I am trying to understand some choices of hyper-loop designers.

The effort seems to be focused on trying to bring a near vacuum (space-like) environment to ground level.

Why would it not be easier to focus instead on containment of a kind of artificial jet-stream (see diagram)?

enter image description here

Note: The loading area in essence represents a sealable door in the tube.

  • Firstly the jet stream would transmit acceleration power to the pod, consequently the size and weight constraints on the engine and fuel are removed (compared to a powered pod).
  • Secondly, as the jet stream is isolated the pod will not need to break the sound barrier in the airstream, (there is some boundary effect to address at the chamber walls and the airstream velocity may exceed the sound barrier, which I am asking about [see questions below]).
  • Thirdly (especially if combined with descent/ascent phases) the pressure differential across the cab may be quite low, corresponding to Pod friction and acceleration (when there is any) ($f_{pod}+m_{pod}a_{pod}$) and should be easier to manipulate through pod design. It’s not this pressure that’s important to high speed, but the internal air-stream speed.
  • Fourthly there is no requirement to maintain a partial vacuum in the tube, simplifying the technology significantly; making it more tolerant to limited leakage.

The pod design can be dedicated more to the problem of reduced friction with the tube walls. For example to achieve lift, the pod could manipulate air flow and/or use magnets to attract itself to the opposite wall (assuming the tube material is ferromagnetic like steel).

The principal question is simply where or by whom has the physics of this “contained jet-stream” approach been looked at. What are the links, keywords I need to use to find out more. Are there any significant conclusions?

I would also like pointers to the physics of the edge effect of high velocity air within the tube (noting potentially a fairly low pressure differential along the tube, or between the tube and the environment), particularly at or above the speed of sound.

  • $\begingroup$ Re, "the edge effect of high velocity air within the tube" Don't forget, you are talking about a high-velocity air stream interacting with hundreds of miles of tube wall. $\endgroup$ Nov 10, 2017 at 14:46
  • $\begingroup$ -1. Not clear. Your 1st questioin seems to be looking for comments on your criticism of the design. I think this is off topic because it is an engineering question. The last 2 paragraphs ask for resource recommendations, which could be on topic. I think you should mention what research you have done already, and what is the source of your information to date. $\endgroup$ Nov 10, 2017 at 15:45
  • $\begingroup$ Ahh, keywords "pipe flow" will get this en.wikipedia.org/wiki/Pipe_flow There's also some history under "pneumatic transport" $\endgroup$
    – JMLCarter
    Nov 10, 2017 at 16:10
  • $\begingroup$ Not sure if I understand your design. Your idea is to use compressed air to push the pod down the tube? If you want to achieve extremely high velocity, that's going to require a lot of air and a lot of energy since there will be significant energy losses in trying to push a high speed flow of air down the length of a long, narrow tube. Also, unless you want the pod to expend lots of energy pushing the air ahead of it, you're still going to have to maintain a good vacuum for the air in front of the pod. $\endgroup$
    – user93237
    Nov 10, 2017 at 20:22
  • $\begingroup$ No DV, but I agree with Sammy above that this is more Engineering SE, maybe search Elon Musk's patents :) $\endgroup$
    – user175021
    Nov 10, 2017 at 21:58

1 Answer 1


You neglect the friction between air and tube walls. To move a cylinder of air through the tube at high speed needs powerful pumps. Yes, for the pod itself the power balance is positive, as it moves in a tailwind. However, the full system will be very inefficient, spending almost all energy on pressing air though a long pipe.

It will be better to suck most of the air out so both the pod and the tube wall will experience little friction.

  • $\begingroup$ Is there an equation or other mechanism that will demonstrate the magnitude of this fricton is large? It should be much easier to suply larger amounts of energy from fixed end station than from a moving pod. $\endgroup$
    – JMLCarter
    Sep 5, 2018 at 21:33
  • $\begingroup$ @JMLCarter: Search for pipe flow: This is a well-researched issue (see this answer here, for example). Make the length of the tube long and the flow speed high enough, and the needed pressure at the pump would easily exceed the burst pressure of any pipe. Not to mention inefficiencies, heat problems and what not. $\endgroup$ Sep 15, 2018 at 9:48
  • $\begingroup$ Thanks for the link. Pressure drop is proportional to pipe length. (and it's back to the drawing board for me.) $\endgroup$
    – JMLCarter
    Sep 16, 2018 at 12:50
  • $\begingroup$ @JMLCarter, pressure drop is also proportional to velocity squared. $\endgroup$ May 3, 2020 at 22:32

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