Incorporation of solids into polymeric matrix to produce highly filled composites is subject to weight errors. In composites used as energetic materials, such as solid propellants, road flares, and car airbags, it is crucial to deliver the exact amount of solids. Usually, slurries are tested for viscosity during mixture. In propellants, pre-mix is prepared by addition of solids into a polyol. After mixture, a diisocyanate is added to give highly filled polyurethane. Burning of pre-mix samples can supply thermodynamic data. This paper presents a sensitivity analysis of simulated thermodynamic responses to solid content, e.g. ammonium perchlorate and aluminium powder. Changes in solid content were evaluated by factorial design of experiments in two-levels with central and axial points. The thermodynamic responses were pressure, temperature and enthalpy . A second order model was obtained for each response. Thermodynamic responses were less sensitive to changes in Al than in AP.