1569 replies to this topic

[Creepin_pyro]

I have some 42mm plastic shell hemispheres. I also have some 52.9mm mortars. This leaves me with a gap of 5.45mm around the shell. I'm assuming the gap is too big. Should I paste some kraft around it to increase the shells diameter?

Also, for shells with the 1/8" fuse hole, is wrapping the visco in some tape, inserting it, and hot-glueing both ends sufficient to hold it in?

Thanks guys

[Richard H]

No it will be fine. For example, a shell to be fired from a 4" mortar might be rolled on a 3.5" case former.

For the small festival ball shells, secure the fuse to a snug fit and then use hot glue either side to secure.

[Creepin_pyro]

Thanks for that. I'm nearly there now... just my lift to sort out now. So far, my shell weighs in at just over 35g. My previous shells have weighed about half that and have launched well with 3-4g lift.

This is the shell - rising comet attached, and the pile of meal I propose to lift it with. The lift is 6.5g of 2-9mm granules.


mods: is this ^ pic too big?

Anything seriously wrong here? Any suggestions would be appreciated.

[adamw]

What's with the time fuse?? Looks a bit toooooo loooonnngggg to me. I am also wondering if the lift power is too 'coarse'?

[Creepin_pyro]

It thought that might be the case I've left the fuse long, so I can adjust it later. Will shorten it a bit... I've never seen any mention of special priming for visco, so I'll just cut at a nice diagonal, and get a blob of meal on there.

I'm waiting on some screens from Steve so I can sort my granules out. I was thinking approx 4mm grains would suit ?

Thanks for the help.

[Richard H]

Use a 10 mesh screen to granulate your lift powder. That will be equal to roughly 2F.

[bobconan]

Jow much goex would you need for a 3 inch shell? How about a heavy 4"shell?

[Phoenix]

The rule of thumb is one ounce of 2FA per pound of shell weight. Basically, one sixteenth of the shells weight. For smaller shells, you might need more. I usually use just under 1/10 of the shells weight of BP for my 1-1/2" shells.

There is a table in the Passfire Reference section that gives typical lift charges.

[Yugen-biki]

The lift charge is something you have to fine tune your self. It depends on the BP quality, BP mesh, "mortar-shell gap" and the shell's wheight. And how high the builder likes it to travel.

Good guide lines for spherical shells are 30 grams for a 3" and 40 grams for a 4" +- 5 grams. Cylindrical shells tends to be higher and this adds on some extra wheight. (I use about 15 grams of lift for a 2" cylindrical shell of ~70 grams.)
The mesh size is not critical. For 2-3" shells a grain size of 2-5mm is something to start with.

One second time fuse is something to start with for a 2-3" shell.


I know a lot of people has a lot to say about lift and time fuses. I can only say one thing:
-You have to find out the correct parameters your self.

Pyro is not an exact science.


EDIT: By the way your shell looks nice :-)

Edited by Yugen-biki, 02 July 2004 - 03:03 PM.

[pyrotechnist]

Hi what end do you put that shell in the mortor with that comet on top. Is the comet going to lift the shell? or egnit in the air when the shell brakes? Thanks.

[Loci]

The Comet will go in facing upwards and will be ignited by the hot rapidly expanding gasses...

[Kembang Api]

Creepin_pyro: Looking at the picture along the break line of your shell, I would recommend that you should add more tape so that the shell will hold a bit longer for the shell to burst and give you a better reports at the same time to let the stars travel out in a longer distance or give a bigger diameter effect.

If you have to have a longer fuse because of your comets have to be hanging there, I would suggest that you should take some of the meal powder inside the fuse and replace with a couple of black fuse (this will act as a quick fuse).

I think you would need at least 5 seconds fuse and about 2 second fuse to ignite the lift charge. This will give you 3 seconds for the shell to travel up from the mortar (20 to 25 meter). Depending on the lift charge quality.

Yugen-biki: You have to find out the correct parameters your self

[Creepin_pyro]

I think you would need at least 5 seconds fuse and about 2 second fuse to ignite the lift charge. This will give you 3 seconds for the shell to travel up from the mortar (20 to 25 meter). Depending on the lift charge quality.

Thanks for the advice, but I really don't understand the last bit. Do you mean 5 seconds to run away from the mortar?

[Yugen-biki]

Kembang Api

Yugen-biki: You have to find out the correct parameters your self

I don't understand.

[Creepin_pyro]

Use a 10 mesh screen to granulate your lift powder. That will be equal to roughly 2F.

I assume you mean by passing the damp composition through a 10 mesh sieve. I haven't tried this method yet, but will give it a go.

If I use a 10 mesh sieve with my pressed, granulated stuff, should I use grains that the 10 mesh holds, or just try to get the grains close to 10 mesh?