I degassed the propellant for 2 hours before mixing in the curative, then again for about 5 minutes before packing into 2" OD PVC casting tubes (vacuum tube). The propellant was hand mixed, the propellant is quite thick and the 700 gram batch would be about as much as my arm strength could endure. I used the same motor casing and nozzle as I had used the day before.
Number of Grains: 3
Length: 9.59" (3.196" each)
Core: .75"
OD: 1.9"
Propellant Weight: 1.1532 pounds (523.1 grams)
Density: 0.0503 lb./cubic inch
Kn: 156, 175, 161
Here is the thrust/time curve from the test.
Here is a capture shortly after ignition.
Click Here for a short video of the test. (692 KB)
As you probably noticed from the graph, this was not a nominal burn. The initial pressure spike was around 1,800 psi. As a result the divergent portion of the graphite nozzle broke off. While the convergent part of the nozzle remained intact, the throat did open up somewhat as a crack extended into the throat.. Without a divergent exit cone, no meaningful data can be extrapolated. However, the motor did produce 205.32 pound seconds of total impulse for an Isp of 178.
I must admit, I was a little hesitant to test this motor as the propellant density was so low. When I trimmed the grains, I noticed a lot of very small bubbles. The low density combined with CuO acting as a burn rate catalyst resulted in a very fast burn. The loss of the divergent end of the nozzle also resulted in overheating of the casing in the nozzle area. That overheating and the high pressure the motor experienced will lead me to retiring this casing.
I'm not entirely sure why the propellant density was so low. It was fresh dry AP and I degassed the propellant for over 2 hours. There must of have been some moisture somewhere that reacted with the Mg to create the small bubbles. To be honest, this wasn't a propellant formula I really intended to use anyway. It was more just an attempt to find a novel way to use up my remaining Mg. I'm not a big fan of fast burn rates, and this is a fairly fast propellant even had it been a nominal burn.