The letter is the total impulse, the first number is the average thrust and the second number is the time delay. The motor in the figure is a class D total impulse engine with an average thrust of 12 Newtons(pounds) and a time delay of 5 seconds.
Total Impulse
The letter indicates the total impulse class of the engine, which is effectively the amount of fuel in the engine. The total impulse is the total momentum change that an engine can impart to a rocket. Total impulse is measured in Newton-seconds (pound-seconds).
The standard impulse class for each letter is shown in the following table.
ClassTotal Impulse Newton-sec
1/4A0.000 – 0.625
1/2A0.626 – 1.25
A1.260 – 2.50
B2.510 – 5.00
C5.010 – 10.0
D10.01 – 20.0
E20.01 – 40.0
F40.01 – 80.0
G80.01 – 160.0
H160.01 – 320.0
Each class is double the impulse of the class below it, so as you increase the class of the engines, you double the amount of fuel each contains and double the amount of momentum you can give to a rocket
Average Thrust
The number following the letter indicates the average thrust of the engine in Newtons (pounds).
Because the amount of fuel in an engine is fixed by the class letter, an engine with higher average thrust burns up its fuel more quickly than one with lower average thrust. As a result, the duration of a burn is roughly equal to the total impulse divided by the average thrust. The graph below shows the typical thrust profile for an engine with an average thrust of about 6 Newtons.
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typical engine thrust profile.
A typical engine starts with an initial high thrust for a fraction of a second, which is useful for getting things moving. It then settles down and burns the remainder of the propellant at a relatively constant rate.
Time Delay
The last number on an engine is the time delay, in seconds, to activation of the recovery system. The propellant in a model rocket typically burns up in about 1 second. At that point, the rocket is still moving upward at a high rate of speed. The time delay allows the rocket to continue up to its highest point before activating its recovery system. The time delay is achieved with a smoke charge that also aides in tracking the rocket. When the smoke charge burns out, it ignites an ejection charge that activates the recovery system.
Time delays are typically 3 to 8 seconds, with short time delays needed for larger heavier rockets and longer delays needed for lighter ones. Do not use too long of a time delay as it may allow your rocket to impact the ground before activating the recovery system. Such impacts endanger the spectators and should be avoided.
Rocket motors marked with a time delay of 0 seconds are booster engines. A booster engine is used in the lower stages of a multi-stage rocket and has no time delay and no ejection charge. Only the top stage in a multi-staged rocket needs an engine with a time delay and an ejection charge.
