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Another thing I want to look at during this test is the actual burn profile while using a regressive grain configuration. In the past, most of my grains were progressive in nature, that's a bad habit I got into simply because I was too lazy to cut the propellant into more grains.
The reason I want to look at these things, is because I've been looking at the thrust profiles of some of my old tests. They seem more progressive than they should be, it could be inhibitor failure, that's really what I'm checking on.
Grain Diameter: 1.77"
Grain Length: 2.4"
Number of Grains: 5
Core Diameter: .87"
Total Propellant Weight: 1.39662
Nozzle Throat Diameter: .625"
Kn: 168 initial, 171 maximum, 141 ending
Density: .0624 pounds per cubic inch
Looking at the thrust curve above, it's clear the burn was still slightly progressive. I wouldn't read a whole lot into the tail off portion, as a couple of the cores were a little off center, and I'm sure erosive burning has some affect as well. The low Isp of 114 seconds isn't unexpected, as the test was performed at a very low Kn, resulting in low chamber pressure and a nozzle that was not optimized for this pressure range.
Burn Time: 1.717 seconds
Peak Thrust: 172.05 pounds
Total Impulse: 158.81 pound seconds
Isp: 113.7 seconds
Estimated Chamber Pressure: 420 psi
Here is what the curve should have looked like had it followed the burn rate formula.
Post firing inspection of the motor revealed no problems with the motor, the inhibitor however, and to my surprise, was burned through on almost every grain. The burning appears to have occurred from the inside, not the outside. I believe it was a result of the slightly offset cores. When the thin side of the grain burned out, the remaining propellant on the thick side burned through the now exposed inhibitor layer. I was going to make some centering caps to prevent the offset cores before I cast these grains, but I was out of the 2" PVC end caps I needed. I'll pick some up before the next test.
So, assuming there was no inhibitor failure during the burn, and I don't think there was, the burn was still progressive. A few explanations come to mind.
1) The propellant doesn't follow the de Saint Robert formula.
2) There is more erosive burning than usually expected.
3) There was inhibitor failure, exposing more grain surface to burning.
4) Chuck Knights "flushing" theory is correct and we see this to a lesser degree in standard KN/SU propellant.
More than likely it's a combination of points 1, 2 and 4 above.