One of my least-favorite aspects of the KN/Sucrose fuel is its hygroscopic nature. It absorbs Florida humidity so rapidly that an engine made with it must be either fired immediately, or carefully sealed to prevent deterioration.
In an attempt to thward the permeating moisture of my area, I am experimenting with nitrocellulose lacquer as a coating for the grains.
My hopes are:
1. The fuel will be protected from ambient moisture for extended
2. The combustible nitrocellulose coating will not retard ignition, perhaps even assist it.
Here is a preliminary test, a small slug of KN/sucrose with a wart of corned black powder set in the end. It is painted with two heavy coats of green 10% nitrocellulose lacquer from Firefox. I got the green because they were out of clear, but am happy about that now because I can tell where the lacquer is and isn't. And the color is appropriate for this "environmentally friendly" fuel, don't you think?
This slug sat around for three days without apparent deterioration. An uncoated piece of the same batch had formed a puddle by the next day. But another coated slug (not pictured) had some cracks in the coating. It soon started getting soft at the cracks, and had seriously deteriorated in 24 hours. This suggests to me that this is an all-or-none process, that the coating must be perfect to start with and must be protected from any insult that might break the seal.
And it should be done quickly. I tried coating a piece that had sat out for an hour. It already had a moist coating, and the lacquer would not stick. Thus on my next experiment, I made sure the grain got coated as soon as it came out of the mold.
I dropped this slug in on top of an ignitor in an open paper tube with a few grains of black powder and applied current. The ignitor and black powder flashed but the grain did not ignite. I tried it again with more BP. Still no ignition. So I put several large grains of homebrew BP in the tube. This time it ignited and burned just fine. Apparently the coating makes it a bit harder to get the initial ignition going.
So I decide to try a larger grain and burn it in a case under pressure.
In other words, a static test. This one is made with a fairly fast-burning
batch or recrystallized KN/Sucrose, burning in open air at the rate of
12 seconds per inch. It was molded in a piece of 1-inch PVC pipe
with 17 inches of printer paper lining the inside, about 5-1/2 turns.
The paper is peeled off after unmolding to yield a slightly undersized
grain that will fit in the engine tube without a struggle
Many apologies that I did not think to take photos of this grain being made or prior to its coating. It was the jade-like mottled color that made me grab the camera. The grain is 4 inches long,1 inch diameter, has a 1/4 inch cylindrical core, and weighs 78 grams.. It is uninhibited, and has four large grains of corned black powder pressed into the head end. They are the ugly black things in the third photo from the left. I figure that if I can get those embedded BP grains to ignite, the candy underneath can't help but burn, spreading flame throughout the case toot sweet.
After the first experiment failed to ignite twice, I want to be really
sure with this one. So I make a Warner Brothers looking ignitor.
My normal ignitor is a thin copper filament soldered between the leads
of a twisted pair of wires salvaged from old network cable. This
is laid on a piece of masking tape, and a small amount of ground-up black
powder poured on it. The tape is folded to seal the powder around
the wire. In this case, I am using a generous amount of powdered
BP and supplementing it with three large, long-burning chunks. Total
BP weight is about two grams.
This ignitor is too large to push up through the 1/4 inch core, so the leads are passed through the core and the engine assembled around the ignitor. Apologies for the bad photos - I forgot to turn on the lights. The engine casing is once used. I find that I can use these pvc cases twice, sometimes three times. When I cut this one open, the "5-minute nozzle"* was intact, the case barely scorched, and since this is a low-tech ignition test why not. Nozzle throat is 3/8 inch diameter, yielding an initial Kn of about 156.
(*Packed Bentonite clay - takes about 5 minutes to make)
Click on the thumbnail below to download video of the burn.
Beware: This a an 800k .mpg file, yielding 4 seconds of video.
Upon ignition, there was a scary "snap" followed by a brief pause, then a quick build to pressure. The thrust-burn lasted about 0.7 seconds, with a little after-burn.
View of case after the burn.
My guess: The powdered BP priming caused the snap, and created a quick flash in the case but not enough to ignite the grain wholesale. Candy at the head-end did get lit, and once it started burning the nitrocellulose coating on the rest of the grain went away pretty quickly.
This is encouraging, so I vow to try it some more.
Click here for experiments of 11/2/01