tip for acurate star rolling

[Givat]

Hi all,

Lately I started rolling round stars in pounds so I searched for a system to calculate the amount of composition I will need for a specific amount of stars, include the prime etc...

I used the "ball volume formula" http://upload.wikimedia.org/math/c/e/e/cee10af73cbd198af616d9a19fc9a09b.png, to calculated how much will be a volume of certain layer of composition on an already rolled star of some size. (the star volume with the layer - the star volume before the layer).

For you who don't want to deal with mathematics to much here is my summary:

 

for 1 m"m layer (which will increase the star diameter plus 2 m"m) = [4*3.1415(0.3r^2+0.03r+0.001)]/3

for 1.5 m"m layer - [4*3.1415(0.45r^2+0.0675r+0.003375)]/3

for 2 m"m layer - [4*3.1415(0.6r^2+0.12r+0.008)]/3

for 2.5 m"m layer - [4*3.1415(0.75r^2+0.1875r+0.015625)]/3

for 3 m"m layer - [4*3.1415(0.9r^2+0.27r+0.027)]/3

for 4 m"m layer - [4*3.1415(1.2r^2+0.64r+0.064)]/3

(r=radius of the star you start with)

 

Now I know the volume of a certain layer over one of my star, so to calculate how much composition I need to all my stars I need to know how much stars I have:

Take 30 of your stars and weigh them. this number you divide by 30 (the amount of stars you weighted) and you got approximately 1 star weigh. now all you need to do is weigh all the stars you want to roll and divide this weigh by 1 star weigh = the amount of stars you are going to roll.

 

So we know how much stars we want to roll and what will be the volume of the layer we want to do (on 1 star), all we left to do is to multiply the number of stars by the volume of on star layer.

 

But how do you make this volume into amount of composition to roll on the stars??

you need to find out what is the "specific mass" of your composition. for charcoal compositions (C6, BP prime...) I calculated it to be around 1.5 gram/c"c^2. for my metallic fueled color stars it's about 2 gram/c"c^2.

 

If you want to know your composition specific mass just make 1/4" stars, calculate how much stars you have (and from this the volume of all the stars together)and how much all this batch weigh and you have your specific mass.

 

I hope this will help other people to make accurate round stars. in this method you don't need to keep on adding composition and using star screen every 5 minutes to make a specific diameter star, jsut calculate how much composition you need in advance, mix it and use all of it.

 

good luck.

[warthog]

Well, I still await my roller but it is supposed to be here in no more then a couple of weeks.

I will have to get the knack of rolling first of course but once I do this is pretty helpful information IMO, Thanks!

[DanielC]

Well, I still await my roller but it is supposed to be here in no more then a couple of weeks.

I will have to get the knack of rolling first of course but once I do this is pretty helpful information IMO, Thanks!

 

 

That's great man. What kind are you getting? I was thinking of building a smaller benchtop roller, but I just got the motor in today.

[warthog]

Well this is a bit off topic but the roller is coming from Mumpyro. Derrick is sort of a friend of mine and I knew he was making his club a bigger one to replace the little one they started out with so I asked if I could buy the little guy from him. He agreed but wanted to replace a few parts so it was like new for me, he is a pretty nice guy that way. He emailed to to say that he finally finished work on the new, bigger one and was almost finished overhauling mine so I guess it is close now. I am pretty excited about it and ready to try my hand at rolling.

[allrocketspsl]

VH718 rolls awesome stars he could give lots of input.All

[Mumbles]

While this works in theory if you're very good at rolling stars, there still is the issue of different stars taking up different amounts of composition. This is of course compounded by being inexperienced and other factors. I don't know if you'll be able to get away from at least a final screening, especially on the hobbyist scale.

[Potassiumchlorate]

I must admit that I suck a bit on rolling stars, even though I think that rolled stars are the best. I often make "raspberries".

[FREAKYDUTCHMEN]

A lot of math, I'm using my experience to choose the amount of needed composition. I can see by the eye how much composition I need to roll a certain amount of stars from a certain size into the wished size. Sometimes I need little more, sometimes a little less, but quite many times I'm lucky that I have mixed the exact amount of star composition. Another thing is that a willow compositions and such will shrink in volume way more than color star comps.

[50AE]

Keep in mind that the it will be approximate. Volume in powder state usually differs than in wet/rolled state, the later usually gets denser.

[Givat]

Another good tip about star rolling:

If you roll 1 m"m thick layer (1 m"m to the radius = 2 m"m to the diameter) you double your weigh of stars.

for example:

if your roll 500 gram, 6 m"m colored stars with 1 m"m layer prime (which will make them 8 m"m diameter) you need about 500 gram of prime (+- because the density of different composition is not the same, but most of them are very close).

 

just throwing some ideas to the room =]

 

50AE, after I roll my stars I calculate the density of it and save this number as the density of the composition the star made from.

 

Mumbles, after 8 m"m I don't screen my stars any more. they grow about the same speed and I only need to take on of the stars out of the roller and see if I got the size I want. (I made 14 m"m stars and got about 1 m"m difference from the same stars in the batch).