text: The combustion time of the aluminium particles in the hot combustion gas varies depending on aluminium particle size and shape. In small APCP motors with high aluminium content, the residence time of the combustion gases does not allow for full combustion of the aluminium and thus a substantial fraction of the aluminium is burned outside the combustion chamber, leading to decreased performance. This effect is often mitigated by reducing aluminium particle size, inducing turbulence (and therefore a long characteristic path length and residence time), and/or by reducing the aluminium content to ensure a combustion environment with a higher net oxidizing potential, ensuring more complete aluminium combustion. Aluminium combustion inside the motor is the rate-limiting pathway since the liquid-aluminium 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: As the particle radius increases the volume (and, therefore, mass and heat capacity) increases as the cube of the radius. However, the surface area increases as the square of the radius, which is roughly proportional to the heat flux into the particle. Therefore, a particle's rate of temperature rise is maximized when the particle size is minimized.
  Common APCP formulations call for 30–400 μm AP particles (often spherical), as well as 2–50 μm Al particles (often spherical). Because of the size discrepancy between the AP and Al, Al will often take an interstitial position in a pseudo-lattice of AP particles.
  Characteristics