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Rocket Engine FAQ

What does the first letter mean?

The code on the model rocket engine typically starts with a letter such as A, B, C etc. For some engines a fraction may precede the letter such as 1/4 or 1/2. This letter indicates the engines total impulse range (commonly measured in newton-seconds). Each letter in successive alphabetical order doubles the impulse of the letter preceding it. This does not mean, however that a given C engine has twice t he total impulse of a given B engine, only that C engines are in the 5.01-10.0 N-s range while B engines are in the 2.51-5.0 N-S range. For instance, a B6-4 engine from Estes-Cox Corporation has a total impulse rating of 5.0 N-s. A C6-3 engine from Quest Aerospace has a total impulse of 8.5 N-s.

What does the first number after the letter mean?

The number that follows the letter indicates the average thrust of the engine measured in newtons. A lower number represents a lower average thrust and usually a longer thrust burn (assuming comparable engines have the same total impulse). So a B4 engine with go slower and burn longer than a B6 engine (assuming that they have the same total impulse).

Clustering engines increases both the average and total impulse and thus the velocity of the rocket. For example if you put two B6-4 engines together in a cluster the average thrust would rise to 12 newtons and the total impulse would be classified as a C engine. Staging engines increases the total impulse but does not change the average thrust. For example, if you have a two stage rocket with a B6-0 for the first stage and a B6-6 for the upper stage the average thrust stays at 6. The two engines together would be equivalent of C6 engine.

What does the last number mean?

The delay component from the picture (4) controls when the ejection charge is ignited and the recovery device is ejected. This is the last number in the model rocket engine classification and simply represents a time in seconds. Thus a B6-4 engine will have a delay after the initial thrust of 4 seconds before the ejection charge is fired. A B6-6 will have 6 seconds of delay.

Picking the right delay is pivotal in having a successful flight. Too short a delay and the rocket is still traveling at a high rate of speed when the recovery device is ejected. This may cause damage to the recovery device and/or rocket. A delay that is too long may result in a late recovery device ejection. This can be disastrous for obvious reasons. Usually a longer delay time will be used on a lighter rocket or the top stage of a staged rocket. A shorter delay is good for heavier rockets.