3D printed fin "mounts" in a level 3 cert rocket

[deadalus52]

I'm in the design phase of my level 3 certification. The base airframe will be a LOC Bruiser EXP. Unfortunately, I think my kit was a mixed bag between versions which means my fin situation looks like this:
This obviously isn't ideal. Now, I do have a 3D printer, and the obvious solution to me is to make something like the pink part in the picture below. A few notes: 1. this is a rough first draft to show the idea, not a final design and 2. the centering ring directly above the fin is not being shown, but it will be there.



I'd be epoxying everything together with aeropoxy, filleting all of the joints, including between the fin and the airframe on both sides. My plan would be to print using PETG at a high infill percentage, probably close to 50%, with 3 shells for added strength. If printed in the orientation above, all of the forces would be perpendicular to the layers, meaning I wouldn't be relying on layer adhesion for strength.

To be honest, I'm not convinced the print would be the weakest part of the fin can, but I am worried about the Tripoli rule against 3D printed fin cans in certification attempts. Before I get too far into the design and build process, I'd like to ask around and get a bearing on whether this would be an acceptable solution

 

[David Schwantz]

I think a rule states that you may not use 3D printed fin cans for a L3 cert flight. But I do not know about 3D printed fin can parts?

 

[krislhull]

What do your TAP members say? That is really what matters.

 

[deadalus52]

What do your TAP members say? That is really what matters.

I don't have a TAP yet, though I have several people in mind, one of whom said he would be willing, though that was a few years ago. My plan was to get at least the design in place before taking up someone's time to look things over.

 

[Nytrunner]

This doesnt violate the printing rule. That prohibits entire one-piece printed fin cans.

A much easier solution would be to cut notches in the centering rings that hold the fins, and just use good internal and external fillets. (You can leave oit the plastic and save a ton of weight that way)

 

[HVArcas]

I don't have a TAP yet, though I have several people in mind, one of whom said he would be willing, though that was a few years ago. My plan was to get at least the design in place before taking up someone's time to look things over.

literally nothing matters besides your TAP. you need to get one before there can be any answer to your question.

 

[dr wogz]

Have you contacted LOC to see about getting the proper fins?

I would just cut new fins..

 

[HVArcas]

are you guys certain this is not some partial-thru-wall deal that used to be popular on large kits from certain mfgrs? (fairly certain it was LOC that i am thinking of but memory could be failing too)

 

[deadalus52]

are you guys certain this is not some partial-thru-wall deal that used to be popular on large kits from certain mfgrs? (fairly certain it was LOC that i am thinking of but memory could be failing too)

I can't tell for sure. I actually talked to the guys that own LOC at nypower a few years back, they were saying the old design threaded something through the fin on the inside of the airframe, but there isn't a cut slot. What I'm wondering is if I got the fins from the version that had a cluster mount. This kit was bought I want to say during a black friday sale in 2017, so it had been kitted up probably way earlier than that. Supposedly they've made major changes to the kit since then.

 

[richP]

Get a TAP. Ask him the same question prior to any more work. You could provide input as to the solution that you want to use, but ultimately, only what he is willing to sign-off on matters.
If it were me, I would get fins with the correct tab size. Your fix would likely be more than enough strength-wise, but the added weight could be better used for other reinforcements.

 

[HVArcas]

I can't tell for sure. I actually talked to the guys that own LOC at nypower a few years back, they were saying the old design threaded something through the fin on the inside of the airframe, but there isn't a cut slot. What I'm wondering is if I got the fins from the version that had a cluster mount. This kit was bought I want to say during a black friday sale in 2017, so it had been kitted up probably way earlier than that. Supposedly they've made major changes to the kit since then.

It kinda sounds like you got a mix of components from different editions of the "same" kit

I would talk to LOC if you wanted a complete stack, as-intended from mfgr

 

[HVArcas]

Get a TAP. Ask him the same question prior to any more work. You could provide input as to the solution that you want to use, but ultimately, only what he is willing to sign-off on matters.
If it were me, I would get fins with the correct tab size. Your fix would likely be more than enough strength-wise, but the added weight could be better used for other reinforcements.

like i said

even the rulebook does not mean much, if the TAP says good, then good. TAP says no-good, then no-good. Some parts of rocketry are based on facts and science. L3 cert approval is not.

