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Aug 19, 2015, 03:34 PM
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burkefj's Avatar
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A cheaper alternative for the Aerotech 24mm RC casing...


This is just for information only and I'm not suggesting you do or do not try this. This is what I have done and I'm just sharing the experience.

One of the big hurdles with using the Aerotech 24mm RC rocket motor reloads is the cost of the motor casing. Since this casing is machined from a single piece of aluminum(it has a solid end) and not a tube, it is double the cost of the standard 24/40 rms casing.

The E-6, D-7 and E-7 are endburning reloads and do not come with forward seal parts since they are not needed in a solid case.

I started thinking whether or not the rc reloads could work in the standard casing with forward closure, so I bought a motor tube and the forward closure and compared dimensions. The forward closure has a little flange that holds the forward o-ring in place in a standard non rc reload. That o-ring butts up agains a .032 fiber washer to seal the front closure. For that reason, it is not possible to use the rc reload as is in the casing, as the internal length is .032 longer than the rc casing internal length. Also, due to the forward closure lip, the liner tube tends to slip over that little flange when pushed on by the nozzle, increasing this gap even further. This prevents proper sealing of the rear o-ring.(when you tighten the rear closure the nozzle flange pushes on the liner tube as it is tightened, not on the propellent grain.

I did a bit more thinking about this and was concerned about any gas blow around into the hollow forward cavity maybe being an issue and how I could get a washer of the right size to fit and an o-ring to seal the threads of the front closure.

What I did was to coat the forward delay closure reservoir with grease, then put a little piece of wax paper over the delay hole to keep epoxy from going into the hole. I then filled the reservoir with jb quick, level with the top of the closure, and placed a greased fiber washer over the top, so that I would be sure it was even with the top and any excess would flow into the washer hole. You can see the result, it makes a nicely sealed forward closure. I greased the closure so that I could later push out the jb weld if I wanted to convert back to a normal closure, or if I had to replace the jbweld if it erroded with use. You could just seal it permanently with jb weld since the closures aren't expensive.

For the spacer o-ring I used a .03 fiber washer from rcs/aerotech and a 1/16" washer


https://www.rocketmotorparts.com/Bun...9_7799687.aspx

https://www.rocketmotorparts.com/Was...9_7823572.aspx

and used the E-6 reload as is with these additional components. I bought 30 of each.

Again, you need the .03 washer to space the reload back far enough that the rear o-ring will seal properly, the forward o-ring helps seal the threads of the forward closure under pressure.

I put some grease in the ejection reservoir just to see if any blow by was occuring and there wasn't. Make sure you grease the forward end of the propellent to keep it from getting a burn spike as it burns through at the end.

After flight the jb weld was fine, I wiped it off and re-used the forward washer and o-ring for three flights just like the rear o-ring. Flights were nominal, clean up was easy.

The loaded motor weighs 6 grams more than a stock reload in the rc casing, and the casing stuck back 7/8" further in the motor mount because my RC RG is not recessed forward for the longer forward closure. In the end I needed to put an extra 7 grams in the nose to balance compared to the RC casing.

Again, this is a non approved motor modification suitable for private fields or tra experimental launches, and use at your own risk, but in my test flights it worked fine for me.
Last edited by burkefj; Feb 05, 2018 at 11:30 PM.
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Nov 12, 2015, 01:35 AM
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clipper's Avatar
Do u find the casing gets any warmer with the end burner loads than the standard center burners?
Thanks Eric
Nov 12, 2015, 09:18 AM
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burkefj's Avatar
Not that I can tell, by the time I land, its still warm but I've never had any issues.

frank

QUOTE=clipper;33175050]Do u find the casing gets any warmer with the end burner loads than the standard center burners?
Thanks Eric[/QUOTE]


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