5 ms·
In orbit you have to slow down to speed up
- brudgers 1y agoA similar thing is true when cornering a race car when measuring time through the corner.
- dotancohen 1y agoHow so?
- anonymars 1y ago"Faster" (higher speed) = wider cornering radius = more distance = slower
- wrigby 1y agoBut you exit going faster, which means you make up time on the straight after the corner.
- deleted 1y ago[deleted]
- oarsinsync 1y agoAssuming there is a long enough straight before the next corner
- rtkwe 1y agoIt's a balance, many of these cars can accelerate and decelerate very hard so the time to get back to the full speed for the next section is fairly short reducing the effect of slowing down. The effect of taking a too wide racing line though means a large multiple in the distance travelled.
- HPsquared 1y agoCars can usually brake and turn harder than they can accelerate. You also tend to spend more time on the straight after the corner, than in the corner itself So you mostly optimise for corner exit speed, especially if the car has particularly slow acceleration and a long straight comes after the corner.
- drivebyhooting 1y agoFor F1 I was under the impression exit speed wasn’t as important as minimizing arc length of the turn.
- deleted 1y ago[deleted]
- dns_snek 1y agoThey call that the geometric racing line and it might minimize the amount of time that it takes you to navigate one specific corner but if the corner is a hard braking zone followed by a long straight, your exit speed will likely be lower which will cost you time all the way to the next braking zone. https://www.youtube.com/watch?v=uIbTPvHFf-w https://www.youtube.com/watch?v=uIbTPvHFf-w https://driver61.com/uni/racing-line/ https://driver61.com/uni/racing-line/
- dotancohen 1y ago> The effect of taking a too wide racing line though means a large multiple in the distance travelled. So a better driver typically takes a shorter route than a less skilled driver? Can a very skilled driver get the Kessel Run down below 12 parsecs?
- rtkwe 1y agoIn ideal scenarios without having to account for other racers yes generally. In reality it's a difficult to talk about mix of driver skill, mechanical car performance and race strategy that determines the actual best line at any given moment though there is still one fastest theoretical line through a corner. On race strategy it's rare for drivers to be pushing their cars to the limits for entire races because tire wear and the stops required to replace them is a major time sink because you can only drive so fast in pit lane so even the 1-2 second stops F1 cars go through today lose drivers position that then has to be regained using the extra performance fresh tires provides. Then driver skill can put the better driver in the correct position to take the correct line more often when you include other cars in the mix and they also know better how to deal with suboptimal routes (eg being force to take an inside route by traffic so you have to know how much harder you need to break to not wreck into another car). On an unrelated side note because I'm just personally annoyed by the 12 pArSecS!? misunderstanding. The 12 parsec run is impressive because Kessel is in a part of space ridiculously dense with hazards so the usual route to it loops through a narrow region where it's relatively safe to travel through. Han's 12 parsec run cut through the dangerous parts through either luck, superior navigation, or he was just lying the commentary is mixed. [0] [0] https://static.wikia.nocookie.net/starwars/images/1/17/KesselRunMap.jpg/revision/latest?cb=20180913173158 https://static.wikia.nocookie.net/starwars/images/1/17/Kesse...
- taneq 1y agoYeah depends on the corner but the general thumb-suck approximation is sound.
- rascul 1y agoIf there's banking, it can change things.
- everyone 1y agoThe distance has no effect.. Its all about speed, you want to take the line that lets you get through the corner while maintaining the highest speed. If you are going faster and spend as little time as possible breaking and accelerating you will gain time. Also a higher exit speed means you will be going faster for the entire straight after the corner making a very big time difference. Your car, depending on how much grip it has + other variables, will have a theoretical minimum diameter circle it can drive around at various speeds. The higher the speed the bigger the circle. Finding your racing line is just a matter of fitting the biggest circular arc inside the space available in the corner. Ideally you want to break in a straight line before the corner and reach the speed your car can drive the circle at at just the moment you enter it. Theres more nuance when it comes to compound corners, FR vs FF cars, oversteer understeer, hills bumps etc. But the basic theory is simply fitting circles. https://ibb.co/VY11TpTM https://ibb.co/VY11TpTM
- bluGill 1y agoYou are generally not taking a perfect circle corner. You can/should be slowing down as you enter the corner and then speeding up even before you exit. In this way you can shorten the distance traveled while getting a higher exit speed - sometimes higher than the largest possible circle corner. Optimizing this for the car/track/conditions is what makes for a great driver.
