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Model Rocket Classification System

guide Propulsion rocket Rockets

With so many different sizes and types of rocket motors on the market today, it can be a daunting task for new and aspiring rocketeers to find the right rocket motors for their first DIY space projects. Knowledge is power, and understanding the model rocket motor classification system can help save you time and money in finding the right motor for your rocket propulsion needs. The following is a breakdown of everything you need to know to understand the model rocket classification system.

How to Read the Rocket Motor Code

Before we can dive into the actual list of model rocket motors, it is important to understand how to read the three-part code that accompanies commercial rocket motors. Each code consists of a letter specifying the total impulse, a number specifying the average thrust, and a final number specifying the time delay between burnout and recovery ejection. For a standard A8-3 rocket motor, the letter “A” corresponds to a total impulse of 1.26-2.50 N-s. The number 8 specifies an average thrust of 8 N, and the final number indicates a 3 second delay before the recovery system or parachute is activated. If there is a fraction in front of the letter, it implies that the motor has that fraction of power of the “A” motor. A 1/4A motor has a quarter of the power of an A motor while a 1/8A motor has an eighth of the power. It is important to remember that the specs within the rocket motor code are merely guidelines and that the actual test data for the total impulse and thrust of any NAR certified motor is available on

Total Impulse

In model rocketry, motors are classified based on their power or total impulse. Total impulse is the product of a rocket motor’s thrust and the duration of time over which said thrust is being produced.  Total impulse is a vector quantity that has units in Newton-seconds (N-s). In laymen’s terms, total impulse is the total change in linear momentum that an engine can impart on its rocket. Because the total impulse is independent of the level of thrust produced, the type of propellant used or how the propellant burns, it makes an excellent indicator for comparing different rocket motors.

Choosing the Right Level of Thrust

After total impulse, thrust plays an important role in determining the potential applications of a given rocket motor. Factors such as the weight of the rocket, its durability and the desired flight path must be considered when choosing the right thrust level. In general, heavier rockets require higher levels of thrust in order to reach the speed necessary to achieve the stability necessary to coast. Lighter rockets on the other hand, can be ripped apart at higher thrust levels and require a lower level of thrust. A low thrust rocket also benefits from a greatly reduced drag force; experiencing four times less drag than a model traveling twice its speed. As a result a low thrust motor will carry a rocket higher than a high thrust motor of equal total impulse, provided that weight is not an issue.

Time Delay and Recovery System

The recovery system or parachute needs to be activated at the apogee or highest point of a rocket’s flight path. This typically occurs a few seconds after the propellant has been burned off and the rocket has been allowed to coast. Activate the parachute too soon, and you risk shredding your parachute. Likewise activate it too late, and it may not be able to save your rocket from the fall. Delay times depend upon the weight, air drag, launch angle, and flight stability of the model.  A computer simulation may be used to approximate the ideal time delay of a model’s flight.

Model Rocket Classification System

As we mentioned earlier, the model rocket classification system is based on the total impulse of the rocket motor. An arbitrary starting value of 2.50 N-s was assigned as the maximum possible value for a category “A” motor. As you move up the tier in power levels, every corresponding level sees a doubling in power, such that a “C” size motor has twice the power of a “B” size motor and four times the power of an “A” size motor. Likewise, at its maximum value, a “C” motor should therefore lift a model rocket twice as high as a “B” motor and four times as high as an “A” motor. It should be noted that each level represents a range. A “C” motor may have a maximum total impulse of 10.00 N-s, while a motor with a measured thrust stand of 10.01 N-s would be considered a “D” motor.  The following table provides a list of model rocket motor classes from A – O: [caption id="attachment_4057" align="alignnone" width="237"]Model Rocket Classification System Model Rocket Classification System[/caption] The above table may be extended as far as necessary to encompass even larger rockets.  You should now know how to read a model rocket motor code and understand the roles that total impulse, thrust and time delayed recovery systems play in model rockets. Understanding the model rocket classification system is only the first step towards building your own rockets.  If you would like to dive further into the realm of DIY space, check out some of our articles below to learn more about model rocketry. Image cc Flickr via Jurvetson

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