11 replies to this topic

[Shake]

HI there;

I've search quite a bit for this with little success...

Is there an equation to figure out how much lift powder to use depending on the size and weight of the shell including the length of the mortar and the relative length of the time fuse?

Trial and error seems a little expensive and time consuming and being about to assemble my first shell this weekend I want to make sure (as close as possible) that the shell gets up high enough and breaks at the right time.

I'm working with a 3" plastic spherical shell (actually 2 7/8") and a cardboard mortar of 3 1/8" ID by 22" long. The shell will have cut stars 9mm X 9mm X 5mm, powdered Rice Krispies and a flash powder break charge.

Any suggestions would be helpful.

Thanks!

Chris

[digger]

HI there;

I've search quite a bit for this with little success...

Is there an equation to figure out how much lift powder to use depending on the size and weight of the shell including the length of the mortar and the relative length of the time fuse?

Trial and error seems a little expensive and time consuming and being about to assemble my first shell this weekend I want to make sure (as close as possible) that the shell gets up high enough and breaks at the right time.

I'm working with a 3" plastic spherical shell (actually 2 7/8") and a cardboard mortar of 3 1/8" ID by 22" long. The shell will have cut stars 9mm X 9mm X 5mm, powdered Rice Krispies and a flash powder break charge.

Any suggestions would be helpful.

Thanks!

Chris

Interesting. I spent ages looking for information on this topic. The problem is no-one knows how your BP compares with commercial stuff, what the grain size is (obviously bigger grain for bigger shells), how well the shell fits the mortar etc so there will have to be an element of testing to find out how much to use.

You obviously do not need to use finished shells for testing, you can always use dummy shells of the same weight and slowly increase the lift (or decrease) until you are happy with the height. You could film the dummy shells going up and work out how long the ideal shell took to reach apogee then dial in the time fuse to suit.

But to give you a starting point if you have decent BP then around 30g - 35g should fire the shell to a respectable height.

Digger

[Bonny]

One of the "rules of thumb" for shell is 1oz of BP for every 1lb of shell weight up to 10 lbs I think, but a good starting point using untested BP would be 1/10" the weight of the shell. This may be too much but better to have the shell too high than risk a low burst. The delay of the time fuse should be about 3sec for a 3" shell, so calculate the length of time fuse according to burn rate. There are tables that list a lot of info available on Passfire. You do have to pay, but the wealth of info is well worth it.

[Shake]

One of the "rules of thumb" for shell is 1oz of BP for every 1lb of shell weight up to 10 lbs I think, but a good starting point using untested BP would be 1/10" the weight of the shell. This may be too much but better to have the shell too high than risk a low burst. The delay of the time fuse should be about 3sec for a 3" shell, so calculate the length of time fuse according to burn rate. There are tables that list a lot of info available on Passfire. You do have to pay, but the wealth of info is well worth it.

Thank you both Digger and Bonny;

This certainly gives me a great starting point, far more than I had before. I could just see myself packing in a bunch o' lift and never seeing the shell again...HA!

[Bonny]

Thank you both Digger and Bonny;

This certainly gives me a great starting point, far more than I had before. I could just see myself packing in a bunch o' lift and never seeing the shell again...HA!

Just make sure the shell is solidly built. What type of BP (pressed and corned or pulverone) are you using? And what size grains?
I used to use -10+20 mesh, then changed to -4+10 for 3: as I was blowing all my shells apart. Partly due to overloading I'm sure, but the larger grains seem to be more gentle on the shell.

[digger]

One of the "rules of thumb" for shell is 1oz of BP for every 1lb of shell weight up to 10 lbs I think, but a good starting point using untested BP would be 1/10" the weight of the shell. This may be too much but better to have the shell too high than risk a low burst. The delay of the time fuse should be about 3sec for a 3" shell, so calculate the length of time fuse according to burn rate. There are tables that list a lot of info available on Passfire. You do have to pay, but the wealth of info is well worth it.

Wow that sounds like a long fuse for a 3" shell.

The usual rule of thumb is half the shell diameter in inches, however this gives a very short fuse for a 3" er. I have used 2.2 seconds for 4"ers and 3 seconds for 6"ers with good results (maybe a little over lifted). Up to now I have only used 3"ers on the top of rockets as headers with an instant fuse (passfire tube).

I do not corn my lift I just mill in about 4% dextrin and then dampen it until it just about holds together with 90% water / 10% alcohol solution. Then I rub it through a 5 mesh screen and dry. I end up with chunks in the region of 10 mesh. A finer mesh would also be suitable for smaller shells.

Edited by digger, 08 July 2008 - 08:29 PM.

[BrightStar]

Wow that sounds like a long fuse for a 3" shell

A typical commercial 3" supposedly takes about 4s to rise to apogee. Having it burst on the way up is safer than on the way down so 3s timing would probably be OK.

I try to aim for 2.5s timing for 3" and an extra 0.5s for every extra inch up to 6".

I agree that 1/10 shell weight is a good starting point for home made lift. The fit in the mortar is a big factor though so test runs are usually needed.

Edited by BrightStar, 08 July 2008 - 11:20 PM.

[pyrotrev]

I've timed loads of commercial shells, and for 3" the delay fuse seems to normally be between 2.5 and 3.1 seconds. In the last 10% of it's flight it's moving relatively slowly so it's not ultra critical.

[digger]

I've timed loads of commercial shells, and for 3" the delay fuse seems to normally be between 2.5 and 3.1 seconds. In the last 10% of it's flight it's moving relatively slowly so it's not ultra critical.

Very interesting to know

[maxman]

I use 3 sec time fuse and 28g pine BP granulated to 3FA equiv and the shells seem to reach their height and sit there just before breaking spot on. Hope this helps

[Shake]

Just make sure the shell is solidly built. What type of BP (pressed and corned or pulverone) are you using? And what size grains?
I used to use -10+20 mesh, then changed to -4+10 for 3: as I was blowing all my shells apart. Partly due to overloading I'm sure, but the larger grains seem to be more gentle on the shell.

I'm using spruce BP with 10% by weight Dextrin, corned to a 10 mesh. I have used this with little 3/4" cylinder shells but only got 70 feet lift out of it....maybe too much dextrin is slowing down the burn?

I have also tried willow BP with the same specs but didn't see any noticeable difference.

[Bonny]

I'm using spruce BP with 10% by weight Dextrin, corned to a 10 mesh. I have used this with little 3/4" cylinder shells but only got 70 feet lift out of it....maybe too much dextrin is slowing down the burn?

I have also tried willow BP with the same specs but didn't see any noticeable difference.

You might want to try reducing the dextrin to 5% to increase the speed. I've only used pulverone once or twice, so I can't really comment on it, although it should burn faster than pressed BP. How much did you use on the 3/4" shells, and how much did they weigh?
My lift BP is willow (and I have some Alder to try) with no dextrin pressed and corned. I use SPF (spruce/pine/fir...average lumber) in all comps other than lift BP and for what I call general purpose BP, to use for blackmatch,fountains, priming, coating burst etc...