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Rosco Bodine
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| Quote: | Originally posted by nitro-genes
| Quote: | Originally posted by Bert
The actual mechanism that causes them to explode is unknown. |
One of these odd things indeed, especially considering the high densities required for these microstars to work properly. While for flashpowder the
opposite is true...
Somehow it must be surface related, since larger microstars behave much different and rarely explode as a whole. The burning outerlayer may be able to
"cook off" a certain volume, given that that inner volume to be heated is not too large or it will not reach this critical temperature...
[Edited on 5-10-2006 by nitro-genes] |
That looks like a thermite type of composition , and it could be that entrapped air and moisture suddenly heated very intensely simply makes the
bindered
granules pop like popcorn , causing a miniature
dust explosion in the locally intense heat .
[Edited on 5-10-2006 by Rosco Bodine]
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Bert
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Don't think so. Too brissant. I've seen a 2mm micro star blow a hole in beer can aluminum. And there is a delay between the ignition of the micro
star and the explosion.
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The_Davster
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Well a CuO/Al thermite can be detonated by blasting cap, perhaps these type of thermites can DDT?
But I have also heard that sodium bismuthate can be used, so perhaps the thermite type reaction is not the only thing at work.
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nitro-genes
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If moisture or entraped air would aid in the explosiveness of these stars then it should be favorable to deliberately entrap air during manufacture.
But the funny thing is they only work when this is not the case! I go with the cook-off scenario, so my guess would be that there should be as little
air possibe present to ensure a good heat conduction. Still, the delay time is so small that it is hardly imaginable that any real heat conduction
takes places. Maybe this is why it only works for stars less than 4 mm with high burning surface to volume ratio...  Anyway the process is not like any other DDT transition, which becomes more likely
with larger quantities of the composition and a lower density. As with flashpowder for example...
And they are pretty powerfull indeed, a 2 mm star can really leave you with badly ringing ears! 
I'll check what "Shimizu" has to say about this...
[Edited on 5-10-2006 by nitro-genes]
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Rosco Bodine
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Sort of veering off topic here , but has anybody tried
loading say a 3/4" bore by 3 or 4 " long paper tube
until it is nearly full of crackling microstars and then loosely fill in the empty space between the microstars with some fast flash , plugged well
and fused ....to
see what sort of salute it might make ?
Empty 12 gauge shotgun shells do have their usefulness
[Edited on 5-10-2006 by Rosco Bodine]
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Bert
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I use cakes that fire inserts made just that way. The effect is a flash break, followed a fraction of a second later by a cloud of micro star
explosions in a spherical pattern.
A lot of people have speculated about this mechanism. I don't think Shimizu addresses it in FAST. It is one of the newer effects, along with AP strobe
(which we THINK we know how works)
These effects whose chemistry is so transient and then end up being quickly dispersed are difficult to analyze. I recall the analysis of the glitter
phenomenon was done by placing cold glass or ceramic plates near to the effect and analyzing the quickly chilled detritus. But micro stars melt into
glass on impact, making this a bit hard to replicate.
Anyway, we're a long ways from MMAN/MMAP now.
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a_bab
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For the Pb3O4/magnalium microstars I can say that the first step is the melting of the whole star; it turns into a melted blob of black matter. The
Pb3O4 alone easily aswell, but the microstar seems to continue the exotermic reaction for a while till the star is fully melted. After a second or two
the melted mass will explode.
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FrankRizzo
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We had a similar discussion on rec.pyrotechnics a while back. I'd intuitively guessed that the crackle reaction was a DDT involving the thermal shock
of the thermite reaction causing the nitrocellulose to go high order. Apparently that's not the case because there are formulas which use dextrin as
a binder
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