BOLT ON FINS

[PSLimo]

Hey Guy's,

I'm looking to scratch build a 12" diameter stubby rocket with detachable fins for easy transport and have a fun low flyer that fits in the car. No speed demon, just White Lightning J's & K's for some low and loud flights in the 1500 to 2000 feet range.

If I upscale a rocket like Bullet Bobby to 12 inches I end up with a 12" diameter x 31" long rocket that fits the bill but has a very wide fin footprint. I was thinking about an aluminum channel to bolt the fins to.


Maybe do a Minion theme.



Has anyone had success with a low flying large diameter rocket with removable fins?

Thanks in advance,

Phil

 

[bjphoenix]

I haven't tried it but I assume the results would be as good as your workmanship. I've found it interesting that things like sounding rockets and guided missiles frequently have the fins bolted on with simple brackets.

 

[dhbarr]

Plenty of room to have channel on the MMT and plates at the slots.

 

[Hobie1dog]

Go for it

 

[Neutronium95]

You might want to check out @bandman444's Mega Squat build. I believe that the fins on that slotted into channels attached to the centering rings.

 

[G_T]

You can see what I did in this thread: https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-3#post-439855 The thread is somewhat lengthy and has various distractions, but you might get some ideas from it.

Jerry O'Sullivan has done this sort of thing before, I think more than once. You can probably find some of his rockets with a bit of search.

Another way to do removable fins is to have essentially 6 (for 3 fin) or 8 (for 4 fin) stub fins mounted through the tube, sort of like conventional fins. They would glue down to the motor tube; conventional through the wall construction. But they only stick outside the airframe perhaps an inch or inch and a half. They form the bolting bracket for the large real fins. There would be a slot between these bracket "fins" for a structural tab of the real fin to stick down in the airframe, to provide a good pocket for the fin to resist bending loads. That would be a lazier way than what I did. I've not seen it done but it is too simple to have not been done by someone, somewhere.

There are many ways to skin this cat.

A disadvantage of any sufficiently strong removable fin approach is additional weight at the back end of the rocket.

You could possibly create an arrangement with a removable spar that bolts onto the airframe, and have the fins be hollow (say, folded out of that corrugated sign material - forget the name) and slide over the spars. At least that would be light. But it also assumes the speed isn't going to be high.

Gerald

 

[PSLimo]

Thanks for the ideas guy's,

From reading your builds it looks like a single channel mount isn't as structurally sound as 2 "L" brackets.

I'm also looking to keep this rocket as light as possible and surface mounting the fin's would help with that problem. I built a low and slow 12" diameter that was 7.5' tall with TTW G-10 fins that required 5 lbs in the nose to be stable and weighed in at 35+lbs. Here's what that one looked like not making 1000' on an AT K-1100T:



This one is almost 3 times shorter so I'm thinking light is key to keeping extra nose weight to a minimum and still be able to fly on J and K's.

How did you guy's bend one side of the brackets to conform with the circumference of the body tube?

Phil

 

[rfjustin]

Thanks for the ideas guy's,

From reading your builds it looks like a single channel mount isn't as structurally sound as 2 "L" brackets.

I'm also looking to keep this rocket as light as possible and surface mounting the fin's would help with that problem. I built a low and slow 12" diameter that was 7.5' tall with TTW G-10 fins that required 5 lbs in the nose to be stable and weighed in at 35+lbs. Here's what that one looked like not making 1000' on an AT K-1100T:



This one is almost 3 times shorter so I'm thinking light is key to keeping extra nose weight to a minimum and still be able to fly on J and K's.

How did you guy's bend one side of the brackets to conform with the circumference of the body tube?

Phil

Where are you getting the nose cone from please?

 

[PSLimo]

I'm going to keep it as light as posible with sonotube and it will be easy enough to blow up a ball inside and glass over it. I've had success with that simple method in the past.

