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After casting, and before the grains were completely cooled, I tried to remove the coring rods from the grains. For some reason, the rods would not slide out. So I waited until the grains were completely cooled and tried again. Still no luck, so I had to tap out the rods with a wood dowel and hammer. Now, there is some risk of cracking a grain when you pound out a coring rod, and I did manage to crack the upper inch of one of the grains. It takes very careful inspection to find cracks in a grain. Of course, I check all my grains after casting regardless.
I did have a grain to spare, so it wasn't all that big a deal. But it did have me wondering why the rods were tight this time, and removed so easily on the casting before. The only explanation I could come up with was the extra heat I used for this batch, perhaps the increased heat melted the wax paper and caused it to stick. So for the second batch I reduced the heat back to 250 degrees. I waited exactly 30 minutes and attempted to remove the cores. That obviously wasn't the cause, as one grain came out fairly easy, the next had to pounded out with a hammer and a steel bar. The pounding had severely cracked this grain.
So I remelted the cracked grain, this time I went back to the trick I use with xylitol, that is to wrap the coring rod in aluminum foil, and tape the seam closed. After casting the grain I allowed it to cool exactly 20 minutes, the propellant seemed hard throughout, yet was still hot to the touch. The steel coring rod slid right out with no apparent friction at all. I'm not sure what helped more, removing the rod sooner or the aluminum foil. Probably the foil, but removing the core sooner helped too I'm sure. I think it's possible with these larger grains the heat is soaking into the coring rods, and melting the wax paper. Earlier tests used smaller grains, so there would have been less thermal inertia in them.
After the grains were trimmed to length, I did density checks on each pair of grains that came from an individual casting stand. One set of grains came in pretty close to normal at .06162 lb/ci, while the other two sets came in lower at .05764 and .0595 lb/ci. I also noticed small bubbles, or voids in the two sets with lower density. The grains with the lower density I believe were from the pour with the lower casting temperature. Not happy with the low density and the voids, I decided to remelt the entire batch and start over again. It was some time before the grains melted, then I removed all the cardboard from the melted propellant.
I decided to pull a vacuum on the propellant before casting this time. I pulled a vacuum for about 5 minutes, swirling the propellant in the melting pot the entire time. Small bubbles would appear and break as the propellant was moved about. I can't say the propellant foamed up at all, there wasn't a noticeable increase in volume either. But it was clear there were some small bubbles in the propellant. I released the vacuum, then pulled it down one more time for good measure.
I used the aluminum foil tape on all the cores this time, again pulling the cores out at the 20 minute mark. The cores came out so easily, the foil was intact and undisturbed on the coring rods, so I was able to reuse the coring rods without even recovering them.
Here are the grains after casting and inhibiting.
Here are the grains after the pyrogen was painted on, the PVC spacer rings are shown below.
Here is the convergent side of the nozzle.
And here is the divergent end of the nozzle.
Here is the assembled motor.
Here is the assembled motor with the nozzle covered with foil tape. The foil tape works very well to seal the motor, although with KN/ER propellant there is no worry about hygroscopic propellant.
Number of Grains: 7
Grain Length: 2.8036" each, 19.625" total
Propellant Diameter: 1.91"
Core Diameter: .76"
Total Propellant Weight: 2.931 pounds, 1329.5 grams
Density: .06196 pounds per cubic inch
Nozzle Throat Diameter: .5"
Kn: 410, 441, 379
Total Impulse: 395.78 pound seconds
Isp: 135.03
Burn Time: 3.583 seconds
Peak Thrust: 359.84 pounds
Peak Chamber Pressure: ~1,270 psi
Well, this test certainly gives food for thought. All in all, a very good test if you forget about the spike at the 1.95 second mark. I'm not sure what caused the spike, but I have a sneaking suspicion. I used a longer motor case than needed, the casing was some 8" longer than the propellant stack. My guess is that a grain or grains were slammed into the nozzle by flowing gases, cracking the grain and causing a moment of high thrust. Either that or I had a grain fracture on its own, but I think that is unlikely.
Other than the spike, a very good burn, chamber pressure would have been around a max of 1,000 psi, right where I wanted it. The burn time was very good as well. The time of the burn above 10% max thrust was about 2.4 seconds, or a burn rate of .2395"/second. The Isp was very good at just over 135 seconds making the motor a strong "K" class.
That was the last of my erythritol. I've got another 8 pounds on the way, but it looks like I should have ordered more right away. I'd like to try this propellant in my SBS-800 motor and see how it does in a 3.5" diameter casing.
Above is a picture of the nozzle after the test. It's a graphite insert and a steel retainer. You can clearly see the damage to the graphite. This isn't the greatest graphite in the world, as it's a medium grain graphite. But I've never seen this type of damage before, not even in composites running at 4,500 degrees. My guess is that this was caused by grain fragments hitting the nozzle. I'm still thinking a grain got caught in the gas flow and impacted the nozzle. The throat was long enough that there was no increase in throat diameter.
One other item to note. After removing the forward bulkhead, there was a nice size pile of residue built up on the forward closure. The pile of was nearly 2" tall, this isn't unheard of when testing a motor in the "nozzle up" position. But it was more residue than expected. I really can't get a good weight on the residue as it was fused to a grain inhibitor and the EPDM rubber liner in that area. Not really a concern of mine, just a note.