gizmothegecko Posted February 2 Posted February 2 Hey everyone, I’m new to building rockets and have a few questions about spherical shell headers, specifically the balance between pasting layers and break charge and how to get the biggest break on a shell without compromising their ability to ignite effectively. How do you determine whether to add more pasted layers to the shell or increase the amount of flash powder in the break charge to achieve a larger break? I assume there’s an optimal balance between the two, but I’d like to understand what that looks like in practice. Additionally, some stars fail to ignite in harder breaks, even with step priming. What’s the best way to prevent this? If stars do end up going blind, is it better to reduce the number of pasted layers or decrease the break charge to improve ignition?
Arthur Posted February 2 Posted February 2 Beware! A too-big break from a small header will simply look sparse. BP based burst charges are slower than flash based bursts which can be better for passing fire to all the stars.
gizmothegecko Posted February 8 Author Posted February 8 Didn't exactly answer my question, but you do have a good point that I need to consider. I think I'll just experiment myself with the layers pasted vs. the break charge used. I'm making 2 inch shell headers at the moment with 6 layers pasted, and 1 gram of 70:30 flash with -8+14 mesh black powder and the stars seem to be a nice peony effect.
Mumbles Posted February 12 Posted February 12 This is kind of a hard question to answer, which may be why you're not getting a lot of responses. There are people in both schools of thought. Both can be made to work. Ultimately, it comes down to getting the internal case pressure up. This can be accomplished both by reinforcing the case (more pasting), or by adding an accelerant or hotter burst to get the pressure up before the case has a chance to fail. There are benefits and limitations to both approaches. There is a physical limit to how much pasting you can put on a shell before it wont fit in a mortar, but if done well, I feel offers a better chance for improved symmetry. Adding more booster is less labor intensive, but can increase the probability of retina blinding centers, the potential for blowing stars blind, and can sometimes overbreak shells. If the symmetry is bad, my first approach would be to add a couple more layers of pasting. If the symmetry is good, but you just want to go bigger, either would probably work. I have a personal preference for booster to be distributed on the burst evenly, not just dumped in the middle and stirred around. To me, it's a recipe for inconsistency, but to each their own. More pasting or using a stronger burst (see below) is probably more of the japanese approach. More booster is probably the approach taken in the Chinese or Spanish styles of building. It may really be more related to hand vs. machine pasting or bulk production. This is all sort of based on BP + booster discussion. I personally use mostly perchlorate based bursts in ball shells, and don't feel comfortable adding a booster to those, and exclusively dial in with pasting or maybe changing bursts or carriers. The burn rate of BP is less sensitive to pressure and perchlorate based compositions. 1
Arthur Posted February 12 Posted February 12 I don't think there is a single unique "right answer" to your question! Everything depends on the powder, the casing with pasting, and what you think of as a perfect burst.
gizmothegecko Posted February 15 Author Posted February 15 Mumbles and Arthur, I appreciate both of your insights. I had assumed this wasn’t an exact science, and as Arthur pointed out, there are many factors that influence pasting and burst charges, making it impossible to rely on a strict formula for determining the right proportions. As Mumbles mentioned, it ultimately comes down to personal preference and the specific effect one wants to achieve rather than a precise balance. I’ll be following Mumbles’ advice and focusing on the desired effect as I continue experimenting. Thanks again for your help!
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