For this next series of tests I am casting the xylitol/KNO3 propellant into 1.5" PVC pipe. It's quick and easy and should provide more than ample inhibition for the outer grain surfaces. The PVC has an actual internal diameter of 1.6", and an outside diameter of 1.9". I'll use the grains in my LW-1 motor. Depending on how the tests progress, I have an nozzle insert retainer for the 2.25" LW-1 motor casing that I can use it to quickly turn a graphite nozzle to test different Kn ratios.
All propellant batches from now on, unless otherwise specified will use a standard mixture of 65% granular KNO3 and 35% granular xylitol. I have two sets of grains curing from batch XL-12 made on Jan. 21, 2005. I cast two more sets of grains on Jan. 24, 2005.
On January 29th I checked on the grains, the grains cast on the 21st were fully cured, the grains cast on the 24th were cured about 3/4" deep, the inside remaining soft. So 8 days will cure a grain that size, 5 days won't. So it looks like we're looking at about a week for an average size grain to cure. The grains cast on the 21st were burned in Static Test 75, while the propellant performed well, the Kn was way to low. For the next test I'll raise the KN to try to increase the performance.
A bit of a mystery popped up yesterday. I noticed my most recent grains had a significantly lower density than previous castings had. All the early tests had resulted in a density of .065 to .067 pounds per cubic inch, the most recent had a density of .0591 to .061 lb/ci. The question is why? So I went back through my early castings to see what I had changed. The only thing I could find was that I had left some unused propellant in the melting pot before starting a new casting.
So I emptied the casting pot and started a new batch. Here are the steps I followed:
A couple of notes.
First, I noticed the propellant was thinner than before. It seems for some reason remelting of old propellant results in a thicker mixture.
Next, I hadn't noticed the small bubbles before, that's not say they weren't there before, but I hadn't noticed them.
I heated the mixture at a higher temperature this time, 250 degree setting as opposed to 225 degrees. It is possible the KNO3 or xylitol has picked up some moisture from the bag being opened and closed. The bubbles could be that small amount of moisture boiling off. That's another reason I went with a higher temperature, to try to boil off any moisture that may be present.
Finally, I also heated the propellant longer, that gave the molten propellant more time to "degas", or boil off residual moisture.
Results:
After the grains cooled, I did a density check using density2 (software link). I found the density slightly better at .06281 lb/ci. Though still not where the early tests were.
So I cast another grain using dried, powdered KNO3 and granular XY. The density was about the same at .06245 lb/ci.
For a last test, I thought I'd try some propylene glycol, it lowered the viscosity of KN/sugar propellant significantly, which also improved the propellant density. So I prepared this batch the same as the last one, using powdered KNO3 and granular xylitol, stirred and heated at melting pot setting of 250 degrees (F). I stirred the mixture for 3-4 minutes after it was melted, then quickly stirred in 2% of the total propellant batch weight in propylene glycol. It seemed to improve the viscosity at first, but after it was all stirred in the propellant went back to a mashed potato consistency.
After these tests failed to provide an answer to the density question. I had to think some more. I believe I have the answer now. The one other variable from the early tests was that all those grains' density was tested after coring rods were removed and the grain ends trimmed. I think what happened was this: The coring rods were removed while the propellant was still soft, as the coring rod was pulled out it drew propellant in behind it, in effect, creating a smaller core by drawing propellant in from the end of the grain. When I calculated grain density, I used the coring rod diameter as the core diameter, not an actual measured diameter. So the density results I thought I was getting in the early tests were in fact, flawed. An actual density in the .062-.063 lb/ci should be about right.
| Test# | Length | Start Pressure | End Pressure | Aver. Pressure | Time | Burn Rate | Density |
| XY-4 | 1" | 351.0 | 788.5 | 569.75 | 4.016 | .2490 | |
| XY-1 | 1" | 346.2 | 798.1 | 572.15 | 4.166 | .2400"/sec | . lbs./cu in |
| XY-5 | 1" | 399.0 | 851.0 | 625.0 | 3.834 | .2608 | |
| XY-2 | 1" | 596.2 | 1091.3 | 843.75 | 3.333 | .3000 | |
| XY-3 | 1" | 846.2 | 1370.2 | 1108.2 | 2.967 | .3370 | |
| - | " | . | . | . | . | . | |
| - | " | . | . | . | . | . |
I thought perhaps I should get some burn rate data from the KN/XY propellant. So I prepared some 1" long CPVC cased strands of standard 65/35 propellant using granular XY and KNO3.
c or a (if you prefer)= .0115, n= .483
All the data points fell right on the plot line, so I'm fairly comfortable with the results in this pressure region of 570 to 1108 psi range. I added a software predicted graph of Static Test 76. If you take a look at the actual thrust curve they are very similar, again, leading me to believe the a and n numbers are pretty close.