preheating), and increase the temperature of the combustion gases (increasing Isp). Common species Oxidizers: Ammonium perchlorate as the primary oxidizer Metal-oxide catalysts as thermite oxidizers High energy fuels: Aluminium (high performance, most common) Magnesium (medium performance) Zinc (low performance) Low energy fuels acting as binders: Hydroxyl-terminated polybutadiene (HTPB) Carboxyl-terminated polybutadiene (CTPB) Polybutadiene acrylonitrile (PBAN) Special considerations Though increasing the ratio of metal-fuel to oxidizer up to the stoichiometric point increases the combustion temperature, the presence of an increasing molar fraction of metal oxides, particularly aluminum oxide (Al2O3) precipitating from the gaseous solution creates globules of solids or liquids that slow down the flow velocity as the mean molecular mass of the flow increases. In addition, the chemical composition of the gases changes, varying the effective heat capacity of the gas. Because of these phenomena, there exists an optimal non-stoichiometric composition for maximizing Isp of roughly 16% by mass, assuming the combustion reaction goes to completion inside the combustion chamber. The combustion time of the aluminum particles in the hot combustion gas varies depending on aluminum particle size and shape. In small APCP motors with high aluminum content, the residence time of the combustion gases does not allow for full combustion of the aluminum and thus a substantial fraction of the aluminum is burned outside the combustion chamber, leading to decreased performance. This effect is often mitigated by reducing aluminum particle size, inducing turbulence (and therefore a long characteristic path length and residence time), and/or by reducing the aluminum content to ensure a combustion environment with a higher net oxidizing potential, ensuring more complete aluminum combustion. Aluminum combustion inside the motor is the rate-limiting pathway since the liquid-aluminum droplets (even still liquid at temperatures 3000 K) limit the reaction to a heterogeneous globule interface, making the surface area to volume ratio an important factor in determining the combustion residence time and required combustion chamber size/length. Particle size The propellant particle size distribution has a profound impact on APCP rocket motor performance. Smaller AP and Al particles lead to higher combustion efficiency but also lead to increased linear burn rate. The burn rate is heavily dependent on mean AP particle size as the AP absorbs heat to decompose into a gas before it can oxidize the fuel components. This process may be a rate-limiting step in the overall combustion rate of APCP. The phenomenon can be explained by considering the heat-flux-to-mass ratio: