(Edit) The following is the text portion of Bill Kimbrough's Girandola Seminar. He provided it to me to convert to a document
and gave permission to post it online. Scanned pictures of the scanned pictures are also appended but not all were in good
enough shape to be included. The original pictures were not available. (Dan Creagan)
Bill Kimbrough's Girandola Construction (Motors)
If there is one secret to girandolas, the secret is the construction of the drivers. When building a
fireworks rocket, the idea is to produce a lot of thrust over a very short time and get the heading up to
break height with as little composition as possible. Rockets normally have a long open center called a
core and burn all of the fuel very quickly, within one or two seconds, and then just coast till reaching
break height. Girandola drivers on the other hand depend on having a continuous thrust over their
entire active phase and even though the thrust is much less than the rocket, the thrust continues for as
long as 10 seconds or more for most drivers. Therefore, most girandola drivers are called end burners
and are just a solid slug of fuel burning on one end with the gases exiting through a nozzle. Girandolas
can be made with cored rockets but their performance is not anywhere near what is normally expected
out of an end burning driver.
I normally use plain 75/15/10 powder for drivers and mill with the best ingredients that I can reasonably
find. I have made powder using fertilizer nitrate but when compared to the technical crystal nitrate I
found the burn rate was around 25% slower. I normally start with willow or hackberry charcoal and
weight out the appropriate amount which in my case is 300 grams. I run the mill for 30 minutes and
then add 1500 grams of potassium nitrate and 200 grams of good sulfur and mill for several hours.. I
have a timer on the mill so that it is run for the same amount of time for each batch of powder. I empty
the mill and then run another batch, trying to use identical materials and identical technique to
minimize variations.
Once I have a five gallon bucket full of powder, I am ready to start making drivers. I used to granulate
the powder by adding water and then running it through a 1/8" mesh screen into a tarp to dry but I
discovered that even though this really helps in the loading and keeps the dust down, I needed to use a
lot more pressure when loading the drivers or the tubes would blow out from overpressure. Now, I just
use the powder as it comes from my mill.
I press my drivers and a 3/4" tube requires 100lb of line pressure on a 6" cylinder to consolidate the
powder appropriately. That translates to 2827 lbs of pressure on the ram and 3768 psi on the powder.
That much pressure tears up the unsupported tubes really badly and therefore they must be loaded
inside a sleeve. I have been lucky in being able to find stainless steel pipe with an inside diameter just
about perfect to fit the outside diameter of the paper tubes I use. To insure that they will slide in and
out easily, I rub "Butcher's Bowling Alley Wax" inside the pipe liberally about each fourth or sixth one.
The nozzle material that I use is bentonite/kyanite 50/50 by volume. Once the bentonite is mixed with
the kyanite, I also add 5% wax to the mix to help keep it consolidated and to protect from easy moisture
penetration. The way I do the wax is to dissolve 50 grams of toilet bowl ring in Coleman fuel (200 gm)
and pour over the mixed nozzle mix, stirring well with gloved hands. I then set it out to dry.
A trick that really helps with the ignition problem in drivers is to press a small piece of black match into
the fuel at the same time you press the fuel grain. The way I do that is to press all the nozzles first into
the tubes so that the tubes have the nozzles only, no fuel or match. I make a box of these nozzled tubes
up and then change the spindle which I use to form the nozzle to a spindle with a longitudinal hole in it
into which I insert a piece of stiff black match with is 1/2 in the fuel grain and 1/2 sticking out of the
nozzle. I then put the nozzled tube back onto the spindle and base and then press in the powder.
The spindle base has a hole which allows me to change the spindle itself and then the spindles are held
in place with two set screws. The base of the spindle has an angle machined into it of about 60 degrees
and then the top of the spindle is about 3/4 inch long and tapered about 2 degrees. The spindle just
barely penetrates the nozzle mix and does not penetrate into the fuel grain much - just really makes a
tiny dimple in the fuel. If you use much of a penetrating nozzle with a really hot fuel, you will get a big
pressure spike and most of the time pick up your girandola from pieces all around you.
The rammer, which makes the inside of the nozzle needs to have and angle of 60-40 degrees
determined by your experience. If the rammer is flat, there is turbulence in the gasses and the tubes
will burn through immediately above the nozzle much faster than when an inside angle is used.
I normally use a flat rammer with a hole in the center to avoid the black match and the hollow spindle
for the first ramming. I then turn the rammer over to just a flat end with no hole to complete ramming
the driver. I normally use a conical shaped scoop to load the nozzle mix and then another size for the
fuel mix as a cone fills evenly and easily. It is important to use the same amount of comp each time you
ram a nozzle or ram a fuel grain. I normally try to press the powder increments so that the pressed
increments each are equal to or less than the diameter of the inside of the tube. The better the
pressing, the less surging in the burning fuel.
One you have completed three drivers, then it is important to test them to see if they will work for your
intended purpose. My initial test is to just stick them upside down in the soft dirt and try them out. If
they don't blow out, have a good burning sound, and burn for about the right amount of time then I will
try to fly them. . I find that a 3/4" tube burn rate should be about 1" per second. Once I'm satisfied they
are about right, I tape a stick on one and try to shoot it out of a rocket launch tube just like a rocket. If
the driver flies on this test, add a paper tube on the top and start adding weight until it won't fly. Then
back off till it just barely flies and that will be your maximum takeoff weight.
I actually use a computerized test stand built out of junk yard parts and find it really valuable for
determining the thrust of the drivers and for that matter rockets. It is amazing how much variation in
thrust there is when you make very simple changes in the way the material is loaded or the angles of the
nozzles or the composition of the fuel.