Cavity Comets

[Sparx88]

I need to see some examples of tooling needed to make cavity comets like these where they are hollow most of the way. I have seen wolter tools like this but they have to small of cavity. I want them to be like the ones in this pic.

 

 

 

 

[Mumbles]

I'm not sure what there is to see. Just imagine the wolter tooling, just with a larger cavity former on the end.

[Sparx88]

I want to make a set with my lathe and just wanted to be sure I have the best qualities in mind before starting. Although it is a pretty simple tool. So I could just increase/decreas dimensions of the end as illustrated here right?;

 

 

 

 

 

[dagabu]

Since you have the drawing, just use a set of calipers to measure the tool as you see and scale it up/down for your needs. The ratio stays the same in all sizes for that one.

 

-dag

[nater]

The cavity pumps from Wolter have a generous cavity, plenty for color comps which typically burn slow. Once the color comp is buttered in and the comet pasted, they can burn a little long if you're not careful.

 

[Sparx88]

Those look nice. The main type of comets I like to shoot are just single shots out of a 1" gun. I like em fast and high. I want to have 2" long by 7/8 dia with the cavity going deep to 1 3/4". The idea is to have 1.5 gram flash bangy at the end of them with room for the color to run most of the way up. I shoot 2" long by 7/8" dia solid comets now and they run out while still heading up. So maybe with the color comp slowing down the burn a bit I'll get a little more height with that bang at the end. It would be nice to be able to make a square or tapered star cavity on a lathe. I like the whole crossette/comet thing.

[nater]

So you want a comet that changes color while ascending and ends with a report? Or do you want a color comet to report like a Thunderking?

The traditional cavity comet is a long duration, color changing star. Typically a charcoal streamer to a bright comp.

Edited February 15, 2014 by nater

[LambentPyro]

Nater, how do you seal off those comets once you insert a star into the cavity?

[nater]

Lambentpyro,

 

The cavity is filled with dampened star comp, not a completed star. Once the cavity comet is completely dry, prepare your color comp and dampen it until it is at the consistency for cut stars, or similar to modeling clay. The damp comp is "buttered" into the cavity, compressing it slightly with your fingers as you go. Then take a knife, small trowel, anything that can smooth the comp out and cut it off at the top of of the comet. Set this aside in a safe place until it is completely dry.

 

Now the comet is pasted in, like a crossette. I use a strip of 30# kraft wide enough to cover the height of the comet plus enough extra to fold over the color cavity with a triangle fold minus about 1/4" and long enough for 3 wraps around the comet. The strip is broken in with paste, like when making a paste wrap for a cylinder shell. Start wrapping the comet about 1/8" to 1/"4 from the solid end, folding the excess paper over the cavity end as you go. Allow the paste wrap to dry completely. Finally, prime the solid end as you normally would for comets. These particular comets were brushed with liquid bandage and dunked in fine grained BP and functioned perfectly.

 

* note - the liquid bandage is mostly nitrocellulose and a solvent. I did not make this into a BP slurry as you normally would, just an experiment to see if it would work. It did, but I have not tested the method thoroughly enough to know if the results are consistent.

 

These pictures are of poor quality. In the first picture in this thread, you can see the star comp embedded in the cavity. My first picture are 1" C-6 comets that I have not filled yet. The following pictures are 1/2" D-1 filled with Hardt Blue #6.

 

 

Filled Cavity Comet

 

Pasted and primed cavity comet

 

I will post better quality pictures as I complete the 1" comets. That batch is C-6 and I intend to fill them with a Strontium Nitrate / MgAl Red. The traditional effect of cavity comets is a long duration, color changing star. Per Mike Swisher, the most traditional is a charcoal streamer to brilliant white. In that respect, I will also be making some filled with his recommended Luce Forte comp, fired high on a rocket to illuminate the field. More to come later this season...