- everyone 1y agoThe distance is irrelevant.. It is true that depending on the car you may gain time breaking and accelerating while turning. But that is a more subtle and advanced concept though (like dealing with elevation changes).. People should understand the big circle first.
- bluGill 1y agoIn the context of winning a race you need to get the subtle and advanced concepts right or you will be in last place. If you are just driving on the street it doesn't matter.
- taneq 1y agoSlow is smooth, smooth is fast.
- Scubabear68 1y agoThis brings back fond memories of Heinlein's juvenile sci fi series.
- ChicagoBoy11 1y agoFor anyone who is remotely interested in this, a considerable chunk of the Gemini program was all about solving some of the practicalities involved with Rendezvous, and it is quite interesting even hearing some of the astronauts come to grips with some of the physics while orbiting in space trying the various types of rendezvous and docking maneuvers that were attempted.
- stoneforger 1y agoThey should really teach physics using KSP.
- delichon 1y agoI doubt SpaceX could put a satellite in orbit with KSP physics. Just the absence of realistic thermal conduction would prevent it. The outer skin temperature typically peaks around 300–600 °C during the densest part of the atmosphere. If you calculate those forces wrong the rocket has a bad day. Best case it is over engineered and has a reduced payload. They might as well do their calculations with pi equal to 3.
- 0_____0 1y agoWhat does thermal conduction affect? Is it mostly practical spacecraft construction, or actually related to orbital mechanics?
- bogzz 1y agoThe FAR mod is touted as being realistic; I haven't played it though.
- iso1631 1y agohttps://xkcd.com/2205/ https://xkcd.com/2205/ comes to mind with your pi approximation. Nobody is saying KSP physics is perfect. Until I played KSP, I had no idea how hard orbit was compared with just going up into space (and generally the greater population thinks the same -- they think that sending New Shephard upto 100km is about the same as sending a Dragon into orbit). I had no idea how you move in orbit, how getting from low earth equitorial orbit to Jupiter takes less energy than getting from the same ship to a polar orbit (and even then that the only real way to change your orbit like that is to go out beyond the moon and back), etc.
- vannucci 1y agoI tried to teach a group of HS students about orbital mechanics as a high school physics teacher using KSP. It was... difficult. Not impossible. But I agree it's an excellent learning tool.
- pfdietz 1y agoThis is a consequence of the virial theorem of mechanics. https://en.wikipedia.org/wiki/Virial_theorem https://en.wikipedia.org/wiki/Virial_theorem This theorem also lets you conclude that as a nondegenerate star becomes more tightly bound (smaller, for a given mass) it must also become hotter. (Why did someone downvote this?)
- terminalbraid 1y agoI'll posit it's because the virial theorem is applicable to the time average of a stable system and is therefore a specific case. The time average is not a requirement for this discussion (i.e. dynamics) and invoking that theorem is unnecessary. Ergo it is not a consequence.
- zara762 1y ago[dead]
- everyone 1y agoI thought I understood all this until I played Kerbal Space Program.
- elephanlemon 1y agoObligatory: https://xkcd.com/1356/ https://xkcd.com/1356/
- ralfd 1y agoI have a hard time imagining physics. For example take a train moving 100 kmh to the north which wants to reverse direction to the south. It has to break and then accelerate again, a very costly operation. Except when the tracks make a turn? But how can a northward momentum change to a southward momentum? The same confusion I have when trying to imagine satellites going around Earth or slingshot maneuvers. Would an X-Wing turn in space differently than in the atmosphere of Hoth? Would it in space just rotate, but keep its forward (now backwards) momentum instead of turning like a fighter jet?
- aaronblohowiak 1y agoplay KSP, it will click after a few days.
- deleted 1y ago[deleted]
- ambicapter 1y agoIn your train example, the rails exert a force on the train as it turns. In orbit, the planets are constantly exerting a force on the satellite.
- Xerox9213 1y agoWhen you brake you generate a ton of heat. Doing a U-turn generates less heat, but still quite a bit. The train will have to slow down depending on the radius of the curve, and even then the turn will slow it down some more. But yeah, less heat generation means kinetic energy is conserved. Cars have to slow down when they turn because it’s too much to ask of the tires to accelerate (throttle) and turn, since turning is in itself acceleration.
