So, I exceeded the limits of a 3D printed fin can

[Buckeye]

I built a Wildman Mach2 but swapped out the fiberglass fins for this 3D printed fin can from Additive Aerospace.

Flight was on an Aerotech K480. RASAero II simulation predicted about 23k feet AGL and a max speed of ~2200 ft/sec (Mach 2.0).

This is how it came back to earth. Apogee was recorded as 17k ft AGL per baro altimeter and GPS. 6,000 ft was lost due to the fins losing their shape.

What caused the warped fins? Aerodynamic forces or aerodynamic heating at Mach 2? The heat of the motor tube or exhaust?

Edit: Only the leading edges and tip edges are damaged. The rest of the fins/can looks OK.

 

[Exactimator]

Looks like fin flutter to me. But instead of the fins straightening back out they held a little bit of the flutter shape.

I don't know a lot about 3D print materials. What would make the red turn chalky white like that? Melting/cooling? Flexing? Both?

 

[bobbyg23]

I built a Wildman Mach2 but swapped out the fiberglass fins for this 3D printed fin can from Additive Aerospace.

Flight was on an Aerotech K480. RASAero II simulation predicted about 23k feet AGL and a max speed of ~2200 ft/sec (Mach 2.0).

This is how it came back to earth. Apogee was recorded as 17k ft AGL per baro altimeter and GPS. 6,000 ft was lost due to the fins losing their shape.

What caused the warped fins? Aerodynamic forces or aerodynamic heating at Mach 2? The heat of the motor tube or exhaust?

Edit: Only the leading edges and tip edges are damaged. The rest of the fins/can looks OK.

View attachment 476457

Mach 2 gets hot. Heat + 3d printed parts =melting

 

[Buckeye]

I don't know a lot about 3D print materials. What would make the red turn chalky white like that? Melting/cooling? Flexing? Both?

That is just light reflection off the plastic. No change in color.

 

[dvdsnyd]

My guess would be aero heating.
Did you see any deviation in the flight profile on the way up?

 

[FlyBy01]

I built a Wildman Mach2 but swapped out the fiberglass fins for this 3D printed fin can from Additive Aerospace.

Flight was on an Aerotech K480. RASAero II simulation predicted about 23k feet AGL and a max speed of ~2200 ft/sec (Mach 2.0).

This is how it came back to earth. Apogee was recorded as 17k ft AGL per baro altimeter and GPS. 6,000 ft was lost due to the fins losing their shape.

What caused the warped fins? Aerodynamic forces or aerodynamic heating at Mach 2? The heat of the motor tube or exhaust?

Edit: Only the leading edges and tip edges are damaged. The rest of the fins/can looks OK.

View attachment 476457

Pure speculation but Mach flutter, Mach heat, and Mach tuck. Thank you for posting because this is really cool. Do you have a plan or ability to refit this rocket for further testing?

 

[Buckeye]

My guess would be aero heating.
Did you see any deviation in the flight profile on the way up?

Flight looked nominal to my eye. I still need to download and analyze the data from the electronics.

 

[Buckeye]

Pure speculation but Mach flutter, Mach heat, and Mach tuck. Thank you for posting because this is really cool. Do you have a plan or ability to refit this rocket for further testing?

Well, all I need is a replacement body tube from Wildman, and I can rebuild the Mach 2 per its original design. I kept this minimum diameter build very simple - no motor retainer, no shock cord anchor. All the effort was in the HED electronics bay. I will put on the stock fiberglass fins next time.

Copying @Landru on this thread so he gets this feedback on the fin can.

 

[OverTheTop]

Sounds like the higher surface area to volume ratio near the edge allowed it to soften during flight. Have you thought about giving the edges a coat of JB Weld to help them out?

 

[Buckeye]

Sounds like the higher surface area to volume ratio near the edge allowed it to soften during flight. Have you thought about giving the edges a coat of JB Weld to help them out?

Yep, that is a good explanation.

Nah, I am not messing around with plastic parts anymore for extreme flights. I like this printed fin can for ease of assembly and accurate alignment, but it has its limits.

 

[jnmiller]

Do you know what filament they use? PLA?

 

[jderimig]

I am shocked it survived at all. Good game.

 

[thzero]

Yep, that is a good explanation.

Nah, I am not messing around with plastic parts anymore for extreme flights. I like this printed fin can for ease of assembly and accurate alignment, but it has its limits.

Unless they were printed with higher temp and stronger materials, its not really that surprising. Even a layer or two of fiberglass would probably have helped.