 

As you can see cavity comets are constructed similar to crossettes and are labor intensive. The effect is unique, and I think it is worth it. Wolter's site suggests that cavity comets can be used to create a color star with a tail. Other pros say this is not reliable and without the paste wrap, the color portion is known to fall out of the star, ruining the effect. Married comets are a better way to create the color head with tail effect, but that is another discussion. I have also been told that a respected pyro has filled the cavity with flash to create a surprise report. Again, Mike Swisher responds that this works sometimes, but can easily fail and is not a reliable means to create that effect. I have only experimented with the traditional method described in Pyrotechnica IX.

Edited February 15, 2014 by nater

[cogbarry]

Sorry if I'm hijacking a thread but I just bought a 3/4" cavity pump and have been thinking about about different comp combos to try. After researching this (including the article in Pyrotechnica IX), I'm under the impression the cavity star needs to be brighter than the streamer outer star.

 

Nater, how did your D-1/Hardt Blue #6 look? I may want to try the MgAl Red as well - maybe with Miller's variation of D-1?.

[Sparx88]

I'm also using the hardt blue comp and it does nice. Though it burns a tad long for the comets I'm making now that have a larger cavity but can be faster with some 325 mesh alum. I don't have an exact recipe yet as I'm still testing but so far it's good as is.

[nater]

"Needs to be brighter" is a strong phrase. Cavity comets are traditionally a charcoal streamer to a bright effect, but they can be whatever you want them to be. All a cavity comet is, is a pumped long duration color changing star.

 

I liked the D1 to Hardt Blue #6, but I am also a sucker for the blue and gold combination. The MgAl Red was nice on the ground, I have not made enough of them yet for a shell or candle.

[cogbarry]

"Needs to be brighter" is a strong phrase. Cavity comets are traditionally a charcoal streamer to a bright effect, but they can be whatever you want them to be. All a cavity comet is, is a pumped long duration color changing star.

 

I liked the D1 to Hardt Blue #6, but I am also a sucker for the blue and gold combination. The MgAl Red was nice on the ground, I have not made enough of them yet for a shell or candle.

Thanks,

I was thinking gold to blue as well. Maybe a little gold/silver to bright red. These will be my first cavity stars but my understanding was that they would be a bit different than a simple color changing star. More like a colored tip with a streamer tail? Obviously, the color would not appear right away.

 

On another note, what are you folks using for a HCB or Dechlorane substitute?

[nater]

I am using the new dechlorane that is out there. It is a flame retardant, which seems odd in a pyro comp but it works. I want to get some saran and try that as a substitution in a couple of blues.

 

The idea behind cavity comets is to be a color changing star. The pasting serves as a fire block, so the comet burns from one end to the other ending with whatever comp is filled in the cavity.

 

Rich Wolter writes on his site that cavity pumps are used to create color stars with a tail. This would work if the comet is not pasted, with a few issues. It would take some work to dial in the burn rates so the amount i. The cavity burns out at the same time as the rest of the comet. You also have the chance of the comp in the cavity separating from the comet, ruining the effect. The pasting helps hold the comet together as well. See Mike Swisher's article in the PGI Anthology about married comets if you want to learn more about colored stars with a tail. He has also been very helpful answering my questions about these cavity comets.

[Sparx88]

Thank you Nater and Mumbles. I was mistaken on some things with these and I think I'm back on track now. I just get excited when things start turning out like they should and then I kind of get ahead of myself before mastering the tech at hand. What can I say, I'm a pyro freak

[cogbarry]

Thanks Nater but do you have a source for the Dechlorane? I used saran as a 1:1 substitute in the following:

 

0.62 potassium chlorate
0.21 paris green
0.08 stearin
0.04 hexachlorobenzene
0.05 dextrin

 

I liked the star but I can't compare it with the original formula as I don't have the Dechlorane.

