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Ammonium Perchlorate Composite Propellant, or APCP, is the fuel of choice for mid-power and high-power rocketry. It consists of an oxidizer, ammonium perchlorate (NH4ClO4), combined with an elastomer (rubber) that acts as both fuel and binder.

APCP is easily cast at reasonably low temperatures and is normally cast into hollow cylindrical slugs of about 60 grams each. Several of these slugs are normally stacked together inside an engine casing, which is a precision-machined aircraft aluminum tube with threaded closures at both ends. Standard sizes for the casing are 24, 29, 38, 54, 98 and 112mm diameter, in lengths ranging from 3cm to 2m.

This technique suffices up to about K class, beyond which larger slugs are used but the same priniciple applies. Commercial and hobby APCP motors are reasonably common up to O or P class. Larger APCP motors are used in some space vehicles, such as the Space Transportation System (Shuttle)'s X class solid rocket boosters.

The reason for attempting to limit individual slugs to 60 grams or so stems from the previous regulatory regime, in which individual grains smaller than 62.5g were exempt from control by the Bureau of Alcohol, Tobacco, and Firearms. However, since the events of September 11, 2001, that agency (now the Bureau of Alcohol, Tobacco, Firearms and Explosives) has embarked on a highly controversial attempt at regulating all APCP motors, regardless of size. Currently (mid-2003) this is a matter of quite some uproar in the high-power rocketry community.

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