Kn

Kn (Klemmung) is the ratio of a propellant grain's burning surface area to the nozzle throat area, at a given point in the burn:

<math>K_n = \frac{A_{burn}}{A_{throat}}</math>

It's a key driver of chamber pressure - roughly, higher Kn means higher pressure, since more burning surface is feeding gas through the same throat. Kn changes throughout the burn as the grain's geometry (and therefore its burning surface area) changes, which is why it's plotted as a curve rather than a single number - see the Kn and Kn Eff traces on BurnSim's graph.