Double Voice Crackers
WARNING AND DISCLAIMER: If you are underage, then consult with your parents or guardians before attempting any of this. You are on your own - I'm not responsible for your actions or harm you may bring to others because of your actions. Making the items described below can result in injury or death to you or people in your vicinity. Some things mentioned here may be illegal to make in your city, county, state, or country so check the laws that apply to you before you attempt anything described here. These notes are not complete on purpose. If you are reading them and new to pyrotechnics, then you are making a mistake. Stop now - this page is not for you. Get a beginning book on fireworks (see Skylighter or American Fireworks News (very quick shipping) for a start) and read up. You can't make any of this work without more information so read up or join a club or ask someone to help you.
After reading a rec.pyrotechnics thread about these, I decided to experiment. Don't let the following fool you into thinking I'm a guru on these - I'm just playing. The initial design came from page 38 of Best of American Fireworks News II (BAFN II). I modified it a bit - and I do not do flash so I added in my own header of Benzolift.
What is a double voice cracker? It is a relatively simple firework that pops twice. Once when the initial fuse burns down and then, after jumping up into the air, it pops again. They are challenging to get to work right but are easy to build at the same time. Here is a picture story of how I build a 5/8" one. Obviously, like any of our hobby pyro devices, there are many ways to build one of these. Feel free to improve on the design and let me know your successes. Be sure to use sturdy tubes!
Note that my version of the construction of these uses stranded tape spiking. You could spike these with twine but that would be a lot of work for such a small device.
The bottom cap is paper with three layers of masking tape. It must be strong enough to let the lift charge develop. I use 2-3 grams of pine pulverone for lift.
The top cap is paper with three layers of stranded tape. It must contain the salute/stars/dragon's eggs that you put in the header. The examples below were garnished with one teaspoon of Benzolift made with Skylighter pulverone.
Both the top and bottom tape ends are secured with a wrap of stranded tape.
The internal fuse is fast visco in the pictures but slow visco could be used. I think it is a little easier to make it work with fast visco because with a poor lift, it will still pop in the air. Click HERE to see a movie of what happens when the lift/delay isn't quite right. There are two videos at the bottom of the page, one with slow visco and one with fast visco. See which you like. A slight improvement on reliability for visco might be to prime the bottom internal fuse end somehow - it occasionally fails to light.
You can control the timing a little by varying the internal plug thickness. I start with 5-6 grams of clay for slow visco and 10-12 grams for fast visco. That is a .5" to about 1" inner clay plug. I ram them fairly hard using 10 moderate whacks with a 1 lb dead blow mallet. The plug has to hold back an explosion so it and the tube should be fairly sturdy. After ramming, use a pair of small scissors to split the bottom fuse as close to the clay plug as you can get. Just split it, don't trim it back. You want lots of area for the fire to take.
Videos of successful launches are at the bottom of this page.
Movie: Slow Visco Launch
Movie: Fast Visco Launch