Tripoli prohibition on the use of 3D printed fin cans for L3 certification

[gldknght]

It's just my opinion, but I would think that the overall rocket performance would have a stronger impact than just the motor impulse. For example, I've flown 3D printed fins on a J to ~430 mph. My 3D printed L3 project sims out to just slightly faster (~450 mph), the fin span is only and inch wider (6" vs 5"), and the fin thickness is increased. As long as the fin isn't taking the motor loads, the fin doesn't know or care about how big the motor is; it only cares about acceleration and aerodynamic loads that it sees. That being said, a mach 2+ N flight with unreinforced printed fins will probably fail, but I feel that a low-and-slow M flight is a much different discussion. Basically what I'm trying to say is, I think speed and span matters more than impulse.

The problem is, without some kind of guideline, how does the RSO know, when the rocket is presented for a safety check, how fast a rocket is going to be going at max Q ? I've seen several level 3 cert flights and never once saw anyone present their L3 cert package or talk about the expected max speed of the rocket.

According to my simulations, my L3 cert flight on a baby M could be going supersonic.There is a pretty good chance 3DP fins will not hold up to those kind of stresses. Aerodynamic heating alone could soften the plastic and cause fin failure.

I think those who have suggested more research is needed by certified fliers are correct.

 

[Wallace]

I too concur with the BoD's decision, I have seen 3D printed parts fail in flight, not once but twice (both were nose cones).

Calm down Francis, the sky is not falling...

 

[Ez2cDave]

At our monthly BoD meeting we discussed the proliferation of 3D printing. We all agree that the technology is exciting and will undoubtedly lead to some great designs. However, for two reasons the board has decided to prohibit the use of 3D printed fin cans for Level 3 certification flights. The two reasons are:
1. It’s impossible to know whether the L3 candidate designed the fin can. Simply printing a shared file doesn’t demonstrate expertise or knowledge, which is what certification represents.
2. The strength of most 3D printed parts is still not strong enough.

The board has not prohibited the use entirely. We would like it if actual launch experience with 3D printed fin cans gets reported so we can better understand what the risks are from this technology. Our L3 certification procedure will be updated with this prohibition.
Thanks,
Steve

Steve,

You posted, "However, for two reasons the board has decided to prohibit the use of 3D printed fin cans for Level 3 certification flights. The two reasons are:

1. It’s impossible to know whether the L3 candidate designed the fin can. Simply printing a shared file doesn’t demonstrate expertise or knowledge, which is what certification represents.

2. "The strength of most 3D printed parts is still not strong enough."

I have a few genuine questions : ( no "sneer quotes" )

(1) "Designing" the fin can . . . What if a flyer can prove that he did "design" it, even if it was made by someone else ? Also, the 3D printer actually "makes" things, not the flyer.

(2) A shared 3D file is essentially the same as ordering components ( not designed or made by the flyer ).

From Tripoli: "Commercially available pre-fabricated fin cans, either as part of a kit or obtained separately, may not be used for level 3 certification flights."

Shared 3D files and the subsequent printed items, made later, are not "pre-fabricated" . . . Does that give them a "loophole" ?

(3) If a flyer uses a kit, he did not design it or make the components, should kits be disallowed for Cert flights ?

(4) If the strength of most 3D printed components are not strong enough, in the interest of safety, why should their use be allowed, at all ?

Frankly, your statement, "The strength of most 3D printed parts is still not strong enough", should have been reason enough to prohibit their use, based on the safety risks alone, and not just in Cert flights !

In my opinion, I think that "plastic" fins ( as "fin cans" or individual components ), as produced by current "home 3D technology", are inherently prone to failure, especially in the upper Total Impulse ranges, and at supersonic velocities. I think that a much closer analysis and, possibly, some form of structural testing is in order, to provide accurate data, in the interest of safety.

Dave F.

 

[Wallace]

What do you use with PETG?

