[HN Gopher] Orbital Mechanics - How do rockets get to where they... ___________________________________________________________________ Orbital Mechanics - How do rockets get to where they're headed? Author : jgrodziski Score : 57 points Date : 2021-04-26 16:14 UTC (1 days ago) (HTM) web link (steemit.com) (TXT) w3m dump (steemit.com) | throwawaysea wrote: | As an aside, I was wondering what this "steemit" website is, and | Wikipedia describes it succinctly | (https://en.wikipedia.org/wiki/Steemit): | | > Steemit is a blockchain-based blogging and social media | website, which rewards its users with the cryptocurrency STEEM | for publishing and curating content, and is owned by Steemit | Inc., a privately held company based in New York City and a | headquarters in Virginia. | | I found their welcome guide | (https://steemit.com/guide/@steemitblog/steemit-a-guide-for-n...) | and while it's great to see new, alternative social media | platforms, it seems a bit complicated. Hopefully they find a way | to make all this easier so that new users give it a chance. | Koshkin wrote: | The change of mass of the spacecraft due to the use of fuel | should not be discounted. | gtolle wrote: | If you'd like to try out some of these concepts on your phone, | I've been working on a side project -- an iOS mobile game called | Solar Express [1]. You can launch a rocket, rendezvous and dock | in orbit, transfer between moons and planets, and land. It's a | bit like a mini-KSP with real orbital mechanics, but more casual | - no rocket building, and lots of delta-V to play with. | | [1] https://apps.apple.com/us/app/solar-express/id1503449353 | lighttower wrote: | Do you have any android version? | aw1621107 wrote: | A few comments: | | There are a few points where I feel the author might have been | taking force/velocity diagrams a bit too literally (and is a bit | sloppy with the difference between force and velocity). | | > the following main velocity vectors acting on it: | | > gravity acceleration | | > thrust | | > rocket's velocity | | and | | > Once in orbit, the spacecraft will have two main forces | exerting their grab onto it: the tangential velocity and the | gravitational pull. | | I'm not sure whether someone new to those concepts would notice, | but it's a potential source of confusion. | | > Gravity Turn or Pitchover is the second maneuver that is | executed as early as possible by using the gimbal of the engines | or by using cold gas thrusters on the nose of the rocket or a | combination of the both. | | Honest question here: do _any_ rockets use their cold gas /RCS | thrusters for the initial pitchover? I feel like they wouldn't be | powerful enough in most cases to pull it off. | | > The velocity vectors are similar to the vertical flight phase | but because the gravity acts on the same vertical plane it makes | the spacecraft change it's pitch without additional input from | the engines, tasking them with the only job of increasing the | speed of the spacecraft. | | Technically, it's a combination of gravity and aerodynamic forces | that cause the pitch to change. Gravity causes the velocity | vector to turn, but doesn't exert a (noticeable) torque. It's | aerodynamic forces that work on the rocket to (hopefully) keep it | aligned with the velocity vector. | | This is why some rockets have fins at their base - additional | drag at the base ensures that the aerodynamic forces keep the | rocket pointed the right way. Rockets without enough drag near | their base or too much drag at their nose will tend to flip right | around with interesting consequences. | | ---- | | That being said, it's one thing to read about orbital mechanics, | but it's hard to beat hands-on experience for really wrapping | your head around things. | | As mentioned in the article, Kerbal Space Program is one | frequently-recommended way to go about this, and does a fairly | good job with the basics. Not to say that the basics aren't much; | you can get quite far with "just the basics", and arguably | they'll work just fine for the most common mission profiles. | | KSP does use a simplified gravitational model, though, taking | into account the gravitational forces of only a single body at a | time, which means it's missing some more interesting features of | full n-body dynamics, such as Lagrange points and low-energy | transfers. If you're interested in those, consider trying the | Principia mod, which adds n-body dynamics, non-uniform | gravitational fields, and more [0]. There's also the Realism | Overhaul mod if you wish to work with more realistic rockets and | celestial bodies [1]. | | [0]: https://github.com/mockingbirdnest/Principia | | [1]: | https://forum.kerbalspaceprogram.com/index.php?