Batch 2017-batch-09

8 grains of 75mm using sorbitol, intended to be a reload for my CTI Pro75 6xl case with Loki nozzle and forward closure. Grain length 4.375" with 0.125" spacing per grain, so we'll make the casting tubes 4.5" long. Delrin coring rods of 0.75" diameter.

To overcome shrinkage noticed in the last batch, fill lines set based on the total 4.5" casting tube length, on the assumption that any shrinkage will provide sufficient inter-grain spacing.

Formula for a batch size of 4800g:

• 65% KNO3, 3120g
• 35% Sorbitol, 1680g
• 1% red iron oxide, 48g, added after melt
• 72 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 15 October 2017.

Results

This batch made for a pretty full Presto, and a fairly long time coming up to temperature. It seems like 5kg might be the most propellant I should try to melt at one time. As an aside, that means for my 6" O project, we'd be melting and casting one grain at a time...

Average casting tube weight 15.3g.

• Grain 1, 542g gross, 527g net
• Grain 2, 548g gross, 533g net
• Grain 3, 552g gross, 537g net
• Grain 4, 546g gross, 531g net
• Grain 5, 544g gross, 529g net
• Grain 6, 540g gross, 525g net
• Grain 7, 528g gross, 513g net
• Grain 8, 538g gross, 523g net

Total propellant load 4218g. That's enough lower than the anticipated mass that I decided to "top off" the grains with more melted propellant from the left-overs in this batch. Will re-weigh after they cool/cure.