Comment by schiffern | Hacker News Reader

Comment by schiffern | Hacker News Reader

How can anyone possibly suggest that this would be cheaper than high-speed rail? If you're going 1150 mph ("average speed twice as fast as a commercial jet"), the curve radius is going to be immense – 90 km to maintain <0.3 lateral g. This will constrain your right-of-way selection vastly more than the HSR project Musk scoffs at.

And, of course, the skin friction on a 2.5 meter tunnel would be immense. Using a duct friction loss calculator I get 285 megawatts of loss over the entire tube. You need two tubes. At 120,000 passengers/day (HSR estimate), it would take 114 kWh per trip. That's worst than the Model S, hardly an system in which "the fundamental energy cost is so much lower" than a car.

No, the "theoretically fastest way" to go from Point A to Point B is a great-circle vacuum train connecting them. A launch loop does essentially that, but exploits the vacuum above our heads instead. It just fits better.

Any hyperloop theory needs to deal with supersonic speeds – LA-SF as the crow flies in 30 minutes is just under the wire for subsonic speeds. Elon said "under 30 minutes." There are mountains in the way.

One of the interesting things about Jacques speculation is that if you move the pressurized air with the capsules they never go super sonic, the air is moving with them and their motion relative to that air is pretty constrained. The duct friction issue however is a serious issue.

It could be a giant hack of course, it could be Elon saying he knows how to do it when he doesn't and getting all these great ideas to pour out. Or he could die and it would be his 'last company' legacy like Fermat's last theorem, sort of "Wow, really elegant way to do that, won't quite fit in the margin here though."

In aerodynamics, everything is relative. If you, plus a bunch of air are moving at supersonic speeds through a pipe, then that looks exactly the same as if you're moving a pipe at supersonic speeds over static you.

I think what Chuck meant was that the speed of sound increases with pressure, so the pressure wave in front of the capsule would cause the transition speed to supersonic to be higher. I'm not sure this is a good thing though from a drag perspective, but could be if it keeps the flow under ~0.8 Mach. Definately the complexity of the fluid dynamics at high speeds can't be neglected. http://en.wikipedia.org/wiki/Drag_(physics)#Wave_drag_in_tra...

You're both right, at the boundary layer between the volume of air you're pushing and the tunnel there is going to be a bit of turbulence given the viscosity of the air, within the bubble you may never exceed the mach number for the air around you. Excellent paper put out by Barrett on designing long range sniper rifles [1] talking about the issues have having the bullet not be deflected by leaving the barrel (where even though its travelling quite fast it has yet to exceed the mach number of the expanding gas around it) and the ambient atmosphere (where the air is 'still' to the now speeding bullet). Needless to say the paper lost me in the fluid dynamics of the barrel air mixing with the outer air but they seemed pretty clear that inside the barrel things were pretty linear up to that point.

Anyway, what ever it turns out to be I'm sure it will be cool.

[1] My Dad is an engraver with an FFL and gets all sorts of interesting things in the mail. Usually to engrave, but the promotional literature is cool too.

The principal of "choked flow" limits flow velocity in a constant section tube to Mach one unless the pressure drop is much higher than 1 atm. The drag force at choked flow would be about 20 tons. Tests show drag in a tunnel with high blockage ratios can be more than an order of magnitude greater than the same shape moving in open air.

Pressuring a system (depending on the pressure) would be more stress and risk than a vacuum. Depending on materials used (concrete for instance - that prefers to be loaded in compression), the vacuum can actually IMPROVE stability and safety. As far as a pressurized "loop" as you describe, it would greatly INCREASE DRAG compared to a vehicle in the open air. What you describe is a 'Pneumatic Tube Transport' (PTT) system (as used in bank drive-up windows). PTT does not scale up well due to the enormous drag of air on the ENTIRE inside surface of the tube. Large PTT systems take a lot of power regardless of use. The energy per vehicle would be greater (but could be reduced a little by using hydrogen or helium that has less drag than air (mostly nitrogen)); however small molecules leak much more than large ones, and the expensive working fluid would have to be replaced.

Google "Evacuated Tube Transport" (ETT) or "Evacuated Tube Transport Technologies" (ET3) (trade marks of ET3 Global Alliance, and et3.com Inc.) on google and youtube -- watch the videos to learn of more advantages. Read the first US patent (5,595,543) to learn exactly how all of what Musk claims (and much more) can be accomplished. NOTE: ET3 (tm) is the product of over 200 experts, companies, and institutions in 18 countries. We hope musk joins our efforts to implement ET3. ET3 is optimized to accommodate BOTH cargo and passengers in car sized vehicles. Initial (local) routes will be at 300-400mph, and ET3 will accomplish over 50 times more transportation (ton-miles and passenger-miles) per KWh than the most efficient electric car or train. ET3 cost is less than a tenth as much as HSR (high speed rail), mostly due to much less vehicle mass It takes 1/30th as much concrete and steel to support 1200lb car sized capsules (400lb vehicles that haul the same 800lb payload as a typical automobile) than to support 100 ton locomotives on elevated structure. ET3 has over 10 times more capacity than HSR at 350mph design speed, and 0ver 100 times more at 4000mph. Refs: search “ doi: 10.3969/j.issn.2095-087X.2011.01.007 “ for a peer reviewed paper published in Journal of Modern Transportation; US patent 5,595,543 (many additional patents in the ET3 portfolio)

Great points! The lateral g load could be reduced by banking the capsule - but you still raise an important limit that even with banking would still cause an unreasonable curve radius.

Launch loops are interesting. They seem to be even more difficult than tether-based space elevators, which have themselves been unable to work due to material strength:weight constraints on the tether. I'd expect the belt supporting a launch loop to have similar constraints, and the dynamic bending of the belt would raise fatigue concerns.

Agree with your speed estimations and that supersonic or near- speeds have very high drag costs and complicated analysis.

>I'd expect the belt supporting a launch loop to have similar constraints [as space elevators], and the dynamic bending of the belt would raise fatigue concerns.

The reason inertial schemes like the space fountain and the launch loop were created is to overcome those material limitations. Aside from the scale, the engineering and material science of a launch loop is relatively straight forward.

The seminal paper, which discusses rotor fatigue and much more: http://launchloop.com/LaunchLoop%3Faction=AttachFile%26do=ge...

I did a quick read and a search and didnt find any discussion of ribbon fatigue. The issue is that the various stressings on the ribbon thermal, bending, tension, and curving through the 28 km loop over many cycles. In general the mechanical analysis of ribbon strength was unconvincing to me.

Usually when comparing speeds of trains and airplanes you factor in the transit time from city center to airport + security checks etc. So the time measured is total travel time. Railway stations are already at city center, and security is less strict so time savings are considerable for the benefit of rail.

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