KNO3 / Erythritol Propellant
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Using powdered KNO3 I expected a very thick propellant, the same as with using xylitol, and I wasn't disappointed as the melted mixture was very thick. Once melted it required spooning and tamping in the molds to settle the propellant. Density wasn't too bad a .0613 pounds per cubic inch, but I think using granular KNO3 will improve the density.
I cast these grains pretty much the same as I cast regular sucrose propellant. I used a single wrap of paper in a PVC casting tube, and wax paper covering the bottom of the casting tube and the coring rod. While it did work, the propellant did seep through the paper and stick to the PVC casting tube some. Next batch I'll use a heavy coated paper to prevent this. Once the grains were trimmed to length I used a single layer of foil tape as an outer inhibitor.
Grain Specs:
Number of Grains: 5
Grain Length: 2.216" average
Grain Diameter: 1.55"
Grain Core Diameter: .57"
Total Grain Length: 11"
Total Grain Weight: 1.099 pounds
Nozzle Throat Diameter: .42"
Kn: 261 initial, 283 maximum, 236 ending
Propellant Density: .0613 lb./cubic inch
Here is the thrust/time curve from the test.
Burn Time: 4.3 seconds (of recordable thrust)
Total Impulse: 79.79 pound seconds
Isp: 72.6 seconds
Chamber Pressure: <200 psi
While these numbers look pathetic, in reality, it's not that bad. The motor burned for several seconds before coming up to pressure and recording thrust. Then at the end of the burn the motor again burned for a couple of seconds, then chuffed, burned for a few more seconds then stopped. So there was a lot of time spent burning propellant without recording any thrust. Obviously the Kn needs to be increased, and a more powerful igniter is in order as well. With the very low chamber pressure I'm a little surprised there was any stable burning at all.
I knew the propellant was a little hard starting, and I considered adding some BP green meal as an ignition aid, I think it would have helped in this case, and I may do that with the next test. I'll bump the Kn up over 300 for the next, that should help stabilize the burn considerably.