Article: 0663
By Robert Krausert, October 2009

Motor Size -vs. Total Rocket Weight (Safe Lift-Off)

Author: Robert Krausert October 2009

Ever wonder if the motor you’ve chosen will be safe for your rocket? Ever wonder how to determine selecting the right motor? This article is a quick method for determining if the motor is acceptable from a minimum thrust for a safe lift off.

This article is to quickly calculate the minimum safe motor size for your rocket. This is not to be considered the final. If your motor is close to the minimum, you need to consider many other factors.

Start off with the motor number. That number is the average thrust in Newtons. Whether a J350 or D12, the number is the average thrust. Using these two example motors, here is how to figure maximum safe rocket weight for lift off.

[ Motor Number / 4.5 / 5.0 ] = Maximum Weight in pounds.

• D12 / 4.5 / 5.0 = 0.533… pounds or 8.5 ounces
• J350 / 4.5 / 5.0 = 15.55… pounds or 248.8 ounces

You must include the motor weight in this calculation. If your rocket weight is 5 pounds and the motor weight is 3 pounds, you must use the combined 8 pounds in your lift off weight.

Hope this helps.