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Comment by parhamn | Hacker News Reader

> The author has dived to the deepest levels of one section of the proposal

> This open source project is very ambitious and planned in good detail

Through reading the article I think his opinion is far from that. The vibe I got was along the lines of "this is so bad that a high school physics student knowing the thermal expansion laws, and a sophomore college student who knows how cantilever beams work could tell you this project is fundamentally fishy". Considering this perhaps his snarky comments are warranted. Musk dropped this project as a critical piece against current plans for this infrastructure and on a very basic level it comes off as 'snake oil'. Expecting a high level of polish doesn't seem too much of a requirement to me.

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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.

The problem with taking that tone is that it is by no means obvious that the article poster actually understood the part of the proposal he is criticising.

That makes it very hard to take his criticism seriously, and doubly so when the tone is so condescending.

I have seen less criticism from the engineering community than from other groups, thought this may be due to a general reluctance of engineers to go on the record unless obligated.

This. On one hand, we hate credentialism and criticize those who arrogantly wave their diplomas around as a substitute for making good arguments. On the other hand, we're enamored enough with SpaceX and Tesla to give Musk the benefit of the doubt, even though:

- The Hyperloop is a very different type of engineering compared to anything Musk and his engineers have done before (competency in one field does not guarantee competency in a very different field),

- The analysis has some serious issues. A trivial example: the thermal expansion issue in this blog post has been brought up in user discussion several times in previously posted Hyperloop articles on HN, and nobody was able to post a good response or justification.

And it only takes one unsolved show-stopper to make everything else you've done irrelevant.

> the thermal expansion issue in this blog post has been brought up

The author missed the "not rigidly fixed" part when he was analyzing thermal expansion issue. My comment:

https://news.ycombinator.com/item?id=6238022

How do they deal with thermal expansion on traditional railways? Same problem no? The track needs to expand, they spread across miles and miles and I never saw a damper area (except that the tracks are disjointed by a quarter to half an inch each, which would be the same with a tube in a tube configuration)...

Most modern railroad use continuous welded rail, so there are no joints like what you're talking about. This is both stronger and quieter, but it can lead to problems when the temperature is too high or too load, because of thermal expansion.

"There are a number of ways to increase the resistance of rail to buckling. The first and most common is the use of rail anchors to limit the longitudinal expansion of the rail. Anchors come in a variety of designs, but they all apply several thousand pounds of clamping force on the base of the rail. When applied properly against each side of the crossties, the anchors prevent expansion of the rail in the longitudinal direction." - http://www.interfacejournal.com/features/03-05/track_bucklin...

For more, and pictures, see http://en.wikipedia.org/wiki/Rail_stressing .

The UK had problems this summer because of buckling: http://www.dailyecho.co.uk/news/10549015.Rail_delays_as_trac... .

They install the railway at the maximum possible temperature for the area. (Or heat it.)

Then when it cools it pulls very hard, but because it has a tons of anchors on the ground it is able to handle the stress.

But it requires a very strong railway track. With the hyperloop the tube is not made that strong, and would tear.

The choice of Rail Neutral Temperature depends on the region. In the US Midwest and plains it's typically around 95 to 100 degrees. In the UK it's stressed to 27°C (81°F), which is the mean summer rail temperature. In neither case is it the maximum possible temperature.

You don't want it to be the maximum possible temperature for the area, because a well-laid track can be in temperatures 40 to 50 degrees above the RNT before it starts to buckle, and a higher RNT is harder to achieve.

Nor do they lay the track or heat it at the hottest possible temperature. They achieve the RNT by stressing the rail. The RNT can also change over time, for example, if winter causes a curved track to become a bit more shallow.

I agree with you and with the linked-to author: I don't see how the proposed hyperloop system can ever economically handle temperature expansion, and the lack of a good explanation or discussion makes it easy to dismiss the seriousness of the proposal.

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