15 replies to this topic

[Green_as_my_powder]

Fellow Pyros,

I thought I'd just share what I've found. I fired a 65g can shell (o.d. 1 3/4") from a mortar tube (i.d. 2") with 12g of lift powder. I retrieved, and fired the same shell, same tube, same amount of powder, but with a single wrap of nylon rope around the top of the shell, bringing the o.d. a little less than 2". With the rope around the top, it would slide down the tube--no pushing required. A video camera was used to calculate height based on airtime.

1 3/4" can ==> 105 m high
~2" can ==> 180 m high

Just thought you'd like to know...

[pyrotrev]

Interesting to see such a big difference. Nice to see someone applying some scientific method too!

[Deano 1]

Fellow Pyros,

I thought I'd just share what I've found. I fired a 65g can shell (o.d. 1 3/4") from a mortar tube (i.d. 2") with 12g of lift powder. I retrieved, and fired the same shell, same tube, same amount of powder, but with a single wrap of nylon rope around the top of the shell, bringing the o.d. a little less than 2". With the rope around the top, it would slide down the tube--no pushing required. A video camera was used to calculate height based on airtime.

1 3/4" can ==> 105 m high
~2" can ==> 180 m high

Just thought you'd like to know...

Thats incredible, and really worth remembering. 75 m could mean the difference of excellent display and what the £;.;ck was that.

[Spyrotechnics]

hmm, aren't those heights a little high for a 2" shell??

[RegimentalPyro]

Hmmm - 12g to lift 65g is probably overkill. I use 15g to lift 120g.

That being said the purpose of this was to test the relationship between height and "snugness of tube fit" - not to lift the shell to a traditional level.

[Spyrotechnics]

yes, fair point

[Green_as_my_powder]

What is the traditional level for a 2" shell? I've been trying for 150 to 180 m for willow stars so they burn out before they hit the ground. Maybe I'm making my stars to big?

[Bonny]

What is the traditional level for a 2" shell? I've been trying for 150 to 180 m for willow stars so they burn out before they hit the ground. Maybe I'm making my stars to big?

I think that is way too high for a 2" shell. Your stars will need to be smaller. The usual rule of thumb is 100' for every inch of shell size +100'. So a 2" shell would break at 300' and a 3" at 400' etc... Many people find this too high, especially on small shells. I break 1" shells at ~150-175' and 3" shells at ~300' (I don't make 2")

[Green_as_my_powder]

Thanks for the info. I'll "aim lower" and save the stars for when I can make bigger shells!

[al93535]

I would like to add, for your guys safety, and to save a hard worked on firework.

You NEED the extra space around the shell. With smaller shells, and especially spherical shells it doesn't matter as much when the fit is really close, or even tight.

Move up to a 4" + shell, and here especially a cylindrical shell and that extra room around the shell is really required! Other wise you get too much of a pressure build up! Its simply best to add a bit extra lift to ensure a good height rather then enlarge the diameter of your shell.

I have had two can shells, a 4" and a 6" give me problems because they were too close of a fit. The 4" blew the mortar apart, and the 6" flowerpotted.

Video of flower pot 6" :

http://smg.photobuck...tfarfalle-1.flv

[Mumbles]

I've had some real snug shells. To the point they wouldn't slide down under their own weight. I think it had something to do with the residue on the club guns, or the metric system. They fit just fine into my HDPE at home. I've never had a shell flower pot, even when I had to physically assist the shell down. They are in a 55 gallon barrel of sand for a bit of reinforcement.

Then again I've been using commercial 2FA, and I know how hot your powder is Al. I could see blowing a few things up with high quality lift.

[pyrotrev]

You can have shells that are a tight fit that work OK, it's just a matter of having the right amount of lift and a well constructed shell. I remember Karlfoxman talking of some Maltese shells which were almost a push fit in the mortar (they tend to have a foam pad at the bottom to act as a seal), and you're talking a shell that's maybe 10Kg+ if it's one of their multibreaks. If your powder's a bit hot, try granulating it more coarsely, the Italian cylinder shells I've fired probably had the lift grains 6mm+.

Edited by pyrotrev, 30 March 2009 - 12:25 PM.

[Bonny]

You can have shells that are a tight fit that work OK, it's just a matter of having the right amount of lift and a well constructed shell. I remember Karlfoxman talking of some Maltese shells which were almost a push fit in the mortar (they tend to have a foam pad at the bottom to act as a seal), and you're talking a shell that's maybe 10Kg+ if it's one of their multibreaks. If your powder's a bit hot, try granulating it more coarsely, the Italian cylinder shells I've fired probably had the lift grains 6mm+.

When loading commercial shells for shows, the 2" shells routinely (almost always) need to be pushed into the mortars. I've even had a few that needed to be pushed very hard, but still worked just fine.

[MDH]

Typically shells only have 1-2 millimeters of room. Sometimes they don't even have to be a tight fit. In consumer shells they put a lifting disk on the bottom of the shell made of cardboard.

[Bonny]

Typically shells only have 1-2 millimeters of room. Sometimes they don't even have to be a tight fit. In consumer shells they put a lifting disk on the bottom of the shell made of cardboard.

Are you referring to the 2" only? I've not yet seen a 3" or larger shell that didn't fall freely to the bottom of the mortar. My own shells (3" and 5") usually have 1/8"+ of clearance all the way around.

Edited by Bonny, 01 April 2009 - 01:54 PM.