- NetMageSCW 1y agoCaveat: when the tires are already at the limit of adhesion (e.g. on an F1 car). In a road car, you are not normally turning at 1g and probably can’t accelerate at 1g so you can turn and accelerate when you have enough margin. It’s just the average driver doesn’t realize how much margin is available.
- yabones 1y agoForwards is up, up is back, back is down, down is forwards.
- geon 1y agoOr in the words of Larry Niven (The integral trees) East takes you Out Out takes you West West takes you In In takes you East
- NitpickLawyer 1y agoDown is where the enemy gate is.
- taneq 1y agoSo precise, he piss on a plate and never splash.
- jayknight 1y agoHow related to this is the helicopter 90-degree phase lag thing?
- nomel 1y agoIt's much easier to reason about when you frame it closer to reality: you're not on a circular path, you're continuously falling, and because you're moving forward, you're continuously missing the earth, with its pull decreasing with distance.
- deleted 1y ago[deleted]
- QuiCasseRien 1y agocould it be possible to flag a thread when you need to pay or register to read an article ?? very annoyoing, the subject looks good, open tab and rohhhhhhhh... paid or register.
- ceejayoz 1y agohttps://archive.is/ https://archive.is/ Paste link, good to go. https://archive.is/qrP0p https://archive.is/qrP0p
- QuiCasseRien 1y agothanks for the pointer, and so very nice article
- embedding-shape 1y agoAllowing paywalls vs not been discussed for a long time. Latest comment from dang about it seems to be this: > The answer is that paywalls are allowed when there are workarounds (such as archive links) which allow ordinary readers to read the article without paying or subscribing, while hardwalled domains (i.e., without such workarounds) are banned. - https://news.ycombinator.com/item?id=43876575 https://news.ycombinator.com/item?id=43876575 https://hn.algolia.com/?dateRange=all&page=0&prefix=true&query=by%3Adang%20paywall&sort=byDate&type=comment https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
- f4uCL9dNSnQm 1y agoThere is the small, tiny issue of people commenting just based on title, without even reading the article. In this case I expected it just links to https://www.youtube.com/watch?v=bcvnfQlz1x4 https://www.youtube.com/watch?v=bcvnfQlz1x4 and didn't even notice in links to Wired.
- HPsquared 1y agoLife and business are often the same.
- lproven 1y agoAs Larry Niven wrote in The Smoke Ring and The Integral Trees... "West takes you In, In takes you East, East takes you Out, Out takes you West, North and South bring you back again." https://forum.nasaspaceflight.com/index.php?topic=3341.0 https://forum.nasaspaceflight.com/index.php?topic=3341.0
- schiffern 1y agoBuzz Aldrin is the original legend. His PhD was literally "how to eyeball your way there in orbit,"[0] and then he actually performed the calculations and procedure he invented (with slide rule and sextant) when the automatic system failed on Gemini 12. He used less fuel than the computer.[1] There's a reason why his nickname among the other astronauts is Doctor Rendezvous. [0] https://dspace.mit.edu/handle/1721.1/12652 https://dspace.mit.edu/handle/1721.1/12652 [1] https://history.howstuffworks.com/historical-figures/buzz-aldrin.htm https://history.howstuffworks.com/historical-figures/buzz-al...
- lproven 1y agoThis is amazing stuff. I didn't know. Thanks!
- ubj 1y agoBuzz Aldrin (who was the second person to walk on the moon) wrote his MIT thesis on methods for astronauts to handle the complexities of orbital dynamics when performing rendezvous maneuvers in orbit: https://dspace.mit.edu/handle/1721.1/12652 https://dspace.mit.edu/handle/1721.1/12652
- hinkley 1y agoDidn’t that become “the book” as well? As in he wrote the book on orbital mechanics?
- cweagans 1y ago> In the hopes that this work may in some way contribute to their exploration of space, this is dedicated to the crew members of this country' s present and future manned space programs. If only I could join them in their exciting endeavors. Imagine writing this 6 years before you were literally standing on the moon.
