Thrust to Weight Ratio Calculator
Enter your motor's average thrust and the loaded mass of the rocket to find the liftoff thrust to weight ratio. A ratio of at least 5 to 1 is the model rocketry rule of thumb for a stable, straight boost off the launch rod.
How to find your thrust to weight ratio
- Enter the average thrust of your motor in newtons (printed on the motor data sheet).
- Enter the loaded liftoff mass of the rocket in kilograms or grams.
- Read the ratio. Aim for 5 to 1 or higher for a safe, stable launch.
Examples
Estes A8 class motor on a light rocket
thrust = 10 N, mass = 0.2 kg
weight = 1.96 N, ratio = 5.1 to 1, safe to launch
Same motor on a heavier rocket
thrust = 5 N, mass = 0.2 kg
ratio = 2.5 to 1, too low to launch safely
Frequently asked questions
What is a good thrust to weight ratio for a model rocket?
The widely used rule of thumb is at least 5 to 1. That gives the rocket enough speed leaving the launch rod for its fins to keep it pointing straight.
How do I calculate thrust to weight ratio?
Divide the motor's average thrust in newtons by the rocket's weight. Weight equals mass times g, where g is 9.80665 m/s^2. So ratio = thrust / (mass x g).
What thrust value should I use?
Use the average thrust from the motor's data sheet, not the peak thrust. Average thrust best represents the force lifting the rocket over the whole burn.
What mass should I enter?
Enter the full liftoff mass: the airframe, recovery gear, payload and the loaded motor. This is the mass the motor must actually lift off the pad.
What happens below a 5 to 1 ratio?
The rocket leaves the rod slowly and can weathercock into the wind or go unstable. If your ratio is low, pick a higher impulse motor or lighten the rocket.
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