No one knows why the most used spacecraft propulsion system works
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The really tricky thing is that the simulations show that basically every aspect of Hall thruster performance is extremely sensitive to the choice of anomalous transport parameter in certain parts of the discharge, but completely insensitive to it in other parts of the discharge. this makes validating models difficult, as many different models could agree in the small region of the discharge where it matters, but completely disagree in other parts of the discharge, and we would have little chance of discriminating between them.
What is the order of the turbulence closure models used in modern work? 1.5? 2? 2.5?
If you are referring to the order of accuracy of the simulations, that's typically 1st to 4th order, depending on method used.
I mean, in the context of Reynolds-averaged Navier–Stokes equations [1], what sort of turbulence model [2] you are using. But apparently you are not working in this context at all, so maybe my question doesn't make sense.
[1] https://en.wikipedia.org/wiki/Reynolds-averaged_Navier%E2%80...
For instance, turbulence in Hall thrusters is likely to be governed by an inverse energy cascade, where small disturbances magnify to larger and larger scales and the dissipation occurs via convection at device-scale instead of by diffusion at kolmogorov scales.
Bonus question: how would you design a planet scale thruster? (Asking for a friend)
Per the second question, you would need a very efficient thruster to limit the amount of mass needed as propellant. If you have an entire planet, you might look into using a photon rocket. Alternatively, you can try a massive solar sail or magnetic sail but I'm not an expert on these things.
Could this be a viable space to launch a start up? With launch costs coming down there could be a lot of demand for more efficient thrusters.
Yes, it is. There's a lot of small thruster startups out there. See Orbion, Thrustme, and Applied Ion Systems for examples.
Basically I am just imagining a ship with a ton of the tiniest realistically produced thrusters side by side - would be some wasted space between the rings but this seems like it could be a solid design (not to mention additional redundancy in case individual thrusters go out)
When you find it, will it not likely be a secondary electromagnetic condition is affecting the permittivity and permissibility of materials in a way that was not previously anticipated? Like, this problem is "understood", right? You know where to look and likely what it will be? The fact Hall Thrusters work with fuel of very small mass, it doesn't seem to change the nature of the problem: detonation and fluid (plasma) dynamics.
The fuel actually has quite high mass compared to a typical rocket engine. We use Xenon (131.293 g/mol) or Krypton (~83 g/mol), compared to air which is (~29 g/mol).
I mean, it's on the phrase diagram. The answer will be the statistics of stable orbits at heat and pressure. Any gradient or annulus in a phase diagram is going to mean that there will be potential for rapid "phase" changes in all directions. Isn't this all just to say that you need instruments, instruments, instruments? You know where to look, what it is doing and why (you have guesses, you'll 'get it' when you see the final answer). Is this blog post not mystifying it instead of saying that's it's cool and we know where to look to make it better?
https://www.researchgate.net/profile/Dima-Bolmatov/publicati...
Doesn't matter, nothing changes. If you have a strange effect something is confining. If you don't know how it is confining you have to entertain all possibilities so you need the stats at each phase
These trees aren't very dense or hot and they can't touch for the same reason. Every leaf that falls from the edge has to be replaced at the center. https://en.wikipedia.org/wiki/Crown_shyness
Near the Quanta things get discrete so coagulation gets chunky, but nothing changes in principle even down at 10^-3 Torr. In fact, the high-pressure, higher-heat phases of Hydrogen aren't that interesting, but the high-heat "moderate" pressure ones are. There is no escape from consequences of the material itself.
> The propellant is extremely rarified and not particularly hot
This type of statement is where you hide blind spots from yourself. You are going to extreme effort to confine, detonate, an exhaust fuel at just the right time. I would wake up everyday asking, how is my fuel less like a rarified gas and more like a hot, dense fluid. Like, your answer is in some tricky phase of the noble gases. There will be some kind of confinement or effective erasure of expected confinement. It will look like all secondary effects once you discover it, like, "oh, neat phase, water also expands when it freezes, now it makes sense". Why is this whole topic mystified? It seems only as hard to understand as air conditioning.
As I understand it, processes you can treat as MHD practically never occur in nature, and are always something that has been carefully arranged by design so the system can be modeled at all. Astrophysicists seem to treat the two terms as interchangeable. Would you say that MHD suffices to model ion propulsion systems? To model anything, anywhere in nature?
In my work, we use the Euler equations for ions (assuming they lack viscosity), with electromagnetic and ionization source terms. Electrons are treated as massless and generally steady with respect to the ions. Both of their equations of motion can be derived starting with the Boltzmann equation and applying simplifying assumptions.
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.
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.
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.
I'd say we understand it about equally well as the weather.
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!)
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.
