The burning rate characteristics of solid composite propellants can be modified via different methods. One of these is the application of different oxidizer particle sizes in the propellant. The effect of the use of fine and coarse particles of the oxidizer hydrazinium nitroformate (HNF) in hydroxyl-terminated polybutadiene (HTPB)-based propellants is reported in this paper. file fine HNF particles were obtained by means of a grinding process. The fine and coarse HNF grades were characterized regarding their hazardous and thermal stability properties, as well as the influence of a cured and uncured HTPB binder on these characteristics. The ballistic properties of three (two monomodal and one bimodal) HNF/HTPB-based propellants show that the use of fine HNF (5-10 mu m mean size) leads to a lowering of the pressure exponent of the propellant, implying that the propellant burning rate call be tuned by Varying the mean size of the HNF grades present in the propellant. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.