"chuffing"

[dave321]

hi,

 

mumbles is right about the "chuffing" phenomena sometimes observed in coloured / white smokes.(also in composite rocket motors)

 

http://www.youtube.com/watch?v=CxQyLciU0yE

 

Chuff: A form of unstable combustion marked by brief bursts of thrust separated by periods of no thrust. Typically, the bursts come faster and become longer as burning proceeds, until stable burning results. The sound of chuffing is similar to that of a steam locomotive starting up. It generally occurs in a composite motor that is ignited too low in the grain.

 

does anyone know how to reduce this effect ?

 

lower compression force ? higher compression force ?

 

it seems to be an internal pressure effect associated with the ash/ resublimed material formed after burning

the above formula already has 15% kaolin in.

 

any ideas ?

 

dave

 

 

 

 

[psyco_1322]

Black powder will start to chuff if you add over 5% mineral oil to it. Not sure if its cause by the same reason, but something to consider at least.

[anapogeetoofar]

Chuffing is a sign of low pressure. Either get rid of any sort of pressurization completely (there has to be some pressure for chuffing to occur) Or increase you're KN/chamber pressure.

 

In your case, it looks like you want to reduce the pressure.

[Seymour]

It generally occurs in a composite motor that is ignited too low in the grain.

 

I know a number of rocketry people from the New Zealand Rocketry association, and have witnessed many chuffing rockets, and have heard some tips (and seem them in action) to prevent chuffing.

 

Other than avoiding the obvious, by tying to ensure that ignition is not at the base of the grain, some sort of priming seems to be very effective.

 

One method that I particularly like is the "copper thermite" method. A small quantity of copper oxide/Aluminium is ignited by the igniter inside the motor. The several thousand degree fireball of Aluminium oxide and copper metal seems to do a thorough job of igniting the entire surface area of the grains rather instantly, and chuffing tends to be avoided!

 

Plus, since it's rather low gas, and high heat, it does it's work with less risk of over pressurization.

[dave321]

I know a number of rocketry people from the New Zealand Rocketry association, and have witnessed many chuffing rockets, and have heard some tips (and seem them in action) to prevent chuffing.

 

Other than avoiding the obvious, by tying to ensure that ignition is not at the base of the grain, some sort of priming seems to be very effective.

 

One method that I particularly like is the "copper thermite" method. A small quantity of copper oxide/Aluminium is ignited by the igniter inside the motor. The several thousand degree fireball of Aluminium oxide and copper metal seems to do a thorough job of igniting the entire surface area of the grains rather instantly, and chuffing tends to be avoided!

 

Plus, since it's rather low gas, and high heat, it does it's work with less risk of over pressurization.

 

syemour,

 

thanks for that, i'll try an alternative ignition arrangement / composition

the device was ignited from the central cavity at the bottom (quarter filled with chlorate/sugar mix)

 

dave