Batch 2017-batch-07

5 grains of 75mm using sorbitol, intended to be z reloads for one of my resurrected CTI 5-grain cases. Grain length 120mm with 2mm spacing per grain, so we'll make the casting tubes 122mm long. Delrin coring rods of 0.75" diameter.

Formula for a batch size of 3200g:

• 65% KNO3, 2080g
• 35% Sorbitol, 1120g
• 1% red iron oxide, 32g, added after melt
• 45 drops Polystep-B1, added before pour using eye dropper

This formula is completely fluid with 225F indicated on the Presto Multicooker, but viscosity is noticeabley lower making for easier pouring at 250-275F.

These grains were cast on 28 September 2017.

Results

Average casting tube weight 16.3g.

• Grain 1, 590g gross, 574g net
• Grain 2, 594g gross, 578g net
• Grain 3, 592g gross, 576g net
• Grain 4, 584g gross, 568g net
• Grain 5, 580g gross, 564g net

Total propellant load 2860g.

Grain 5 had an obvious "dip" in the surface, presumably where a bubble came up during cooling, certainly enough to account for the lower mass on that grain.

All grains had far more "empty space" at the top than expected or desired, making the shrinkage while cooling very obvious. I think as the grain size gets larger, it might make sense to calculate fill lines based on the full length of the casting tube and just let the shrinkage provide the inter-grain spacing, rather than trying to incorporate such spacing explicitly? Just need to avoid filling grains so much that they "overflow" when putting in the coring rod. More thought needed here.

Otherwise, the grains look fine, can't see any reason not to burn/fly them!