Hyperloop De-hyped

It’s here!

The plans for Elon Musk’s much hyped hyperloop concept, which is supposed to radically reduce the travel time between Los Angeles and San Francisco, and create a fifth mode of transport after planes, trains, cars and boats, have finally been revealed:

As predicted by many, the concept features a compressed tubular and modular system that shoots people and vehicle-carrying pods from one location to the other.

As Musk states on the outline, the engineering concept depends on a new approach to dealing with the friction problem associated with high speed travel:

Short of figuring out real teleportation, which would of course be awesome (someone please do this), the only option for super fast travel is to build a tube over or under the ground that contains a special environment. This is where things get tricky. At one extreme of the potential solutions is some enlarged version of the old pneumatic tubes used to send mail and packages within and between buildings. You could, in principle, use very powerful fans to push air at high speed through a tube and propel people-sized pods all the way from LA to San Francisco.

However, the friction of a 350 mile long column of air moving at anywhere near sonic velocity against the inside of the tube is so stupendously high that this is impossible for all practical purposes.

Another extreme is the approach, advocated by Rand and ET3, of drawing a hard or near hard vacuum in the tube and then using an electromagnetic suspension. The problem with this approach is that it is incredibly hard to maintain a near vacuum in a room, let alone 700 miles (round trip) of large tube with dozens of station gateways and thousands of pods entering and exiting every day. All it takes is one leaky seal or a small crack somewhere in the hundreds of miles of tube and the whole system stops working.

However, a low pressure (vs. almost no pressure) system set to a level where standard commercial pumps could easily overcome an air leak and the transport pods could handle variable air density would be inherently robust. Unfortunately, this means that there is a non-trivial amount of air in the tube and leads us straight into another problem.


The approach that I believe would overcome the Kantrowitz limit is to mount an electric compressor fan on the nose of the pod that actively transfers high pressure air from the front to the rear of the vessel. This is like having a pump in the head of the syringe actively relieving pressure.

It would also simultaneously solve another problem, which is how to create a low friction suspension system when traveling at over 700 mph. Wheels don’t work very well at that sort of speed, but a cushion of air does. Air bearings, which use the same basic principle as an air hockey table, have been demonstrated to work at speeds of Mach 1.1 with very low friction. In this case, however, it is the pod that is producing the air cushion, rather than the tube, as it is important to make the tube as low cost and simple as possible.

You can read more of the details to Musk’s concept here. But generally speaking, the design breakthrough seems to be that the pod would feature a compressor on its nose that “ingests oncoming air for levitation and to a lesser extent propulsion”.

He believes the economics can work, and even compete with the $68.4bn proposed California high speed rail. There would also be notable energy efficiencies to be gained:

A breakdown of the costs for building ‘capsules’, meanwhile, looks like this:

So there you have it.

Time for some solid critique from the engineering community, no doubt. But let’s hope it comes in constructive form rather than in petty Musk/Tesla bashing mode.

This piece has been cross-posted from FT Alphaville with permission.

2 Responses to "Hyperloop De-hyped"

  1. Mike H.   August 13, 2013 at 4:05 pm

    David B…not a bad question about the breakdown scenario but I think that if similar reliability standards used for the airline industry were applied to the hyperloop then you could reasonably expect breakdowns infrequently enough for this to not be a big worry. As for a medical emergency, use the airline industry as an analogy here as well.

  2. Ashish   August 14, 2013 at 12:45 am

    Sounds cool. However, where does this fit in as far energy efficiency/ climate change etc is concerned? Isn't high time we start planning for a less carbon intensive world?