Help Graphing a Parabola

I'm doing a project for orbital mechanics and I'm stuck on graphing a parabola using parametric equations. I think I know the what the problem is: I need it to graph the trajectory of the parabolic orbit, but I'm adding a value so large the function doesn't graph like a parabola should. Any help on how I can correctly graph this?

if true
  % code
end
if
r_p=1.275620000225780e04;
a=-1.019309762925967e11;
Re=6.378e3;
q=-(r_p+a);
       q2=q+a;
           T=[-20000*pi,20000*pi];
     x_graph=(-1*(T.^2))+q;
     y_graph=T;
     plot(x_graph,y_graph,'b');
     hold on
     hold on
     grid on
       plot(q,0,'ro') %center of the earth
       plot(Re*cos(T)+q,Re*sin(T),'--') %plots the surface of the earth
            hold on
%          hold on
%          lineseg([q 0], PeriVect_new)
%          lineseg([q 0],PeriVect_OG)
%          hold off
           hold off
hold off             
hold off
end

[Jan]

You define 2 points also. How could this create a parabola?

[Mike B]

What are you talking about? The point defined appears on the plot, it has nothing to do with the formation of the parabola.

[Jan]

See my answer.

Answer 1

[Jan]

T is a [1 x 2] vector. Then x_graph and y_graph have 2 elements also. With these two points, you can either draw 2 points or a line, but not a parabola. See your code:

r_p = 1.275620000225780e04;
a   = -1.019309762925967e11;
Re   = 6.378e3;
q    = -(r_p+a);
q2   = q + a;
T    = [-20000*pi,20000*pi];
x_graph = (-1*(T.^2))+q;
y_graph = T;
disp(x_graph)
disp(y_graph)

Perhaps you want:

T       = linspace(-20000*pi, 20000*pi, 1000);
x_graph = (-1*(T.^2))+q;
y_graph = T;
plot(x_graph,y_graph,'b');