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My 2nd Gerb CATO :(


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I have started to get back into my pyro adventure and made my very first couple of Gerbs recently.


My first Gerb was great. it was only a 10mm ID of about 50mm High. Used my modified 1 Ton Arbor to press granulated BP into it and used a 6mm Drill bit by hand to make a nozzle. Lasted like 5 seconds but worked as expected.


Today I made a 19mm (3/4") Gerb, about 90mm in length (around 3.5"). The same granulated BP. Hand drilled a 10mm Nozzle in and fused. Lit it and it started ok, just like my first Gerb but then she exploded. Scared me! lol, but no injuries or fire thank god. Being night time still haven't seen anything of my tube...


Mine happened very much like this one

https://www.amateurpyro.com/forums/index.php?app=core&module=attach&section=attach&attach_id=4593 ( From this thread)


I suspect the reason that the 2nd one failed, was that I was putting too much comp in at a time and pressing it in my arbor. I will try again and hopefully not make the same mistake twice. The 2nd Gerb had approx 20grams of comp in it.


I do have a couple of questions about Gerbs in general if someone could please help me. I haven't been able to find any answers googling as yet....


There is always talk about ramming and pressing Gerbs. What happens if you just fill up the tube with loose comp? does it just spit out sparks low and slowly?? Does it just burn the tube to the ground???


I have seen that most people say the nozzle needs to be about half of the ID of the tube. Bigger holes mean slower longer burns. Smaller nozzles mean faster higher burns. Even though I had a 10mm Nozzle, could of my BP burning cause dross build up which would have blocked the nozzle and caused the CATO? The Cato happened no more than 2 seconds after the Gerb started up. I don't think Dross buildup was my issue.


Can you Press a Gerb with too much pressure? I did read that if a crack has appeared in a tube after pressing it can also cause a CATO. I have to make my own tubes but I do find my tubes are very strong. Compared to the consumer 1.4G Chinese fireworks. their tubes are so flimsy...


If I have a 19mm(3/4") Tube, How many presses should it take to fill up an inch of that tube with comp? Just after a good rule of thumb for the future.


I personally suspect to much comp in each press caused my CATO.



So far, I'm so much better at the chemistry side of my adventure...

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A Gerb is just a slow rocket, if you look in Lancaster's book he gives lots of examples of using BP or meal powder and adding increasing charcoal to slow them as the size increases. While a 10mm tube will make a fine gerb with just powder, a 20mm tube my need 20+% of charcoal mixed into the comp to reduce the speed to a gerb rather than a rocket cato. Also the type of charcoal added is important, some use BP +10% willow charcoal AND 10% pine or hardwood charcoal, the willow slows the burn and the hardwood burns so slowly that the tail grows longer.


A rocket uses the fastest BP that doesn't cato, a gerb is a much longer slower burn with charcoal burning in the tail for visual effect.

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If I have a 19mm(3/4") Tube, How many presses should it take to fill up an inch of that tube with comp? Just after a good rule of thumb for the future.
Maybe one press for every half diameter, so every 10mm. Or 2-3 per inch.


Can you Press a Gerb with too much pressure?
No. But the tube without a rocket sleeve can only take a certain force before it breaks on bends. And that will cause troubles, explosions that is.
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Tried my 3rd Gerb out tonight, Same tube as the 2nd one that exploded.


This time I took 15g of my BP mix. Added 1.5g Milled Horticultural Charcoal and 3.5g of my homemade pine charcoal(-20 + 40 mesh).


So much quiter, lots of orange sparks and so pretty and no explosions!


Thanks for the tips dangerousamateur and Arthur :D Very much appreciated :)

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Nice that you had success. Every change in the additives will change the effect. Fine pyro charcoal will slow the burn inside the tube, different mesh cuts of hard charcoal will change the size and shape of the plume as they burn in the air. Larger diameter tubes will need more slowing charcoal and more plume charcoal. There is a whole world of different gerbes/fountains. Everything is propelled with BP (probably as meal) but there will be so many choices of additives, assorted charcoals are useful as are fine metals and even microstars.


Remember that a rocket is the fastest powder that doesn't cato in the intended tube to get the lifting thrust,-yes they need more slowing as the diameter increases. A gerbe is the same only slower -no thrust is needed but the effect duration is what's needed. A rocket may burn for 1 to 4 seconds, a gerbe may burn for 15 to 60 seconds.


Look at https://www.lemaitreusa.com/products/pyrotechnics/prostage-ii/jets-gerbs-waterfalls-saxons/long-duration-gerbs1/

This shows one commercial range of gerbes and jets where both the height and duration are defined. This firm is well regarded for consistency.

Commercially achievement is when all gerbes lit at one time end at the same time and have the same height. Typically they are called gerbes if they burn for more than one second but jets if they burn for less.

It takes a lot of trial and testing to get heights and durations defined

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