BP Motors: What's the variation of burn time, per NFPA 1125?

[lakeroadster]

BP Motors: What's the variation of burn time, per NFPA 1125, or is there one?

The total impulse must not have a standard deviation greater than 6.7%.
The ejection delay must not vary more than 1.5 second or 20% (whichever is greater, up to 3s) from average.
The average thrust must not vary more than 20% (or 1N for model rocket motors, 10N for high-power motors, whichever is greater) from average.

 

[jsdemar]

NFPA 1125, Model Rocket Motors.... min of 10 tested samples:

• Total impulse: 6.7% max standard deviation.
• Average thrust: 20% or 1N whichever is greater
• Time delay: 20% or 1.5 sec whichever is greater. Not to exceed 3 secs from labelled value.

NFPA 1125, HPR Motors.... across min of 2 tested samples:

• Total impulse: 6.7% max standard deviation.
• Average thrust: 20% or 10N whichever is greater
• Time delay: 20% or 1.5 sec whichever is greater. Not to exceed 3 secs from labelled value.

 

[rharshberger]

Based on Johns post, burn time is a function of determining the averages only, it is not called out for specific tolerances.

 

[jsdemar]

Based on NFPA 1125:

• The burn time is from 5% of peak thrust, at both the beginning and end of the curve.
• Average thrust is the total impulse during the 5%-defined burn time, divided by the burn time.
• The total impulse is measured over the whole thrust curve (not the 5%-defined burn time).

This means that the letter for the motor class is based on the area under the whole curve, but the value after the letter is based on the average thrust during the truncated burn time.

A common error is to measure average thrust by dividing Total Impulse by the "last action" burn time. This gives a different value than the correct NFPA 1125 method. For motors with long tails, the correct method gives a better indication of the useful average thrust.