Model rocket motor classification

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 and 1/4-A engines are also available), up to O as the largest. A is from 1.26 Newton-seconds to 2.5 Ns, and each class is then double the total impulse of the preceding class, with B being 2.51 to 5.00 Ns. The designation for a specific engine looks like C6–3, with the letter representing the total impulse of the engine, the number before the dash representing the average thrust in Newtons, and the number after the dash representing the delay from engine burnout to the firing of the ejection charge (a gas generator composition, usually black powder, designed to deploy the recovery system). So the C6–3 engine would have between 5.01 and 10 Ns of impulse, produce 6 N average thrust, and fire an ejection charge 3 seconds after burnout.

Class	Minimum Impulse (Ns)	Maximum Impulse (Ns)
A	1.26	2.50
B	2.51	5.00
C	5.01	10.00
D	10.01	20.00
E	20.01	40.00
F	40.01	80.00
G	80.01	160.00
H	160.01	320.00
I	320.01	640.00
J	640.01	1280.00
K	1280.01	2560.00
L	2560.01	5120.00
M	5120.01	10240.00
N	10240.01	20480.00
O	20480.01	40960.00