Surface mounted fin strength

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

elibrowner03

New Member
Joined
Feb 1, 2023
Messages
2
Reaction score
1
Location
Los Angeles
I am designing the fins for my university's rocket club; it is a hybrid rocket, has an apogee goal of 10k feet, and has a 6 inch diameter. In the past, we have used tab slot fins made of plywood that fit into two centering rings in the body tube, and then we do a T2T fiberglass layup over it. The rocket is subsonic (M < 0.8), and I was curious if the fins would still be strong enough if we instead just surface mounted the fins with the T2T layup, as the result seems a lot cleaner. Also, if this change would potentially make the fins a lot weaker, would opting for a delta shaped fin significantly increase the strength of the connection between the fins and the body tube since there is more shared surface area?
 
Any reason not to run through-the-wall tabs? Unless it's minimum diameter, surface mounting wouldn't really see any benefit other than saving you from cutting some fin slots.

Considering the subsonic performance, any inclusion of t2t would realistically be extra weight with no benefit. That is unless you're expecting a really really rough landing.

Throug-the-wall with no tip-to-tip composite is plenty strong, plenty fast to build, and plenty simpler than adding a non-beneficial composite layup!

Braden
 
Last edited:
Here's a 5.5" body tube with a 75mm motor tube, and plywood fins.

Been to 10,000 ft. Survived a tree landing and two 2 weeks of bad weather before its retrieval.

Just a little something different on the base of the fins to make sure I had good purchase of the epoxy.

No glassing inside or outside for the fins or body tube. West System and J-B Weld epoxy for fin attachment.

The "cheek" block between the fins is for the 1515 rail button.

You may want to do a Fin Flutter check. Even a subsonic ride will still create a flutter issue if the
plywood thickness is minimal and the fin stands tall.

As "Upscaler" pointed out - no tip to tip needed if you go through the wall. Save on weight and hassle.


IMG_7557.JPG IMG_7562.JPG IMG_7574.JPG


IMG_6970.JPG IMG_6975.JPG
 
The rocket is minimum diameter. This is a yearly project, and last year the process of using TTW fins was very clunky. Essentially, the fins slotted into this cylindrical base that went on the inside of the rocket (picture 1). To align the base, they used a jig that slotted into both the cylinder and the fins to align everything (pictures 2 and 3). At the end of everything, they had to deal with the extra weight and space that was taken up by the base that held the wings in place, and had to dremel out a big part of it so that there wasn't too much extra weight. And then to add onto everything, they did a fiberglass layup over it. Looking at it now, it seems like an overall inefficient and overkill process.


fin base.jpgfin aligner.jpgfin aligner2.jpg



The main thing I am worried about is that by removing the aforementioned TTW system, I will be significantly weakening the fins, but looking at the general responses it doesn't seem like that will be an issue. Are there any other benefits from using surface mounted fins for a minimum diameter rocket besides getting to avoid the process listed above?
 
You didn't say what the tube material is. The epoxy might be strong but if you're using a sonotube that material will be weaker than the epoxy bond. Another factor is the size of fillets against the tube, I think this is more important than surface fiberglassing.
 
Crazy idea but I have wondered if it would work.

could you print the fillets with a 3D Printer, glue them in with epoxy. It would provide more surface area for contact with the body tube, but I don’t know if the plastic would be stronger than the epoxy alone. You could oversize them for even more contact surface, but that would probably mean more drag.
 
Back
Top