This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Yes and no. They are the same size. Composite motors have 2 to 3 times
the power of comparably size BP motors. Balsa-finned 18mm powered models
tend to loose body parts in quantity when launched with a D21 or E25.
The ejection charges seem to be hotter, as well (IMHO). The same holds
true for Aerotech 24mm motors. Care should be taken before launching a
24mm-based model on an E15, let alone an E30. I have an old MegaSize that
I fly on E15-10's. Works great. The Estes Saturn V flies well on E15's,
too. E30's tend to shred all but the strongest D models, though. E30's
also tend to relocate motor mounts to someplace OUTSIDE of the rockets, as
well. If I plan to use E's in an Estes model I make it a point to reinforce
the motor mount, especially for EM-2060, EM-2070 and EM-5080 mounts.
You also want to use an engine block (a 2050 adapter ring works
great) in addition to the metal clip. IMO, I would also reinforce
fin/body tube joints. Five minute epoxy fillets work great.
Generous cyano fillets also seem to work well. White glued fins
don't seem to survive E15/E30 launches with any consistent success
(i.e., the failure rate tends to be > 50% :-). Many modelers also
recommend that stronger 24mm motor tubing, such as that from LOC
or Aerotech, be used for models flying with composite motors. The
stronger tubing holds up better to the ejection charges of the composites.
There are now several D and E reloads available for the 18 and 24 mm
casings that might not over power 'standard' model rockets. The RMS
motors have a variety of reloads available, some with fairly low average