Average Thrust
Average thrust is a measure of how slowly or quickly the motor delivers its total energy, and is measured in newtons. The “6” in our example motor tells us that the energy is delivered at a moderate rate (over about 1.7 seconds). A C4 would deliver weaker thrust over a longer time (about 2.5 seconds), while a C10 would deliver a strong thrust for a shorter time (about 1 second).
The average thrust printed on the motor may differ greatly from the actual average thrust of the motor. You should check the Certified Motors for an accurate value. Just click on the motor designation for a particular motor to get a sheet with the actual as-tested numbers for every NAR certified motor.
As a rule of thumb, the thrust duration of a motor can be approximated by dividing its total impulse by its average thrust.
Keep in mind that you cannot assume that the actual total impulse of a motor lies at the top end of its letter’s power range — an engine marked “C” might be engineered to deliver only 5.5 Newton-seconds, not 10.
Time Delay
The rocket is traveling very fast at the instant of motor burnout. The time delay allows the rocket to coast to its maximum altitude and slow down before the recovery system (such as a parachute) is activated by the ejection charge.
The time delay is indicated on our sample motor is 3 seconds. Other typical delay choices for C engines are 5 and 7. Longer delays are best for lighter rockets, which will coast upwards for a long time. Heavier rockets usually do better with shorter delays — otherwise the rocket might fall back down to the ground during the delay time.
Motors marked with a time delay of 0 (for example, “C6-0”) are booster engines. They are not designed to activate recovery systems. They are intended for use as lower-stage engines in multi-stage rockets. They are designed to ignite the next stage engine immediately once their own thrust is finished. Often their labels are printed in a different color to help prevent you from using them in a typical rocket. In a multi-stage rocket, you would usually select a very long delay for your topmost engine.