Motor Designation
Model rocket motors are designated by code starting with a letter followed by a number then a dash and then a final number.
For example:
C6-3
In the above 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 in seconds after the motor's thrust ends until the ejection charge fires.
Total Impulse
The total impulse of the motor is the total thrust over the entire time that the motor provides thrust. Total impulse is measured in units called Newton-seconds (Ns).
The letter in a motor's designation specifies its total impulse within a range and the range doubles for the next letter. For example, an A motor has a total impulse in the range of 1.26 to 2.50 Ns while a B motor has a total impulse in the range of 2.51 to 5.00 Ns.
| Letter | Total Impulse (Ns) |
|---|---|
| 1/8A | 0–0.3125 |
| 1/4 A | 0.3126–0.625 |
| 1/2A | 0.626–1.25 |
| A | 1.26–2.50 |
| B | 2.51–5.00 |
| C | 5.01–10.0 |
| D | 10.01–20.0 |
| E | 20.01–40.0 |
| F | 40.01–80.0 |
| G | 80.01–160 |
Average Thrust
The first number in the designation is the average thrust of the motor in Newtons (N). For the C6-3 motor, the average thrust is 6 N.
Delay
The number after the dash in the designation is the delay, in seconds, between the motor burnout (when the thrust ends) to the firing of the motor's ejection charge. The ejection charge fires out the top of the motor in order to expel the parachute or other recovery system from the rocket.
Rocket motors with a delay of 0 are booster motors which don't actually have an ejection charge. Instead, the propellant is allowed to burn through the top of the motor to ignite a staged motor above it.
A few motors have the letter P instead of a number for the delay. These are plugged motors with no ejection charge. They are used for special purposes such as in rocket-powered cars.
Suffix
Occasionally, a suffix of one or more letters will follow the designation. The suffix is added to further describe the motor. For example:
E20-7W
In the above case, the "W" identifies the motor as one based on Aerotech's White Lightning propellant.
Common suffixes include:
| Suffix | Manufacturer | Meaning |
|---|---|---|
| C | Aerotech | Classic Enerjet Propellant |
| FJ | Aerotech | Black Max Propellant |
| T | Aerotech | Blue Thunder Propellant |
| T | Estes | Mini Engines (13mm diameter) |
| W | Aerotech | White Lightning Propellant |
Motor Class
Sometimes a motor will be described using a percentage such as:
27% D
In this case, the number represents how far along the range for the letter that the motor falls. In this case, the 27% D motor's total impulse is near the start of the range for D motors.
A 1% D would be barely above a 100% C motor in total impulse while a 100% D motor would almost be an E motor.