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I'm speculating a little that at higher Kn's, the propellant will perform with a thrust curve of the inverted "V" shape. That's the reason I designed the Kn with a somewhat regressive nature.
Here are the numbers:
Grain Dimensions: 1.6" ID, 1.973" long, .5" core
Number of Grains: 6
Total Grain Length: 11.84"
Total Grain Weight: 1.2791 lb., 580.2 grams
Kn: 271 initial, 291 maximum, 207 ending
Nozzle Throat: .435" diameter
Exit Cone: .76" diameter
Expansion Ratio: 1:3
Density: .05892 lb/ci
Here is the thrust/time curve from the test.
Here is a graph of the software predicted thrust curve. It's not too far off from what the actual curve looks like.
Here is the actual Kn curve of the motor.
Burn Time: 3.417 seconds
Total Impulse: 142.85 pound seconds
Isp: 111.68
Peak Thrust: 69.75 pounds
Chamber Pressure: 345 psi (estimated)
If you look at the thrust/time curve from the test. The first point of note is the drop and peak shortly after ignition. I used a new motor casing, and it's very long. So I had to add a length of wire to my igniter, I think what happened is the igniter wire caught at the splice in the nozzle, blocked the nozzle for a moment and was then ejected. The theory is backed up by the fact I couldn't find the spent igniter. So I think we can disregard that anomaly.
The next point of interest is of course the long burn, with a significant increase in the Kn, I had expected a shorter burn and higher chamber pressure. While the burn is a little shorter and the chamber pressure higher, it's not a huge change in either. It looks like I may have to go well over a Kn of 300 to get chamber pressures high enough to increase the propellant efficiency (Isp).
Also note, the burn curve followed the Kn fairly well. I don't think I need a regressive Kn profile, at least not at these pressures.
After disassembling the motor I noticed something interesting. One grain stayed at the upper bulkhead end of the motor (the end pointing down) and the other 5 grains moved to the nozzle end. I'm not sure at what point that happened, but it looks like it would have had to have happened at ignition. There was about 15 inches of free room in the casing, I did say it was a long motor! That could be the reason the thrust build up was a little slower with this test. I suppose I should put a dead end at the forward closure end to keep the grains a little tighter to the nozzle end.