What is the primary reason most ammonium perchlorate composite propellant (APCP) rocket motors are central-burning?

The primary reason most ammonium perchlorate composite propellant (APCP) rocket motors are central-burning is related to the burn rate characteristics of the propellant itself. Ammonium perchlorate has a specific burn rate that tends to be too low for effective use in endburning configurations. In a central-burning design, the propellant is ignited at the center and burns outward toward the casing, which allows for a more effective and sustained combustion process.

This centralized combustion provides a more consistent thrust profile and is more efficient in utilizing the energy produced by the APCP. In contrast, an endburning motor relies on the propellant to ignite and burn from one end to the other, which necessitates a higher burn rate to maintain effective thrust. Because the burn rate of APCP does not suit this design well, central-burning configurations are preferred.

While there may be advantages to other designs in different contexts or specific applications, the unique combustion characteristics of APCP make central-burning the most practical and effective choice for most rocket motors using this type of propellant.