Notes from paraffin fuel grain pour (August 17, 2003)
Fuel Grain 081703A
99% Paraffin
1% Graphite
237.7 gram fuel mass
Procedure
First attempt at making paraffin fuel grain. Paraffin was heated in a ‘Sunbeam’ crock-pot. Thermostat was set at 300 degrees Fahrenheit.
Ingredients:
759 gram paraffin slab (160 deg hurricane paraffin wax)
7.6 grams of graphite powder
Paraffin took approximately 15 minutes to melt and was constantly stirred.
While paraffin was melting the 0.49” aluminum dowel (used to create the central port in the fuel grain) was inserted into the unfinished graphite nozzle blank. The mandrel was sealed into the nozzle throat using a small amount of adhesive putty in a ring shape. The 2.0 inch (I.D) cardboard tube was then placed onto the nozzle shoulder and then the graphite premix chamber was placed on top of the cardboard tube. The seams between the tube and graphite pieces were covered with electrical tape to prevent the molten paraffin from seeping through.
The entire assembly was placed in a foil tray and the molten paraffin was ladled in. The ladle was fairly big and clumsy so the paraffin had a tendency to spill down the side of the cardboard tube. This was not a big issue since the outer layers of cardboard have to come off anyway in order to fit the fuel grain inside the 2.07 inch (I.D.) aluminum combustion chamber. In the future it would be better if we had a more accurate way to transfer the paraffin.
To speed the cooling process the fuel grain was then place it a refrigerator at 37.2 degrees Fahrenheit for 15 minutes. Some shrinkage of the paraffin was expected upon cooling so the grain was overfilled by about 0.5”, using the graphite premix chamber as a riser for the molten paraffin. After the 15 minutes the paraffin had dropped about 1.5 inches in height and had a large ‘mushy zone’ between a solid paraffin wall on the cardboard tube and one on the aluminum mandrel. More molten paraffin was added. This time to about 0.5” from the top of the graphite premix chamber. It was assumed that the molten paraffin would re-melt the solidified surfaces and create a good bond. The assembly was then placed back in the refrigerator and allowed to cool for 2 hours.
Upon removal the grain was completely solidified and somewhat longer than needed. The graphite premix chamber was removed from the assembly and the paraffin trimmed to size. The coloration of the top and bottom of the grain was similar indicating that the settling of the graphite powder in the molten paraffin was not significant.
The mandrel was easily removed from the paraffin at this time using a pair of vise grips on the larger diameter ‘grip section’ at the top of the mandrel. The inner port surface seems quite smooth and there is not visible reason why this fuel grain cannot be used for the initial test firings.
Action Items:
- Better (cleaner) way to transfer molten paraffin from crock-pot to cardboard liner.
- Determine exact shrink rate so we know how much to overfill into riser section.