5 ms·

"No one knows why this device works" --> "There are still some small gaps in our understanding of the details of this device's performance"

by topaz0 5y ago

1970-01-01 5y ago
Pretty much this. Also, we still do not 100% understand aircraft lift and Bernoulli's principle.
- shadowgovt 5y ago
  Or the physics of lithium ions, which is why sometimes a new battery design will just spontaneously catch fire (because the time it takes it to denature into something dangerous exceeds the time the designers spent on practical testing).
- kcolford 5y ago
  What are you talking about?? We completely understand aircraft lift. Bernoulli's principal is a nearly irrelevant footnote that gives a small boost to efficiency. Aircraft lift is caused by the upward angle of the wings forcing air downwards as the jets move the plane horizontally forwards. This "angle of attack" is usually very small so it's hard to perceive with the naked eye when looking at a commercial aircraft.
  - 1970-01-01 5y ago
    That's not totally correct. See here: https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
    - nofunsir 5y ago
      It says it in the article. Lift is created by turning a fluid. Just as a spoon in a faucet, it’s all how you “throw” the fluid in the direction opposite the direction of lift. This is angle of attack.
      - H8crilA 5y ago
        The year is 2312, and we still have to combat the Bernoulli-airfoil misinformation. Even though all you have to do is stick your hand out of the window of a self-driving drone and twist it around a bit.
        postingposts 5y ago
        I find it hilarious that in the future everyone is more capable than the present at inventing things and also simultaneously these same people are incapable children who can’t use anything manual. It’s concerning that people dream that way.
        sheepybloke 5y ago
        This sounds like an XKCD comic!
        nofunsir 5y ago
        https://xkcd.com/803/ https://xkcd.com/803/
- sandworm101 5y ago
  >> do not 100% understand aircraft lift Do you mean turbulence? There are some blank spots there but we have very good equations to approximate the behavior, equations good enough for practical aviation.
  - puzzledobserver 5y ago
    From what I understand, the challenge is the following: Even though we know precisely how to calculate the lift generated by a wing, all attempts to reduce those equations and their predictions to "ELI5" arguments leads to incorrect explanations. Basically: We can explain aircraft lift to a college student, but not to a 5 year old. Or, we can explain aircraft lift using equations, but not using informal arguments.
    - sandworm101 5y ago
      Air pushed down. Plane go up. More air pushed down, more plane go up. Most everything else is about preventing and controling turbulence, keeping the air moving smoothly around stuff.
nine_k 5y ago
The "small gap" here is an order of magnitude difference between theoretical and practical performance.
- contravariant 5y ago
  No it's not, the gap is an order of magnitude between a pen and paper calculation, or a fudge factor in a more sophisticated simulation. I'd say we understand it about equally well as the weather.
  - jessriedel 5y ago
    I don't think the weather is a good analogy, at least if you're just referring to the unpredictability that comes from chaos. That the weather is unpredictable (when projecting out a few multiples of the relevant Lyapunov time) is an unavoidable consequence of chaos. Indeed, if you could hypothetically run a full atomic-level quantum simulations, the wavefunction would contain different branches where different weather events happened; there is literally quantum uncertainty in the outcome, even if we had a complete theoretical description and understanding of the different outcomes. In contrast, the situation with a Hall effect thrust is that we cannot explain a repeatable effect. And that could change with better theoretical understanding. It's in that sense that Hall effect thrusters are "not understood", which I think is a reasonable thing to call it. (And if you are talking about some aspect of the weather that, unlike the chaotic aspects, is empirically repeatable but not predictable from our models, then I think it would be fair to characterize that as not understood too!)
archermarks 5y ago
In this case, it's different than lift. We can run CFD simulations of wings and get accurate estimations of lift/drag etc. We cannot do that for Hall thrusters without already having run the thruster, as we need to calibrate the anomalous transport parameters to make the simulation match experiment. This makes the simulations much less useful in design than they are in other fields, which is a real drawback. Another point to make is that testing these devices is expensive. They are low thrust propulsion systems that are designed for thousands of hours of continuous operation. They need to be run in very high vacuum, and for higher power devices there are only a handful of vacuum chambers in the world sufficient for the task. Additionally, Xenon is expensive (dramatically moreso since the Russian invasion of Ukraine), so developing better predictive simulations is critical.
- topaz0 5y ago
  Thanks for the context. My comment was not intended as an indictment of the post. It's an interesting problem and the missing physics is clearly not trivial. All I mean to say here is that I think saying "no one knows" trivializes all the knowledge that people (like you!) do have.
cestith 5y ago
It is, per the article, a large enough gap that designs from scratch can't predict performance and that simulation isn't always fully reliable. I don't know that the author is looking for a metaphysical answer to all of existence here. It would be nice to understand the functioning of something that moves billions of dollars of material around well enough that design is less dominated by trial and error.
- topaz0 5y ago
  To be clear, I'm not saying the gaps are not interesting or economically important, and I am not belittling the work that the tweeter does. I'm willing to entertain the possibility that my rephrase is too extreme. But I'd still argue "No one knows why X works" is an unhelpful frame that could give the casual reader the wrong idea.
slingnow 5y ago
Exactly. I see more and more of this clickbait-style title applied to scientific articles. I instinctively no longer read them. The fact that this is ALSO a twitter thread is just icing on the cake. It's about one step away from "We thought scientists knew how rockets worked. You won't believe what we uncovered next"
- topaz0 5y ago
  For what it's worth, the rest of the thread is a good read.