And not being an L3 flier, what’s so different between a 10G J boost and a 10G M boost? Torque and lateral forces on components because the parts are bigger?

Charles: I've been slowly (as in when I find time) doing some destructive testing concerning exactly that. General consensus seems to be Loctite super glue of some sort, but I personally won't use any form of "super" glue (not even super duper glue) for structural purposes. Glenmarc Rocketpoxy looks promising, I've had nothing but good results using it on injection molded nose cones, although I can't say with any certainty exactly what "type" of plastic they are made from. Since it only has a Shore D of 85 it seems to have a similar amount of "give" as PETG printed parts.A 3d printing "pen" seems to also look promising, but since I don't have one...My current thoughts are to chemically bond parts, but also back up said bonds with mechanical fasteners...

 

[Locksmith]

Steve,


(4) If the strength of most 3D printed components are not strong enough, in the interest of safety, why should their use be allowed, at all ?

Frankly, your statement, "The strength of most 3D printed parts is still not strong enough", should have been reason enough to prohibit their use, based on the safety risks alone, and not just in Cert flights !

In my opinion, I think that "plastic" fins ( as "fin cans" or individual components ), as produced by current "home 3D technology", are inherently prone to failure, especially in the upper Total Impulse ranges, and at supersonic velocities. I think that a much closer analysis and, possibly, some form of structural testing is in order, to provide accurate data, in the interest of safety.

Dave F.

That statement is why I dont like blanket statements like 3d printed. As I posted earlier its just as much an art as it is a science. If you take two builders who use the same glue with same materials and flyer A fails cause his fins fell off and flyer B doesn't and has multiple successful flights, should tripoli ban the glue and materials that both used since it failed and is un safe?
Its ok if you don't know or understand what 3d printing is but dont use a blanket statement since 1 flyer might be using a PLA filament that would melt if left out in the sun to long and flyer 2 is using a Carbon Fiber infused Pollycarbon that can easily with stand tempratures in excess of 300 degrees with short burst to over 500 degrees and out perform the strength of aluminum. All of which can be printed at home in a fairly cheap 3d printer.

Even if 2 people are using the exact same filament and exact same printer the end product will be different,1 person may have spent hundreds of dollars on filament dialing it in to get the best quality and strength. where person 2 just bought it slapped it on the printer.

Now I agree that fin cans in general should not be used in lvl3 and it should end with that not what the fin cans are made of.

 

[rharshberger]

Calm down Francis, the sky is not falling...

Nope the nose cone was....[emoji3]

 

[Steve Shannon]

Steve,

You posted, "However, for two reasons the board has decided to prohibit the use of 3D printed fin cans for Level 3 certification flights. The two reasons are:

1. It’s impossible to know whether the L3 candidate designed the fin can. Simply printing a shared file doesn’t demonstrate expertise or knowledge, which is what certification represents.

2. "The strength of most 3D printed parts is still not strong enough."

I have a few genuine questions : ( no "sneer quotes" )

(1) "Designing" the fin can . . . What if a flyer can prove that he did "design" it, even if it was made by someone else ? Also, the 3D printer actually "makes" things, not the flyer.

(2) A shared 3D file is essentially the same as ordering components ( not designed or made by the flyer ).

From Tripoli: "Commercially available pre-fabricated fin cans, either as part of a kit or obtained separately, may not be used for level 3 certification flights."

Shared 3D files and the subsequent printed items, made later, are not "pre-fabricated" . . . Does that give them a "loophole" ?

(3) If a flyer uses a kit, he did not design it or make the components, should kits be disallowed for Cert flights ?

(4) If the strength of most 3D printed components are not strong enough, in the interest of safety, why should their use be allowed, at all ?

Frankly, your statement, "The strength of most 3D printed parts is still not strong enough", should have been reason enough to prohibit their use, based on the safety risks alone, and not just in Cert flights !

