71 replies to this topic

[AdmiralDonSnider]

In my recent pyro life I especially focussed on bombette cakes and did some investigation in commercial cakes. I found out that the construction principles of the bombettes (cake inserts) are always the same. The results of my studies can be found here: http://www.pyroguide...and_Comet_Bombs

The units always contain a portion of free comp in powdered form in addition to the stars. In some cases the units contain microstars/small granules and powder. However, with the lack of scientific instrumentary I never succeeded to analyse the chemical properties of the free powder comp, but in most cases it looks like it contains fine metals.

I focussed on a simple break of stars: the unit (http://img161.images...mage=piccq8.png) then contains some comparably large (pumped) stars and again: some free powder (generally about 1g) . Does anybody know what kind of comp that is? Flash?
I assume the powder largely contributes to the break energy of such a small (and rather thick-walled) device. The simple fact is I fired some units where I tried to employ meal D as a "substitute" of free comp: they never gave a good break.

How do I successfully break such small inserts? What is the deal with this small portion of comp? Any ideas/experiences?

P.S. It´s a pity that no classic fireworks source contains valuable information about how to construct cake inserts. Sources like passfire (the version I know) and others - e.g. some UKPS users that succeeded in making some good cakes - focussed on salute inserts where getting a good break isn´t the issue.
However I also try to avoid a method of construction similar to Fulcanelli´s insert shell where the case is filled with a mix of stars and grain powder - the question is: why does no commercial bombette need to employ grain charges for a good brake...?

Thanks a lot!

[portfire]

Excellent work there AdmiralDonSnider

The powder will most certainly be flash and I'd say the standard 70/30. The reason for using flash powder, is it's shear breaking force and enabling a wide burst, BP will just not cut it in such small inserts and shells for that matter.

Did you test the powder.i.e. A small loose pile?

Edited by portfire, 01 September 2008 - 10:05 AM.

[Sambo]

I once split a small cake that failed to fire (visco was damaged where it was threaded between the tubes). The inserts contained what i would describe as slow flash, very similar to the nitrate flash used to boost shells. I wouldn't have thought it would be used commercially though due to a possible nitrate Al reaction.

I have successfully used nitrate flash to break small kinder egg shells for rocket headers though. Heres the vid.

[Mortartube]

I agree that the burst is most likely to be standard flashpowder. I have made Bombettes by rolling a tube the right size to go into the mortar and covering one end with masking tape and putting in a little resin to form a plug. When set, put in stars and flash, then glue in a pillbox star into open end of the the bombette tube with pva glue. I used tigertail (for the pillbox star that would form the rising tail) and timed the burn to get the right length of pillbox spool.

The burst wasn't perfect, but I only played about once and never revisited this project. I believe with a little tweak it will work. Mine were 24mm O.D bombettes if I recall correctly.

[pyrotrev]

Most of the bombettes I've seen broken open (due to cake failures - opening such a device up unless you've soaked it VERY thoroughly would be VERY dangerous, not to mention a bit illegal) seemed to use a flash mix that used fine bright aluminium rather than the ultra-fine dark ali used in salutes. Whether this is to slow it down a bit, for better storage or purely an economic decision I don't know.

[AdmiralDonSnider]

Thanks a lot for the help! Seems that everyone here agrees the free powder is flash in one or the other variety. That´s what I assumed but I never tried a flash break in a bombette. Up to now I´ve described the comp as "additional prime or star composition" but that had most likely been a mistake.

I will securely try to burn a bit of the misterious silver-grey powder to find out some of its properties. Due to its color and the silver residue it leaves on ones fingers I assume the powder indeed is some kind of weaker (compared to 70/30) flash composition employing more or less fatty bright Al. As most cakes are chinese in production and as the chinese are known to use these kinds of flash the assumption could turn out true...

Unfortunately the charge is not the only issue when making bombettes. Ones case design can be responsible for what you say it´s your loads fault and a heavy charge does not help if ones clay blows out easily. I keep working on all these...

