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Rocket Motor Information

How To Interpret Rocket Motor Codes

Sport rocket motors approved for sale in the United States are stamped with a three-part code that gives the modeler some basic information about the motor’s power and behavior:

C6-3 Rocket Motor

  • A letter specifying the total impulse (“C”);
  • A number specifying the average thrust (“6”); and
  • A number specifying the time delay between burnout and recovery ejection (“3”).

 

Total Impulse

Total impulse is a measure of the overall total energy contained in a motor, and is measured in Newton-seconds. The letter “C” in our example motor above tells us there is between 5.01 and 10.0 N-sec of total impulse in this motor.

In a typical hobby store, you will find motors in power classes from 1/8A to D. However, E, F, and some G motors are also classified as model rocket motors. Modelers certified for high power rocketry by the NAR can purchase motors ranging from G to O.

Each letter represents twice the power range of the previous letter, total available power increases rapidly the further you progress through the alphabet.

Hobby Rocket Motor Information
Classification Impulse Range Impulse Limit Category
Model Rocket 1/8A 0.3125 Micro
1/4A 0.625 Low Power
1/2A 1.25
A 2.5
B 5
C 10
D 20
E 40 Mid Power
F 80
G 160
High Power H 320 Level 1
I 640
J 1280 Level 2
K 2560
L 5120
M 10240 Level 3
N 20480
O 40960

 

Average Thrust

Average thrust is a measure of how slowly or quickly the motor delivers its total energy, and is measured in Newtons. The “6” in our example motor tells us that the energy is delivered at a moderate rate (over about 1.7 seconds). A C4 would deliver weaker thrust over a longer time (about 2.5 seconds), while a C10 would deliver a strong thrust for a shorter time (about 1 second).

The average thrust printed on the motor may differ greatly from the actual average thrust of the motor. You should check the engine data sheets at https://www.nar.org/standards-and-testing-committee/nar-certified-motors/ for an accurate value. Just click on the motor designation for a particular motor to get a sheet with the actual as-tested numbers for every NAR certified motor.

As a rule of thumb, the thrust duration of a motor can be approximated by dividing its total impulse by its average thrust.

Keep in mind that you cannot assume that the actual total impulse of a motor lies at the top end of its letter’s power range — an engine marked “C” might be engineered to deliver only 5.5 Newton-seconds, not 10.

 

Time Delay

The rocket is traveling very fast at the instant of motor burnout. The time delay allows the rocket to coast to its maximum altitude and slow down before the recovery system (such as a parachute) is activated by the ejection charge.

The time delay is indicated on our sample motor is 3 seconds. Other typical delay choices for C engines are 5 and 7. Longer delays are best for lighter rockets, which will coast upwards for a long time. Heavier rockets usually do better with shorter delays — otherwise the rocket might fall back down to the ground during the delay time.

Motors marked with a time delay of 0 (e.g., “C6-0”) are booster engines. They are not designed to activate recovery systems. They are intended for use as lower-stage engines in multi-stage rockets. They are designed to ignite the next stage engine immediately once their own thrust is finished. Often their labels are printed in a different color to help prevent you from using them in a typical rocket. In a multi-stage rocket, you would usually select a very long delay for your topmost engine.

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    • Safety
      ▼
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      • Model Rocket Safety Code
      • High Power Rocket Safety Code
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    • Rocket Motor Information
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      • Certified Motors
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        • Motor Related Lists
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      • Manufacturer Notifications
      • Report a Malfunctioning Motor
    • Contest Flying – NAR Competition
      ▼
      • National Scoreboard
      • US Team Selection Scoreboard
      • Find a Contest
      • Register an NAR Launch
      • Competition Results Entry
      • Team Registration
      • Teams List
      • Contest Events
      • Competition Meets
      • US National Model Rocket Performance Records – 8/2/19
      • International Competition
      • Competition Guide
      • U.S. Model Rocket Sporting Code
      • Rules Revision Process
    • Sport Rocketry Magazine
      ▼
      • Magazine Submissions
      • Write It and Get Paid
      • Advertise in Sport Rocketry
      • Sample Issue
      • Contact Sport Rocketry
    • Member Resources
      ▼
      • Members Only Forum
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        ▼
        • Sport Rocketry Magazine
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        • The Model Rocketeer Magazine
        • Model Rocketry Magazine
      • NARTREK Skills Program
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        ▼
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