[HN Gopher] Orbital Mechanics - How do rockets get to where they...
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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.
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(page generated 2021-04-27 23:00 UTC)