Test 1
We decided to start out with a Sorbitol mix. Following Scott Jolley's lead, we procured a sample of Polystep-B1 surfactant to make the propellant pourable.
A quick burn simulation suggested 3 grains in an Aerotech 38/360 case with 0.250 nozzle throat would be something like an H125 with Kn in the low 300's, fairly even burn rate, using around 177g of propellant. It looks like this is in a burn rate plateau, as simulating with the stock nozzle size of 0.180 inches yielded nearly identical results, just with higher chamber pressures.
Aerotech 38mm grains are 1.875 inches long, and our casting tube measured 1.260 inches inside diameter. We used a Tru-Core casting set with 0.5 inch diameter mandrel. The plan was to cast one long grain and then cut it into 3 grain-length pieces, so we started with 6 inches of casting tube weighing 4.75 grams hoping to get at least 5.625 inches of good grain after saw / knife kerfs.
Formula
Since we need 177 grams, let's make 200 grams of propellant in this batch.
- 65% Potassium Nitrate (130g)
- 35% Sorbitol (70g)
- 1% Red Iron Oxide, added post-melt (2g)
- 1 drop per 100g Polystep-B1 (2 drops)
Process
The grains were cast on 30 May 2015.
We measured out the potassium nitrate and sorbitol into a sealable plastic container and shook vigorously to break up the clumps in the sorbitol and get a good physical mix.
The mix was dumped into a Presto Multi-Cooker and heated with the dial set to something like 225 degrees F. Once it started to melt, we stirred almost constantly with a silicone spatula.
Once the sorbitol was all melted, we added the red iron oxide and stirred until it was all blended in. Then we added 2 drops of surfactant and continued to stir. The surfactant clearly reduced viscosity as shown by lifting the spatula and watching how the material dripped off.
We started to pour the propellant into the casting tube on the stand, but fairly quickly the mixture got thick as it cooled. So we put it back on the heat, with the control turned up to something like 275. Stirring constantly, we noticed it start to smoke and immediately pulled the power and started to pour again. This time we got the casting tube filled pretty easily.
We set up some folded card stock in a piece of aluminum angle and poured the remaining propellant in to form a test strand for open air burning.
Clean-up was done using water and paper towels, including boiling water in the multi-cooer to help soften the residual propellant. Pretty easy to clean up actually.
About 4 hours later, the card stock was pulled off the test strand, which proved to still be a bit on the pliable and "tacky" surface side.
Observations
After casting, further reading on Nakka's site indicates initial melt is at about 91C, or 196F. Fully melted at 125C, or 257F. Discoloration at 175C, or 347F.
Nakka says wait 24 hours before trying to pull the core rod. The propellant will remain pliable after cooling, thus can be deformed if handled sooner.