Definition of burnout

Is it when the motor/engine is completely spent or when the motor engine has reached it's peak thrust?

It's when all of the propellant is exhausted. Sometimes it's a little hard to define empirically, however, because some motors peak very quickly then have a relatively small sustained thrust for a longer period of time. See the Estes A10-0T thrust curve, for all practical purposes it's done at about .25 secs but the actual burn time is closer to 1.05 secs. This kind of thrust curve can play havoc with electronics that try to use burnout to trigger events, such as an airstart ignition.

ArthurSull said:

Is it when the motor/engine is completely spent or when the motor engine has reached it's peak thrust?

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The offical NFPA 1125 specification is when the thrust drops down to 5% of the peak value.

John

Thanks for the clarification. It would probably be more useful if the spec was for 95% of the total impulse, but it is what it is.

Johnly said:

The offical NFPA 1125 specification is when the thrust drops down to 5% of the peak value.

John

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I think of burnout as process where the motor goes from burning fuel to produce thrust. Say, A running motor.....
to a point where the motor "shuts down". this is inherantly characterized with instability, and big changes in production of thrust.

To say that its a number is necessary for a certifier. But in real world "working" it really will vary big time based on motor engineering and propellant types.

For example some propellants and motors, such as a fynocl with a moderate burn rate, will have a longer trailing tail of burnout. A bates will just shut down, with maybe a crackle or puff.

For me, i consider burnout where there is not any motive thrust being produced, but burning may still be occuring. MFG and NFPA obviously differ from that in thier thrust curves.

ClayD said:

For example some propellants and motors, such as a fynocl with a moderate burn rate, will have a longer trailing tail of burnout. A bates will just shut down, with maybe a crackle or puff.

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Not necessarily. Here's a finocyl thrust curve (with a moderate burnrate): https://pro38.com/products/pro150/motor/images/29920O3700-P.pdf
Here's a bates: https://pro38.com/products/pro54/motor/images/2833L805-P.pdf

cjl said:

Not necessarily. Here's a finocyl thrust curve (with a moderate burnrate): https://pro38.com/products/pro150/motor/images/29920O3700-P.pdf
Here's a bates: https://pro38.com/products/pro54/motor/images/2833L805-P.pdf

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i do not know enough about the geometry of either of those motor to know the burn rate. HOwever, a finocyl has both the fin geometry, and the core geometry to taylor the burn profile. All of mine have had trailing curves.
Bates, is not intended to have a tail. it is meant for a neutral burn. The L curve you linked has some initial instability, and is probably from erosive burning. thus the tail. design it correctly without the erosive burning, and the tail will go away.

also Bates geometry is a specific ratio.... a regressive burn motor is not a bates....