This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
From: firstname.lastname@example.org (Kevin Reed)
A hybrid motor as sold for model rocketry uses a solid fuel grain and a
liquid oxidizer -- in the case of commercial model motors, nitrous oxide.
A composite motor uses a solid oxidizer -- ammonium perchlorate -- mixed
with a rubber binder/fuel to make a unified solid grain.
I can't think of any 24mm hybrids on the market; the smallest, I think,
has an "I" rating and fits into a 54mm mount.
There are two companies currently manufacturing them commercially,
Aerotech and Hypertek. One system loads the oxidizer tank before loading
the motor in the rocket, while the other fills the tank after the rocket
is in launch position.
Hybrids have a couple of advantages over composites: one is that there is
virtually no fire hazard transporting or storing the motor: without the
oxidizer in direct contact with it, the fuel grain is almost inert. It is
also not covered bythe same DOT shipping restrictions, because the tanks
are DOT certified and the fuel grain poses no environmental or fire
[Editor's note: The Jan 1996 issue of High Power Rocketry magazine has an
excellent article comparing the Hypertek and Aerotech hybrid systems.]