This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Aerotech makes the following 'standard' retail motors in -4 and -7
second delays. The first two motors are the same size as Estes A-C motors.
The next two are the same size as Estes D motors. There are some other
24mm motors that are available from Aerotech that are longer than
Estes D motors. Some of these 'non-standard' Aerotech 24mm motors are listed
after the four 'standard' ones. The F and G motors may be obtained from
some dealers and mail order sources, but are not packaged for retail sale.
Motor Size Power Same Size As D21 18x70mm 20NS Estes/Quest/MRC A-C E25 18x70mm 22NS Estes/Quest/MRC A-C E15 24x70mm 40NS Estes D motor E30 24x70mm 40NS Estes D motor F32 24x124mm 80NS Non-standard size F44 24x101mm 70NS Non-standard size G42 24x144mm 90NS Non-standard size G55 24x177mm 125NS Non-standard size