The most expensive number in engineering • Hacker News Board

Re: The most expensive number in engineering

#41

Post by UncleMeat » Mon, May 31, 2021, 6:29 PM UTC

snowwrestler wrote: post #34

Grossly increasing the factor of safety is a subtle way that science fiction stories connote a feeling of very advanced technology. For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space. Then later another ship falls from orbital height and plows through San Francisco without losing its hull shape. In Star Wars the Mi…

I don’t think this is science fiction doing this to connote the future in many of these cases.

The cars in the “fast” franchise also survive ludicrous damage. The bodies of action heroes survive falls from ridiculous heights and blows and stab wounds that would kill somebody with ease.

This is more a property of action movies (and adventure movies, to a lesser degree).

Re: The most expensive number in engineering

#42

Post by formerly_proven » Mon, May 31, 2021, 6:30 PM UTC

santiagobasulto wrote: post #24

For context, the FS of elevator cables is ~10 (depending of the country). EDIT: What's usually limiting in elevators (and that's why they say "max 4 people") are the breaks.

> EDIT: What's usually limiting in elevators (and that's why they say "max 4 people") are the breaks.

Even here there's a safety factor (self-limiting packing density of people in Western countries... "Uh... I'm gonna take the next one").

Re: The most expensive number in engineering

#43

Post by dhosek » Mon, May 31, 2021, 6:31 PM UTC

bjt2n3904 wrote: post #11

Obligatory Calvin and Hobbes comic: https://www.gocomics.com/calvinandhobbes/1986/11/26

Calvin's dad is my role model as a father. My 7-year-old children believe that in the past the world used to be black and white.

Re: The most expensive number in engineering

#44

Post by codeflo » Mon, May 31, 2021, 6:32 PM UTC

snowwrestler wrote: post #34

Grossly increasing the factor of safety is a subtle way that science fiction stories connote a feeling of very advanced technology. For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space. Then later another ship falls from orbital height and plows through San Francisco without losing its hull shape. In Star Wars the Mi…

About the Abrams movie: Too bad the same factor of safety wasn't applied to the buildings, where that should have been even cheaper.

Your point is good though. Just like we're now starting to put wi-fi chips into absolutely anything just in case, why wouldn't an advanced civilization simply use their super strength nanoparticles for everything? Why go out of your way to use worse materials?

Re: The most expensive number in engineering

#45

Post by Gibbon1 » Mon, May 31, 2021, 6:39 PM UTC

rossdavidh wrote: post #33

whatshisface wrote: post #15

Earlier quoted context omitted.

> It makes sure we calculate the loads correctly and use appropriate materials. No, it make sure that nobody dies when you calculate the loads incorrectly and use inappropriate materials.

I think the idea is that, if you calculate the load incorrectly enough, or use inappropriate enough materials, the safety factor will not save you. But, if you have done those things correctly, then the safety factor should be enough to save you from normal unknowns, unexpecteds, etc.

Yeah four things going on.

  Design errors
  Probabilistic nature of the loads applied.
  Material defects
  Fatigue
  Deterioration

All structures have a service life and it's the service lifetime an experienced engineer is trying to hit.

For the impeller in a rocket turbo pump the service life is like 5 minutes. For the impeller in a hydro electric dam it's 50 years.

The other thing that one of my professors pointed out was that 80% of engineers end up designing once off designs. Where the NRE cost is a lot more than the material costs. Shaving the safety factor is false economy.

Re: The most expensive number in engineering

#46

Post by GCA10 » Mon, May 31, 2021, 6:41 PM UTC

I'm liking the Robert Norton chart about 2/3 of the way down, showing how safety factors need to be adjusted quite radically once we think about how reliable or rickety our estimates might actually be.

Particular kudos to thinking harder about whether we've truly tested the actual environment where our product might be used.