"No one knows why this device works" --> "There are still some small gaps in our understanding of the details of this device's performance"

by topaz0 5y ago

"No one knows why this device works" --> "There are still some small gaps in our understanding of the details of this device's performance"

1970-01-01 5y ago

Pretty much this. Also, we still do not 100% understand aircraft lift and Bernoulli's principle.

shadowgovt 5y ago
Or the physics of lithium ions, which is why sometimes a new battery design will just spontaneously catch fire (because the time it takes it to denature into something dangerous exceeds the time the designers spent on practical testing).
kcolford 5y ago
What are you talking about?? We completely understand aircraft lift. Bernoulli's principal is a nearly irrelevant footnote that gives a small boost to efficiency. Aircraft lift is caused by the upward angle of the wings forcing air downwards as the jets move the plane horizontally forwards. This "angle of attack" is usually very small so it's hard to perceive with the naked eye when looking at a commercial aircraft.
- 1970-01-01 5y ago
  That's not totally correct. See here: https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
  - nofunsir 5y ago
    It says it in the article. Lift is created by turning a fluid. Just as a spoon in a faucet, it’s all how you “throw” the fluid in the direction opposite the direction of lift. This is angle of attack.
    - H8crilA 5y ago
      The year is 2312, and we still have to combat the Bernoulli-airfoil misinformation. Even though all you have to do is stick your hand out of the window of a self-driving drone and twist it around a bit.
      - postingposts 5y ago
        I find it hilarious that in the future everyone is more capable than the present at inventing things and also simultaneously these same people are incapable children who can’t use anything manual. It’s concerning that people dream that way.
      - sheepybloke 5y ago
        This sounds like an XKCD comic!
        nofunsir 5y ago
        https://xkcd.com/803/ https://xkcd.com/803/
sandworm101 5y ago
>> do not 100% understand aircraft lift Do you mean turbulence? There are some blank spots there but we have very good equations to approximate the behavior, equations good enough for practical aviation.
- puzzledobserver 5y ago
  From what I understand, the challenge is the following: Even though we know precisely how to calculate the lift generated by a wing, all attempts to reduce those equations and their predictions to "ELI5" arguments leads to incorrect explanations. Basically: We can explain aircraft lift to a college student, but not to a 5 year old. Or, we can explain aircraft lift using equations, but not using informal arguments.
  - sandworm101 5y ago
    Air pushed down. Plane go up. More air pushed down, more plane go up. Most everything else is about preventing and controling turbulence, keeping the air moving smoothly around stuff.

nine_k 5y ago

The "small gap" here is an order of magnitude difference between theoretical and practical performance.

contravariant 5y ago
No it's not, the gap is an order of magnitude between a pen and paper calculation, or a fudge factor in a more sophisticated simulation. I'd say we understand it about equally well as the weather.
- jessriedel 5y ago
  I don't think the weather is a good analogy, at least if you're just referring to the unpredictability that comes from chaos. That the weather is unpredictable (when projecting out a few multiples of the relevant Lyapunov time) is an unavoidable consequence of chaos. Indeed, if you could hypothetically run a full atomic-level quantum simulations, the wavefunction would contain different branches where different weather events happened; there is literally quantum uncertainty in the outcome, even if we had a complete theoretical description and understanding of the different outcomes. In contrast, the situation with a Hall effect thrust is that we cannot explain a repeatable effect. And that could change with better theoretical understanding. It's in that sense that Hall effect thrusters are "not understood", which I think is a reasonable thing to call it. (And if you are talking about some aspect of the weather that, unlike the chaotic aspects, is empirically repeatable but not predictable from our models, then I think it would be fair to characterize that as not understood too!)

archermarks 5y ago

In this case, it's different than lift. We can run CFD simulations of wings and get accurate estimations of lift/drag etc. We cannot do that for Hall thrusters without already having run the thruster, as we need to calibrate the anomalous transport parameters to make the simulation match experiment. This makes the simulations much less useful in design than they are in other fields, which is a real drawback. Another point to make is that testing these devices is expensive. They are low thrust propulsion systems that are designed for thousands of hours of continuous operation. They need to be run in very high vacuum, and for higher power devices there are only a handful of vacuum chambers in the world sufficient for the task. Additionally, Xenon is expensive (dramatically moreso since the Russian invasion of Ukraine), so developing better predictive simulations is critical.

topaz0 5y ago
Thanks for the context. My comment was not intended as an indictment of the post. It's an interesting problem and the missing physics is clearly not trivial. All I mean to say here is that I think saying "no one knows" trivializes all the knowledge that people (like you!) do have.

cestith 5y ago

It is, per the article, a large enough gap that designs from scratch can't predict performance and that simulation isn't always fully reliable. I don't know that the author is looking for a metaphysical answer to all of existence here. It would be nice to understand the functioning of something that moves billions of dollars of material around well enough that design is less dominated by trial and error.

topaz0 5y ago
To be clear, I'm not saying the gaps are not interesting or economically important, and I am not belittling the work that the tweeter does. I'm willing to entertain the possibility that my rephrase is too extreme. But I'd still argue "No one knows why X works" is an unhelpful frame that could give the casual reader the wrong idea.

slingnow 5y ago

Exactly. I see more and more of this clickbait-style title applied to scientific articles. I instinctively no longer read them. The fact that this is ALSO a twitter thread is just icing on the cake. It's about one step away from "We thought scientists knew how rockets worked. You won't believe what we uncovered next"

topaz0 5y ago
For what it's worth, the rest of the thread is a good read.