Many solid rocket propellants are based on a composite mixture of ammonium Perchlorate (AP) oxidizer and polymeric binder fuels. In these propellants, complex three-dimensional diffusion flame structures between the AP and binder decomposition products, present on the length scales of the heterogeneous mixture, chive the combustion via heat feedback to the surface. Changing the AP crystal size changes the burn rate of such propellants. Large AP crystals are governed by the cooler AP self deflagration flame and burn more slowly while small AP crystals are governed more by the hot diffusion flame with the binder and burn faster. This allows control of composite propellant ballistic properties via particle size variation. Most composite propellants based on AP contain a bimodal or trimodal particle size distribution of AP. A typical formulation would contain 85% solids (AP) with 15% binder (eg. HTPB). The AP would consist of a mixture of coarse and fine AP (say 62% 200 um and 38% 25um).
Smooke, M.D., Yetter, R.A., Parr, T.P., Hanson-Parr, D.M., Counter-Flow Diffusion Flames of Ammonium Perchlorate with Gaseous Fuels, Sixth International Microgravity Combustion Workshop, NASA Glenn Research Center, Cleveland, OH, CP-2001-210826, pp. 185-188, May 22-24, 2001.