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No one knows why the most used spacecraft propulsion system works
- perihelions 5y agoHere's a mirror: https://nitter.net/lougrims/status/1516013489722216450 https://nitter.net/lougrims/status/1516013489722216450 And: the figure in tweet 15 comes from this paper (I believe the tweeter is the author): https://tel.archives-ouvertes.fr/tel-02165921 https://tel.archives-ouvertes.fr/tel-02165921
- the2and7 5y agoBlogging on Twitter has to stop.
- kamranjon 5y agoWhy?
- ducktective 5y agoit's hard to read?
- geoffeg 5y agoI actually find it easy to read since the format requires users to break things up in to what feels like shorter, bite-sized pieces.
- ducktective 5y agocause they've burnt our attention spans
- junon 5y agoMore like I'm not really interested in hearing another quack talk about their opinion as though it were fact, chipping away at my very limited time.
- dylan604 5y agoWe used to call those sentences. Grouping multiple sentences together to form paragraphs were not difficult to read.
- recursive 5y agoA thousand paper cuts. For one, if I click on any of the text, anywhere, it takes me out of context to somewhere else. Another: It can use up to ~20% of my screen space to show content I care about. Most of the rest goes to menus and trending nonsense.
- everyone 5y agoAbout 15% of the page is available for the actual content. The rest is distracting clutter (eg. the share bar thing repeated over and over again after every sentence almost) or just void. 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.
- frosted-flakes 5y ago> Please don't complain about tangential annoyances—things like article or website formats, name collisions, or back-button breakage. They're too common to be interesting. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
- deleted 5y ago[deleted]
- everyone 5y agoOne should never stop pointing out egregiously bad things. If you stop complaining about something you start accepting it, and eventually it becomes one of those extremely stupid things everyone just takes for granted.
- ianai 5y agoWas it this common before about 2016? It seems like society only recently pulled into this trend.
- frosted-flakes 5y agoPlease just follow the site guidelines. Thanks.
- ianai 5y agoSounds like some of us would like to change the guidelines to be against twitter reposts to HN. Might not be agreeable to everyone but surely we can agree that should be a valid opinion to voice.
- frosted-flakes 5y agoIf you'd like that to happen please bring it up with dang. Don't complain about it in the comments. But don't hold your breath, it's not likely to happen.
- 5y ago
- crate_barre 5y agoThe main problem with Twitter blogs is that it’s like the scariest infinite-scroll. On a regular blog post you can get a sense of how long it is and just about where someone should be hammering home a point. But one tweet after the other feels like an infinite lazy loaded slideshow. I get no sense if I mostly got the point of it or if I should keep scrolling. It has a chance of becoming an insane dark pattern, especially if they jam ads in between.
- _Microft 5y agoBlogging on Twitter is preferable to "not blogging at all" and "blogging on an obscure domain that almost nobody ever discovers". 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 (maybe ignore it if neither of that sounds like an acceptable choice).
- meragrin_ 5y ago> Blogging on Twitter is preferable to "not blogging at all" and "blogging on an obscure domain that almost nobody ever discovers". 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.
- OGWhales 5y ago> Blogging on Twitter is preferable to "not blogging at all" and "blogging on an obscure domain that almost nobody ever discovers". 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.
- Someone 5y agoFrom an author’s perspective, why split their posts between platforms depending on their length? 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.
- hedora 5y agoLinking from twitter to a seld hosted blog makes sense for long term control over distribution of their content, and discoverability of their other work. I wish RSS were more popular.
- criddell 5y agoGet a better client.
- meragrin_ 5y agoGet a better medium. If I have to spend more time than it takes to read it to figure out how to read your content, you are doing it wrong.
- criddell 5y agoOr you could just ignore it. The person was writing for their Twitter audience. If you aren't on Twitter, then maybe you aren't who they are writing for. 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?
- EricE 5y agoMeh - every service has it's pro's and con's. Thankfully for Twitter there is threadreaderapp: https://threadreaderapp.com/thread/1516013489722216450.html https://threadreaderapp.com/thread/1516013489722216450.html
- dang 5y ago"Please don't complain about tangential annoyances—things like article or website formats, name collisions, or back-button breakage. They're too common to be interesting." https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
- 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 agoPretty much this. Also, we still do not 100% understand aircraft lift and Bernoulli's principle.
- shadowgovt 5y agoOr 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 agoWhat 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 agoThat'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 agoIt 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.
- shadowgovt 5y agoIt's fun to pause and reflect how much of engineering is like this. 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.
- cs702 5y ago> coming up with clever ways to build large things out of small things that are actually, not theoretically, stress-tested. This reads like a definition of engineering.
- mech422 5y agoThat sounds like a fun project! :-) 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 ?
- sandworm101 5y ago>> There is not enough collisions to get nice statistical Maxwell-Boltzmann distributions. So you need to simulate the ions as individual particles. 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."
- archermarks 5y agoIt's pretty unlikely that anomalous transport comes down to quantum fields. The instabilities that probably cause it are complex but are classical in nature.
