BP engines specifically for swing testing

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mooffle

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So maybe this a problem no one besides myself needs solved but in all my years of rocketry I didn't think to do this until now. My dad was old school and wouldn't fly a design unless it passed a swing test and with very few exceptions I inherited this part of the hobby from him.
(Yeah, I know things like the Alpha throw this off...)
Anyway, half the time I finish a rocket build and never have the proper engine handy for a swing test. Any good motor is meant to be launched, just like halloween candy on my dinner plate. The problem then becomes I have to go buy motors to balance test, but what about those leftover motors from last launch?

The solution for myself was to fill a bunch of old casings with epoxy clay as measured against other live engines and the Estes engine mass chart. Here's a quick rundown of the process:
1- I gave the casings a quick rinse (only 10-15 seconds) to remove burnt BP residue so the clay can adhere to the inside. If I did this again I -probably- wouldn't do this step again. the rinse worked great but the ends of the tubes flared a little bit. I let them sit overnight and sanded them down the next afternoon with no issue but as you can see the C5 lost its nozzle in the process and I had to mold one.
2- Run a round file around the inside a couple times, again to get rid of the residue and I probably could have just started here.
3- I wrote the mass and marked the CG of actual engines on these casings and yes, the motor codes match the mass to alleviate my own confusion.
4- Mixed the clay and packed with a piece of old launch rod. I wanted the C5-3 ends to be distinguishable hence the nozzle part. Even though the casing is just about completely packed the CG is not in the middle so knowing where the business end is helps. The clay is also fairly hard and even when packing didn't push out the nozzles or squish through like those play-doh toys.

One thing to note- I only have one of each impulse class- Estes engines with the same letter are ridiculously close in mass and the margin of error when I was measuring two B4-2's and a B6-6 was negligible for my purposes.

Afterward I set these things aside in my shop separate from other engines in a jar on my build desk. Something about having these specifically set aside is really nice so I don't have to scramble to test stuff. Hope this helps someone!
 

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Neat idea! I had drill out 18mm motors with a 1/2 drill bit to make stands. Gets them clean enough for wood glue to grab.
 
great idea.

Murphy’s law being what it is, might want to color them with a red felt pen, I can see myself accidentally picking up one and sticking it in a rocket and wondering, “why won’t this igniter work?”!
 
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