Model Rocket Motor Sizes Explained: What the Letters, Numbers and Codes Mean

How to read a motor code

Take the code C6-5. The letter C is the impulse class, which describes how much total energy the motor delivers. The 6 is the average thrust in newtons, and the 5 is the delay, in seconds, between the end of thrust and the ejection charge that deploys the recovery system. A higher thrust number means more push but a shorter burn for the same class.

Each letter in the series roughly doubles the maximum total impulse of the one before it, so a B motor has about twice the power of an A, and a C about twice a B. That is why a jump of one or two letters can change a rocket's flight dramatically. Always check the instructions that came with your kit or motor for what it is designed to use.

The numbers describe averages and ranges, not guarantees. Conditions such as temperature, rocket weight and how well the motor is installed all change the actual flight, and manufacturers publish performance data for each motor so you can compare them. If you plan to fly more than a few times, keeping a short note of each motor you used makes it easier to see which ones suit your rockets best.

Motor diameters and fit

Motors are made in standard diameters, with 13 millimeter, 18 millimeter and 24 millimeter being common in hobby rockets. A motor must fit the motor mount tube in the rocket body, and you cannot safely force a motor into the wrong size. Kits list their motor mount size, so choose from the compatible ones.

Smaller tubes suit small, lightweight rockets and bigger ones suit heavier models. A rocket designed for 18 millimeter motors will not safely take a 24 millimeter motor. Never modify a motor or its casing, and never use a motor outside what the manufacturer recommends.

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Choosing the delay

The delay number sets when the ejection charge fires. For a good recovery, the charge should fire near the peak of the flight, when the rocket is moving slowly. Too short a delay deploys the parachute while the rocket is still moving fast, which can damage the recovery system. Too long, and the rocket may be falling quickly when it opens.

Kit instructions usually suggest the right motors and delays for each rocket. Heavier rockets typically need shorter delays, since they reach their peak sooner, while lighter rockets can use longer ones. Follow the manufacturer's recommendations instead of guessing.

Wind and altitude also influence the choice. On a breezy day, a rocket with a longer delay or a bigger parachute drifts farther, so builders often pick a smaller motor or a larger recovery area. Check the weather, pick an open field with plenty of space and never fly in high wind, near power lines, trees or buildings.

Matching a motor to a rocket

The most important factors are the rocket's weight and the motor's thrust. A motor needs enough thrust to lift the rocket off the pad at a safe speed, but not so much that the model is overstressed. Kit makers list a recommended motor range, and that is the safest guide for beginners.

Do not choose a motor just because it promises a higher flight. A bigger motor adds speed and altitude, but it also raises stress on fins, body and recovery gear and can carry the rocket farther away. Start with the recommended motors and build experience before moving up.

Weight is easy to overlook. Paint, glue, nose weight and a heavier parachute all add up, and a rocket that is a little heavier than the instructions assume may need a different motor than the box suggests. Weigh your finished rocket if you can, and compare it with the weight range given in the instructions before picking a motor.

Safety basics

Model rocket motors contain propellant, so handle them according to the manufacturer's instructions. Store them away from heat and flame, keep them away from children and never try to take them apart or tamper with them. Launch only where it is allowed, in open space, and follow the safety code published by recognized model rocketry organizations and your local rules.

Use only commercial motors made for model rockets, along with the igniters designed for them. Check the packaging and any local regulations for what you may buy and use. If you are unsure, ask at a hobby shop or a local rocketry club.

Where to go from here

After a few successful flights, you can look at larger kits and higher-power options, but those come with extra rules and may require certification in some places. Many hobbyists join a local club, where experienced members help with launches and safety.

Keep a flight log with the rocket, motor, weather and result. It helps you learn what works and builds a record you can use later. Our rocket kits guide and beginner articles can help you choose a first kit that fits your goals.

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Frequently asked questions

What does C6-5 mean? C is the impulse class, 6 is the average thrust in newtons and 5 is the delay in seconds before the ejection charge fires.

Is a bigger letter always better? No. Use the motor range recommended for your rocket, since a motor that is too strong can damage it.

Can I use a different diameter motor? Not unless the maker offers a proper adapter that is designed and approved for that exact use. Never force a motor into a mount.

What delay should I choose? The one the kit instructions recommend for your rocket's weight and motor. Check the instructions for each model.