Basic Rocket Motor Knowledge

About Rocket Motors

Rocket motors come in many sizes. They are measured in total impulse and are designated by letters of the alphabet. Each letter designates a range of total impulse and each motor size contains twice the total impulse of the previous letter. For example a B motor has a potential total impulse twice that of the total impulse of an A motor; a C motor has twice the potential total impulse of a B motor, etc. Rocket motors larger than G are considered High Power; use of them requires certification by Tripoli and/or NAR.
Low Power Rocketry
1.25Ns ⟹ 2.5Ns >2.5Ns ⟹ 5Ns >5Ns ⟹ 10Ns >10Ns ⟹ 20Ns
Mid-Power Rocketry
>20Ns ⟹ 40Ns >40Ns ⟹ 80Ns >80Ns ⟹ 160Ns
High Power Rocketry
Certification Level 1
>160Ns ⟹ 320Ns >320Ns ⟹ 640Ns
Certification Level 2
>640Ns ⟹ 1280Ns >1280Ns ⟹ 2560Ns >2560Ns ⟹ 5120Ns
Certification Level 3
>5120Ns ⟹ 10240Ns >10240Ns ⟹ 20480Ns >20480Ns ⟹ 40960Ns

Their potential power is indicated by a letter. However a motor of any given energy range might be configured to deliver only the middle of energy range rather than the full amount. That’s because you don’t always want the full amount. The motor manufacturer’s charts give you this information. The letter is followed by a number. The number tells you what the average thrust is. An example would be an I170 or an I240.
We are used to speaking in terms of horsepower and such, however, that’s only part of the equation, the amount of time applied given the amount of horsepower is needed to figure the total energy used. In rocketry, we use a different measurement and it is related to the total energy of the motor, i.e., power and time. Rocket motors are rated in Newton-seconds, however if you divide it by 4.45 and you will get pound-seconds. In other words so many pounds of force for one full second.
Hopefully, you are not lost and have acquired a good grasp of the concepts discussed to this point. To help you better comprehend all of this, here we will give you a little contrived comparison that makes it simple to understand.
Think of the term Newton•seconds (or pound•seconds) as a way to express the amount of energy in four ounces of gasoline.  For this example, assume that one ounce of gasoline has 50.7 Newton-seconds of energy — in reality, gasoline possesses significantly more energy — and therefore four ounces of gasoline have 202.8 Newton•seconds (or 45.6 pound•seconds) of energy. With it, you could propel a car very fast for a brief time or slowly for a longer time. Either way you’ll burn up four ounces of gasoline and consume the same amount of energy (202.8 Newton•seconds or 45.6 pound•seconds). The choice is in how to use the energy, fast and brief, or slow and long.
Let’s assume you want to use an F motor to launch a rocket and you’re choosing between a F25 motor or a F50 motor. Both are powered motors and both motors have approximately the same 77 N•s (Newton•seconds) of
F50:ThrustF25:ThrustAeroTech® F50 —vs— AeroTech® F250. (Seconds)Thrust (Newtons)
energy in this example. As stated previously, the number after the letter refers to the motor’s average thrust, so the two F motor examples have a thrust of either 25 or 50 Newtons (not Newton•seconds). Divide 25 or 50 Newtons of thrust by 4.45 and you’ll get pounds of thrust. So the F25 produces an average of 5.6 pounds of thrust and the F50 produces an average of 11.2 pounds of thrust. If you had  a one pound rocket you can now see the ratio of thrust to the weight of the rocket. The choice is again, fast and brief or slow and long. In general, the safe minimum lift-off weight ratio is 5:1. (ie. 5 lbs. of thrust for every 1 lb. of rocket.) Because the total energy contained in both these F motors is 77 Newton•seconds (or 17.3 pound•seconds) it means the F25 will burn for 2.3 seconds and the F50 for 1.3 seconds. A lot of thrust for a brief period of time or half as much thrust for twice as long a period. For some detailed motor statistics go to www.thrustcurve.com.
Interpreting Rocket Motor Codes
Model rocket motors approved for sale in the United States are stamped with a three-part code that gives the rocketeer some basic information about the motor’s power and behavior. For example, an Estes B motor might be stamped:
  • Impulse Class >2.5Ns ⟹ 5Ns  
    • The letter B specifies the impulse class.
  • Average Thrust 6 Newtons  
    • The number 6 tells the average thrust is 6 Newtons.

         6 Newtons equates to 1.3 pounds of thrust.

  • Ejection Delay 4 Seconds  
    • The 4 gives the delay time between motor burnout and the firing of the ejection charge for recovery.
In High Power Rocketry, the codes vary slightly from one manufacturer to the next. Sometimes they have a letter or description indicating a propellant type that gives off different colors or type of smoke. Yes, in high power you can choose color too.
Delay Time
The rocket is traveling very fast at the moment its motor quits burning. The timed delay allows the rocket to coast to its maximum altitude and slow down before its parachute is ejected by the ejection charge. The ideal delay time for a rocket and a given motor can be calculated. Better yet, use OpenRocket — a free rocket modeling and simulation program — to predict both its altitude and coast time rather accurately.
Booster Motors
Model rocket motors marked with a time delay of 0 (e.g., “C6-0”) are booster engines for multi-stage rockets. They are designed to ignite the next stage’s engine immediately once their own thrust has been exhausted. Booster motors have no ejection charge! Never use a booster motor when motor ejection deployment is needed for recovery.