Comment by lolcraft | Hacker News Reader

Comment by lolcraft | Hacker News Reader

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Hum. I got a different impression with regards to the FEM graphs of the beams' stresses. Dr. Drang seemed more bothered by the fact that they were there because it was too amateur, and too much pretty graphs for their own sake, since any civil engineer could do the calculations in their sleep. Not because of technical problems, per se. (Honestly, that might be a social faux pas among physical engineers, and a noob move from Musk, but I'm not a civil engineer nor Musk so I don't give a shit.)

Apart from that, it seems, as I gathered from comments here, that there was a solution to thermal expansion, and that's both not fixing the tube to the pylons, and having the trajectory curve so they can bend safely under expansion. This confusion rose from the ambiguous meaning of "nearly straight tube". The Hyperloop seems to have decent tolerances for turning, so maybe that could work.

But the most solid arguments against Hyperloop, I think, come from the economics side. It's less efficient than a good HSR, like the French or the Japanese ones. It's not SF to LA, it's "somewhere near SF" to "somewhere near LA". Its cost is probably wildly underestimated. It looks as if the only failure modes are either "tubes explode causing chaos", or "crash and die horribly"; not necessarily an exclusive choice.

> any civil engineer could do the calculations in their sleep

The calculations can only be performed in Finite Element Analysis (FEA) software, doing this type of analysis (accurately) is impossible without computers.

>Its cost is probably wildly underestimated

There are also significant economic benefits to the hyperloop, in that it may be less expensive to maintain and power, while requiring much less rolling (floating?) stock. Rails are very expensive to maintain, largely because of the heavy loads they carry, and because they are entirely exposed to the elements.

I can't speak for civil engineers, but in mechanical engineering no one uses FEA to solve a cantilever beam. That's like using a supercomputer to calculate 3 times 4. Cantilever beam has bean studied for centuries and there are easy-to-use (and easy-to-prove) formulas for calculating its stress and strain.

From the post I was replying to:

>Hum. I got a different impression with regards to the FEM graphs of the beams' stresses. Dr. Drang seemed more bothered by the fact that they were there because it was too amateur, and too much pretty graphs for their own sake, since any civil engineer could do the calculations in their sleep.

smnrchrds, you are correct that simple structures have been studied, and equations describe their characteristics very accurately; what you are missing is that the FEMs in question were used to analyse a complex structure's response to vibration. I would challenge you to find any large bridge designed in the western world last 10 years without FEA.

> any civil engineer could do the calculations in their sleep

This sentence of Dr. Drang's article refers to FEA analysis of "applying a 1 g lateral acceleration to the three-pillar model and plotting out various stresses in the pillars". It's located directly below those plots and after his criticism of vibration analysis has finished. I agree that vibration analysis would be very hard without FEA, but applying a constant 1g lateral force to a cantilever beam could be easily done by paper and pen (well, maybe also a calculator) and is an overkill for FEA.

"(Honestly, that might be a social faux pas among physical engineers, and a noob move from Musk, but I'm not a civil engineer nor Musk so I don't give a shit.)"

Besides, it's the idea that's important.

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