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Delay grain.


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#1 Crimson0087

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Posted 16 October 2018 - 06:47 AM

Any advice on making delay grains. My last motor I believe that the burn somehow bypassed the delay and ignited the BP. Looks like the delay grain continued to burn after the BP went off. Also I was just using kno3 and sugar mixed with baking soda? Any suggestions?



#2 Davidg1

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Posted 16 October 2018 - 01:51 PM

Is the core too deep? I had a similar issue with whistle rockets until I
lessened the core depth. If was great watching 4" shells bursting at less than 15' altitude.

#3 CaptHaywire

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Posted 22 October 2018 - 04:59 AM

I found the pressed KNO3 - Sugar will not tolerate the pressure from the motor exhaust and was not reliable for me. I then moved to KNO3 - Epoxy but this mix tended to flame out even when adding RIO. Finally, I settled on Ammonium - Perchlorate and Devcon epoxy in a 60/40 ratio, a 70/30 ratio worked but burned to aggressively. Depending on mesh a 1/2 plug gave me around 6 seconds or so delay. Great care must be taken to seal the delay grain so it is not breached by the exhaust at high pressure. I hope that helps.


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#4 dagabu

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Posted 03 November 2018 - 07:55 PM

Delay grains are counter intuitive, you need a longer delay grain to contain the thrust inside the motor casing so that you have at minimum, 1 full ID thick. People seem to think they want to make the delay burn longer... That's the wrong direction, think of how you can make the delay grain LONGER and burn at a rate that gives you the delay timing you want. 


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#5 CaptHaywire

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Posted 04 November 2018 - 12:00 PM

No matter the length of the grain it's the seal between the inhibitor and the delay that is critical. High combustion heat along with pressure will force its way on the outside of the delay travelling up the side and set off the ejection charge. I am building PVC motors and use epoxy to seal the delay into the motor. The 70/30 or 60/40 AP combination yields a delay that will not go out and is tough enough at 1 full ID thick as dagabu pointed out to withstand great pressure without cracking. I also rough up the inside of the inhibitor so the epoxy will adhere better and sandwich the delay between fiber washers then an insert is placed above it to hold everything inside so it doesn't blow out.

 

I am surprised there is little information on this topic in the APC forum.



#6 dagabu

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Posted 09 November 2018 - 01:44 PM

...I am surprised there is little information on this topic in the APC forum.

 

Because we stay away from PVC due to its nature to fragment into slivers, it's being plastic and it's inevitable softening when heated blowing out nozzles. 

 

Paper is better in every way and since this is a pyrotechnic forum, we all tend to stay with consumable items that biodegrade.  


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