Going to wikipedia's article on solid-fuel rockets, I come across some graphs of thrust curves for certain bore-hole geometry. Right now I'm interested in the simplest bore-hole: a circular hole right in the center of the fuel grain:
This thrust-curve seems totally wrong. For a circular hole with a linear burn rate, the radius should expand linearly. Since C = pi*d, the circumference should also increase linearly. (Burn rate is proportional to the circumference of the exposed fuel grain.) Therefore, the mass flow should also increase linearly. Therefore, the thrust should also increase linearly.
(That all assumes that all else is equal. I don't know much about solid-rocket calculations. I've tried working with them twice over the years, and I came to the conclusion that solid rocket calculations are even more complex than liquid rocket calculations, which really bugged me cuz solid rockets appealed to my sense of simplicity due to having no moving parts. I'm not sure if combustion pressure goes up linearly, but it seems it would have to...)
Anyway, if all this is true, then the thrust curve should not even be a curve at all. It should just be a straight line going up at some angle...maybe steeply, maybe shallowly, but linearly.
The graph shown is radically different than that. Am I missing something?
What is the thrust curve for a solid rocket with a simple circular hole, and why?