# Getting the Velocity Vector in an Elliptical Orbit

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

• Do you have the radius vector or just the scalar length?
– uhoh
Jan 20, 2020 at 4:47
• 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. Jan 20, 2020 at 9:34
• 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.
– user7073
Jan 20, 2020 at 14:58
• @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. Jan 20, 2020 at 15:06
• Do these answer your question? Converting Orbital Elements to Cartesian State Vectors or Calculating velocity state vector with orbital elements in 2D
– user20636
Jan 20, 2020 at 23:51