Hey guys, I'm in my last year of my physics degree, and part of my dissertation project involves mathematica, modelling the orbital behaviour of a spacecraft using Ion propulsion. I have 2 coupled differential equations being solved, r''(t) and o''(t), in an NdSolve function (o is pheta in my case, modelling the change in angular momentum). It solves this with no problem, and plots the results fine. My next step is to attain a parametric plot, in which my 2 functions are cos(o)r and sin(o)r, to get the trajectory of my spacecraft along its journey. I have attemped to copy the syntax in the "get help" section and the when evaluated, it just returns a blank graph. What am i doing wrong!? ParametricPlot[{Evaluate[r[t] /. TotalThrust]* Evaluate[Cos[o[t]] /. TotalThrust], Evaluate[r[t] /. TotalThrust]* Evaluate[Sin[o[t]] /. TotalThrust]}, {t, 0, 10}, PlotRange -> All] (TotalThrust is the label given to my NDSolve function) Thanks in advance! Mike

Unfortunately i need help again! It seems trivial on paper, however im struggling to get it in terms of code. Based on my ODE solutions, i now need to find the value for time t such that r(t) = 5.2. I assume its a simple piece of code, but im having trouble getting it to work. Thanks again!

Wow, that actually worked perfectly, thanks alot man!

Try not to use Evaluate. It isn't needed. So the input to your plotting function is: {(r[t]Cos[o[t]] /. TotalThrust), (r[t]Sin[o[t]] /. TotalThrust)} Evaluate this and look at the result. It's a list of lists. That is r[t]Cos[o[t]] /. TotalThrust produces a list. Lists are those curly brackets. To get rid of them use r[t]Cos[o[t]] /. TotalThrust[[1]] as shown in the article linked in the previous post. ParametricPlot[{(r[t]Cos[o[t]] /. TotalThrust[[1]]), (r[t]Sin[o[t]] /. TotalThrust[[1]])}, {t, 0, 10}]

Need help with parametric plot of coupled differential equations