How & what? Motorcode  


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Motor table

Impulse:
On every rocket motor there will be a printed code telling you how much thrust it will produce and the duration of the delay phase. Examples of motor codes of some popular motors are A10-3, B4-4 and C5-3. The rocket motors are divided into different total impulse ranges that are identified by means of a letter. In the table below you can find the letter codes that correspond to the impulse ranges:

'Level-0' engines
Class Total Impulse (Ns)
1/2A 0,63-1,25
A 1,26 - 2,50
B 2,51 - 5,00
C 5,01 - 10,00
D 10,01 - 20,00
E 20,01 - 40,00
F 40,01 - 80,00
G 80,01 - 160,00

    The motorcode consists of:
  • A letter specifying the total impulse ("C5-3")
  • A number specifying the average thrust ("C5-3")
  • A number specifying the time delay between burnout of the engine and ejection of the recovery system("C5-3")

Total Impulse

Total impulse is a measure of the overall total energy contained in a motor, and is measured in Newton-seconds. The letter "C" in our example engine above tells us that there is anywhere from 5.01 to 10.0 N-sec of total impulse available in this engine.


Average Thrust

Average thrust is a measure of how quickly the motor delivers its total energy, and is measured in Newtons. The "5" in the example motorcode tells us that the energy is delivered at a moderate rate of 5 Newtons (over about 2.1 seconds). A "C6" would deliver stronger thrust (6 Newton) over a shorter time (about 1.7 seconds), while a "C10" would deliver an even stronger thrust (10 Newton) for only one second.


Theory
As a rule of thumb, the thrust duration of a motor can be approximated by dividing its total impulse by its average thrust.
Let's say: C5 = 10 N.s, then thrust duration = 10 / 5 = 2 seconds.
So an A motor, with a total impulse of about 2.50 Ns, can produce a thrust of 2.5 Newton for 1 second, or of 1.25 Newton for 2 seconds, or 5 Newton for 0,5 second. A few graphs explain this below

Examples of idealised thrust-time curves of A motors.
Notice that the enclosed areas all have the same surface area.

B motors have a total impulse that may be twice as big as that of an A motor. Examples of some idealised B motors appear below.

Examples of idealised thrust-time curves of B motors.

Notice that the enclosed areas have about the double amount of surface area as compared to an A type motor! The number behind the letter, in the code, tells us the average thrust in Newtons. In the previous examples you can see this quite easily. At the B-engine example the average thrusts, from left to rigfht, are 5 N, 2,5 N en 10N. So they are respectively a B5, a B2,5 and a B10 engine.

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 10Ns.


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.
(This is excactly the time between the end of the thrust fase and the ignition of the ejaction fase. See drawings in "How does a modelrocket enging work?" )

The time delay as indicated on our sample motorcode 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 (e.g., "B6-0") 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 (one-stage) rocket. In a multi-stage rocket, you would usually select a very long delay for your topmost engine.


Thrust-Time curves in practice
In practice the thrust-time curves of a motor will be slightly different from those given in the examples above. A motor will always produce a lot of thrust right after it is ignited. In this way it helps the model to accelerate to a safe speed of at least 50 km/h before it leaves the launch pad. At these speeds the fins will stabilize the model safely for the rest of the flight. An example of a typical motor curve is given below. The dotted line represents the thrust-time curve of an idealised (theoretical) motor.

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