Inter-fin Gussets

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JohnCoker

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I building a heavy (for me) rocket, and thought I would try something different to reinforce the fins inside. In addition to the normal fillets, I added gussets between them, locking them in place. This is easier to install and lighter than 2-part foam, and hopefully it will provide the same benefit.

fin-gussets.jpg
 
That looks great. Those fins will snap off before they break loose, I would bet.
 
Looks like you saved yourself the trouble of doing full length internal fin fillets. Me bets the rocket will give you some fine service. Kurt

I did internal fillets as well, before adding the gussets.

Are they three independent 120° segments running between fin tabs or full rings with half-slots to lock into matching half-slots on the fin tabs

These are three independent segments, set about halfway along the fin roots.

Anyway, it was easier than 2-part foam, which also would have been problematic with this rocket because of wire routing holes in the bulkhead forward of the fins.
 
These are three independent segments, set about halfway along the fin roots.

Anyway, it was easier than 2-part foam, which also would have been problematic with this rocket because of wire routing holes in the bulkhead forward of the fins.

I like the gussets. Did you do anything to make it easier to locate the gussets and hold them in place while the epoxy cured?


Steve Shannon
 
I building a heavy (for me) rocket, and thought I would try something different to reinforce the fins inside. In addition to the normal fillets, I added gussets between them, locking them in place. This is easier to install and lighter than 2-part foam, and hopefully it will provide the same benefit.

View attachment 340436


Yes, I echo Steve Shannon's question about how you located the secured the gussets.

The idea is to immobilize the fin tab, so that it won't shear tangentially to the circumference of the motor tube? I wonder what other constraints on the design or construction made your choose three annular sectors over dados recessing into a centering ring?
DADOedCR.png
 
The idea is to immobilize the fin tab, so that it won't shear tangentially to the circumference of the motor tube? I wonder what other constraints on the design or construction made your choose three annular sectors over dados recessing into a centering ring?
Correct on the ultimate reason.

The main reason for the thirds is that it's at the center of the fin root. I have used dadoes in the forward center ring when the fins went that far, but in this case there were wires through the bulkhead so the fins didn't contact either the forward or after centering ring.

Holding them in place wasn't hard; they were cut precisely to fit. But if you find they tend to fall through, a drop of CyA at a couple of spots will hold them in place for epoxying.

The holes were so I could fit them in place by putting a finger through them and pull them up if the went down too far.
 
Correct on the ultimate reason.

The main reason for the thirds is that it's at the center of the fin root. I have used dadoes in the forward center ring when the fins went that far, but in this case there were wires through the bulkhead so the fins didn't contact either the forward or after centering ring.

Thanks for posting this. Food for thought. I wonder about a design that eschews adhesive except for the joints between centering rings and air frame, using only interlocking mechanical joints to secure the fins and to transmit thrust from the motor.
 
Thanks for posting this. Food for thought. I wonder about a design that eschews adhesive except for the joints between centering rings and air frame, using only interlocking mechanical joints to secure the fins and to transmit thrust from the motor.

The Black Sky Optimal 38 was like that. It had 4 inch polycarbonate body tube (an odd size, not the hobby standard) all held together with screws to aluminum centering rings and an aluminum fin can that is identical to the Hawk Mountain fin can. Nosecone was typical molded plastic, like Ace, but sized to fit the unusual size of BT.


Steve Shannon
 
The Black Sky Optimal 38 was like that. It had 4 inch polycarbonate body tube (an odd size, not the hobby standard) all held together with screws to aluminum centering rings and an aluminum fin can that is identical to the Hawk Mountain fin can. Nosecone was typical molded plastic, like Ace, but sized to fit the unusual size of BT.


Steve Shannon

I could not find any information about this rocket. I am not familiar with Black Sky. I'd be interested in seeing the design.

So far, this is what I have done

viperfincan.png

If I can figure out a way to key both ends of the fin tab into the centering rings, I think I will be able to get away without any adhesive between the fin tabs and the motor tube.
 
I could not find any information about this rocket. I am not familiar with Black Sky. I'd be interested in seeing the design.