 

Regarding the cavity stars, I've read Pyrotechnica lX and Xl and various articles, forum posts, etc. Some of the material I read was from Mike Swisher. My question on the description of the stars effect as a "color changing star" is due to the fact, that I think of "color changing star" as a star that simply changes color. If this star does what I think it does, it starts burning the outer comp only at first, eventually it burns into the cavity but the outer layer is still burning as well. If the outer layer is a streamer, you will have a streamer with a colored tip. Only the tip changes color although the streamer may not really have a tip/flame envelope that shows before the cavity begins burning. I've seen a comercial shell (yung feng) which I believe is a spider shell. It begins as a streamer, then has a red tip. When the star goes out, the pattern left in the sky a set of long tails with a red ball at the end. It reminds me of a jokers hat. My guess is these are cavity stars.

 

I know of married stars but it seems the effect described above is similar once the cavity begins burning and the stars produced with a cavity pump will have a length equal to the diameter, that may make the shell construction a bit easier.

 

Someone correct me if I'm wrong but I think refering to this effect as a simple "color changing star" leads to confusion. I actually thought about using this tool to make simple color changing stars. I thought about "marrying" two of the stars together with cavitiy sides joined and not pasting anything. However, I realized that the shapes involved would make it difficult or impossible to get a clean changeover and at some point both comps would be burning.

Edited February 26, 2014 by cogbarry

[nater]

Dechlorane is available from Cracker, but is not to be confused with hexachlorobenzene.

 

Cavity stars do not get a clean change over from color to color, it is more gradual. With mine, the tail that is burning from whatever part of the comet that supports the cavity is minimal. It is a small amount in comparison to the filled cavity, and I think the brightness of the color comps drown out the little tail that would be burning.

Edited February 26, 2014 by nater

[cogbarry]

Thanks,

The comp I refered to called for hexachlorobenzene (I can't get), but some of the other blues (including the hardt comp you refered to, I think) calls for dechlorane. The question is, does a 1:1 substitute to saran produce comparable results? The comp I used with saran had PG in it which I've heard will produce a good blue without a chlorine donor at all.

[Mumbles]

I've had good luck replacing HCB with both dechlorane and saran in colored stars. The use of HCB in colored strobes is still without a good replacement. I have some leads I'm working on. Saran isn't suitable in my experience, and I never got much chance to try with dechlorane. My few attempts were not overly successful though.

[ddewees]

Thanks,

The comp I refered to called for hexachlorobenzene (I can't get), but some of the other blues (including the hardt comp you refered to, I think) calls for dechlorane. The question is, does a 1:1 substitute to saran produce comparable results? The comp I used with saran had PG in it which I've heard will produce a good blue without a chlorine donor at all.

Shimizu B49 is a nice blue, and doesn't require any hard to obtain chems.

[cogbarry]

Thanks ddewees, I see it doesn't use chlorate either. The last batches of blue I've made are Ofca's Royal Blue. These stars were tested side by side with the previous comp I mentioned and no one could tell the difference between them. Again, the HCB in the previous comp was replaced with saran if that makes any difference.

 

Mumbles, TR makes some nice strobes, I wonder what he uses. Of course he may have HCB.

[Mumbles]

Sorry, I meant colored strobes specifically. I know TR makes use of white strobes often. You can make decent white strobes without it. There are a couple of formulas for them floating around the web using barium nitrate, sulfur, and magnalium as a base mixture with some additives. To turn them green or red or other colors, you tend to need a chlorine donor and HCB works best so far. For the time being, there are always AP strobes, but those have their own set of issues.

 

I believe TR makes an aqua strobe occasionally, and it very well may contain HCB. Given our luck, it's one of the few formulas he has that he's sworn to secrecy about though.

[pyrokid]

TR told me he uses AP for some of his strobe stars.

[cogbarry]

I believe there are some colored strobe stars in the following:

 

[Wiley]

Nater, that's an interesting point you made about the liquid bandage. Have you tried mixing it with BP green mix and using it as a prime for fuses (such as in timed reports)? It doesn't appear to have water in it, so if it dries strong enough, that might make an affordable "sort of" NC lacquer.