Ammonium perchlorate composite propellant (APCP) is a modern solid rocket propellant used in both manned and unmanned rocket vehicles. It differs from many traditional solid rocket propellants, such as black powder, not only in chemical composition and overall performance, but also by the nature of how it is processed. APCP is cast into shape, as opposed to powder-pressing as with black-powder. This allows for manufacturing regularity and repeatability which are necessary requirements for use in the aerospace industry and hobby rocketry.

Commercially available ACAP rocket engines come in the form of reloadable kits or in pre-assembled single use motors. Typical reloadable motor kits include mostly disposable parts that are assembled into re-useable hardware. Reloadable motor kits are less expensive than single-use motors of equivalent power.