38mm Custom Motor
 
January 11, 2003
 


Project Parameters:

Where to Start:

I set out to build my first "big" motor from scratch. I decided that since I was already flying 38mm motors, that this would be a good size to start with. First to find what size liners and casting tubes were available. A quick search found that the most common sizes available are designed for .083 inch walled aluminum tube. John "The Liner Guy" Lyngdal sold me some liner and casting tube sets and I was on my way. I got some Aluminum 6061 round stock on Ebay and the drawn tube from Online Metals. Now it was time to design the parts!

Click on the images above to download a dimensioned PDF drawing of the threaded aluminum nozzle


Same for the stainless steel nozzle insert- note the O-ring located on the insert. The insert has a sleeve that fits in the liner.


And the threaded forward closure- nothing fancy for now!


And the .083 6061 aluminum casings- nozzle and forward closure screw in either end.


The finished pieces

Another viewpoint

Knurling tool to the rescue! Sure makes it easy to take em back apart!

Click the image above for the design of the propellant with Sorbitol and KNO3- Thanks to Richard Nakka's Spreadsheet!


 

Conclusion:

The hardest part was machining the stainless steel. This might have been due to my inexperience however. Once I learned to just "take it slow" it went very well. The threads took a little "warm up" first to get them down before making the final pieces. I now cut just .003" at a time and get very clean threads requiring no clean up. If you make one of these, do plenty of testing to insure everything fits right before firing it up with propellant. Use it (like everything else on these pages) at your own risk! Note that the nozzle has been designed to fit 4 2.2 inch long 65/35 KNO3/sorbitol grains. This will need to be adjusted for your own motor based on your grain geometry and propellant type.

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