In my opinion, I think that "plastic" fins ( as "fin cans" or individual components ), as produced by current "home 3D technology", are inherently prone to failure, especially in the upper Total Impulse ranges, and at supersonic velocities. I think that a much closer analysis and, possibly, some form of structural testing is in order, to provide accurate data, in the interest of safety.

Dave F.

Post 23 in this thread does a good job of answering your first question.
This decision was made to close the loophole in the current wording prohibiting commercial fin cans.
Kits are fine, as is purchasing individual components; they still require good traditional construction techniques that demonstrate expertise, which something like commercial fin cans or a file that’s printed on a 3D printer might not demonstrate. Keep in mind too that this is just for the L3 certification flight, nothing else. Eventually we’ll figure out better rules regarding this technology. The experiences that people share regarding 3D printed components will help.

 

[Bill Hanson]

This is not entirely an "apples to apples" comparison, but I just finished up doing post-flight inspections for about 80 rockets during this year's Spaceport America Cup. While they were not technically L3 rockets due to many teams not being certified, these were all 10K or 30K launches (plus a couple of 100K flights) on M or better motors. Also, these rockets had to carry an 8 kg payload, which introduced some different failure modes, like the high percentage of teams who chose to put their payload in the nose cone having their nose cones destroyed on landing.

We did not have any 3D printed fin cans, and only a couple of fins which used 3D printing as part of their process, and these were as part of a sandwich of other material. Several teams (~10) used 3D printed nose cones. About half the teams used 3D printed av-bays of various designs, split between PLA and PETG with a couple of ABS or nylon.

Other than damage due to hard landings and ballistic returns, none of the fins incorporating 3D printing failed, and none of the printed nose cones failed. We had a large percentage of broken av-bays, even under normal landings. Most of those failures were PLA, and many of those had low infills. There was one that poured out of the av-bay in about nickel-sized pieces -- PLA with 10% infill. Besides the obvious issues with poor choices for infill, the PLA in particular suffered due to the very hot conditions of ~100 degrees, and the temperature inside the rockets were much higher from sitting on the rails in the sun.

While the plural of anecdote is not data, and realizing this doesn't directly address the fin can issue, I think this is a pretty good snapshot of some of the issues.

 

[Wallace]

What if someone was able to show/prove they understand enough about material selection, software settings and structural design necessary to safely implement said. Printed objects are not necessarily inherently weak or failure prone, it's nothing more than a tool that needs to be used correctly to avoid potential problems.

 

[Wallace]

That is a certified scale btw...

 

[Wallace]

In other words, I knew better than to print them out of PLA using vase mode.

 

[Wallace]

They are beefy, but certainly not overkill.

 

[Steve Shannon]

This is not entirely an "apples to apples" comparison, but I just finished up doing post-flight inspections for about 80 rockets during this year's Spaceport America Cup. While they were not technically L3 rockets due to many teams not being certified, these were all 10K or 30K launches (plus a couple of 100K flights) on M or better motors. Also, these rockets had to carry an 8 kg payload, which introduced some different failure modes, like the high percentage of teams who chose to put their payload in the nose cone having their nose cones destroyed on landing.

We did not have any 3D printed fin cans, and only a couple of fins which used 3D printing as part of their process, and these were as part of a sandwich of other material. Several teams (~10) used 3D printed nose cones. About half the teams used 3D printed av-bays of various designs, split between PLA and PETG with a couple of ABS or nylon.

Other than damage due to hard landings and ballistic returns, none of the fins incorporating 3D printing failed, and none of the printed nose cones failed. We had a large percentage of broken av-bays, even under normal landings. Most of those failures were PLA, and many of those had low infills. There was one that poured out of the av-bay in about nickel-sized pieces -- PLA with 10% infill. Besides the obvious issues with poor choices for infill, the PLA in particular suffered due to the very hot conditions of ~100 degrees, and the temperature inside the rockets were much higher from sitting on the rails in the sun.

While the plural of anecdote is not data, and realizing this doesn't directly address the fin can issue, I think this is a pretty good snapshot of some of the issues.

