Static Test 67

KP/IO/Epoxy

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This is another test of propellant CXP-22, as found in the Experimental Propellants 4 page. For this batch I decided to use the T-4 motor casing, as it is a easier to make a graphite nozzle for this size casing, and I wanted to do a little smaller batch, as I'm just about out of KP. I decided to try to optimize this batch a little better by mixing the KP and iron oxide together by ball milling first. So the pre-weighed chemicals were ball milled for 30 minutes, then added to the epoxy and hardener that were also pre-weighed and pre-mixed.

The propellant was cast into 1.5" PVC casting tubes, lined with a single layer of .045" thick EPDM rubber to reduce the diameter, and a single layer of paper as a bonding layer. The coring tool was a .5" wood dowel, wrapped in wax paper.

I'm using a different brand of epoxy, this is Mr. Fiberglass brand epoxy. Richard Nakka has experimented with KN/Epoxy using Mr. Fiberglass brand with good results, hopefully the same will hold true here. I really should do some strand burn rate tests at pressure, but I'm not convinced my strand burn rate tests are entirely accurate. When I go to test this propellant in the motor, I'll be sure to keep my head extra low just in case.

Here are the grain numbers:

1) 2.65" @ 116.1 grams

2) 2.42" @ 105.8 grams

3) 2.54" @ 110.3 grams

4) 2.45" @ 106.2 grams

Total Propellant Weight: 434.4 grams, 0.9576 pounds

Total Grain Length: 10.06"

Diameter: 1.46"

Core: .51"

Density: .06479

Nozzle Throat: .5"

Kn: 142 initial, 166 max

The measured density is better with this batch than the propellant that was used in static test 55, however, it's still not where it could be. I did notice very small bubbles in the propellant when a small sample was sliced off with a razor blade. We're talking near microscopic in size, no voids were found.

In prepping the grains for installation in the motor, I decided to use two wraps of foil tape over the paper for a little heavier inhibitor layer. I also spaced these grains 1/4" apart by using another wrap of foil tape at each grain joint. The wrapped grains were then wrapped in one complete layer of EPDM rubber and inserted into the motor.

Burn Time: 1.367 seconds

Total Impulse: 145.72 pound seconds

Isp: 152.17

Peak Thrust: 310 pounds

Notes on the test: Not a bad burn, the motor handled the test with no adverse affects. If you look at the graph of the burn, notice the progressive nature of the curve. If the test followed the Kn, it would be a much flatter curve. I'm sure this progressive curve is due to the high pressure exponent of KP based propellants, once the pressure starts building, the burn rate increases, it's kind of like a the snowball effect.

The Isp of 152 isn't too bad, in fact, it's good enough that I'd say if I were buying KNO3 at $3 a pound for technical grade, I'd spend the extra 50 cents a pound on the KP, the epoxy is cheap enough too, just $35 for 5 quarts, that would make a lot of motors! One could drop the Kn just a little I think, and still get a good burn and more than likely a longer burn. I also think the propellant could be degassed prior to casting and you'd slow the burn down quite a bit more.