I wish social scientists would do the same in controlled studies of human behavior -- which are then extrapolated to the ways that people make real-world decisions. A particularly vexing examples involves the way that psychology students make decisions in short experiments involving small amounts of money or other rewards. (Endless variations on the "marshmallow test," etc.)

Knowing what a college student will/won't do for a whimsical $5 reward says almost nothing about how an adult on the brink of poverty will balance bigger, more difficult decisions. Yet we apply a 95% confidence level to the college-student experiment and think we've learned something about the power of all financial incentives

Re: The most expensive number in engineering

#47

Post by unethical_ban » Mon, May 31, 2021, 6:43 PM UTC

iscrewyou wrote: post #12

Factor of Safety or F.S. for short was something us civil engineers were taught to never forget. You got grades deducted if you solved the problem correctly but forgot to include it in the very last line. It makes sure we calculate the loads correctly and use appropriate materials. You can't fix a bad design. The Arkansas bridge that has been in the news lately probably would have collapsed if it wasn't for the F.S.…

Aren't all the major bridges in New York built with ridiculous safety factors? It's why these century old bridges built for carriages and small trucks in a city of a million, can deal with 2021?

It's fascinating to me, and I feel like over-spec'ing certain chokepoints in infrastructure makes sense like this.

Re: The most expensive number in engineering

#48

Post by proggy » Mon, May 31, 2021, 6:45 PM UTC

UncleMeat wrote: post #41

snowwrestler wrote: post #34

Grossly increasing the factor of safety is a subtle way that science fiction stories connote a feeling of very advanced technology. For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space. Then later another ship falls from orbital height and plows through San Francisco without losing its hull shape. In Star Wars the Mi…

I don’t think this is science fiction doing this to connote the future in many of these cases. The cars in the “fast” franchise also survive ludicrous damage. The bodies of action heroes survive falls from ridiculous heights and blows and stab wounds that would kill somebody with ease. This is more a property of action movies (and adventure movies, to a lesser degree).

The loose industry term for this is “plot armor” [1]. There is no explicitly stated reason for the hero(es) being nigh invincible (be it person, spaceship, car, etc.). The only reason why the character survives is that it has a reason to continue existing for sake of the plot.

So yes, I suppose you can rationalize in your head that most ships are made out of super strong materials in science fiction, but unless that’s clearly laid out, you may just be rationalizing writer’s convenience.

[1] https://tvtropes.org/pmwiki/pmwiki.php/Main/PlotArmor

Re: The most expensive number in engineering

#49

Post by glitchc » Mon, May 31, 2021, 6:51 PM UTC

UncleMeat wrote: post #41

snowwrestler wrote: post #34

Grossly increasing the factor of safety is a subtle way that science fiction stories connote a feeling of very advanced technology. For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space. Then later another ship falls from orbital height and plows through San Francisco without losing its hull shape. In Star Wars the Mi…

I don’t think this is science fiction doing this to connote the future in many of these cases. The cars in the “fast” franchise also survive ludicrous damage. The bodies of action heroes survive falls from ridiculous heights and blows and stab wounds that would kill somebody with ease. This is more a property of action movies (and adventure movies, to a lesser degree).

Seems disingenuous to lump a science fiction movie where the focus is future technologies to an action movie with exaggerated physics and lack of real damage. In action movies, bad guys die with one bullet while the hero finds his way to safety (and survives!) despite having 10 or more lethal bullet wounds.

For the record, Star Wars is not science fiction nor has it ever been portrayed as such. It's very much an action adventure set in space.

Re: The most expensive number in engineering

#50

Post by 05 » Mon, May 31, 2021, 6:51 PM UTC

snowwrestler wrote: post #34

Grossly increasing the factor of safety is a subtle way that science fiction stories connote a feeling of very advanced technology. For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space. Then later another ship falls from orbital height and plows through San Francisco without losing its hull shape. In Star Wars the Mi…

> For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space.

Subverted in Futurama: https://www.youtube.com/watch?v=7GDthiBGMz8