Batch 2021.11.12

5 grains of 75mm KNER, intended to be a reload for a Loki 76/6000 case. with Loki-style #36 short nozzle. Grains to be cast 5.25" long using Delrin coring rods of 0.75" diameter. Liner calculates to be 28 inches long. Will "glue" grains in liner using clear silicone and o-ring spacers.

Nakka's SRM spreadsheet suggests this could be an 84% L968, with 3473g of propellant, a 4.88 second burn time, and max pressure of 655psi.

Will mix a 3500g batch, the formula for which is:

• 65% KNO3, 2275g
• 35% Erythritol, 1225g

I started by setting the multi-cooker to 300F indicated for this batch, but had to turn that up to 350F to get the entire batch melted, or the heating element would keep turning off. It still took a very long time to all melt. Once it did, I dialed back to 300F indicated. It's true that the ambient temperature was lower in the shop than on other days I've cast sugar.

The 5.25" grains were poured to a 122mm fill line, and then an 0.75" Delrin coring rod was twisted down into the goo puncturing foil over the coring rod hole at the grain base. A casting cap was then placed over the top to hold the coring rod centered and protect the grain during cooling.

The first four grains poured easily. There was enough propellant sticking high on the sides of the pot that I couldn't figure out how to break loose, though, that I paused and added 3 old 38mm KNER grains to the pot and waited for them to melt and get mixed in before casting the last grain. The last grain was also problematic in that the casting cap didn't want to go down over the casting tube, perhaps due to a bit of KNER splash already hardening on the outside of the tube?

I noted that there were lots of small bubbles visible on the surface of the melt, all of which were popping easily at the time of the pour. I did the usual thing of twisting the coring rod as it went in and saw bubbles come to the grain surface, then tapped each grain on the table top after the cap was on to hopefully further settle the propellant. It's hard to imagine vacuuming KNER helping this, as the KN wants to settle to the bottom of the pot, requiring re-stirring to homogonize the mixture before pouring... which necessarily lets more air back in?

Results

The grains came out very poorly, with lots of cracks and voids. My guess is that since the shop was colder than normal, and I could see massive amounts of propellant solidifying on the sides of the multi-cooker, that rapid and/or differential cooling as the propellant was being poured into cold casting tubes and cold Delrin coring rods were inserted was the root cause.

The only credible choice is to set them aside to be re-melted in a future batch.