"...if it's a LOX rocket the injector at least needs to be stainless steel
to handle the temperature."
I have had good results with aluminum injectors on several motors under 1000
lbf and 375 psi chamber pressure. Also, my SDSU students did a brass
co-axial injector (~500 lbf, 900 psi Pc). I only mention this because I find
drilling ~1 mm orifices in stainless steel (even with a CNC mill) to be a
pain, but others may be up for the challenge.
--- Carl
