Exactly what causes the burst of sparks just after burnout..?

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Tim51

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Hello all

At burnout (or rather very shortly after) motors often appear to 'cough' sparks. This is a shot of CTI Redlightning doing this. On other occasions, particularly with Blue Streak, I've noticed a short dark streak of smoke between burnout and the white tracker smoke. What is happening inside the motor at this point? Does anyone know what exactly causes either of these phenomena?

Just curious..


Burnout sparks.png
 
Interesting! I've seen the black smoke but not sparks.

My theory on the black smoke is it's the liner/casting tubes getting torched as the propellant gets too close to the edge... I haven't heard of c-slots or moonburners doing it though. The sparks might be bits of propellant falling off as the propellant burns out from under them, but that's also just a wild guess.
 
Interesting! I've seen the black smoke but not sparks.

My theory on the black smoke is it's the liner/casting tubes getting torched as the propellant gets too close to the edge... I haven't heard of c-slots or moonburners doing it though. The sparks might be bits of propellant falling off as the propellant burns out from under them, but that's also just a wild guess.
Sounds like a feasible hypothesis.
I've only seen the black smoke on Blue Streak propellant - have you noticed it with other loads?
 
Does the amount of sparks vary with propellant type? I'm wondering if it is slag coming off the nozzle from a high solids propellant.
 
Does the amount of sparks vary with propellant type? I'm wondering if it is slag coming off the nozzle from a high solids propellant.

That's a good point - I haven't noticed it from the ground on, say, White Thunder, Smokey Sam or Green 3, but I having said that I have no onboard footage of those flights to give close up evidence. (Red Lightning and Blue Streak are my favourite propellants). I'll have to experiment with a wider range of propellants with the rockets I have cameras on to test that. Since my original post I too was wondering whether the loss of thrust shakes something loose and the drop in pressure at the base of the rocket draws it out into a trail behind the rocket.
 
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... Since my original post I too was wondering whether the loss of thrust shakes something loose and the drop in pressure at the base of the rocket draws it out into a trail behind the rocket.

This sounds like a significant possibility. Consider that you have a delta in the acceleration from e.g. 10g's towards aft to 2-3g's forward (deceleration). You still have gas volume exiting the casing (delay grain/smoke), so things that get released by the acceleration change would then be pushed out...

:2:
 
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You're seeing small bits of partially combusted/still burning propellant coming out as the pressure quickly drops and the remaining combustion is less efficient/complete. Pretty common in solid motors.
 
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