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Motors for model rockets and high powered rockets are classified by total impulse into a set of letter-designated ranges, from A (the smallest, though 1/2, 1/4, and 1/8-A motors are also available), up to O as the largest. A is from 1.26 newton-seconds to 2.5 N·s, and each class is then double the total impulse of the preceding class, with B being 2.51 to 5.00 N·s.

The designation for a specific motor looks like C6-3. In this example, the letter (C) represents the total impulse of the motor, the number (6) before the dash represents the average thrust in newtons, and the number (3) after the dash represents the delay from motor burnout to the firing of the ejection charge (a gas generator composition, usually black powder, designed to deploy the recovery system). So a C6-3 motor would have between 5.01 and 10 N·s of impulse, produce 6 N average thrust, and fire an ejection charge 3 seconds after burnout.

Class	Total Impulse (Metric Standard)	Total Impulse (Imperial Standard)
A	1.26-2.50 N·s	0.29-0.56 lbf·s
B	2.51-5.00 N·s	0.57-1.12 lbf·s
C	5.01-10.00 N·s	1.13-2.24 lbf·s
D	10.01-20.00 N·s	2.25-4.48 lbf·s
E	20.01-40.00 N·s	4.49-8.96 lbf·s
F	40.01-80.00 N·s	8.97-17.92 lbf·s
G	80.01-160.00 N·s	17.93-35.96 lbf·s
H	160.01-320.00 N·s	35.97-71.92 lbf·s
I	320.01-640.00 N·s	71.93-143.83 lbf·s
J	640.01-1280.00 N·s	143.84-287.65 lbf·s
K	1,280.01-2,560.00 N·s	287.66-575.30 lbf·s
L	2,560.01-5,120.00 N·s	575.31-1150.60 lbf·s
M	5,120.01-10,240.00 N·s	1150.61-2301.20 lbf·s
N	10,240.01-20,480.00 N·s	2301.21-4602.40 lbf·s
O	20,480.01-40,960.00 N·s	4602.41-9204.80 lbf·s

The letter-scale continues past class-O, beyond the domain of high-power rocketry. Motors and vehicles of such size and power are generally considered the domain of amateur rocketry. (In this context, the term amateur refers to the rocketeer's independence from an established commercial or government organization.)

In many countries, the sale, possession, and use of model rocket motors is subject to governmental rules and regulations. For example, in the United States, high-powered motors (above 160 newton seconds) are restricted to citizens who possess the requisite certification and permits.

• Aerotech website
• Estes website

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