Model rocket engines are usually labeled with something like this: “B5-6.”
- The letter is an indication of average impulse, measured in Newton-seconds. If the engine were expended in an instant, this would be the force it delivers.
- The number following the letter is the average thrust of the engine. This is measured in Newtons. At any given time, the force from the engine is equal to this.
- The number after the dash is how many seconds after launch until the ejection charge fires. The ejection charge fires a clay plug into the parachute, deploying it. Engines with a 0 after the dash are booster engines used in multistage rockets, and have no ejection charge.
Engines are also color coded. A single stage engine would have green lettering, while an upper stage engine would be in purple. Booster engines are in red, and plugged engines are in black. Plugged engines are for R/C gliders, and don’t use an ejection charge.
The following is a table of engine classes and fuel. Engines that supply more than 160 Newton-seconds, or engines that contain more than 62.5 grams of propellant are considered high powered engines, and require a special permit.
Class--Ave Force--Propellant
A –- 2.0 –- 1.3
B –- 3.8 –- 2.5
C –- 7.5 –- 5.0
D –- 15 –- 10
E –- 30 –- 20
F –- 60 –- 41
G –- 120 –- 82
H –- 240 –- 163
I –- 480 –- 326
J –- 960 –- 652
K –- 1920 -- 1305
L –- 3840 –- 2609
M –- 7680 –- 5219
N –- 15360 –- 10438
O –- 30720 –- 20875
P –- 61440 –- 41751