/topic/155700... | alexdory wrote: | Hey, I am the original author, I know I made some | simplifications, that blog was meant to attract new people to a | STEM group written on a blockchain, and to promote science and | tech for everyone. It was a few years ago, I found this thru a | friend of mine who reads ycombinator daily and I thank you for | the clarifications and for the time dedicated to write it. What | a lovely surprise, have a great week! | formerly_proven wrote: | tl;dr | | The rocket knows where it is at all times. It knows this because | it knows where it isn't. By subtracting where it is from where it | isn't, or where it isn't from where it is (whichever is greater), | it obtains a difference, or deviation. The guidance subsystem | uses deviations to generate corrective commands to drive the | rocket from a position where it is to a position where it isn't, | and arriving at a position where it wasn't, it now is. | Consequently, the position where it is, is now the position that | it wasn't, and it follows that the position that it was, is now | the position that it isn't. | | In the event that the position that it is in is not the position | that it wasn't, the system has acquired a variation, the | variation being the difference between where the rocket is, and | where it wasn't. If variation is considered to be a significant | factor, it too may be corrected by the GEA. However, the rocket | must also know where it was. | | The rocket guidance computer scenario works as follows. Because a | variation has modified some of the information the rocket has | obtained, it is not sure just where it is. However, it is sure | where it isn't, within reason, and it knows where it was. It now | subtracts where it should be from where it wasn't, or vice-versa, | and by differentiating this from the algebraic sum of where it | shouldn't be, and where it was, it is able to obtain the | deviation and its variation, which is called error. | Jtsummers wrote: | http://w3.uwyo.edu/~jimkirk/guidance.html | | Has the wav file as a link. Entertaining to hear randomly from | my phone because it somehow got into my iTunes library in the | 00s. | edrxty wrote: | Here's the rap remix version: | https://www.youtube.com/watch?v=_LjN3UclYzU | NikolaNovak wrote: | I _honestly_ cannot tell (at least until I read the article), | if that 's an honest and correct summary; a language barrier; | or a George Carlin sketch :-> | edrxty wrote: | Honestly the real answer is far worse. It's a ridiculous | meme. | dylan604 wrote: | Reads like Douglas Adams. | [deleted] | generalizations wrote: | You could read this, or you could go play with KSP for an hour. | afterburner wrote: | Except KSP simplifies orbital dynamics, such that my favourite | aspect, Lagrange points, is missing (or merely artificially | inserted). | aw1621107 wrote: | There's always Principia for your n-body needs [0]. | | [0]: https://github.com/mockingbirdnest/Principia | baq wrote: | i agree. though a hundred hours would do more good. | | either way, the article reaches about the same conclusion in a | quite direct way. | golergka wrote: | I came here to leave this comment exact comment. KSP just gives | you a better... can I call this "feeling"? of all these | mechanics than any theoretical explanation ever could. | uoaei wrote: | "intuition" | Syonyk wrote: | > _...for an hour._ | | This is a whopper of a lie. :p | | KSP is not "an hour" of time. KSP is "Oh, hey, I'll sit down | for a quick Eve mission at 8PM, hrm, that didn't work, ... ugh, | more struts ... right, I'll solve that with a few more boosters | ... oh _bleep_ how can that be a relay sat blackout NOW? ... | how can it possibly be 6AM? Is that the sunrise? " | | Wonderful game, absolutely worth spending money on if you have | any interest at all in space. But "an hour," you will not spend | on it. | sand500 wrote: | relevant xkcd | | https://xkcd.com/1356/ | Arnavion wrote: | It's hard to recommend KSP now. | | I installed it last weekend out of nostalgia. Turns out you're | automatically opted in to analytics that includes identifying | information about your specific machine. Opting out is not | possible; the game shows you a button which is apparently | supposed to open a web page where you can delete the | information collected so far but not opt out. (I say | "supposedly" because the button didn't do anything for me when | I clicked it. It's what I gathered from reading about it on | forums.) | | So you have to discover that you have to edit two config files | to suppress some of the analytics, then download some fan-made | Unity Analytics DLLs, that were reverse-engineered from the | originals but are no-ops, to suppress the rest. | | Fuck that shit. | dr_orpheus wrote: | I took a spacecraft mission design class in college. Every time | I tried to search for information, the first 6 google results | were from KSP before I got to something based on a real | spacecraft. ___________________________________________________________________ (page generated 2021-04-27 23:00 UTC)