ID vs Length...VS Choke dia. Fountains

hi guys, another one to pick your brains...ive done lots of fountains in the past - but nothing really all that big....I'm hoping to construct a larger series, and could do with some advice.

am i correct in understanding that the generally accepted theory, is that a fountain's choke should be 1/3rd of its ID ???? so a 30mm ID Tube requires a 10mm Choke ?

My next question relates to the length of the tube.....take these 2 examples...

A short tube, with an 30mm Id, and a 10mm choke....say is 100mm long...so when the comp has burned to halfway, the volume of the empty void is 35 cubic CM.

a long tube, with an 30mm id, and a 10m choke...lets say this one is long, 300mm. Now, when the comp as burned to half way, the volume of the empty void is 106 cubic cm.

So the pressure in the tube(at halfway) is 1/3rd of that of the shorter tube - so by that fact, should the choke be made smaller, in order to retain pressure for longer??

i guess what im asking is there a magical guide for working out choke dia....but taking into account the length of the tube, and the diam ?

am i correct in understanding that the generally accepted theory, is that a fountain's choke should be 1/3rd of its ID ???? so a 30mm ID Tube requires a 10mm Choke ?

I don't have my books with me for references but the range generally quoted for small to medium fountains is between 1/4 and 1/3 of the case ID.

My next question relates to the length of the tube..... should the choke be made smaller, in order to retain pressure for longer??

No, for most end-burning fountains the assumption is that they reach an equilibrium state where the internal pressure is governed by the rate of gas generation (proportional to the burning surface area exposed) and the flow resistance created by the nozzle. This pressure should remain pretty much constant throughout the burn, regardless of the internal volume. The length of the case generally determines the burn duration, ultimately limited by the flame resistance of the tube and erosion and clogging of the nozzle.

i guess what im asking is there a magical guide for working out choke dia....but taking into account the length of the tube, and the diam ?

Is anything in pyro ever that easy? :-)

Edited by BrightStar, 04 November 2009 - 07:30 PM.

Masterly answer from Brightstar!

All I have to add is that I tend towards smaller chokes, as you get a higher plume - but there is a point at which it will go bang!

The fierceness (??) of the composition also bears on this. A very strong comp that might suit a small gerb will burn too fast for a fat one.

Experiment is the only true guide I think...

less than 100g at a time mind. Obviously.

less than 100g at a time mind. Obviously.

But of course

less than 100g at a time mind. Obviously.

supermarket dross as a lot less than 100g! lol

To elaborate on what Phil said. Basically the larger the ID of a fountain (or rocket) the slower the mix needs to burn as the increased surface area produces gas more quickly when it burns and therefore creates increased internal pressure.

With gunowder based mixes if you think of saltpetre as an accelerator and charcoal as a brake you can add more charcoal to slow things down a bit or more saltpetere to speed things up, to a point.

As long as all the bits add up to 100 parts by weight using this method and the mesh size of the chemicals is constant you can tinker, altough in larger fountains you may get away with using a mix that explodes with fine mesh chemicals but burns fine by using a larger mesh size, or even by just adding a proprtion larger mesh charcoal to an exploding mix.

If a mix explodes immediately upon ignition it may need taming by as much as 5% (I.e reducing the saltpetre by 5 parts per 100 and increasing charcoal by the same amount.) If it burns a little first then explodes, try a 2-3% change.

Much of it is trial and error but once you get in the right sort of area where it works without explosion you can play with the mix to optimise it.

All good points above...

On the subject of optimum length, I've noted the longest usable case is about 10 times the ID given a reasonable wall thickness (~4mm with 3/4" ID).

This works as a maximum with the relatively cool burning BP + charcoal mixes. With hotter barium nitrate + Al mixes and the even hotter and slower perc based colour gerbs, I've had to shorten the cases to 3/4 or even 1/2 of this to prevent burn-through. Dissecting the remains post-firing helps show how well the tubes hold up.

What case lengths do you guys usually use?

Pressure is "gas generated vs gas exiting" so no volume factor. Duration is controlled ultimately by the char resistance of the paper of the case and the material of the nozzle.

The largest commercial fountain I have seen is the Kimbolton 1 Kilo fountain and that is a cone with a paper nozzle about 1/2 inch DIA

The longest burning fountains I have seen were some Chinese "fountains" in big five pointed star cases containing about 11 tubes about 10mm Bore all fused to go in overlapping sequence. -For their overdressed appearance they were fully functional and stable on the ground, also had a different function in sets of tubes in the sequence.

Edited by Arthur Brown, 05 November 2009 - 10:16 AM.

What case lengths do you guys usually use?

I think you can get away with longer than 10 times the ID.

Without consulting my notes, I think my 'thin' gerb is 1/2" inside dia, 1/4" choke 10" long although my 'fat' one is 1" Inside, 3/8" choke 8" long which is well within that rule of thumb...