So far, this is what I have done

View attachment 340585

If I can figure out a way to key both ends of the fin tab into the centering rings, I think I will be able to get away without any adhesive between the fin tabs and the motor tube.

This is one place where small threaded rods running alongside the fin boxes and a removable aft centering ring might be nice.
 
This is one place where small threaded rods running alongside the fin boxes and a removable aft centering ring might be nice.

Yeah, as it happened, for that rocket I could have done something like that -- should have done something like that -- but I was considering a tail cone that would have made sliding on the rear CR complicated.

The design I am working on now has fins that conform to a tail cone and I am trying to figure a way around full length slots, or fabricating the tail and fin can as a separate component and coupling it to the airframe. I think the latter is where I am going to end up.
 
I apologize to John Coker for my role in hijacking his thread. Jlabrasca, if you want to start a new thread about an adhesive free rocket I’d be happy to post there.


Steve Shannon
 
I apologize to John Coker for my role in hijacking his thread. Jlabrasca, if you want to start a new thread about an adhesive free rocket I’d be happy to post there.


Steve Shannon

Not sure it was a thread-jack -- but I will leave quietly <grin>. Thanks, again, to John Coker for posting. Its an idea I will try to steal.
 
John C.,

An interesting idea. Got me thinking there may be even better materials for weight savings. Why one hole in each instead of two? Is there some kind of aircraft mesh that would be lighter and serve the same purpose?
 
John C.,

An interesting idea. Got me thinking there may be even better materials for weight savings. Why one hole in each instead of two? Is there some kind of aircraft mesh that would be lighter and serve the same purpose?

One hole because I needed to get a finger in there to push/pull them into place. Larger or multiple holes would save more weight, but this is only 1/4" plywood anyway.

I'm not familiar with "aircraft mesh," but plywood has excellent mechanical properties for its weight, so it would probably be hard to do better with non-exotic materials.

Blacksky went out of business before the Age of the Internet. Scott Bartell was a creative guy who introduced rails, incorporating an accelerometer and a baro sensor in a flight computer and canvas phenolic "Optimal" kits.
 
I'm probably not using the right term. But I mean the stuff jumbo jets are made of. It is a kind of honeycombed aluminum, very strong but very light.
 
The bottom piece is more difficult to machine from a manufacturability point. You are requiring milling to a depth radially. Outside of CNC mill/router you'd have to rotate that part three times for each slot in addition to milling a centering ring. He has a shorter tool path per part and faster cycle time. Some CNC tools can't cut circles well and will make a Z right through the center depending how old or what kind it is. And I've had certain complex airfoil parts on small scale that weren't even machinable, it didn't mean they were bad, but it really limits the material selection. Printed materials don't compare to solid milled metal always. John here can trace a template and make it wood in a garage and a guy without a mill couldn't make your part as easy. If his piece was steel metal on a CNC mill it would save hours compared to yours. John made his part cheaper to make per hour. And each section is replaceable. John beat you in design for manufacturability. Your piece may have slight structural advantages,easier to install for user, but it is not useful for center of fin joint. Just rambling as an undergrad mech here. Actually I think John could stamp his design with presses and really beat you on costs. You'd have to machine or print yours.
 
Thanks, again, to John Coker for posting. Its an idea I will try to steal.
Those slots will have high stress concentrations on a thin section of material, it could fail there. Outside of a FEA its hard to know. Holes generally increase stresses locally. I don't want to be mean I think you broke his idea and made it hard to make. The only thing I see is easier user install and it doesn't work in center of fin joint.
 
That is some super clean and creative work. However, ply fins through the airframe to motor tube with good glue joints are not the weak link. The fin will snap first outside the airframe. I saw it happen at URRF 4 on an L powered rocket when the motor ejected before the altimeter could trigger the main. Perfectly good 7 inch blue tube rocket with a fin sheared outside the airframe. That rocket had flown on M motors... no problem.
 
Thanks all for the interesting comments. I hadn't seen the idea done before and indeed it sparked a good discussion.
 
Andrew, the bottom piece can be made with a razor saw and a chisel. No need to over think it or over do it.


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