 

[DaveW6DPS]

You can't begin construction until the TAP approves your plan, but it is a good idea to get a TAP or two involved early in the design phase to head off issues and answer question like this before you waste much time going down the wrong road.

 

[OverTheTop]

As others have said, check with your TAP. They are quite variable and the ones you need to appease. Looks like a good idea from my perspective. I would use a very light infill personally. Should be plenty strong.

 

[HVArcas]

You can't begin construction until the TAP approves your plan

Depends on the TAP!

 

[Nytrunner]

Depends on the TAP!

Indeed so.

Ive spoken with TAPs that follow the rules to letter/spirit, and at least one TAP that believes an expensive shortcut gone wrong is a great way for the flier to learn their lesson (ie, contacting him with a prebuilt rocket/design)

 

[HVArcas]

Indeed so.

Ive spoken with TAPs that follow the rules to letter/spirit, and at least one TAP that believes an expensive shortcut gone wrong is a great way for the flier to learn their lesson (ie, contacting him with a prebuilt rocket/design)

i got no problem with that - however!

and this cannot be overstated

the TAP is the flier of record for the cert attempt. You know how if you land on something cause some damage, or heaven forbid come in ballistic and maim/kill, and it is your fault? When it comes to cert flights, it is the certifier who is on the hook.

 

[DaveW6DPS]

Depends on the TAP!

Presumably the TAP will know and follow the rules.

 

[deadalus52]

Thanks for the advice, everyone! I'll get in contact with a TAP.

 

[Steve Shannon]

i got no problem with that - however!

and this cannot be overstated

the TAP is the flier of record for the cert attempt. You know how if you land on something cause some damage, or heaven forbid come in ballistic and maim/kill, and it is your fault? When it comes to cert flights, it is the certifier who is on the hook.

The TAP is not the flier of record for a cert flight. Nor is the TAP on the hook for the flight, although we would pay attention to bad judgement on the part of the TAP. The candidate is the flier and the flier is always ultimately responsible for the flight.

Regarding the OP’s question, the use of 3D printing in the way he proposed does not violate the 3D printed fin can rule for cert flights, but it may not be an optimal design because it adds mass where least desirable. What size of motor mount tube would fit up next to the fin tabs?

 

[HVArcas]

The TAP is not the flier of record for a cert flight. Nor is the TAP on the hook for the flight, although we would pay attention to bad judgement on the part of the TAP. The candidate is the flier and the flier is always ultimately responsible for the flight.

Regarding the OP’s question, the use of 3D printing in the way he proposed does not violate the 3D printed fin can rule for cert flights, but it may not be an optimal design because it adds mass where least desirable. What size of motor mount tube would fit up next to the fin tabs?

Well shoot. Sorry for the misinformation, though it is well-circulated misinformation round these parts.

(I'd delete the post but now it seems more produnt to leave it and hope folks keep reading)

 

[Dustin Lobner]

The guys at LOC are fantastic, just email them and get the right stuff.

 

[Handeman]

The TAP is not the flier of record for a cert flight. Nor is the TAP on the hook for the flight, although we would pay attention to bad judgement on the part of the TAP. The candidate is the flier and the flier is always ultimately responsible for the flight.

Regarding the OP’s question, the use of 3D printing in the way he proposed does not violate the 3D printed fin can rule for cert flights, but it may not be an optimal design because it adds mass where least desirable. What size of motor mount tube would fit up next to the fin tabs?

+1

That is my first question, what size MMT would fit with those fins. I would go with that size MMT and R&R the CRs if I couldn't open them up to fit the larger MMT. You can always adapt the motor down.

 

[Steve Shannon]

Well shoot. Sorry for the misinformation, though it is well-circulated misinformation round these parts.

(I'd delete the post but now it seems more produnt to leave it and hope folks keep reading)

It may be that you’re thinking of the mentors for Junior certifications or TMP flights. Those flights must be signed by adults because of NFPA 1125/1127.