Inearlierworkithasbeenshownthatspheresofvarioussizescanberandomlypackedtosimulatethemorphology ofammoniumperchlorate (AP)/binderheterogeneouspropellants.Amodelisnowformulated in whichpropellants dee nedinthiswaycanbeburnt,allowingforcompletecouplingbetweenthegas-phasephysics,thecondensed-phase physics, and the unsteady nonuniform regression of the propellant surface. The gas-phase kinetics is represented by a two-step model with parameters e tted to experimental data for the one-dimensional combustion of pure AP and the one-dimensional combustion of a e ne-AP/binder blend. In the discussion of the pure AP e ts, the issue of intrinsic instabilities arises, and these are explored using nonlinear direct numerical simulation and a numerical linear-stability strategy. Results for two-dimensional heterogeneous burning show that surface regions where the AP and binder can mix regress more rapidly than those dominated by AP; local extinction can occur where the binder dominates. Time variations in the integrated e ux-based equivalence ratio are shown to be large. Variations in the average burning rate with pressure are consistent with the (three-dimensional ) experimental record. Nomenclature cp = specie c heat D1;2 = reaction rate constants dj = diameter of the jth particle class E = activation energy f = surface function M = mass e ux Nj = number in the jth particle class n1;2 = pressure exponents in the reaction rates