I was trying to devise a method to pressure test the clay plugs, since they were failing during motor firing. I discovered something odd. The clay plug from a used motor (I’ll have to test on a freshly rammed one soon) was clearly permeable to air. I connected the other end to a bike pump and dripped a little water on the clay plug to show just how bad the problem is. I have no clue if this is normal yet, but I assume not. I plan to take an existing Estes motor and check the clay plug for the same permeability.
In addition to the problems with the permeability, when I removed the plug from the tube there was clearly no sign of the outward convex bulge that should be locking the clay plug into place in the tube. Normally the pressure of the ramming will bulge the clay outward against the tube, creating an interference fit that will prevent the plug from coming out under pressure. I don’t see any visible bulge, so either it’s a lot less than I thought, or the plug wasn’t rammed properly. Also, the clay doesn’t look particularly compact to my eyes. I think these are all problems that need to be solved before I make any more motors.
I suspect that I’m deficient on two fronts: first, clearly the composition of the clay is off. It looks far too grainy which might mean I should be using less grog in my mix. Second, it’s clear that I’m not fully compacting the clay during the ramming process, so I’ll probably try and test ramming force at home using my 1-Ton press to see if that improves things at all.