# Tech Info - What Motor Size Should I Use?

## What Motor Size Should I Use For My Rocket?

For safety, use a 5:1 Thrust to Weight Ratio.

The following formula is used to make sure you have selected a motor that will provide enough thrust to lift your rocket quick enough so the fins will stabilize the rocket after it leaves the launch rod.
Example:

• A G80 motor produces an average thrust of 80 newton seconds. This is a metric measurement, to convert to english divide by 4.5, you get 18, or 18 pound seconds.
• So now we know a motor with an average thrust of 80 newton seconds will produce 18 pound seconds of thrust. Now divide 18 by 5 to get the 5:1 ratio required.
• After dividing 18 by 5 you get 3.6. Therefore 3.6 pounds is the maximum weight a rocket should be , using a motor with an average thrust of 80 newton seconds.
• Yes, it's the same for an F80, G80 or H80, these motors will all have the same average thrust but performance will differ due to their burn times. A 5:1 ratio is used by many RSO's to determine if a rocket could be launched safely. Although the formula will give you a safe starting point, it will not select the perfect motor for your rocket.

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