Rising Tails for Bombettes

[AdmiralDonSnider]

The story:

 

I´m currently making 30mm bombettes using a special tool that allows me to press a pre-perforated clay plug into casings of 1mm wall-thickness. I got them working as desired, so I decided to try adding some rising effects which are so common in commercial units.

 

For this purpose my tooling forms 3mm of empty space below the clay plug. This cavity can be filled with dampened composition which is "buttered" in place with a flexible spatula to make sort of a squat comet or pillbox star.

 

For yesterday´s test I decided to use some tailed comp coming from Hardt, a bright streamer which basically is a medium fast burning charcoal star with titanium added: KNO3 54, C airfl. 25, S 7, Ti 20-40 mesh 7, Dextrin 7. Once the comp was buttered into the end, I left it alone until dry and primed the thing using NC-based BP slurry and a dip of 5FA.

 

The problem:

 

I fired four of these bombettes and the rising tail never succeeded. Instead there was sort of a starmine at the beginning of each units flight (probably going 5 meters high), telling me that the tracer comp had been ignited well, but somehow fragmented or separated from the unit.

 

I tried to nominate possible reasons for that behavior:

 

a.) Wrong comp: My bright streamer comp had been a bad choice, because it burns too fast, is too brittle or both. If it burned too fast, the squat tracer star burns out very early and also looses his integrity, which causes pieces to separate.

 

b.) Tracer destroyed by lift: I also figured out that the tracer comp could possibly have been destroyed or fragmented by the shock of the lifting charge. However, even very plausible at first sight, this theory is hard to believe in practice, mainly because in the case of bombettes there is a perforated disk between the lift and the unit meaning that the bombette is pushed out by the disk, not by the direct action of hot gases. Against this background it´s hard to believe that the force exerted by the disk had destroyed the star.

 

c.) Wrong method for attaching the tracer: As mentioned I´m currently applying damp composition using my fingers and a spatula. This, when dried, is expected to give an encased, rock hard star, even if the star is only 3mm in thickness. This method is very comfortable and seems to be used in commercial units also. I´m trying to avoid the use of mechanical pressure to apply the tracer comp.

 

 

I´m very thankful for any idea which helps me avoid a long trial and error process! Thanks for any help.

Edited August 20, 2009 by AdmiralDonSnider

[Sambo]

You could try a binder like gum arabic, stars bound with this are rock hard. Another option would be to bind with NC or use the poly glue star composition.

Pressing a damp composition on top of a clay plug may cause the clay to absorb some moisture and expand slightly, the bond between the clay and comp may be broken causing it to break up on lift. You could try adding some wax to your clay to make it less moisture absorbent?

[FrankRizzo]

Don,

 

Try loading the bombettes with the other end down. The star comp should light fine from the lift blow-by if well primed, and you'll be able to determine if the comp is too brittle to survive the lift or if the comp is burning too quickly.

 

Edit: Also, how are you securing the star comp into the casing? You may need to figure a way to bond the comp to the tube.

Edited August 20, 2009 by FrankRizzo

[AdmiralDonSnider]

Thanks to both of you!

 

@ Sambo: I´m conscious of the fact that my clay will absorb a certain amount of moisture, especially because my clay mix contains 50% bentonite. However, by now I´m using a minimum amount of water/alc to damped the tracer comp, makes a consistency somewhere between pumped and cut stars.

 

However, I tried to verify the "moisture-gap"-theory and disassembled another unit that uses the tracer comp from Hardt. The clay plug came out rock hard and the tracer star was still adhering well to it - there was no gap. Concerning moisture absorption, the color of the clay mix located next to the star was slightly deeper, telling me that some minimal water absorption had taken place as expected. If it really is this what breaks the performance I´ll have to consider treating my clay, but I consider this tedious and try to avoid it.

 

I´ll try a NC-bound version in any case, as this sounds like a good idea.

