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I cast four grains for the T-3 motor, using standard 65/35 KNSU with propylene glycol, I added 1% of the total in powdered MS (magnesium sulfate) to the dry KNO3 and sugar and mixed by shaking for several minutes. The grains were cast into PVC casting stands lined with a single layer of tagboard. To make sure the cores were properly centered, I used PVC end caps that were bored out on the lathe to hold the tops of the coring rods in place. The grains were wrapped in foil tape to complete the inhibition of the grain outer surfaces.
A couple of things I noticed: First, the grains had some dark brown specs in them. I'm not sure what could have caused that. Perhaps the MS reacted somehow with something in the propellant. Second, the grains seem damp. Is it possible the MS makes the propellant even more hygroscopic? Perhaps that's the reason for the slower burn rate, simply more moisture is retained.
Grain Data:
Number of grains: 4
Grain Diameter: 1.5"
Total Grain Length: 9.76" (2.44" each)
Total Propellant Weight: 0.927 pounds
Core Diameter: .625"
Density: .06507 pounds/cubic inch
Kn: 222 initial, 244 maximum, 223 ending
Total Impulse: 114.48 pound seconds
Isp: 123.5
Burn Time: 1.5 seconds
Peak Thrust: 110.72 pounds
Peak Chamber Pressure: 560 psi (estimated)
Hmm, well that was interesting. It does seem to slow the burn significantly, and the Isp of 123 isn't too bad considering the fairly low chamber pressure. I wish I knew who it was that came up with the MS idea, I'd like to credit him. I'll do some more tests with the MS, but after this test it would seem to be a viable burn rate inhibitor.
The burn was just a little rough, I'm not sure why, but you could hear it in the actual burn and see it in the recorded data. It's not bad enough to give me any large concerns, and I'll see if it shows up in the next tests.
As for the thrust curve, it still looks to me like the propellant burns a little more "peaky" than expected, not as bad in this case though. What we do see is a fairly long trailing off of thrust. It's an even, symetric tail off of thrust. I believe it's from erosive buring. I see this in most of my motors to one degree or another. I see it more in longer burning motors, as is the case here. But I suppose that stands to reason, longer burn motors would give more burn time for erosion of the propellant closest to the nozzle. Further evidence is the lower propellant segment inhibitors are more burnt than the upper segments.
One could argue a larger core would help reduce erosive burning. While that's true, I'm not so stuck on perfect thrust profiles. I'm more interested in the total impulse delivered by the motor, and the burn time.