It's about one step away from "We thought scientists knew how rockets worked. You won't believe what we uncovered next"
Key language: individual particles. A while ago I had this explained to me by a KSP nerd and aerospace scientist. "They don't know why there is extra drag between the electrons and the fuel, but it likely has to due with quantum fields. The engineers building thrusters don't like talking about that stuff. It works. We don't know why it works but we know that it works and can measure how it works. Nuff said."
https://nitter.net/lougrims/status/1516013489722216450
And: the figure in tweet 15 comes from this paper (I believe the tweeter is the author):
I mentor a high-school team that built a high-arc ball shooter this year. They committed to driving the function from distance to flywheel velocity using a theoretical approach: they pulled in all the Newtonian mathematics for shot angle, arc path, and the ratio of linear to angular velocity the flywheel would impart the ball leaving the chute. Put it all in one equation. I recommended it'd be simpler to do some practice shots at different distances and build a look-up table from distance to flywheel velocity (with linear interpolation for distances between the measured ones).
To their credit, their approach worked great at all but long ranges; my guess is their angle was slightly off, or they didn't account for air resistance or how back-spin on the ball might torque the path in flight. But the teams that nailed the challenge this year all did the dumb thing (took several shots at several distances and built a black-box lookup table).
And that's just for Newtonian physics over small distances and speeds with relatively mundane objects.
Theoretical modeling is vital to engineering... It's the thing that lets us extrapolate and scale. It gives us any hope at guessing at what a new design will do without having to commit the resources to just building it and seeing if it explodes. But a lot of practical engineering is coming up with clever ways to build large things out of small things that are actually, not theoretically, stress-tested.
If you're doing it again next year, maybe you could do some measured shots with lookup table and compare to the existing system? Might help find the error ?
This reads like a definition of engineering.
After opening Yet Another Twitter Thread that has turned an intellectually-satisfying meal into individually-gift-wrapped teaspoons fit for a toddler, I gave up and closed it.
TIL they are the real thing, in use for decades.
So yes, you could use a particle accelerator to accelerate ions, and it might be quite efficient, but you would need a ton of power if you wanted a reasonable amount of thrust.
- If something has momentum, it has mass.
- If something isn't leaving an engine now, it's condensing.
- Electromagnetic fields can recruit and organize a conductors' permissively and permeability. Hall Thrusters use electromagnetic confinement.
- If there is a delay in exhaust, there is condensation. If there is condensation without a holding tank then there is a pulse holding the condensation.
- Pulse waves require detonating fuel at the right time to feed the pulse and never feed the lull. Also, detonating is... different. Careful control of timing is needed to maintain a pulse. Finer detection is needed for tuning the pulse. It will take work to understand where this tiny detonating mass has friction in its electromagnetic confinement.
- Hall Thrusters once understand more can possible be tuned to run with, dipole, quadrupole... tuned pulse waves.
A Hall Thruster is a "smaller" version of this https://www.youtube.com/watch?v=Ws4kbgfpKCw however, the fluid dynamics of the Hall Thruster have interactions at the Quanta maybe so those will be that hardest to detect and analyze. But, once recorded and analyzed the knowledge should help fine tune these engines.
Let me link this again because it is just a great presentation on the fluid dynamics behind these effects https://www.youtube.com/watch?v=Ws4kbgfpKCw
Also Could nitrogen be a passable Hall effect thruster fuel?
I envision refilling propellant by the craft dipping into the upper atmosphere and scooping up some air.
You have to scroll like crazy to read a very small amount of text
The text overall is extremely long and skinny, like a receipt, thats a highly suboptimal aspect ratio.
Comments are formatted exactly the same as the article, the article doesnt seem to end properly just morphs directly into comments.
Single sentences floating alone in a sea of crap is not a good way to read.
It has a chance of becoming an insane dark pattern, especially if they jam ads in between.
Short blurb in tweet with link to actual content? You know, like they used to do?
> Consume the content with threadreaderapp if you don't like the UI or use Nitter if you don't want to log in to Twitter.
Or perhaps, people should choose the correct medium to begin with? To me, you might as well be saying I should do X because someone else is trying to hold a "video conference" with me via snail mail and photos.
There is a third option, linking to the blog from twitter. I'm surprised so many people would bother splitting long form writing into a bunch of separate tweets, as it is even more obnoxious than reading them in that format.
Nitter is great, I use that a lot.
Twitter isn’t perfect, but it’s where the readers are, so they publish there, even if it doesn’t have the best UI for them or their readers.
I wish RSS were more popular.
If they have it, I think “self-hosted” isn’t a realistic option for the majority of Twitter users. Their other content will likely be distributed between Facebook, Instagram, etc.
Twitter users also can have some long term control over what they wrote: users that want that can regularly download a machine-readable copy of their feed. I doubt many use that kind of backup, though.
If you routinely find yourself on Twitter reading threads, it's worth the time to find a better client. If you aren't, then why complain about a rare posting that catches your interest?