Static Test 89

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This test is an attempt to determine if increasing the opacity of an erythritol propellant grain will change the odd burn rate behavior the propellant exhibits. To that end I made a batch of 65% KNO3/35% ER and added 1% of the total in powdered charcoal. The KN and ER were weighed into a container first, the batch weight was 440 grams, to that I added 4.4 grams of homemade (near air float) willow charcoal. The dry mix was blended by shaking in a closed plastic container.

After setting up my casting stands and coring tools, I preheated my cook pot at a setting of 275 degrees F. The dry mix was added to the cook pot and melted and cast in the usual manner.

I spooned out a couple of small strands, they quickly cooled and hardened. I burned a strand in the open air. It seemed to burn a little better than the propellant without the charcoal.

The grains were spaced using 1/4" thick PVC pipe sections, with a gap cut into the ring to allow combustion gases to flow around the grains. The same procedure was used in the last test, static test 88.

I added a small amount of my green meal to the forward (down) end of the motor to aid ignition. I think with the addition of the charcoal the motor will light faster without having to use a paint on ignition aid.

Number of Grains: 4

Grain Diameter: 1.535"

Core Diameter: .775"

Total Grain Length: 10.11"

Propellant Weight: 0.82519 pounds

Density: 0.05923 pounds per cubic inch

Kn: 422 initial, 445 maximum, 417 ending

Burn Time: 2.25 seconds

Total Impulse: 85.16 pound seconds

Isp: 103.2 seconds

Peak Thrust: 89.9 pounds

Peak Chamber Pressure: 800 psi (estimate)

The propellant again didn't ignite easily, it burned for 6 or 7 seconds before the motor came up to pressure. Again we see the "crook" in the thrust curve at about 40 pounds of thrust, this relates to a chamber pressure in the 400 psi range. That's the point the thrust should have leveled off, instead, it again increased dramatically to a high peak. It is interesting to note the difference in pressure where this occurred between this test and the last test. Of course, the Kn in this test was some 20% lower, but even while the difference in pressure is great, the end result is the same. This would seem to further indicate that pressure alone is not cause of the high peak burn profile we're seeing here.