I'll let you in on a little secret right now, this test didn't go so well, and ended in the loss of the motor...
I was a little hurried getting the motor finished as I wanted to get it tested this evening yet, and the sun was getting low in the sky. But I really don't think it was my haste that lead to the demise of the motor. I calibrated my load cell for the larger, higher thrust motor in my shop, then loaded up and headed out to my test site.
Once set up, I started recording the data, video recorder was on.
5,4,3,2,1 and ignition.
The motor lit instantly.
Click Here for a video of the test. 3.26 MB
Here's the motor about .5 seconds into the burn. All is well at this point.
At about .8 seconds into the burn, there is a problem.
Oh yeah, there is a problem!
After the propellant burned out, my test stand continued to burn. Using my bucket of water I put out the fire.
Here's the thrust/time graph from the test.
Here is the casing after a little clean up.
It's pretty apparent there was a burn through in the casing wall. The damage in the picture looks a lot worse than it really was, as most of this damage likely occurred while the remaining propellant burned and enlarged a smaller hole. So what could have caused the burn through? It could have been a weak spot in the aluminum, but I rather doubt that.
More likely the cause was twofold. First, I used a thin walled cardboard tube as a thermal liner. The thin cardboard has worked in the past, but this was a larger motor. I should have stuck with EPDM rubber as a thermal barrier. The second part of the problem may have been inhibitor failure. The reason I think I had some inhibitor failure was a slightly higher thrust than expected, leading me to believe there was more propellant burning than should have been. If an outside surface of a grain was burning, it would have impinged on the cardboard thermal liner, quickly burning through it and overheating that spot on the aluminum casing. Had I used EPDM as a thermal liner, it likely would have held up to the inhibitor failure and the burn would have been more or less normal.
The good news is that the motor bulkhead and nozzle are fine, the only thing lost is the casing which is the easiest part to make. The bad news may be my test stand load cell, it took a lot of heat and is likely toast, but I'll have to check it out to see.
So EPDM as thermal insulation it will be, as well as a heavier inhibitor.