# 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 ½A, ¼A, and ⅛A motors are also available), up to O as the largest. Class 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 Class B being 2.51 to 5.00 N·s. Engines classified beyond O are in the realm of amateur rocketry. In this context, the term amateur refers to the rocketeer's independence from an established commercial or government organization.[1]

The impulse designation for a specific motor is based on the alphabetic code. For example, the letter (M) would represent the total impulse of between 5,120.01 and 10,240.00 N·s of impulse.

## Model rocket motor codes

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.

## Motor impulse by Class

Class Total Impulse
(Metric Standard)
Total Impulse
(Imperial Standard)
Aerospace Vehicle
or Rocket(s)
US Requirements
A 1.26–2.5 N·s 0.28–0.56 lbf·s
B 2.51–5.0 N·s 0.56–1.12 lbf·s
C 5.01–10.0 N·s 1.13–2.25 lbf·s
D 10.01–20.0 N·s 2.25–4.50 lbf·s
E 20.01–40.0 N·s 4.50–8.99 lbf·s
F 40.01–80.0 N·s 8.99–17.98 lbf·s
G 80.01–160.0 N·s 17.99–35.97 lbf·s
H 160.01–320 N·s 35.97–71.94 lbf·s Level 1 Certification required from Tripoli or NAR
I 320.01–640 N·s 71.94–143.88 lbf·s
J 640.01–1,280 N·s 143.88–287.76 lbf·s Level 2 Certification required from Tripoli or NAR
K 1,280.01–2,560 N·s 287.76–575.51 lbf·s
L 2,560.01–5,120 N·s 575.51–1,151.02 lbf·s
M 5,120.01–10,240 N·s 1,151.02–2,302.04 lbf·s Level 3 Certification required from Tripoli or NAR
N 10,240.01–20,480 N·s 2,302.05–4,604.09 lbf·s
O 20,480.01–40,960 N·s 4,604.09–9,208.17 lbf·s
P 40,960.01–81,920 N·s 9,208.18–18,416.35 lbf·s FAA/AST Permit or License required
Q 81,920.01–163,840 N·s 18,416.35–36,832.70 lbf·s
R 163,840.01–327,680 N·s 36,832.70–73,665.39 lbf·s
S 327,680.01–655,360 N·s 73,665.40–147,330.79 lbf·s
T 655,360.01–1,310,720 N·s 147,330.79–294,661.58 lbf·s
U 1,310,720.01–2,621,440 N·s 294,661.58–589,323.16 lbf·s Apollo launch escape rocket
Space Shuttle Main Engine
V 2,621,440.01–5,242,880 N·s 589,323.16–1,178,646.31 lbf·s
W 5,242,880.01–10,485,760 N·s 1,178,646.31–2,357,292.62 lbf·s
X 10,485,760.01–20,971,520 N·s 2,357,292.63–4,714,585.25 lbf·s
Y 20,971,520.01–41,943,040 N·s 4,714,585.25–9,429,170.49 lbf·s Delta II
GEM-40 SRB
Z 41,943,040.01–83,886,080 N·s 9,429,170.50–18,858,340.99 lbf·s
2Z 83,886,080.01–167,772,160 N·s 18,858,340.99–37,716,681.97 lbf·s
3Z 167,772,160.01–335,544,320 N·s 37,716,681.97–75,433,363.94 lbf·s
4Z 335,544,320.01–671,088,640 N·s 75,433,363.94–150,866,727.88 lbf·s
5Z 671,088,640.01–1,342,177,280 N·s 150,866,727.89–301,733,455.77 lbf·s Space Shuttle SRB
6Z 1,342,177,280.01–2,684,354,560 N·s 301,733,455.77–603,466,911.54 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.)

## Governmental regulation

In many countries, the sale, possession, and use of model rocket motors is subject to governmental rules and regulations. High-Power rockets in the United States, are only federally regulated in their flight guidelines by the FAA. However, the motor manufacturers and National Rocketry Organizations have established a self-regulating industry through the outside help of National Fire Protection Association - NFPA. This self-regulation of industry requires a user to become certified for use before a manufacturer will sell him a motor. In North America, the two recognized organizations that provide high-power certifications are Tripoli Rocketry Association and the National Association of Rocketry, both of these organizations have three levels of certification which involves building progressively more complex and higher powered rockets and taking a test of safety rules and regulations. The U.S. government regulatory documents surrounding high-powered rocketry is FAA FAR Part 101. With the general Association bodies using TRA and NFPA 1127 safetycodes. In Canada the Canadian Association of Rocketry has a four-step certification process, but all three organizations accept the other's certifications if a flyer shows up at a high-power launch and wishes to fly under their sanction. Some states restrict the maximum impulse to a G motor, and have adopted NFPA safety codes for launch site requirements.

Level 1 certification qualifies you to purchase and use H and I engines, Level 2 certification J, K, and L engines and Level 3 certification M, N, and O engines. Note: Canada adds another step in between, and has a Level 4 which is the same as US Level 3.

In the late 1990s, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives began demanding that individuals obtain a Low Explosives Users Permit (LEUP) to possess and use high-powered motors. On February 11, 2000, Tripoli Rocketry Association, Inc. and the National Association of Rocketry filed suit in the United States District Court for the District of Columbia claiming that the BATF applied "onerous and prohibitive civil regulations" against sport rocketry hobbyists due to the Bureau's improper designation of Ammonium Perchlorate Composite Propellant (APCP) as an explosive. APCP is used in most high-power rocket motors.

In 2009, the court ruled in favor of the hobby organizations and ordered the BATF to remove APCP from its list of regulated explosives.[2]

## BATFE Lawsuit

In the late 1990s, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives began requiring individuals to obtain a Low explosive Users Permit (LEUP) to have a high-powered rocket motor in their possession. Following a 9-year long lawsuit brought by TRA and NAR against the BATFE, these laws were overturned in a federal court in 2009, and U.S. citizens no longer require governmental approval to purchase APCP solid rocket motors.[3]