How to determine lift charge

[MacTim]

All, Sorry to be a PIA but trying to get around "how to determine the amount of BP for a lift Charge". I make small cylinder shells which weighs in around 57 grams to 100 grams. Is there a formula to determine the amount of lift is needed for a particular weight. I know the answer is out there with all the expertise people have in this field would appreciate throwing this ole dog a bone.

Thanks\

 

Tim

[Arthur]

My understanding is that shell lift can be best tested by trial and error based on the long standing guide of half ounce of lift per pound of flying shell, however this is a rough amount as small shells need more and large shells need less than this. Based on this a 1" insert may want about it's own weight of lift and may need to be a very good fit in the tube. The fit in the tube is particularly important for small diameter shells, while a 16" shell may have a half inch gap all round in it's mortar a 1" shell may want only 5 thou all round -adjustment by wraps of glued paper (or sandpaper!)

Edited January 21, 2022 by Arthur

[MacTim]

Firebreather,

Thanks for the comeback on this issue, My problem is I have to drive 50 miles just to put my shells to the test. In my neck of the woods just whispering "fireworks" instills fear in the hearts of fire department personnel and insures a stiff sentence should one be caught with such items.

I realize that the quality of your BP has a lot to do with how hot it is and that is a variable I have dealt with by using quality components to make my mix, With that said you now know what I am up against and wish to get good results on the range without spending half my day driving to and from

Thanks again

Tim

[Arthur]

Should you wish there are clubs and associations in America that hold meets where fireworks are welcome.

 

Western Pyrotechnics Association is holding one such meet south of Vegas in the warm in February.

 

Several books and pamphlets are available online or on (WWW) abe books in your place.

[Mumbles]

The rule of thumb is an ounce per pound, which is 6.25% of the shell weight. In my experience, this really only starts to become applicable for larger shells that weigh at least a pound probably. There's also some implied, but rarely stated rounding assumptions that often happen as well.

 

If I were you, I'd go heavier on the lift. It should work, and you can always dial it back a bit next time. I'd personally probably use 15% of the shell weight if I were you. 10% is more than likely about right, but with the travel time and all that, I'd err on the side of caution the first time at least.

[MadMat]

For smaller shells, you should start out closer to 10% of the shell weight. Of course, that will more than likely need tweaking. One of the things to look for is a lot of sparks or fire shooting out of the mortar tube when the shell leaves. Since this will happen no matter what, you need to do a comparison to various amounts of lift charge. What you are looking for is the lift BP still burning after the shell has left. If this is the case your BP is either way too slow, or you used way too much. If you ARE using too much, it is only wasting your BP. If you have a 3 inch tube, you can try doing "baseball tests". Seriously, a baseball fits a 3 inch tube very nicely and since all (official) baseballs are the same weight, it is a very good indicator of how good of a lift charge your BP makes. Decent BP will give you at least around a 6.5-8 second flight from the boom to the thud @10% of the baseball's weight. If that's what you get, 10% of your shell weight (or close to it) will work nicely. If you get significantly shorter flight times, try using more. If that still doesn't help, you need to take stock of your BP making process. As your shells get bigger that percentage has a tendency to go down

Edited January 21, 2022 by MadMat

[Carbon796]

I would definitely follow Mumbles sugestion of 10 to 15 percent. It's always worked well for me. When building smaller/lighter stuff.

 

The 1/2oz of lift, per pound of shell. Only applies to cylinder shells, once they are north of 10#. For example a 12# cylinder shell would require a minimum of 11oz of lift. 10oz for the first 10# and and an additional 1/2oz for each pound over 10#.

[Arthur]

Given a 50 mile drive each way, I'd consider preparing several test shots then carefully recording the results of tests. Maybe you could shoot 10 shots at a time then go back and revise them by reference to your experiment recording.

[MacTim]

Outstanding advise from the pros, I have prepared about 10 cylinder shells, each with a different weight of lift charge. Packing up a lunch and headed to the hills with the hope that things don't go south on me. The advise from all is a welcome read. I had never thought of what Pyrotechnician said about the amount of sparks that exit the tube upon ignition, makes sense sort of like in metallic cartridge reloading,have to tailor the charge to the weight of the projectile to get optimum performance. In this case BP is still burning when the shell exits the tube, maybe a bad analogy but spot on for diagnosing problem. What I will do is assemble up to the point of adding the lift charge and when range time comes along finish the shell starting with 10% BP and work my way up to optimum performance for the weight of the shell. Who knows might work!!

 

Tim

[Arthur]

Some single shots are made by inserting lift and load into a tube then pushing in a square of board, AF measure = tube bore. This just closes the bore to prevent things moving and helps contain the first microseconds of combustion to ensure that the powder reaches full pressure and the folded corners cause friction to keep the lift gasses inside until the star exits

 

Sometimes the construction is crucially important.

Edited January 22, 2022 by Arthur

[6afraidof7]

Outstanding advise from the pros, I have prepared about 10 cylinder shells, each with a different weight of lift charge. Packing up a lunch and headed to the hills with the hope that things don't go south on me. The advise from all is a welcome read. I had never thought of what Pyrotechnician said about the amount of sparks that exit the tube upon ignition, makes sense sort of like in metallic cartridge reloading,have to tailor the charge to the weight of the projectile to get optimum performance. In this case BP is still burning when the shell exits the tube, maybe a bad analogy but spot on for diagnosing problem. What I will do is assemble up to the point of adding the lift charge and when range time comes along finish the shell starting with 10% BP and work my way up to optimum performance for the weight of the shell. Who knows might work!!

 

Tim

Tim, how did your testing go? Any video's/pics?

[MacTim]

Sorry 6afraidof7 haven't had a chance of hitting the range, lots of stuff to do prior to playing. Will put shoulder to wheel and try harder to get out. Pending