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Standard Motor Sizes and Impulses.

The numbers on a motor, eg C or K, equate to a certain 'class' of total impulse. The larger the letter, the larger the impulse or thrust.

Total Impulse (Ns) Equivalent Motor Class
0.00 - 1.25 ½A ¼A
1.26 - 2.50 A
2.51 - 5.00 B
5.01 - 10.00 C
10.01 - 20.00 D
20.01 - 40.00 E
40.01 - 80.00 F
80.01 - 160.00 G
160.01 - 320.00 H
320.01 - 640.00 I
640.01 - 1280.00 J
1,280.01 - 2,560.00 K
2,560.01 - 5,120.00 L
5,120.01 - 10,240.00 M
10,240.01 - 20,480.00 N
20,480.01 - 40,960.00 O

There are a number of standard motor diameters, detailed below.

'Micro' Motors - mainly 1/2A
The standard model size, catering for A, B and C motors, also some D class AP motors
The larger model size, featuring C, D and E motors
The transition size from model to HPR, featuring F, G, H motors. Also a common hybrid rocket motor diameter
The most common mid/high power diameter, catering all the way from G to J
The start of the larger HPR sizes, featuring J to L/M.
An uncommon diameter, mainly occupied by hybrids.
The large and expensive HPR solids and hybrids. K - N
The most common size for large (>L) HPR motors.
The realm of Cesaroni O class goodness