- sandworm101 5y agoTwo particles travel through an aperture going in the same direction but at different speeds. The aperture is so big and the particles so small that they shouldn't collide or even notice each other. But somehow on the other side of a aperture they appear to have interacted, one accelerating the other. That's starting to look very much like a double-slit type of situation.
- archermarks 5y agoIn this case, we know they would be able to interact via electromagnetic waves. So we would not need to invoke quantum wavefunctions in order to get the right behavior. The tricky bit is predicting the type of wave, their growth, and their propagation characteristics.
- flubloppoma 5y agoI think a better title would be "Hall Thrusters Understood So Well Better Instruments Needed to Measure the Friction of Quanta" - 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 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 https://www.youtube.com/watch?v=Ws4kbgfpKCw
- archermarks 5y agoHall thrusters do not involve any form of detonation or condensation. They're run on inert rarified gases. There are ionization instabilities in Hall thrusters for which I suppose you could make an analogy to detonation. Rotating spoke instabilities in Hall thrusters do resemble RDEs, to an extent. However, these are low frequency modes and as the post discusses, they are insufficient to explain enhanced transport. The instabilities that cause anomalous transport are probably high-frequency modes which occur faster than the characteristic timescales of ionization.
- thriftwy 5y agoI always assumed that these dim blue spacecraft engines shown in many computer games are pure fantasy. They needed some kind of engine so they painted these tubes emitting nicely looking blue haze. TIL they are the real thing, in use for decades.
- archermarks 5y agoThey're really cool to see running. Often not so dim either.
- archermarks 5y agoI actually work directly on this problem as part of my PhD. I work in designing simple "closure models" for Hall thruster anomalous transport that can be implemented in simulations to improve how well they match with experiment. It's a really interesting problem, and like the OP alludes to, it probably comes down to turbulence and plasma instabilities which are really hard to measure and even harder to include self-consistently in a device-scale simulation. 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.
- sampo 5y ago> I work in designing simple "closure models" What is the order of the turbulence closure models used in modern work? 1.5? 2? 2.5?
- archermarks 5y agoDepends on what you mean by order i suppose. The simplest are algebraic (zero-order) models where we prescribe the anomalous transport as an explicit function of other plasma parameters. More complex ones are coupled systems of hyperbolic PDEs, which are typically first-order in space and time. If you are referring to the order of accuracy of the simulations, that's typically 1st to 4th order, depending on method used.
- johnbcoughlin 5y agoI answered the GP based on my understanding of "closure" meaning different things for moment models vs turbulence. I think they're referring to energy partition scaling in saturated turbulence.
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- mrfusion 5y agoSay why can’t we make a small scale circular particle accelerators and push ions out with that? Would that be more or less efficient?
- archermarks 5y agoThe main constraint is power usage. In principle you want to accelerate your fuel particles as fast as possible. At a fixed electric power though, the faster you accelerate each individual particle, the less particles you can accelerate. This conserves propellant but reduces thrust. Consider that you could use a photon rocket, basically a flashlight, where your propellant is moving at the maximum possible speed. This will have great propellant efficiency but you won't move much unless you have TW of power. Since you want to get somewhere in a finite time and don't have unlimited power, in practice you want something with lower propellant efficiency. 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.
- belter 5y agoTwitter multiple post conversation are madness. At least to preserve your sanity and avoid to create a login with your real phone number... https://nitter.net/lougrims/status/1516013489722216450 https://nitter.net/lougrims/status/1516013489722216450
- mrfusion 5y agoHave we given up on the vasmir thrusters? Did Hall effect win out? Why is that?
- archermarks 5y agoVASIMR is a specific type of magnetic nozzle thruster built by Ad Astra and which has never flown. Hall thrusters have been flown since the 70s and 80s and have been rapidly developed since the mid 2000s to fill a variety of niches. Magnetic nozzles are cool and will likely be very important for low power thrusters and future high power thrusters, but at this point VASIMR doesn't have many concrete advantages over Hall thrusters except perhaps in flexibility of propellant choice.
- mrfusion 5y agoCould Hall effect be used to accelerate ions fast enough to fuse? Could this be another Avenue to achieving break even fusion? 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.
- archermarks 5y agoNo. Hall thrusters are low temperature plasma devices, and operate in a completely different plasma regime than fusion devices. The ions dont get that fast, only a few tens of thousands of meters/second. The electrons only reach 10-100 eV, a far cry from the hundreds of thousands of eV in fusion devices.
- Melatonic 5y agoGod damn I hate twitter. This is super interesting but having to go down tweet after tweet seems so dumb.
- disqard 5y agoI'm with you -- I consider myself a curious autodidact and voracious reader. 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.
- EricE 5y agothreadreaderapp - saves you from Twitter hell: https://threadreaderapp.com/thread/1516013489722216450.html https://threadreaderapp.com/thread/1516013489722216450.html
- npteljes 5y agoI use the Privacy Redirect addon for the same reason. On the rare occasion I want to visit twitter, I get redirected to nitter which unrolls these threads quite nicely.
- temptemptemp111 5y ago
- deepsun 5y agoSuch a pity Twitter takes niche of LiveJournal for mid-size essays.
- Sakos 5y agoNow I'm wondering how we developed Hall thrusters if they weren't able to simulate or calculate a realistic amount of thrust.