Propagation Index
A simple method for assessing the ability of a particular composition to burn is the "Propagation Index," originally proposed by McLain and later modified by Rose. The original McLain expression was
where PI - the Propagation Index -- is a measure of a mixture's tendency to sustain burning upon initial ignition by external stimulus. The equation contains the two main factors that determine burning ability - the amount of heat released by the chemical reaction (AH) and the ignition temperature of the mixture. If a substantial quantity of heat is released and the ignition temperature is low, then reignition from layer to layer should occur readily and propagation is likely. Conversely, mixtures with low heat output and high ignition temperature should propagate poorly, if at all. Propagation Index values for a variety of compositions are given in Table 1.
Rose recommended modifying the original McLain expression by the addition of terms for the pressed density of the composition and for the burning rate of the mixture. He reasoned, especially for delay compositions compressed in a tube, that ability to propagate should increase with increasing density, due to better heat transfer between grains of composition. Burning rate should also be a factor, he argued, because faster-burning mixtures should lose less heat to the surroundings than slower compositions.
Table 1. Propagation Index values for a Pyrotechnic Mixtures