This article was provided by Ned Gorski.
If you look in the end of most black powder rockets, or at the end of a gerb (fountain), you’ll see a clay nozzle recessed into the end of the paper tube.
A nozzle is a mechanical device with a hole in it, which controls and directs the flow of a liquid or gas as it passes through it. Think of the nozzle you put on the end of your garden hose. It controls the water flow, builds up higher pressure in the hose than would normally be there, and projects the water out in a nice stream. A rocket nozzle does essentially the same thing with the combustion gasses from the rocket motor. This is what propels the rocket skyward.
Typically, the nozzle in a rocket, and the solid plug at the top of the rocket motor’s fuel grain, is a rammed (hand pounded with a mallet) or pressed (with a hydraulic or mechanical rocket press) mixture of wax, clay, and grog. Some folks use only clay in their rocket nozzles.
I did that for a while, but found that the clay nozzles were very susceptible to shrinkage/expansion, depending on the day’s humidity. One time I pressed a bunch of wheel drivers with only bentonite clay nozzles, here in the Midwest hub of humidity. Then, when I got out to Gillette, Wyoming, which was so dry my lips started cracking, my nozzles got so loose in the rocket tubes that I could turn them with my finger. (I quickly added a ring of Elmer’s glue where the clay nozzle met the tube to secure them.)
Some folks expect their rocket nozzle apertures (hole) to close a bit with the clay’s expansion. So, right before flight, they open the hole up to the correct diameter with a hand-twisted drill bit. Adding wax makes the clay much less prone to this problem. Also, the clay alone, when pressed, forms a smooth, glossy surface; and nozzles and plugs have been known to get blown out of the tube by the pressure of the fuel burning. The grog in this rocket nozzle mix really helps the nozzle ‘bite’ into the side of the rocket tube and resist blowout.
The grog also helps the rocket nozzle resist erosion of the hole during motor burn, whereas without the grog, the clay can wear away some and the nozzle aperture (hole) opens up some during the motor burn, which reduces pressure and thrust.
The technique I use to formulate these ingredients and mix them together is similar to the one David Sleeter recommends in his Amateur Rocket Motor Construction book.
I get the wax that I like to use from the canning supplies department of my grocery store. It reads "Household Paraffin Wax, for canning, candlemaking, and many other uses." (I’m not sure why they don’t list rocket nozzles on the box as one of those uses.)
I either use bentonite clay from Skylighter or Hawthorne Bond Fireclay. They are both very fine, powdered, dry clay. (When I first started making rockets, I imagined that ‘clay’ should be like putty, or that I had to turn the dry clay powder into a ‘play-dough’ by adding water. We live and learn. No water is ever added to the clay.)