I've been reading a lot on hybrid rocket engines over the Xmas break and although I am nowhere near ready to actually start building a hybrid engine (apart from that micro hybrid I started YEARS ago maybe!) I wanted to do something practical and decided to turn out a graphite nozzle on the lathe. Mainly to get a feel for how graphite machines ... which the answer is.. astonishingly easily! It cuts very well with little regard for speeds and feeds and responded well to standard HSS tooling.
Although I am working through the maths in the hybrid book I am looking at, I'm not actually at the point where I have a design. So this nozzle is just made with similar dimensions to a Laval nozzle drawing in the book with a 30 degree angle for the nozzle side and a 45 degree angle for the combustion side. I'm pretty pleased with it as a test piece and its certainly given me a feel as to what is possible in graphite.
Above is the 45 degree side that would face the combustion chamber.
The 30 degree nozzle side and the arbitrary sized throat.