38mm fin can for 54mm rocket?

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Well-Known Member
Jul 8, 2009
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I've got some spare parts - phenolic tubing, centering rings, etc. - and am thinking about building a 54mm diameter rocket with a 38mm motor mount. It would be dual deploy and fiberglassed for flights on everything between G and J. Fin options include glassed plywood or G10, but I'm also looking at an Acme fin can (https://giantleaprocketry.com/hpdefault.asp). I don't particularly like the bulge that would be added to the airframe by using the 54mm fin can, so I'm curious if anyone has attached the 38mm fin can to a motor mount tube housed inside a 54mm main body tube? If so, I'd love to see pictures of the assembly process, especially how you fill in the gaps in the fin slots made by the airfoil shape of the fin can. Thanks,

Brett Keller
NAR 86412 L2
MDRA 0156
This answer is based on anecdotal evidence only.;) I have been told the 38MM fin can is meant to be used with an outer tube like a 54MM tube.
Hopefully others will chime in with pictures.
A 38mm MMT in a 54mm body tube is a great fit, but not with an Acme can. To stick an Acme fin can to the inside requires a 2.6" body tube. Picture Giant Leap's Escape Velocity.

To fill the slot gaps left by the Acme fins and the required slots for it you do it with thickened epoxy, the consistency of peanut butter.
The 38mm ACME Fincan I have would fit fine inside a 54mm LOC Airframe. I plan to eventually use it that way, but just haven't gotten around to it yet (all the parts for it are sitting in my basement.... go figure)
Hm. So are their significant differences in OD's of Acme fin cans? Or significant differences between LOC and PML 54mm tubing ID's?
I'm fairly certain the sizes of the fin cans are not that drastic, but....

The 38mm fin can has an ID of slightly over 1.64" or around 42-43mm. To fit inside a 54mm airframe, the OD is no more than 54mm (duh?). This yields a maximum wall thickness of 5.5-6 mm to fit inside the 54mm airframe. I don't have my fin can with me, but I'm quite certain the wall thickness is less than 5.5mm (more like 3-4 mm).

The GiantLeap website, however, says it can be adapted for use in 2.56" or 3" airframes, but it is a general statement that appears on both the 38 and 54mm fin can units, so I'm not sure.