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I wrote this post when I've supposed to be working on some Kalman Filter models at work, and it really shows. > Additionally, in orbit there is no force opposi

I wrote this post when I've supposed to be working on some Kalman Filter models at work, and it really shows. > Additionally, in orbit there is no force opposing gravity There is when we treat the vehicle in a rotating reference frame centered around the earth origin, as is typically done for orbital calculations. It is completely correct to treat the vehicle in such a manner. And is of course exactly how the Karman Line was first proposed to be calculated.

dboreham 3y ago
> supposed to be working on some Kalman Filter models I think we all knew that.
dabluecaboose 3y ago
Gravity is a centripetal force that is constantly accelerating an orbiting body towards the orbited body. There is no counter-force that cancels it out. An object traveling in a circle is constantly accelerating, and by F=MA must have a force constantly acting against it. You can use coordinate frames to magick up some imaginary force that explains what happens in a limited, non-inertial frame, but that does not mean the force exists. There is no magical force that appears to lift an object up and away from the Earth once it's traveling fast enough. I've never once used an imaginary centrifugal force in any of my orbital calclulations, for what it's worth.
- deleted 3y ago
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- SonicScrub 3y ago
  I swear to God, the amount of misunderstanding around the concept of centrifugal force is insane. Relevant XKCD by the way: https://xkcd.com/123/ https://xkcd.com/123/ Sometimes it's more convenient to do physics in a rotating reference frame. There's nothing magical about it. For the given problem of predicting whether or not a vehicle held steady above Earth by a combination of both it's "Keplerian Force" and it's aerodynamic lift, converting to a rotating reference frame makes the math very easy. It's perfectly fine to conceptualize physics likes this. You could also think of it in an inertial frame. That's fine. Go ahead. But one isn't more correct than the other. > I've never once used an imaginary centrifugal force in any of my orbital calculations, for what it's worth. The equation for determining an orbital speed for given altitude can be derived from an inertial reference frame calculating the acceleration required to sustain circular motion or from a force balance in a rotating reference frame. You can start from first principles given either and derive the exact same equation. So you could choose to if you so desired. And again, as per the above example, sometimes considering the rotating frame makes the problem more intuitive. Neither is more correct than the other. Strict doctrine to one as being "more correct" only limits understanding of how problems can be solved.
  - dabluecaboose 3y ago
    The "Kepler force" is not really a force. It's an expression of inertia in (as I mentioned) a limited, non-inertial reference frame. It's fine to conceptualize but IMO misleading especially when explaining the fundamentals to novices. As for the ubiquity of using a rotating frame, I'm not saying it's not done. I'm just saying it's not necessarily typical for orbits to de facto be considered in such a frame.
    - SonicScrub 3y ago
      Ok, but you made a comment stating that what I said is inaccurate. It's not. It's just a different philosophy for analyzing rotating systems. And importantly, it's how Karman conceptualized his dynamic model for an aerodynamic space-plane that led to the derivation of the Karman Line in the first place. Describing the Karman Line through such a rotational reference is perfectly acceptable. There's no reason at all to keep our thinking about free-body diagrams and physical models to strictly inertial frames. Doing so in no way makes a conceptual framework more accurate than any other. I really hate this way thinking about centrifugal force and rotational frames. It's a breeding ground for pointless internet arguments like this one instigated by the most frustrating of pedantry. Spreading this idea that using centrifugal force is somehow wrong only acts to restrict understanding of physics. It also handicaps you from solving problems in ways that could be better served by changing frames.
      - deleted 3y ago
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      - dabluecaboose 3y ago
        It is inaccurate for any reference frame you should expect the laymen you were addressing to understand. I stand by my statement. You can invent a reference frame that will make anything "technically correct" Edit to address your edit: And a hearty "fuck you" to you as well, you smug, pedantic arse
- kmm 3y ago
  After doing some cursory Google searches on relative motion in orbit, establishing a rotating frame seems standard to me. Even the original Clohessy-Wiltshire paper[1] starts by doing that. As a physicist, though admittedly not working in orbital mechanics, I can assure you working with non-inertial frames and their concomitant pseudoforces is nothing out of the ordinary. Whether or not these forces are "magical" or even "real" is more of a philosophical question, secondary to the fact of their utility. 1: https://arc.aiaa.org/doi/10.2514/8.8704 https://arc.aiaa.org/doi/10.2514/8.8704
  - dabluecaboose 3y ago
    Appreciate the link! I actually dabbled with Clohessy-Wiltshire equations in some higher-level orbital classes years ago. I haven't done anything with spacecraft rendezvous since college so it definitely fell off my radar. As for the "existence" of the force, I suppose you're right in that it can get quite philosophical, as even our "Earth-Centered Inertial" reference frame isn't truly inertial.