Thanks for that. Those are good examples. We have not limited the use of 3D printed fins, sleds, nosecones or any other parts other than fin cans, and those are only restricted for L3 certification flights. We want our members to explore these technologies safely and we want to learn from them so we can better advise other members, Prefects, and TAPs.

 

[Steve Shannon]

What if someone was able to show/prove they understand enough about material selection and structural design necessary to safely implement said. Printed objects are not necessarily inherently or failure prone, it's nothing more than a tool that needs to be used correctly to avoid potential problems.

I suspect that eventually we’ll be able to accommodate such proof of ability. We’re just trying to figure this all out.

 

[Cnorm]

If you want a good laugh, check out the Tripoli Facebook group. Some entitled snowflake is so triggered he's spamming the group with sarcastic posts about what he can and can't use in his rockets.

Grown men throwing very public temper tantrums. Heh.

 

[Wallace]

I'd be more than happy to share my, admittedly non-scientific findings. Actually if you look closely at that tail cone it has an offset a couple inches from the end that does not belong there, suspect it was an SD card related issue. Went over the original file in Autodesk and S3D and it does not appear to have come from either. Point is, since that was a 34 hour print I'm going to attempt a "repair". If I'm not satisfied w/how it turns out, I'll move on to destructive testing with video to share. This would also be an appropriate time to give props to the original designer, he's know here on TRF as Pointy_end_up, Thank you Sir for the outstanding research, design and for sharing.

 

[Ez2cDave]

What if someone was able to show/prove they understand enough about material selection, software settings and structural design necessary to safely implement said.

Wallace,

One big problem is that the people who would be shown the data / proof are inexperienced in the technology, themselves.

My suggestion would be for both Tripoli and NAR to commission "outside experts" to properly examine, analyze, and test the techniques, methods, materials, and designs being used to quantitatively determine their limitations and suitability.

The main problem, as I see it, is when relatively-thin, plate-like, cross-sections are employed, namely "Fins".

Dave F.

 

[Ez2cDave]

If you want a good laugh, check out the Tripoli Facebook group. Some entitled snowflake is so triggered he's spamming the group with sarcastic posts about what he can and can't use in his rockets.

Grown men throwing very public temper tantrums. Heh.

At this rate, RSO's may have to begin providing "Safe Spaces" on the field, before long . . .

LOL !

Dave F.

 

[Wallace]

That statement is why I dont like blanket statements like 3d printed. As I posted earlier its just as much an art as it is a science. If you take two builders who use the same glue with same materials and flyer A fails cause his fins fell off and flyer B doesn't and has multiple successful flights, should tripoli ban the glue and materials that both used since it failed and is un safe?
Its ok if you don't know or understand what 3d printing is but dont use a blanket statement since 1 flyer might be using a PLA filament that would melt if left out in the sun to long and flyer 2 is using a Carbon Fiber infused Pollycarbon that can easily with stand tempratures in excess of 300 degrees with short burst to over 500 degrees and out perform the strength of aluminum. All of which can be printed at home in a fairly cheap 3d printer.

Even if 2 people are using the exact same filament and exact same printer the end product will be different,1 person may have spent hundreds of dollars on filament dialing it in to get the best quality and strength. where person 2 just bought it slapped it on the printer.

Now I agree that fin cans in general should not be used in lvl3 and it should end with that not what the fin cans are made of.

Well said.

 

[Wallace]

Wallace,

One big problem is that the people who would be shown the data / proof are inexperienced in the technology, themselves.

My suggestion would be for both Tripoli and NAR to commission "outside experts" to properly examine, analyze, and test the techniques, methods, materials, and designs being used to quantitatively determine their limitations and suitability.

The main problem, as I see it, is when relatively-thin, plate-like, cross-sections are employed, namely "Fins".

Dave F.

That's a double standard in my eyes. Who's testing every part in every one of all "manufacturers" kits?

