Calculating apoapsis from position and velocity

[ellipsis]

I have a body of negligible mass orbiting a body of considerable mass, with that larger body fixed at the origin. I am given the orbital state vector (position and velocity), and need to return the apoapsis of orbit. Other orbital information would be useful, e.g. energy, angular momentum, eccentricity, etc.

For simplicity, the system is assumed to be in a plane... I am not concerned with the third position/vector component.

What I have so far:

  energy = norm(v)^2/2-S/norm(p);
  angmom = dot(p,v);
  eccen = sqrt(1+(2*energy*angmom^2)/S^2);
  semimajor = 1/(2/norm(p)-norm(v)^2/S);
  apoapsis = semimajor*(1+norm(eccen));

Sorry about the formatting, it's in Matlab language. I am far too lazy to do the latex for such a simple question. The above formulae do not return the correct results.EDIT: Huh... it may work now. ?

 

[mfb]

The above formulae do not return the correct results.

What is correct, what do you get, and where do the formulas come from?

 

[ellipsis]

mfb: The apoapsis changes drastically in different parts of the orbit, and I know the physics code is already correct, so it must be this. The formulae are thrown together from various sources, like Wikipedia and a paper I found on these forums.

The above may or may not be correct, I'm not sure. Mathematically, how does one normally calculate orbital parameters given position and velocity (and standard gravitational parameter, S)

 

[tony873004]

http://orbitsimulator.com/formulas/OrbitalElements.html
Here's a link to a calculator I made in javascript. You can view the source. It's primarily from the method from "Solar System Dynamics" by Murray & Dermott.
Apoapsis is Q in the calculator.

 

[ellipsis]

Wow Tony, you made that? I found that when searching for an orbital simulator, but didn't use it because the orbital elements were not obvious. Thanks.

 

[tony873004]

If you hover the mouse over a text box, it tells you in English what each box means, in case you don't like Greek variables.