Looking At The Hyperloop, Part 2

Thunderf00t has released another video on the Hyperloop.

In it he takes the fans of Elon Musk to task and references the Hyperloop Alpha Document.  I took a look at it and from a professional’s standpoint, it’s a very interesting diagram, but not in a good way.

As I pointed out, the essential ideas around vactrains have been around for a long time.


In that post is discussed the maglev, vacuum and operational concerns of the system, but didn’t actually look into the alpha document itself, because quite frankly the Hyperloop had all those other issues that looking a the paper itself was unnecessary and I didn’t even think to look for it.  Well now I have and there are a lot of things that just don’t make sense to me. Here it is.


Looking at this makes me wish that I could chase down my old University professors. They are gone, so I’m going to have to take a stab at this all by myself.


Of all the pretty pictures in the Hyperloop document this simple diagram is probably the most interesting.  It’s the only one with real numbers.  But do those numbers make sense?  More importantly how are those numbers reflected in  real hardware.

First of all is that axial compressor .  The inlet pressure is 10^-3 Torr or 100 pascals.  That’s what I would normally think of a right at the border of where I start thinking about secondary pumps. Specifically the turbomolecular pump.


I’m going to point out that a Turbomolecular pump can’t compress anything. They operate By pushing individual molecules of gas down into the pump volume. So they spin very fast and are very delicate.  They have some very advanced technologies to enable them to remove gas from vacuum at low pressures, but while they have blades they are not a compressor.

Now here’s why I wish that my old university professors were still around. Back in the day I was going to college, Lycoming was still a going concern down the street,  Sikorsky was right across the street and Pratt was nearby. UB used a lot of adjunct Professors(it’s motto then was “educating for the real world”)    and by the very nature of things many of them had turbo machinery backgrounds.  Also the companies would give stuff to UB, so we had a wind tunnel and bits and pieces of aircraft engines lying around.

One thing that I learned is that compressor design is tricky, very tricky. If you don’t get everything right the thing doesn’t work, or bad stuff happens.  Compressor design requires good knowledge of airflow over the various blade shapes in the operating regime of the compressor. That requires wind tunnel testing of blade shapes with good instrumentation to obtain the gas property values before and after the blades. and the blades have to run in what’s known as a cascade because the blades affect each other.

Jet Engine Detail Design: The Compressor




The problem for the Hyperloop designers is that there is no data for gas flow at near supersonic flow in near vacuum.  The big problem is that not only the does the data not exist, but it may be actually be very difficult to get it. When your up pressure is already near zero, getting a down pressure for a wind tunnel is going to be impossible.


The only way I can think of to get the kind of data is to fly the test object almost into space and try to get data of design a long vacuum tube and have a test model cart in it running at near Mach 1. There’s also the problem of finding pressure instrumentation that works accurately in near vacuum. you are going to need to have Pirani and cold cathode gauges all over the moving high speed test article.


Then there’s the problem of dealing with the heat build up inside the compressor.  That’s a lot of heat that has to go somewhere.  That probably means that the cooling will have to start inside the compressor, which complicates an already very complicated device.

In any case the compressor design alone is probably a billion dollar project with a high degree of uncertainty of success at the other end of it.

Then there’s the intercooler. I’ve worked with intercoolers of various sizes, and seen thousands of designs for working heat exchangers.  This one baffles me.  First of all you have air at a high temperature and both low pressure and low flow rate.  That is not a good thing.  As a heat transfer medium, air doesn’t work very well.  So in order to get rid of the heat you are going to need a very high delta T and lot of surface area.  The problem with that is that means you need  a large cooler with a lot of pipes.  Which means a large pressure drop through the cooler.  When your entrance pressure 2.1 kpa absolute, there’s vary little pressure to drop. I wouldn’t be using water to cool, I would use liquid nitrogen. And cooling by spraying right into the airflow. That way you get maximum cooling for the least amount of pressure drop.

Which also addresses the biggest issue that you have in vacuum, that being that any heat you generate has nowhere to go.  So any heat that is created in the pod, stays in the pod.  The only way to deal with that is to make the pod more massive, which increase the power requirements which leads to more heat build up.

Which leads to another issue, vehicle rigidity.  It might not seem like a big deal, but vehicle are not rigid bodies. They respond to stresses by bending and flexing and some of that flexing can be dramatic, as anybody looking at an airplane wing in flight can see.  Every vehicle flexes, which can be bad when you are moving fluids around and expect the fluid to stay where it is wanted. As many engineers have found out the hard way.  How much will the pod flex?  depending on how it’s constructed, possibly quite a bit, especially around those huge doors that need to be stiff and remain sealed gas tight. To say nothing of the vibrations around the  compressor and other machinery.

Which brings me to my final issue, oscillation.  One thing that the paper seems to have missed is that air bearing are springs. As would be a maglev.  The fact is that oscillation has been a problem for high speed vehicles from the beginning.  The phenomenon known as “hunting” was and is a serious issue for designing high speed trains.  I explain some of that here.


How much would the air suspension oscillate?  I don’t know, but I do know that if that number is not zero it’s going to get real ugly very fast because if an oscillation starts there’s no margin for error and once the Pod’s ski’s actually touches the tube wall, the consequences are going to get ugly very fast as the ski is torn off or over heated due to friction and the pod proceeds to hit the tube wall at nearly Mach 1.  So in order for the Hyperloop to work at all the inside of the entire 1000km tube is going to have to be a highly precise machined surface with no imperfections or burrs allowed.

The Hyperloop has had a lot of “hype” and very little solid engineering behind it.  Mr. Musk should have had a reality check done before pushing it. There are too many fuzzy questions, many of which have no engineering experience behind them and multiple potential catastrophic failure modes with seemingly no redundancies.  To believe that a system like this would be cheap and easy is stretching reality in ways that make no sense.

In order for something like the Hyperloop to work it would take a 20 to 40 year multi billion dollar engineering effort.  For a transportation system for which there is no great need.  The Hyperloop is just a dream in a pipe.






  1. MadRocketSci · July 31

    What in the world?! Am I correct in interpreting that diagram to say that they intend to derive thrust from a turbine operating in near vacuum? Why would you do that? I though the whole point of operating in a vacuum was to get rid of all the aerodynamics (drag, but that also includes much to push against). I thought you had a highly magnetized track underneath you and would be interacting with that for thrust.


    • jccarlton · August 1

      It’s actually a compressor and it’s main function is to scavenge air for the air bearing in the skis. In no way is this likely to be anywhere near as easy as the Hyperloop people would have you believe. Especially since the engineering to prove this out, just doesn’t exist.


  2. Pingback: The Hyperloop Attracting Money, Getting Students To Do The Engineering | The Arts Mechanical

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