Alternative to Internal Fillets

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Skywriter

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A four-fin rocket needs sixteen internal fillets. That’s a lot of fillets, and none of them will be easy. All of them will be messy. What can you do? Go buy some skinny (depending on the size of your rocket) wooden dowels and a ¼-inch dowel. Cut the dowels into lengths that are about six inches longer than your fin base. Get yourself setup with some thick epoxy, e.g., RocketPoxy, and stand your rocket upside down. Spread out a line of the epoxy on a flat piece of wood or cardboard and roll a skinny dowel in the epoxy. Quickly transfer the dowel into the bowels of the engine department, sliding it down along one of the joints and keeping it tight against the juncture. Pull it up an inch, cut it flush, and push it back down. Insert the ¼-inch dowel and press it against the skinny dowel to be sure it is in tight. Repeat for the rest of the fins. You might be able to do all the internal fillets with one big batch of RocketPoxy in less than 30 minutes without a giant mess. I’ve done this several times and it actually seems to be stronger than just dabbing in those fillets, but I wouldn’t use this for the external fillets just because it would look weird.
 
many ways to tackle this.

first there is the method you describe.

then, depending on the force the rocket will experience and the materials in question, and "double dip" or even "triple dip" with that thick epoxy could work. 110% it will work for cardboard / plywood construction as the resulting bond will be more than sufficient.

or, you could build "epoxy dams" or "epoxy pockets" with thin strips of bass wood or balsa. working that method right now on a moderate sized fiberglass rocket.

could always drill holes and inject a thinner epoxy such as West or System Three. this really works best on FG builds, though i bet it could work on cardboard / wood.

note that all of these assume popper prep techniques are followed.

let us know which you choose.
 
Expanding foam is another possibility, although I would still do the fillets to the MMT in any case.

Note that epoxy is stronger in compression than tension, so arguably the body tube inner fillets are the most important for "pulling out". The MMT fillets are most important for side forces. So I guess I'm saying all 12 (3 fin) or 16 (4 fin) internal fillets are important.
 
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Another alternative is to have interlocking CRs with your fins.

  • make an extra (or two or..) CRs and add notches.
  • Add equal notches on the fin tabs.
  • Glue the CRs to the MMT tube, and ensure they are properly spaced & aligned for the fins & notches in the fin tabs.
  • Glue in MMT & CR assembly, ensuring you have it all lined up with the fin slots on the body tube.
  • Insert & glue fins.

The interlocking notches on the tabs & CRs give you a bit more gluing options & a mechanical structure, rather than relying solely on a glue butt-joint.

there are advantages & disadvantages. It's up to you to determine which out-weight what..
 
@JohnCoker

Sure, however, if this is a 3" cardboard and plywood rocket, I wouldnt kill myself with a ton of internal fillets. If there is a failure with the MMT fillet and external fillet in place, its doubtful that the inner body fillet would save the day.
 
Another alternative is to have interlocking CRs with your fins.

  • make an extra (or two or..) CRs and add notches.
  • Add equal notches on the fin tabs.
  • Glue the CRs to the MMT tube, and ensure they are properly spaced & aligned for the fins & notches in the fin tabs.
  • Glue in MMT & CR assembly, ensuring you have it all lined up with the fin slots on the body tube.
  • Insert & glue fins.

The interlocking notches on the tabs & CRs give you a bit more gluing options & a mechanical structure, rather than relying solely on a glue butt-joint.

there are advantages & disadvantages. It's up to you to determine which out-weight what..

I really like that method. It has to planned out, but it works really Well.
 
Sure, however, if this is a 3" cardboard and plywood rocket, I wouldnt kill myself with a ton of internal fillets. If there is a failure with the MMT fillet and external fillet in place, its doubtful that the inner body fillet would save the day.
Yeah, this is a perennial question: how much is too much overbuilding? Since we don't know the exact properties of the materials we use nor the exact stresses they will encounter, we're always using our judgement. To me, skipping fillets to save time isn't not overbuilding, just taking a shortcut.
 
Yeah, this is a perennial question: how much is too much overbuilding? Since we don't know the exact properties of the materials we use nor the exact stresses they will encounter, we're always using our judgement. To me, skipping fillets to save time isn't not overbuilding, just taking a shortcut.
Interlocking with fewer fillets is a good balance between strength and weight; but then again fillets don't have to be uber-heavy.
 
