I have a known graph:*(Courtesy: Richard Nakka. http://www.nakka-rocketry.net)*

The graph shows pressure (in psi) compared to surface area (Kn). We can disregard MPa on the r/h side.

My software calculates the Kn (which is shown along the X axis). The Kn determines the pressure.

What I need to do is to determine the chamber pressure (Y axis) given the calculated Kn. If it was a straight line then I think that I've done something similar before, but this is a curve.

I've tried to break it down it to segments:

Select Case Kn Case 150 to 200 Kn = Kn * 3 End Select

Ive used the number 3 (ratio?) because 600psi/200Kn = 3. But that doesn't work for the lower end: 150 x 3 = 450 which according to the graph should be 400psi. The ratio number does change over the scale, but what I've done so far is *below reasonable*, but better than nothing. I don't need it to be perfect either - just somewhere in-between is fine.

The calculation in the end is about possible over-pressurisation and will give a recommendation. It doesn't need to be perfect but I'd like a bit more accuracy than I have.

I thought I knew this sort of stuff (maybe once). I think I'm missing something simple, but I've spent a few hours trying to solve this, and I'm really stuck trying to work it out.

*[EDIT]* I did look for solutions on the web but I couldn't understand the formulas.