Exterior Fin Attachment

The Rocketry Forum

Help Support The Rocketry Forum:

Alby

Well-Known Member
Joined
Jul 12, 2010
Messages
235
Reaction score
9
There is no reason behind this, but I'm looking to attach fins on a 4" rocket that'll fly 38mm H and I motors
to the exterior body tube vs. a motor tube through-wall setup. My plan is to use either G5000 Rocketpoxy or
Loctite 9340 epoxy for the fillets and attachment to the body tube.

In any case, my main question is if anybody feels this won't provide enough strength and I really should go with
the attachment point to the motor tube so I can have fillets on the tube and body. Or if an attachment just to the
main body tube is good enough.

Thanks,
 

Tractionengines

Well-Known Member
TRF Supporter
Joined
Aug 29, 2020
Messages
228
Reaction score
137
Location
Northeast Ohio
Surface mount with fillets works on minimum diameter rockets, so you should be ok. Just less belt-n-suspenders. Also, TTW gives you some alignment help, depending how it's built.
 

Alby

Well-Known Member
Joined
Jul 12, 2010
Messages
235
Reaction score
9
Surface mount with fillets works on minimum diameter rockets, so you should be ok. Just less belt-n-suspenders. Also, TTW gives you some alignment help, depending how it's built.
So your definition of "minimum diameter" means a 4" diameter rocket shouldn't be a problem?
I know for small low powered rockets that is the default attachment plan. But I wasn't sure just how
large you can go before it's a problem.

I did see that on Joe Barnard @ bps.space did a video on exterior mounted fins, which worked great.
Although he fiberglassed them, where as I'll just be relying on the epoxy fillets.


 

tsmith1315

Well-Known Member
Joined
May 16, 2020
Messages
955
Reaction score
761
Location
Doerun, GA
Minimum diameter, meaning there is no motor mount. The airframe is the motor tube, thus fins must be surface mounted.

Any diameter/impulse is fair game, it's up to your construction skills to see that they stay on!
 

afadeev

Well-Known Member
TRF Supporter
Joined
Sep 21, 2017
Messages
1,782
Reaction score
1,008
So your definition of "minimum diameter" means a 4" diameter rocket shouldn't be a problem?
I know for small low powered rockets that is the default attachment plan. But I wasn't sure just how
large you can go before it's a problem.
The diameter of the airframe is not a critical variable in your analysis.
You need to worry about strength of fin-to-airframe bond when you plan on exceeding speed of sound (Mach 1, 2, 3+), or when your storage transportation methods have a history of inflicting more damage on the rockets than the actual flight (BTDT). Or if you plan to descent at high rates of speed (30+ fps, for whatever reason), and expect fins to take a big hit upon landing.

I did see that on Joe Barnard @ bps.space did a video on exterior mounted fins, which worked great.
Although he fiberglassed them, where as I'll just be relying on the epoxy fillets.
Joe B's videos have fantastic editing and visual quality, on all of them.
But the actual rocketry part of the videos, the building techniques and material prep choices, are somewhat questionable.

If you want to learn about building high-power rockets, the advise from one of the following videos would carry much greater weight:

 

Alby

Well-Known Member
Joined
Jul 12, 2010
Messages
235
Reaction score
9
The diameter of the airframe is not a critical variable in your analysis.
You need to worry about strength of fin-to-airframe bond when you plan on exceeding speed of sound (Mach 1, 2, 3+), or when your storage transportation methods have a history of inflicting more damage on the rockets than the actual flight (BTDT). Or if you plan to descent at high rates of speed (30+ fps, for whatever reason), and expect fins to take a big hit upon landing.



Joe B's videos have fantastic editing and visual quality, on all of them.
But the actual rocketry part of the videos, the building techniques and material prep choices, are somewhat questionable.

If you want to learn about building high-power rockets, the advise from one of the following videos would carry much greater weight:



Thanks. I'll watch these videos right now. :)
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,540
Reaction score
2,907
Location
Melbourne Australia
Minimum diameter, meaning there is no motor mount. The airframe is the motor tube, thus fins must be surface mounted.
Not quite correct. "Must" is overstating the requirement. You can slot the airframe and have the fins glued into the slots. You don't get much thickness but at least the fin gets to rely on something other than peel strength and tension strength to hold it on. It is how I build all my HPR rockets. No tip-to-tip necessary. Between slotting into the airframe and having some fillets there is plenty of strength to keep them in place during flight.

There is an example here:

Here is the dry fit test:
DryFit.JPG
 
Last edited:

Flyfalcons

Well-Known Member
Joined
Apr 14, 2015
Messages
2,546
Reaction score
762
Can you, yes. But through the wall is so much more highly desirable for a reason.

 
Top