What is the best nozzle configuration?

I've been making some of my own rocket motors. I have noticed different recommendations for nozzle shape. I have been using a 15 degree conical
exit cone. I have seen articles that recommend ~ 18 degrees. Some recommend a bell shaped exit cone. Which is the best to give me max performance? What should the pressure side of the nozzle look like?
Thanks for the input,
KT
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The exit angle of the nozzle is probably the least important dimension. More important is the expansion ratio of the nozzle.

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Ok, So how do I figure that out and what is the best expansion ratio?
KT
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snipped-for-privacy@pacbell.net wrote:

I'm sure that books have been written on that question.
The optimal expansion ration will depend on several things including:
Chamber pressure Exhaust composition Expected ambient pressure
If you haven't discovered the program Propep, you can find it here: http://www.ibiblio.org/pub/archives/rec.models.rockets/PROGRAMS /
PROPEP will calculate optimal expansion ratios for a given set of parameters (propellant composition, chamber pressure, ambient pressure).
If you don't want to bother with all that theory, use an expansion ratio between 3 and 5.
-JT
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Thanks, I'll check out the book.
KT
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snipped-for-privacy@pacbell.net wrote:

In a nutshell, you just want to make sure that the exit area of the expansion portion of the nozzle (bell) is such that the pressure in the exhaust gas at the exit is equal to the ambient pressure. If the pressure is too high, this is called an under-expanded condition. If the pressure is too low (i.e. the nozzle exit is too large), then this is called over-expanded.
This is the reason the bells on the Saturn's 1st stage weren't too large, but the 3rd stage bell was pretty big. Also, some ICBMs (and I believe the upper stage on the Delta lifters) have a shroud that comes down to extend the nozzle during flight, as the ambient pressure drops.
Dave
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