Phil

 

[PSLimo]

I found a simpler form to fiberglass for the nosecone. Should be no problem looking through the pile of 12" sonotubes and find a perfect match.



I'm also thinking of a printed channel to bolt the fins to that matches the curve of the tube. I'd need a 6" long channel that should be doable with a smaller printer.

Just need to find someone to print them.

Phil

 

[bjphoenix]

I stumbled across a photo for you. Check this thread- scroll down to post #174.
https://www.rocketryforum.com/threads/wildman-darkstar-gallery.15580/page-6

 

[PSLimo]

Thanks, that's exactly what I need.

How he curved the one side of the bracket is another question.

 

[jqavins]

Here's what I think I would try.

As for conforming the brackets to the tubes, my first inclination is not to worry about it. Attach the inner brackets with a thickened epoxy that will fill gaps, and just don't sweat the outer ones where the joint to the curved surface is bolted. Or, for the outer brackets, lay a barrier on the tube such as kitchen plastic wrap then bolt the brackets down with thickened epoxy in the gaps. For both of these, I'd be using aluminum angle.

Better, but probably impractical, would be to do some sheet metal work and make the brackets with one curved side and one straight. Hammer half of a strip onto a form then bend the strip, but that requires tools most of us don't have (the forms and a brake bender).

Third would be to 3D print the brackets, but for me I'd rather have aluminum.

 

[BABAR]

Okay, crazy idea from an L-0 rocketeer, but I found the engineering problem intriguing. Here is a solution that uses no external hardware

the fin slots are cut all the way to the tail end of the rocket.

each fin has two L Brackets on EACH side (4 per fin). The L brackets are bolted TO the fins, the free or flange sides stick out 90 degrees, each of these segments is tapped to hold a bolt, so you don’t need a nut.

you need three centering rings, the bottom and middle are structural and permanently Bolted to the motor mount. The top one is “loose”, placed last, and functions to limit the space above the motor mount to create A sealed compartment ABOVE the mount to retain Ejection charge in that compartment and Force out nose cone and laundry.

at the launch site, the Fins are placed it the motor tube slots.

the motor mount is lowered from the FRONT end through the tube, the slots in the sides of the body tube will fit over the parts of the fins already INSIDE the body tube.

when it gets to the L brackets, the motor mount is rotated so the holes line up with the bracket holes. There is a cut out of the inside edge of each fin to allow the bottom plate to rotate. The middle centering ring is ABOVE the upper fin L bracket so should be free to rotate.

six screws from above and six screws from below (may need washers) , four For each fin.

the top plate is then placed over the motor mount tube and lowered until it rests on the three brackets attached TO the body tube, and bolted in place.

okay, I just realized you will need external bolts to attach the L brackets for the upper centering ring to the body tube, so you will need three exterior bolts. These are permanent.

you will also need probably three interior bolts to attach the upper centering ring to the motor mount (last thing you do at launch site) to keep the fins and motor mount from sliding out the back of the tube!

just an idea.

 

[PSLimo]

I stumbled across a photo for you. Check this thread- scroll down to post #174.
https://www.rocketryforum.com/threads/wildman-darkstar-gallery.15580/page-6

That's exactly what I need to surface mount and I contacted the builder and those curved brackets are from Giant Leap but only come in 3" & 4".

I should have clarified that I'm looking to surface mount the fins to keep the build as light as possible. Otherwise I'll need too much weight in the nose to fly on J's & K's which is my goal.

A 3D printed bracket might work and would only need to be 6" long and extend about an inch on each side. Anyone out there that 3D prints for a fee make conforming brackets like this?

 

[bjphoenix]

That's exactly what I need to surface mount and I contacted the builder and those curved brackets are from Giant Leap but only come in 3" & 4".

I should have clarified that I'm looking to surface mount the fins to keep the build as light as possible. Otherwise I'll need too much weight in the nose to fly on J's & K's which is my goal.

