more propellant!

[web/gag.com] / rockets / research / 2017-batch-07.mdwn

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+# 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!