This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
The big changes have come in motors. Expendable composite fuel motors
are now available in B-G range for model rockets. These motors
use ammonium perchlorate for oxidizer and rubber as the fuel, similar to
the rocket boosters on the space shuttle, allowing them to pack two to
three times the power in the same space as a black powder motor. The B
motors from Apogee are the same size as Estes mini-motors (13x45mm).
The C motors (also from Apogee) are 18x50mm, while the D motors are the
same size as Estes A-C motors (18x70mm). The D is a full D (rated at 20
Newton-seconds versus the Estes 24x70mm D of about 17 N-s). E motors range
in size from 18x70mm to 29x124mm. All of the motors give Estes kits an
incredible ride, if the models hold together. These kits require stronger
construction methods and materials than typical model rockets. Put an
Aerotech D21 or E25 in your old Big Bertha at your own risk!! You're
likely to end up with a model with no fins (i.e., a complete 'shred').
Another new trend is 'reloadable' motor technology. With reloadables
you have a metal motor casing that you manually reload with solid fuel
pellets, delay and ejection charge for each flight. The casing is
reusable. Reloadable motors are available in everything from 18x70 mm
(with B - E power), 24mm, (with D - F power), 29mm, 38mm, and much
larger. Again, you can get all the way up to 40,000+ Newton-seconds of
The latest technology to hit the High Power rocket scene is the hybrid
rocket motor. Hybrid motors use components from both liquid and solid
ful rocket motors. Two companies have currently certified hybrid motors
with Tripoli, Aerotech and Hypertek. The advantage of hybrid motors is
that they use totally inert fuel grains, such as a cast polymer plastic
or compressed paper pulp, which do not have any DOT or ATF restrictions.
With both motors, nitrous oxide is used for the oxidizer.