A 3D printed bracket might work and would only need to be 6" long and extend about an inch on each side. Anyone out there that 3D prints for a fee make conforming brackets like this?

View attachment 538867

The next time you are in your local home store/hardware store check to see what size aluminum angle they have. I think I would try buying a piece of aluminum and look for a way to form one leg to conform to the body tube.

 

[G_T]

Run a strip of wood at an angle through a tablesaw, to cut a shallow curve in it. Glue that on to give you a flat surface for the bracket. Then regular angle brackets work fine. But your brackets will probably break off if you just try to glue them down. Aluminum is hard to glue adequately. If you need info on ways to improve gluing aluminum, ask. But it is still risky. You specified big fins. Think about your structure and the loads carefully. By the time you adequately attach your brackets you are adding a lot of mass to the back of a stubby rocket.

 

[BABAR]

It’s not orthodox, but two plywood strips wide enough to hold say three bolts can be combined glued and SEWN onto the outside of the body tube. Dental floss with thin CA has been my “go-to” for holding my helicopter hinge assemblies as a hub, you pre-drill the holes in the strips and thru the tube.

Done right neither the tube nor the strip attachment will be a failure point.

 

[DigBaddy]

Would being able to fold the fins in toward the body help create the clearance needed for transport? There are indexed hinges or locking hinges that might make that possible. Never tried it. Just a crazy idea that hit me.

 

[PSLimo]

Thanks again for all the creative ideas.

I'm trying to stay as light as possible and still have removable fins. This is a very stubby build at 12" diameter x 32" tall.

1/4" plywood is about 2.8 grams per square inch. The weight savings in not doing TTW fins is about 336 grams. 3/4" x 3/4" aluminum L brackets are about 4.2 grams per inch so 8, 6" long brackets only adds 200 grams plus the hardware.

After reading all the post I think the two best options are:

1. Figure out a way to round one side of an L bracket to contour to the BT.

2. Find someone that can 3D print brackets to contour to the BT.

I'm back to square one, lol.

 

[jqavins]

I really don't think you need to worry about the conformation of the part along the tube. I ran a calculation for your 6" radius tube and a 1" straight piece, tangent to the tube. At the far end, it would be "lifted" from the tube by about 0.083", or less than 3/32. That's easily filled in with (as I wrote above) thickened epoxy. And a full inch along is probably more than you'll use, so the gap is even smaller.

I don't know about the difficulty in bonding aluminum, but I assume the solutions involve surface prep before applying the adhesive. If that's true, and if the adhesive is an epoxy, then it should still work with the epoxy thickened to fill up the small gap.

 

[bjphoenix]

I had an idea to slot the airframe as if you were doing TTW fins, and put the pieces of angle on the inside with one leg sticking out. Then bolt through the angle leg and airframe. That way it wouldn't be a problem if the angle leg didn't conform to the radius of the tube, you could fill the gap with epoxy. It would complicate the motor tube construction.
What I've done with TTW fins is to glue the front centering ring on the motor tube but just slide the rear ring on, then put it all in the body tube and glue the front ring. Then the rear ring can be pulled back out to give access for installing fins. Then the last thing I do it put the rear ring back in and glue it.

 

[Raketenolli]

1/4" plywood is about 2.8 grams per square inch.

If you're really worried about mass back there, go for foam-core fins with carbon fiber.

Oliver

 

[PSLimo]

Hey Guy's,

Thanks for all the ideas and input. I decided to keep it light and surface mount the fins.

I started a build thread here:

https://www.rocketryforum.com/threads/12-bullet-bobby-upscale-4-6.175170/

 

[lakeroadster]

Why not TTW fins, that bolt internally?

• The internal webs and front centering rings are integral with the body tube and motor tube,
• Bolt the fins to the internal webs,
• The rear centering ring slides over the motor tube, slides into the body tube, then bolts to the internal webs.

Once assembled, everything externally is smooth, and the loose fins are smooth also. Won't even damage the seats in the mini van .