This is the result of my first motor. I used 1/2 inch aluminum rod from home depot for the nozzle and forward closure. The case is 1/2 inch seamless aluminum tube ( 6061 ) from McMasters with a .049 inch wall. The forward closure and nozzle are "turned" on my 1/2 inch drill press. I found that a carbide cutting tool clamped to the bed of my drill press made for an accurate straight cut to size the parts for threading. A 7/16 32 thread tap and die were used to thread each part. The nozzle is based on designs that Bill Bullock sent to me, and has a 30 degree convergence angle, with a 12 degree divergence. My original design called for O-rings where the parts meet as can be seen in the image but Bill pointed out that the threaded fit would be adequate for sealing. He was right!
I heard a number of concerns when I posted a question on Arocket regarding using aluminum for a nozzle so I made this version with a graphite nozzle insert in case the other nozzle melted. I turned the 1/2 inch graphite rod on the drill press as well (again based on Bill's design).
This is what it looks like disassembled with both nozzle options. The graphite nozzle is quite a bit lighter.
Detail shot of the nozzles.
New propellant grain with kraft paper inhibitor
Oh, yeah the propellant... Well the original plan was to "cast" the propellant in a paper sleeve to inhibit burning. Unfortunately it was not so easy to do at this scale. I made a form, lined it with a paper cylinder, but the paper got wrinkled when I tried to get the fuel in there. When I did get things working right, I was getting air pockets. After a bit of experimentation, I came up with a technique for rolling the propellant into rods.Click Here to learn how to "roll your own".
So far so good! An easy motor to build (Built first one in about 5 hours, I think the next will be much faster as I was learning how to create threads etc and it took a few tries to get it right!) and cheap to fly. Time to light her up and see what happens!
Static Test Results:
Well, I guess there is no going back to single use motors and water putty! The consistency of these motors was very cool. Each one had exactly the same burn characteristics and plume. I videotaped each test and even made a small load cell to document the results. I was surprised to find how similar they were. My previous experience has been with water putty nozzles and they look pretty inconsistent in comparison.
The aluminum nozzle turns out to be fine for small motors (1.25 inch long or less graing) with short burn time (.25 seconds) and lower temperatures. I have since switched to making motors with just graphite nozzle inserts since I found that larger motors started to severely erode the all aluminum nozzles. Click here to see my latest nozzle design
The "rolled" grains turn out to work very well. I was able to prep a motor for testing in about 3 minutes and I was cleaning the threads each time. I will probably stick to this method of preparing the grain since it was so easy. It took much less time to roll a batch than to load individual casings at this size. This would be a good time to thank Jimmy Yawn since I am sure this would not have occurred to me if I hadn't studied his fuel prep method! Click here to see what I mean!
This is a static test of the graphite nozzle. Click on the image to see a short AVI movie of it.
This is a static test of the aluminum nozzle. Click on the image to see the AVI
I modified my load cell to accomodate these motors, at the bottom of This Page you can see the graphed results.
Having fun now! Lots more to do but going well so far. Thanks to all the folks at Arocket, Richard Nakka, Bill Bullock, Jimmy Yawn and everyone else for all the help and great web information!
To see my latest 1/2 inch motor design click here
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