[Sambo]

Pyrotrev I believe you are right with the bright Al flake. I put This together earlier comparing 70/30 and the bombette flash.
Although, I mixed a gram of 70/30 using bright flake Al it burnt alot slower than the flash from the bomette I think there must be something else present, maybe sulphur to increase the burn rate slightly.

Edited by Sambo, 01 September 2008 - 05:40 PM.

[MDH]

No, it's just standard 70/30. The flake aluminum you are talking about is common in the consumer pyrotechnics industry. It's very cheap and much safer to use.

The use of Nitrate flash DOES exist in consumer fireworks although not in more regulated countries. Nitrates and aluminum exist in many places even in more regulated countries however (Such as fountains, stars, certain kinds of sparklers).

Edited by MDH, 01 September 2008 - 08:02 PM.

[dr thrust]

sambo, your bright flake al May be of the Stearin coated variety, if so it'll be slower than uncoated, ive used both for flash cores, you can tell the difference

[Arthur Brown]

Every bonbette leaves a bit of paper tube, Every fired insert shows signs of the contends. Get or refer to Vogel Quallitative analysis and do the spot tests for Aluminium etc.

http://www.abebooks....ative inorganic

You don't need a pristine copy as lab books get dirty too easily. Also the older editions will rely on chemical tests wheras the newer books may suggest expensive instrumental techniques.

[Sambo]

sambo, your bright flake al May be of the Stearin coated variety, if so it'll be slower than uncoated, ive used both for flash cores, you can tell the difference

Yeah i think it must be, has anyone got a supplier for uncoated flake Al, was Phil's stuff coated?

Edited by Sambo, 02 September 2008 - 06:33 PM.

[AdmiralDonSnider]

As most of you anticipated the powder from the bombette was flash - no doubt. It uses bright uncoated flake, no dark pyro. However I tried to replace it 1:1 with a 70/30 flash using coated Al. The unit broke violently and most of the stars (silver chlorate, easily ignitable) were blown blind.
I haven´t had the time to test some more different loads, but I assume I just have to use less flash.

Maybe someone has experience with the correct amount of flash to break a 2mm walled unit...

[bigtonyicu]

As most of you anticipated the powder from the bombette was flash - no doubt. It uses bright uncoated flake, no dark pyro. However I tried to replace it 1:1 with a 70/30 flash using coated Al. The unit broke violently and most of the stars (silver chlorate, easily ignitable) were blown blind.
I haven´t had the time to test some more different loads, but I assume I just have to use less flash.

Maybe someone has experience with the correct amount of flash to break a 2mm walled unit...

I wonder if you just didn't shatter the stars, can you give use some specs on the shell (star size, OD of the shell, flash qty.)

[Bonny]

As most of you anticipated the powder from the bombette was flash - no doubt. It uses bright uncoated flake, no dark pyro. However I tried to replace it 1:1 with a 70/30 flash using coated Al. The unit broke violently and most of the stars (silver chlorate, easily ignitable) were blown blind.
I haven´t had the time to test some more different loads, but I assume I just have to use less flash.

Maybe someone has experience with the correct amount of flash to break a 2mm walled unit...

I've had chlorate stars blown blind by bursting too hard. I remember reading of someone who uses (IIRC) a slow flash as prime to ensure good ignition with very hard breaks. Also as bigtonyicu asked, some specs would be a great help. If you are using 70/30 then a small amount can break bombettes pretty hard. I've made 1" salute shells with 2mm wall and used 10g 70/30 and they make quite a boom.

[Creepin_pyro]

"I remember reading of someone who uses (IIRC) a slow flash as prime to ensure good ignition with very hard breaks."

I'm convinced the Chinese use a similar technique in some bombettes. Looking at some tiny stars in crosssection they definitely have an outer layer of dirty flash. The stars were packed in the casing very tightly Poka style and there was a small amount of 'dirty' flash loose, not quite filling up the voids between the stars.