"Build it to fly, not to crash"
I don't think anyone is advocating building a rocket so strong that it survives a ballistic recovery. I agree that in-flight stresses are likely less than transportation and landing stresses, but they're also part of the lifecycle of a hobby rocket.
 
Another alternative is to have interlocking CRs with your fins.

  • make an extra (or two or..) CRs and add notches.
  • Add equal notches on the fin tabs.
  • Glue the CRs to the MMT tube, and ensure they are properly spaced & aligned for the fins & notches in the fin tabs.
  • Glue in MMT & CR assembly, ensuring you have it all lined up with the fin slots on the body tube.
  • Insert & glue fins.

The interlocking notches on the tabs & CRs give you a bit more gluing options & a mechanical structure, rather than relying solely on a glue butt-joint.

there are advantages & disadvantages. It's up to you to determine which out-weight what..
Yeah, Giant Leap has a "Groove Lok" that you slide over your MMT. Unfortunately, it's only for 38mm MMTs. You have to realize that it's going to push your fins out enough so that it looks a little funky unless you grind your fin base down.
 
A fine question to ask is: "how strong does it need to be?"

If there is a force great enough to be concerned about the fin 'pulling out', one should just chase the orangutan away from the rocket, and don't leave it unattended in places like that.

Likewise the inner tube bond is mostly a but-joint, that when loaded in shear, plays an important role in fin stiffness. A simple test will tell you if it is stronger than the fin: bend the fin until it breaks. if the inner bond joint fails through the adhesive first, then it could use a better bond geometry. If the material around it fails or the fin snaps then it is more than strong enough. I just butter the edge with thickened epoxy [like laying a brick] and stick it in there, works just fine.

I find most people prefer to over-build rather than test, to each his own :)
 
I prefer to use thin epoxy.
Before I place the rocket horizontally to dry, I like to run/pour/inject the epoxy down the root where the fillets will be.
I'll do 4 at a time, let dry, rotate 90 degrees and run epoxy down 4 more fillets... until completed.
I'm not extremely concerned about a "perfect" all-matching radii. It's just not that critical.
The purpose is to widen the bonding width of the adhesion surface.

Preparation of the surface is by far the most important step of the internal fillets.

Fillet.JPG
 
sorry, don't follow / understand your reply..
The Groove lok pushes the fin out a little just like your notched center ring would because they both elevate the fin off the MMT. This leaves a portion of the tab, particularly the front, sticking out unless the fin base is trimmed. Also, your notch would need to be deep to reduce this effect, which would compromise the strength of the CR initially and you would have to notch all 3 rings to avoid a rocking effect. Got it? And, unlike the groove lok, placement of the notches would be (too?) critical. One of the original solutions was with Aerotech's Fin-lock ring, which worked great for mid-level rockets.
 
A four-fin rocket needs sixteen internal fillets. That’s a lot of fillets, and none of them will be easy. All of them will be messy. What can you do? Go buy some skinny (depending on the size of your rocket) wooden dowels and a ¼-inch dowel. Cut the dowels into lengths that are about six inches longer than your fin base. Get yourself setup with some thick epoxy, e.g., RocketPoxy, and stand your rocket upside down. Spread out a line of the epoxy on a flat piece of wood or cardboard and roll a skinny dowel in the epoxy. Quickly transfer the dowel into the bowels of the engine department, sliding it down along one of the joints and keeping it tight against the juncture. Pull it up an inch, cut it flush, and push it back down. Insert the ¼-inch dowel and press it against the skinny dowel to be sure it is in tight. Repeat for the rest of the fins. You might be able to do all the internal fillets with one big batch of RocketPoxy in less than 30 minutes without a giant mess. I’ve done this several times and it actually seems to be stronger than just dabbing in those fillets, but I wouldn’t use this for the external fillets just because it would look weird.
There's not a hobby rocket on the planet that NEEDS internal fillets. Build your fin can, slot the tube and insert that assembly into the rocket. Internal body tube /fin fillets are a waste of time and unnecessary. Building your fin can gives you mmt/fin fillets and external bt/fin fillets. That's all needed.
 
The Groove lok pushes the fin out a little just like your notched center ring would because they both elevate the fin off the MMT. This leaves a portion of the tab, particularly the front, sticking out unless the fin base is trimmed. Also, your notch would need to be deep to reduce this effect, which would compromise the strength of the CR initially and you would have to notch all 3 rings to avoid a rocking effect. Got it? And, unlike the groove lok, placement of the notches would be (too?) critical. One of the original solutions was with Aerotech's Fin-lock ring, which worked great for mid-level rockets.