dboreham 3y ago

> supposed to be working on some Kalman Filter models I think we all knew that.

dabluecaboose 3y ago

Gravity is a centripetal force that is constantly accelerating an orbiting body towards the orbited body. There is no counter-force that cancels it out. An object traveling in a circle is constantly accelerating, and by F=MA must have a force constantly acting against it. You can use coordinate frames to magick up some imaginary force that explains what happens in a limited, non-inertial frame, but that does not mean the force exists. There is no magical force that appears to lift an object up and away from the Earth once it's traveling fast enough. I've never once used an imaginary centrifugal force in any of my orbital calclulations, for what it's worth.

deleted 3y ago
[deleted]
SonicScrub 3y ago
I swear to God, the amount of misunderstanding around the concept of centrifugal force is insane. Relevant XKCD by the way: https://xkcd.com/123/ https://xkcd.com/123/ Sometimes it's more convenient to do physics in a rotating reference frame. There's nothing magical about it. For the given problem of predicting whether or not a vehicle held steady above Earth by a combination of both it's "Keplerian Force" and it's aerodynamic lift, converting to a rotating reference frame makes the math very easy. It's perfectly fine to conceptualize physics likes this. You could also think of it in an inertial frame. That's fine. Go ahead. But one isn't more correct than the other. > I've never once used an imaginary centrifugal force in any of my orbital calculations, for what it's worth. The equation for determining an orbital speed for given altitude can be derived from an inertial reference frame calculating the acceleration required to sustain circular motion or from a force balance in a rotating reference frame. You can start from first principles given either and derive the exact same equation. So you could choose to if you so desired. And again, as per the above example, sometimes considering the rotating frame makes the problem more intuitive. Neither is more correct than the other. Strict doctrine to one as being "more correct" only limits understanding of how problems can be solved.
- dabluecaboose 3y ago
  The "Kepler force" is not really a force. It's an expression of inertia in (as I mentioned) a limited, non-inertial reference frame. It's fine to conceptualize but IMO misleading especially when explaining the fundamentals to novices. As for the ubiquity of using a rotating frame, I'm not saying it's not done. I'm just saying it's not necessarily typical for orbits to de facto be considered in such a frame.
  - SonicScrub 3y ago
    Ok, but you made a comment stating that what I said is inaccurate. It's not. It's just a different philosophy for analyzing rotating systems. And importantly, it's how Karman conceptualized his dynamic model for an aerodynamic space-plane that led to the derivation of the Karman Line in the first place. Describing the Karman Line through such a rotational reference is perfectly acceptable. There's no reason at all to keep our thinking about free-body diagrams and physical models to strictly inertial frames. Doing so in no way makes a conceptual framework more accurate than any other. I really hate this way thinking about centrifugal force and rotational frames. It's a breeding ground for pointless internet arguments like this one instigated by the most frustrating of pedantry. Spreading this idea that using centrifugal force is somehow wrong only acts to restrict understanding of physics. It also handicaps you from solving problems in ways that could be better served by changing frames.
    - deleted 3y ago
      [deleted]
    - dabluecaboose 3y ago
      It is inaccurate for any reference frame you should expect the laymen you were addressing to understand. I stand by my statement. You can invent a reference frame that will make anything "technically correct" Edit to address your edit: And a hearty "fuck you" to you as well, you smug, pedantic arse
kmm 3y ago
After doing some cursory Google searches on relative motion in orbit, establishing a rotating frame seems standard to me. Even the original Clohessy-Wiltshire paper[1] starts by doing that. As a physicist, though admittedly not working in orbital mechanics, I can assure you working with non-inertial frames and their concomitant pseudoforces is nothing out of the ordinary. Whether or not these forces are "magical" or even "real" is more of a philosophical question, secondary to the fact of their utility. 1: https://arc.aiaa.org/doi/10.2514/8.8704 https://arc.aiaa.org/doi/10.2514/8.8704
- dabluecaboose 3y ago
  Appreciate the link! I actually dabbled with Clohessy-Wiltshire equations in some higher-level orbital classes years ago. I haven't done anything with spacecraft rendezvous since college so it definitely fell off my radar. As for the "existence" of the force, I suppose you're right in that it can get quite philosophical, as even our "Earth-Centered Inertial" reference frame isn't truly inertial.