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I have a 2-D two body set up. I am using the vis-viva equation to get the velocity scalar of my elliptical orbit. I have the Semi-Major Axis, the radius, the eccentricity vector. I am sure I can derive more information. I just don't know how to figure out how to get the velocity vector. I need speed and direction rather than just speed.

thank you

ryanq.feeney
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  • Do you have the radius vector or just the scalar length? – uhoh Jan 20 '20 at 04:47
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    This is something you can find on the Kepler orbit wikipedia page. If you have trouble with any of the expressions there, feel free to ask for clarification. Right now it seems like you could do a little more effort to figure this out on your own first. – Alexander Vandenberghe Jan 20 '20 at 09:34
  • I agree with @AlexanderVandenberghe, but, if you're still stuck, also feel free to contact me directly (this would be an interesting question to look at on a live video stream). You mention "semimajor axis" and "radius" as two separate things, which confuses me a little. You also say "eccentricity vector", but eccentricity is usually just a number (scalar), not a vector. –  Jan 20 '20 at 14:58
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    @barrycarter, in orbital mechanics, eccentricity is definitely a vector, and the eccentricity you usually see when talking about orbital elements is just the magnitude of that vector. The radius is normally just the vector from your main focal point (central body) to your satellite. – Alexander Vandenberghe Jan 20 '20 at 15:06
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    Do these answer your question? Converting Orbital Elements to Cartesian State Vectors or Calculating velocity state vector with orbital elements in 2D –  Jan 20 '20 at 23:51
  • @barrycarter more about eccentricity vector; 1, 2, 3 – uhoh Jan 21 '20 at 00:02
  • Hello. I do have the radius vector. And I have the eccentricity vector from which I derive the scalar eccentricity. I have had a lot of trouble getting the actual direction of the velocity of the satellite though getting the speed is easy. As for @barrycarters confusion the semi-major axis is exactly what is is defined as in a keplar orbit. As for the radius I should have said 'radius vector' meaning the position of the satellite relative to the central body. Lastly, I did see JCRM second link and I'm a little confused on it. Will continue in the next comment... – ryanq.feeney Jan 23 '20 at 21:50
  • ...My next comment was pretty dumb. I'm getting there though. More to come – ryanq.feeney Jan 23 '20 at 23:19

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