Solid rocket motors are normally classified by their total impulse by a lettered classification scheme. This includes everything from model rockets up through the Space Transportation System and beyond.

The total impulse of a motor is essentially the product of its thrust and burn time (this is a simplification). Since thrust is normally measured in Newtons and time in seconds, the usual unit is the Newton-second (N-s).

Motors of G class and smaller are currently essentially unregulated in the US. Motors up through J class require a Level 1 high-power rocketry certification from one of the two sanctioning bodies -- the National Association of Rocktery and the Tripoli Rocketry Association. Motors up through K class require a Level 2 certification and motors larger than this require a Level 3 certification. At the time of this writing, there are fewer than 500 Level 3 certified individuals in Tripoli and a comparable number in NAR. Please note that the above certification requirements apply to launching rockets using these motors. There are separate regulations for posession of low explosives such as the ammonium perchlorate composite propellant used in most solid rocket motors, and which are far too complicated to go into here.

The standard classes are:
From (N-s)        To (N-s)        Class        Comment
0.625            1.25             1/2 A
1.25             2.5              A
2.5              5                B
5               10                C        common model rockets
10              20                D
20              40                E
40              80                F
80             160                G
160            320                H        level 1 certification required
320            640                I
640           1280                J        level 2 certification required
1280          2560                K
2560          5120                L        level 3 certification required
5120         10240                M
10240        20480                N
20480        40960                O
40960        81920                P        largest common hobby motors
81920       163840                Q
163840      327680                R
327680      655360                S
655360     1310720                T
1310720    2621440                U
2621440    5242880                V
5242880   10485760                W
10485760  20971520                X        Space Transportation System (shuttle) SRB
20971520  41943040                Y
41943040  83886080                Z

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