- austin-cheney 1y agoI have always wondered about this with regard to Newton's Third law: For every action there is an equal and opposite reaction. So if a craft departs from rendezvous with another craft it must do so by pushing away from that other craft. That means it is equally pushing on both crafts. If the rendezvous was in orbit does that mean departing from rendezvous pushes both crafts out of orbit? If so does the other craft have to correct for this to reestablish orbit or is orbit self-correcting as if in a third body scenario? I ask because Earth is an third body scenario between the sun and Jupiter. Jupiter has enough gravity to occasionally pull the Earth slightly (not significantly) out of orbit from the Sun, but Earth's orbit to the Sun is self-correcting due to the difference in mass between the Sun and Jupiter. Quick web searching reveals Jupiter's pull on Earth is only approximately 0.005% of the Sun's after accounting for both mass and distance, but that number rises to 0.011% after accounting for syzygy with the moon.
- gpm 1y ago> So if a craft departs from rendezvous with another craft it must do so by pushing away from that other craft. More commonly we push away propellant, but pushing on the other craft is an option. > If the rendezvous was in orbit does that mean departing from rendezvous pushes both crafts out of orbit? Assuming we're pushing on the other craft, it means both crafts will change orbits, but in opposite directions. If we're talking about an instantaneous push in the direction of travel one craft will move to an orbit where it is closer to earth one half rotation later, and the other craft will move to an orbit where it is farther from earth one half rotation later. Both, either, or neither craft could exit orbit like this, but one would be exiting orbit by crashing into the thing its orbiting around (e.g. earth) and the other would exit by reaching escape velocity and flying off into the distance. > I ask because Earth is an third body scenario between the sun and Jupiter. Jupiter has enough gravity to occasionally pull the Earth slightly (not significantly) out of orbit from the Sun, but Earth's orbit to the Sun is self-correcting due to the difference in mass between the Sun and Jupiter. Quick web searching reveals Jupiter's pull on Earth is only approximately 0.005% of the Sun's after accounting for both mass and distance, but that number rises to 0.011% after accounting for syzygy with the moon. Yes, Jupiter constantly (not occasionally) perturbs earths orbit, and technically earth constantly perturbs jupiters orbit (though the influence in that direction is completely negligible), but as you note its not enough for either to reach escape velocity or crash into the sun, and it appears to more or less even out over time.
- sandworm101 1y agoSpend years reading and doing calculations before you understand orbital rendezvous. Or spend a couple days learning to dock in kerbal space program, with the real-sized earth.
- top_sigrid 1y agohttps://archive.is/qrP0p https://archive.is/qrP0p
- antognini 1y agoOrbital mechanics can be counterintuitive. There was a fairly basic orbital mechanics problem that caused some debate among physicists back in the 1970s. The problem went like this: You are in a spacecraft orbiting the Earth. You distribute a bushel of apples throughout spacecraft so that they are at rest with respect to the spacecraft. After a long time, where do the apples end up? Hannes Alfvén (more famous for Alfvén waves) argued that they would bunch up together in the middle of the spacecraft. But Michel Hénon (correctly) proved that half would end up in the bottom front corner, and the other half in the top back corner. I wrote up a blog post explaining the solution a few years ago: https://joe-antognini.github.io/astronomy/apples-in-a-spacecraft https://joe-antognini.github.io/astronomy/apples-in-a-spacec...
- hinkley 1y agoI’ve often wondered if there is some VR or AR projection that one could use to make this more intuitive. Could we make a HUD that allowed average commercial pilots to get rated for space taxi work, or will it always remain the domain of navy pilots?
- hydrogen7800 1y agoI always thought this was unnecessarily made to sound counterintuitive, and I think there is a way to explain it simply without trying to make it sound like rocket science. Instead of "speed up to slow down", it's "speed up to go higher". I think anyone can get on board with that. You'll come back to the same point you were when you accelerated, but later than before since you went into a higher orbit. I like the included animations, since it also makes it clear that accelerating or decelerating from a circular orbit makes it elliptical. Your don't just jump to a higher or lower circular orbit.
- RataNova 1y agoOnce you realize you're not "switching lanes" but reshaping your entire path, it starts to make way more sense
- RataNova 1y agoHonestly, orbital mechanics is the ultimate "just because it feels right doesn't mean it works" kind of problem
- noodlesUK 1y agoI know it's a bit worn out but if you want to learn about practical orbital mechanics and have fun doing it, Kerbal Space Program is the best tutor there is. It's a simplified physics model so stuff like Lagrange points don't work, but there's no better way of getting an intuitive understanding of how bodies move in space.