.
Summary and Conclusion
After introducing several forms of high-speed transportation, a comparison is made of MagLev and Hyperloop. There is then a detailed discussion of Hyperloop’s problems, and how MagLev can be used to fix them. It could be possible to use Hyperloop, or something like it, to provide shortcut tubes between distant parts of a MagLev network. After exploring several options, the conclusion is that such tubes (“wormholes”), without any switches and containing MagLev rails, would work best. As an afterthought, there is a description of a way of adding Hyperloop’s front compressor to a MagLev vehicle, when it runs in a wormhole.
 Levin, Joshua Zev, Open Letter to Elon Musk about Hyperloop (Accessed October 9, 2013)
http://www.LeviCar.com/Hyperloop/MuskOpenLetter.html Back to text
 China working toward 600 Mph Trains Through Very Low Pressure Underground Tubes for 2020 to
2030 (Accessed October 9, 2013)
http://nextbigfuture.com/2010/08/china-working-towards-600-mph-maglev.html Back to text
 Neff, Todd, Who Needs Hyperloop? This Guy Is Building Something Bigger
(Accessed October 9, 2013)
http://mashable.com/2013/08/25/hyperloop-daryl-oster/ Back to text
 Subbaraman, Nidhi, NBC News, Hyperloop sounds crazy ... but simulation says it just might work
(Accessed October 9, 2013)
http://www.nbcnews.com/technology/hyperloop-sounds-crazy-simulation-says-it-just-might-work-4B11202787 Back to text
 Hyperloop Alpha Specification (.pdf) (Accessed October 9, 2013)
http://www.teslamotors.com/sites/default/files/blog_images/hyperloop-alpha.pdf Back to text
 Gubisch, Michael, London, Aircraft paint suppliers explore sharkskin coating (Accessed
October 9, 2013)
http://www.flightglobal.com/news/articles/aircraft-paint-suppliers-explore-sharkskin-coating-381646/ Back to text
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