It is always a question people do when they start making fireworks shells. How much lift powder do I need to lift my shells up to the "correct" altitude?
Maybe there is not a perfect answer for this questions, because this may depend on multiple factors. The main ones are the quality of the powder, whe length of the morta, the clearance (space between the mortar intertnal walls and the shell) and obviously the weight of the shell.
I'm not a specialist but by my experince, usually you need to have mortars as tall as at least 5 to 7 times it's diameter, so, suppose you have a 75mm diameter mortar, you need it to be at least 375mm tall. As in gun barrels, usually as more length it has, more precision and distance you can get from the firing.
About the space between the mortar internal walls and the shell inside, I would recommend a space of about from 5mm to 7mm all around the shell.
Now, if you have a good black powder for lift (check this link about making a good quality black powder) basically calculate about 8% of the shell weight. So, let's suppose your shell weights 100grams, the lift amount would be 8grams. I was able to lift my shells with just a 5% ratio of lift powder, so, let's say 8% has some margin for not so good black powders, but you can try from 5% to 8%.
I made another post were I show some important information about Japanese shells, including the weight of lift powder they use, check it out here
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