- The first letter is the total amount of energy in the motor. Here a unit called the Newton-second (Ns) is used. 4.45 Ns = 1 Lbs (pound-second). So, an "A" motor has 1.25 to 2.5Ns in it. Not a lot, but it can send a rocket dozens of feet up. A "B" motor has up to 5Ns, a "C" 10Ns, etc. A "1/2A" has up to 1.25Ns.
- The second figure, a number, is the mean thrust in N. For example, a B6 motor puts out 6 N, more or less. You can then figure out the burn time: B(10Ns) / 6 = 1.7s.
- The last number is the delay time, from when the motor burns out until when the charge fires that pops the nose cone off and opens the 'chute and allows you to get and fly your rocket again. So, a D21-7, which I will use in my "3M" rocket to send it to the speed of sound, has 20Ns total energy, 21N mean thrust, and a 7-sec delay. Easy, huh?
This short word idea for a hard idea is not mine; it comes from here.