I have a related question. How do people on this list actually make the
shape of their nozzles? It's all well and good to be able to write out the
equations, but how do you turn it into hardware?
I expect the most common answer is "I make a conical nozzle on a lathe.
That produces sharp angles in the throat so then I turn the throat into an
ad-hoc curve using sandpaper." That's all I've ever done.
Another answer might be "I program the equations into a CNC mill." How
easy is that to do? Do you use any particular software? Is there a
particular approximation like parabolic that is best suited for doing that
coding?
Another answer might be "I work out the diameter at N points along the
length, cut N disks out of plywood, stack them, turn them on a lathe to a
smooth surface, and use that as a mandrel." For anyone who's done that,
how long did it take? Was it worth it to you instead of just making a
conical nozzle?
One reason I'm interested is that your final hardware is going to be only
an approximation of the equation that is an approximation of ideal. How
good of an approximate equation you need is limited by how close you can
approximate the equation in hardware.
Bob
On Wed, Dec 7, 2016 at 8:29 AM, Graham Sortino <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
Hi All -
Thanks for everyone’s feedback on this. I really found it interesting that
there doesn’t seem to be a generally accepted approach to designing the
parabola of a nozzle contour and that this sparked so much debate. Perhaps
this would make a good inclusion in the next version of Rocket Propulsion
Elements?
The reason I originally asked the question is that I wanted to manually
calculate a parabolic curve function that closely approximates the one that
RPA produces. I want to manually calculate this mainly to make sure ensure
I’m comfortable with the approach used by the software and can
compare/contrast any differences. I’m not as interested in reproducing the
method of characteristics since it seems that Rao’s approximation is “good
enough”, which is what I’m aiming for (eg. rather than theoretically
perfect). It also seems like the method of characteristics implementation
is non-trivial. My goal is the 80 degree truncated bell design.
The closest example of doing the parabolic approximation was a post from
Henry Spencer that linked to a 2004 PDF produced by Narayanan Komerath who
is an aerospace professor. Here is the link to the post
https://web.archive.org/web/20090806205105/http://www.adl.
gatech.edu/classes/ae3021/ae3021_f06_9.pdf (see pages 11-15)
The math in Komerath’s seems straightforward but after checking it a few
times I’ve been unable to reproduce it. You can find my attempt here in a
Google Sheets file: https://docs.google.com/spreadsheets/d/
12UIVRPVQXPRIUxpTFkRjNFguH611V1GvDIq-Glvj5wA/edit?usp=sharing. My guess
is that I’ve made some incorrect assumption when calculating the first
unknown P or the X’,Y’ variables.
If anyone has any suggestions as to how to solve this I would like to post
this somewhere so that others can use it as a reference point for building
their chamber geometry.
Kind Regards,
Graham