 

[OverTheTop]

I too concur with the BoD's decision, I have seen 3D printed parts fail in flight, not once but twice (both were nose cones).

As with most things, details matter. Personally I have flown 3D printed nosecones since about 2014 and have had no problems with the ABS. Typically L motors are flown, around Mach. My recent O-M two-stage had a 3D printed (ABS again) NC that successfully flew to M2.14.

If designed right it will work.

 

[Ez2cDave]

That's a double standard in my eyes. Who's testing every part in every one of all "manufacturers" kits?

Wallace,

Those parts are not manufactured using 3-D printer technology.

3-D-printed components, particularly relatively thin, flat cross-section items, such as fins are known to have less structural strength and be prone to thermal weakening.

The materials they are using have already been tested by years, if not decades of use. Their inherent reliability is already quantified.

Ask yourself these questions :

(1) Is a plastic, 3-D printed fin as strong as . . . ?

(a) A Fiberglass fin of the same thickness.
(b) A Carbon Fiber fin of the same thickness
(c) An Aircraft Plywood fin of the same thickness
(d) An Aluminum fin of the same thickness

(2) Is a plastic, 3-D printed fin as resistant to thermal deformation as . . . ?

(a) A Fiberglass fin of the same thickness.
(b) A Carbon Fiber fin of the same thickness
(c) An Aircraft Plywood fin of the same thickness
(d) An Aluminum fin of the same thickness

(3) What changes to a plastic, 3-D printed fin would needed to make it as strong, and as thermally-stable as . . . ?

(a) A Fiberglass fin of the same thickness.
(b) A Carbon Fiber fin of the same thickness
(c) An Aircraft Plywood fin of the same thickness
(d) An Aluminum fin of the same thickness

Final thought : With safety being paramount in Rocketry, why would anyone, in good conscience, choose to use a sub-standard method or sub-standard materials and risk an accident ?

Dave F.

 

[cwbullet]

Dave,

Good questions. Before I start, I agree with the policy.

I will say that I can make a cone as thermally stable as a loc precision cone. Fins may be another story. I have only printed a few. It is better to be safe than sorry. It only takes one bad flight to lose a field.

 

[Ez2cDave]

That statement is why I dont like blanket statements like 3d printed. As I posted earlier its just as much an art as it is a science. If you take two builders who use the same glue with same materials and flyer A fails cause his fins fell off and flyer B doesn't and has multiple successful flights, should tripoli ban the glue and materials that both used since it failed and is un safe?

TRIPOLI and NAR have to "codify" rules and regulations, in the interest of safety, to protect people, and the Hobby itself, against flyers using "questionable" methods and materials that could, potentially, compromise safety. The number of Rocketeers who are "professional-level" 3-D printing experts is greatly out-numbered by the "hoards" of "me, too" Rocketeers just starting out in 3-D printing because prices have dropped on equipment or because the "cool thing" in Rocketry right now is 3-D printing.

So, while the near-experts, experts, and professionals are "restrained" by the rules and regulations, we are simultaneously protected from the "Hey, watch this" crowd. Safety trumps innovation, until innovation meets the safety requirements !

Dave F.

 

[Ez2cDave]

Dave,

Good questions. Before I start, I agree with the policy.

I will say that I can make a cone as thermally stable as a loc precision cone. Fins may be another story. I have only printed a few. It is better to be safe than sorry. It only takes one bad flight to lose a field.

Hi, Chuck !

Nose Cones are not as much of a problem, except at MACH+ velocities. Even then, an aluminum tip would probably provide adequate protection for "most" flights.

The issue here is Fins, due to their inherent thinness, possibility of flutter, softening with heat, and possible shattering, if brittle.

Dave F.

