Test 3

First attempt using Erythritol. Simulation suggested that putting 5 long grains in a 38/720 case would yield a reasonable motor.

Formula is a simple mix of 65% KNO3 and 35% Erythritol. Both used in granular form directly from stock. Because Scott Fintel's site indicated near infinite pot life in the multi-cooker, We decided to melt enough for 8 grains, or a 520 gram batch size.

Before starting this batch, we cut our two delrin mandrels in half to get 4 single grain mandrels.

Tru-Core casting set with four bases each set up with 0.5 inch mandrels. Four pieces of casting tube cut to grain length of 2.250 inches.

Process

The multi-cooker was preheated to about 250F with the lid on. Temp was increased during melting to 300F indicated.

We poured/scooped one grain at a time, leaving the multi-cooker heating and putting the lid back on between grains to keep the propellant at temperature. The propellant was really hard to pour into the 38mm grains with mandrel already in place. Lots of drips over the side. We reduced heat to about 225F while waiting for the first 4 grains to solidify. It was easy to pull the coring rods after an hour or so. The caps were a bit hard to remove.

Given how much trouble we had pouring into grains with the mandrel already in place, we paused to fabricate some new casting bases using the CNC router and left-over hickory flooring material. The resulting bases are used with aluminum foil to hold a casting tube each. The casting tubes are marked with a fill line on the inside ahead of time, then the mandrels are pushed into place after the propellant is poured, penetrating the foil bottom. A cap is then placed over the mandrel and casting tube to keep the mandrel centered. This worked a lot better! We called this test 3B.

Results

  • Test 3, Grain 1, 61.78g net
  • Test 3, Grain 2, 61.54g net
  • Test 3, Grain 3, 61.84g net

  • Test 3, Grain 4, 61.89g net

  • Test 3B, Grain 5, 60.36g net

  • Test 3B, Grain 6, 61.23g net
  • Test 3B, Grain 7, 60.40g net
  • Test 3B, Grain 8, 60.79g net

Observations

The new casting bases worked very well, but we did notice a problem with the caps. When the propellant is carefully controlled to completely fill a grain, it touches the bottom side of the flat caps when still hot and wet. When it cools and contracts, this leaves a thin skin above a ring of mostly air. Cutting the "skin" off the top and scraping a bit to leave each grain with a slightly concave top so there'd be burn space between grains left us with remarkably consistent looking and weighing grains. In future, we might want to either fill the casting tubes slightly less, or perhaps switch to caps with an explicit air space above the poured propellant in hopes that all bubbles would "surface" leaving the propellant solid and with a bit of air gap at the top of each grain's casting tube.