Actually, the notches are in both, and allow the fin tab to butt up against the MMT as intended / mentioned. (the slot in the CR aligns with the slot in the tube) I also mentioned 'extra' not 'in place of' in regards to the CRs.. Having this tab & slot connection is a mechanical connection, in which the slot limits the movement of the tab, in 2 degrees of freedom. The fin lock or groove lok only limits in one degree (sideways)

Look at the fin & CR as shown in the link below (scroll down to the fin & CR):
That is what I'm trying to illustrate, and that has been used successfully, and in various forms. Look also at LOC's Goblin, how the fins have tabs that slot into slots in the CRs. Mind you these are at the ends of the fin tabs.

And yes, I do agree that the placement of the notches are somewhat critical, especially in the CRs, as they will take the side-loading / take it away from the shear of the fin tab butt joint to the MMT tube

All of this is ultimately up to the builder, and their skills in getting things laid out, fabricated, and the assembly lined up. The fin lok or groove lok are shallow, and alignment of them to the tube fin slot also takes more care to ensure proper alignment; usually with a jig or the fin itself. A slot (as I described) in teh CR will show how aligned you are with the fin slot in the tube, as the two are next to each other, and their alignment can quickly & easily be seen by just the parts themselves.. (The CR comes in contact with the inner side of the BT..)

http://www.erockets.biz/semroc-lase...tes-super-der-red-max-upgraded-sem-fes-9705p/
 
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There's not a hobby rocket on the planet that NEEDS internal fillets. Build your fin can, slot the tube and insert that assembly into the rocket. Internal body tube /fin fillets are a waste of time and unnecessary. Building your fin can gives you mmt/fin fillets and external bt/fin fillets. That's all needed.
Hi Jim! Thanks for your comment. Based on my experience, internal fillets are the first to give way, which would seem to indicate that they are a key stress point. I've been flying rockets for more than 65 years. Wildman voids his warranty if you don't have internal fillets. I'll keep the internal fillets.
 
Actually, the notches are in both, and allow the fin tab to butt up against the MMT as intended.
Ok, I I think I see what you're saying; Notch the CR and notch the fin...right? That obviously wasn't doable when I used the Groove Lok. But again, that would take a lot of precision work.
 
Fin cans are inherently strong with minimal adhesives. Over building fin cans is one of the more comical activities in this hobby.
 
Fin cans are inherently strong with minimal adhesives. Over building fin cans is one of the more comical activities in this hobby.
On the flip side of the coin, if you have a strong internal fillet (e.g., the Zephyr) you don't really need external fillets. Mine has landed hard and the internal fillets held fine without external fillets.
 
for the record, my LOC goblin came in hard & sheared a fin (fin folded over, along one side of the fin slot). The fin levered over, shearing the butt glue joint to the MMT. The little tabs are the extremes of the fin tab broke. And, a layer of the fin tab wood was still glued in place, away from the direction the fin folded over.

And this was all with Yellow wood glue. The wood gave before the glue did..
 
Actually, the notches are in both, and allow the fin tab to butt up against the MMT as intended / mentioned. (the slot in the CR aligns with the slot in the tube) I also mentioned 'extra' not 'in place of' in regards to the CRs.. Having this tab & slot connection is a mechanical connection, in which the slot limits the movement of the tab, in 2 degrees of freedom. The fin lock or groove lok only limits in one degree (sideways)

Look at the fin & CR as shown in the link below (scroll down to the fin & CR):
That is what I'm trying to illustrate, and that has been used successfully, and in various forms. Look also at LOC's Goblin, how the fins have tabs that slot into slots in the CRs. Mind you these are at the ends of the fin tabs.

And yes, I do agree that the placement of the notches are somewhat critical, especially in the CRs, as they will take the side-loading / take it away from the shear of the fin tab butt joint to the MMT tube

All of this is ultimately up to the builder, and their skills in getting things laid out, fabricated, and the assembly lined up. The fin lok or groove lok are shallow, and alignment of them to the tube fin slot also takes more care to ensure proper alignment; usually with a jig or the fin itself. A slot (as I described) in teh CR will show how aligned you are with the fin slot in the tube, as the two are next to each other, and their alignment can quickly & easily be seen by just the parts themselves.. (The CR comes in contact with the inner side of the BT..)

http://www.erockets.biz/semroc-lase...tes-super-der-red-max-upgraded-sem-fes-9705p/

Good Post. Odd that this didn't show up yesterday.
 
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