 

[Bat-mite]

TRIPOLI and NAR have to "codify" rules and regulations, in the interest of safety, to protect people, and the Hobby itself, against flyers using "questionable" methods and materials that could, potentially, compromise safety. The number of Rocketeers who are "professional-level" 3-D printing experts is greatly out-numbered by the "hoards" of "me, too" Rocketeers just starting out in 3-D printing because prices have dropped on equipment or because the "cool thing" in Rocketry right now is 3-D printing.

So, while the near-experts, experts, and professionals are "restrained" by the rules and regulations, we are simultaneously protected from the "Hey, watch his" crowd. Safety trumps innovation, until innovation meets the safety requirements !

Dave F.

Very well put. There are requirements for cert flights that do not extend beyond the cert flight. Like oddrocs and use of EX motors, etc.

 

[JohnCoker]

I think a few people are missing the "for certification flights" part of the message. Once you're a proven L3 flier using traditional techniques, all power to you for experimenting. I for one am planning to get into composite and metal printing as soon as the price comes down a bit more. (I use FDM printing for non-structural parts already.)

 

[OverTheTop]

Wallace,

Those parts are not manufactured using 3-D printer technology.

3-D-printed components, particularly relatively thin, flat cross-section items, such as fins are known to have less structural strength and be prone to thermal weakening.

The materials they are using have already been tested by years, if not decades of use. Their inherent reliability is already quantified.

Ask yourself these questions :

(1) Is a plastic, 3-D printed fin as strong as . . . ?

(a) A Fiberglass fin of the same thickness.
(b) A Carbon Fiber fin of the same thickness
(c) An Aircraft Plywood fin of the same thickness
(d) An Aluminum fin of the same thickness

(2) Is a plastic, 3-D printed fin as resistant to thermal deformation as . . . ?

(a) A Fiberglass fin of the same thickness.
(b) A Carbon Fiber fin of the same thickness
(c) An Aircraft Plywood fin of the same thickness
(d) An Aluminum fin of the same thickness

(3) What changes to a plastic, 3-D printed fin would needed to make it as strong, and as thermally-stable as . . . ?

(a) A Fiberglass fin of the same thickness.
(b) A Carbon Fiber fin of the same thickness
(c) An Aircraft Plywood fin of the same thickness
(d) An Aluminum fin of the same thickness

Final thought : With safety being paramount in Rocketry, why would anyone, in good conscience, choose to use a sub-standard method or sub-standard materials and risk an accident ?

Dave F.

Questions (1) and (2) are irrelevant mostly. You are not comparing like for like. Question (3) is what we need to be asking. Being fit for purpose is the requirement from an engineering standpoint. If we had to have same (whatever) in products then you wouldn't have things like Mag (magnesium alloy) wheels. Steel wheels use far less material by thickness and volume than the alloys. The alloy wheels have more volume but are still ligther and have less rotational inertia, so from an engineering POV they are better.

 

[dwightr]

Additive manufacturing is improving every day. GE is printing fuel nozzles for its LEAP turbofan engines and about 35% of its Advanced Turboprop Engine will be additive manufactured. Certainly highly stressed parts but carefully designed and far from hobby priced printers.

 

[Ez2cDave]

Questions (1) and (2) are irrelevant mostly. You are not comparing like for like. Question (3) is what we need to be asking. Being fit for purpose is the requirement from an engineering standpoint. If we had to have same (whatever) in products then you wouldn't have things like Mag (magnesium alloy) wheels. Steel wheels use far less material by thickness and volume than the alloys. The alloy wheels have more volume but are still lighter and have less rotational inertia, so from an engineering POV they are better.

I was comparing known "suitable materials" versus 3-D "plastic" fins in all questions. Also, Question #3 is based on #1 & #2.

Most, if not all, wheels are Aluminum Alloy, nowadays. Magnesium wheels have the unfortunate tendency to catch fire and burn in a vehicle fire. ( I turn 58 this year and I know what "mag wheels" are - LOL !)

BTW - You left out the exotic Carbon-Composite Wheels . . . "Curb-Shot" one and it's "toast" !

Dave F.