 

@FrankRizzo: I considered that myself and it seemed to be a particularly good idea - like an adoption of a technique used in case of aerial shell tracers, which are always on the top side but always light. Furthermore, it is a very good way to protect the star side from being damaged.

 

But there is one major point of concern (I might be wrong however): the perforated disk, that separates the unit from the lift charge, meaning that there is only a small (2mm) hole where the fire jets through. And this jet would be directed at the cardboard plugged end... Just wonder if it would fill the "chamber" between the lower disk and the upper wad/disk with fire and hot gases in a way assuring ignition of the star´s end, even if well primed.

I guess I´ll have to give it a try.

Edited August 21, 2009 by AdmiralDonSnider

[andyboy]

I've noticed that comps with high charcoal content are more liable to break up if not pressed well.

[AdmiralDonSnider]

This may indeed add to its behavior, Andyboy.

 

But I figured out that the main reason for the separation is the burning rate of the composition I chose for the tail. Today I did a dry run using the clay plug+star I disassembled to see if there was some moisture. It took the tracer comp less than a second to burn out completely (it fragmented before it did that, I guess that´s what makes my mines) and the 5FA coat probably adds to the quick burning as well.

 

I reviewed the comp from Hardt and was surprised that it contained as much potassium nitrate and sulphur as Shimizu´s C6, which is expected to give a very fast burning star in chrysanthemums. That´s way too fast for a tracer comp I guess.

 

I´ll give Tiger tail a try soon, which should burn considerably slower. I assume perchlorate color would work well, but I prefer tailed stars for the ascending effect. Seems that I will have to look for tailed comps with a slow burning rate. Any ideas?

 

I should probably also increase the lenght of the tracer cavity by the adjustment of my next tooling; 3mm turned out a bit sparse for many comps.

Edited August 21, 2009 by AdmiralDonSnider

[Mumbles]

Assuming the bombettes are designed to have at least a couple seconds of delay before they burst, it might be of interest to try some brocade compositions. 3mm shouldn't take all that long to burn up, even with something so slow burning. It may be a rather cool effect to fire a volley, have them all burst, and leave a nice shimmering wall of the brocade.

 

If you don't want to have to apply hydraulic pressure, perhaps a dowel will do. After you have it in there, pressing down on it with a dowel or rod might give a little more even compression and tighten it up a bit. Just hand pressure is what I would suggest at first.

[AdmiralDonSnider]

That´s indeed a good suggestion, Mumbles. Aluminum tails would also be nice, e.g. produced by silver wave, which is another classic for the kyoku-do. Just a pity it consumes a lot of flitter.

 

By now I´ve some units drying that use Veline Green (with some Ti) as the tracer effect. This comp should pack well and burn long enough, so I can see if the 3mm dextrin bound tracer star design is the major problem or not. I will report the results as soon as I fired them.

 

In the meantime thanks to all of you!

 

P.S.: I forgot to mention the delay time before the burst, which is 1,5 sec.

Edited August 22, 2009 by AdmiralDonSnider

[AdmiralDonSnider]

By now I´ve some units drying that use Veline Green (with some Ti) as the tracer effect. This comp should pack well and burn long enough, so I can see if the 3mm dextrin bound tracer star design is the major problem or not. I will report the results as soon as I fired them.

 

Yesterday´s tests showed that the design works well given:

 

a.) the composition doesn´t burn too fast and therefore does not disintegrate during flight, and

 

b.) the comp has been compacted well by means of firm hand pressure, or better, like Mumbles proposed, by means of a dowel, which aids to the compaction.

 

Veline Green was a good choice and burned all the way up. Of the two units I fired, one tracer star fragmented slightly while rising (I wouldn´t say it was an unsightly effect, as parts of the comp adhered well until the burst); the other, which was more carefully compacted during manufacture, worked perfectly well.

 

I´ll start experimenting with tailed comps now. I´ll also give gum arabic (and NC) a try, just to see if the added cohesion of the star prevents fragmentation.

Edited August 23, 2009 by AdmiralDonSnider