 

[ghostfather]

I use printed plastic fins and fin cans, but not for extreme flights that go to Mach 2. It is very evident that you had heating at the tips, but plastic by itself isn't stiff and strong enough to withstand the fin flutter at that speed. Crazy that it re-hardened to show the flutter.
While Additive Aerospace makes good stuff, I wouldn't recommend it for extreme stuff. Smaller rockets with G or H motors are OK.

That said, I do use printed fins as the core of a composite construction. I've used a layer of fiberglass on the outside surface of small fins printed with a symmetric Von Karman profile on a minimum diameter rocket, and you probably could have used something similar on that fin can. I would reinforce the leading edge and outside edge with a thin strip of fiberglass, and then go tip-to-tip with another layer. Doesn't have to be heavy fiberglass, probably 60 gr cloth is fine. The fiberglass/epoxy makes the construction much more rigid, and will help against deformed edged due to heating.

I've been playing around with much larger fins, where I print half a fin in Von Karman profile split vertically, laying flat on the print bed, and then laminating (with epoxy) two sides of the fins together around a layer of carbon fiber as core, and then a layer of fiberglass on the outside surface. That way I get a very rigid CF core to the fin, which meets the fiberglass at the edges, and stresses are taken up by the fiberglass skin. Even if it does heat up enough to slightly melt the printed plastic core, it should keep it's shape, and be stiff and strong enough for extreme flights.

 

[Buckeye]

Fiberglassing and epoxying the fin can defeats the purpose of the printed part, IMO. If I need to do all that, I might as well use the stock G10 fins in the kit!

 

[ghostfather]

Fiberglassing and epoxying the fin can defeats the purpose of the printed part, IMO. If I need to do all that, I might as well use the stock G10 fins in the kit!

Stock G10 fins are so ..... "flat".
I print fins to get an airfoil profile, using an M1 NACA symmetric profile for rockets going subsonic, and a symmetric Von Karman profile for Mach+ flights.

So there really is purpose behind my madness...... I think.

I sometimes use G10 plate as a core to attach fins, as the root edge is usually a bit thicker than out at the tips. Works great for Thru-the-Wall attachment, I just print the void in the fin, and glue it to the G10.

You may have noticed that Landru's fin can uses a profile much like the fins on the Nike Smoke sounding rocket. These were flat surfaces, having sharp leading and trailing edges. That was easiest to engineer and manufacture back in the 60's, and it also gets thinner from the root to the outside edge. Landru's fin cans do the same, a nice design for Mach+ flights, just the wrong material. That's why I suggested improving his basic design by adding a layer of fiberglass. That probably would have withstood the flight with a K motor without melting or fluttering.

 

[Bill Hanson]

Do you know what filament they use? PLA?

Description says “PET plastic” which appears to be slightly stiffer and higher temp resistance than PETG.

See this site I found with a useful comparison chart.
https://www.filamentive.com/pet-vs-petg-filament-whats-the-difference/

 

[thzero]

That's why I suggested improving his basic design by adding a layer of fiberglass. That probably would have withstood the flight with a K motor without melting or fluttering.

Yup! I did it on my sets of printed fins, and are planning two layers on my I-J level 2 build.

 

[ghostfather]

Description says “PET plastic” which appears to be slightly stiffer and higher temp resistance than PETG.

Both PET and PETg are loosely classified under the Co-Polyester family of filaments. There are a number of additives to make them stiff, durable, the melt and flow temps, etc. I print lots with a HiTemp formulation made from Eastman's Amphora 5300 resins, which is a bit more resistant to heating. There are so many sorts of co-polyesters.

While probably having a higher melting temperature than PLA, The PET used in the fin can wasn't enough to handle the heating of the extreme Mach+ flight.

 

[Landru]

I built a Wildman Mach2 but swapped out the fiberglass fins for this 3D printed fin can from Additive Aerospace.

Flight was on an Aerotech K480. RASAero II simulation predicted about 23k feet AGL and a max speed of ~2200 ft/sec (Mach 2.0).

This is how it came back to earth. Apogee was recorded as 17k ft AGL per baro altimeter and GPS. 6,000 ft was lost due to the fins losing their shape.

What caused the warped fins? Aerodynamic forces or aerodynamic heating at Mach 2? The heat of the motor tube or exhaust?

Edit: Only the leading edges and tip edges are damaged. The rest of the fins/can looks OK.

View attachment 476457

THAT IS AMAZING! Finally a flight with guts to push the can to its limits.

There has been quite a bit of debate on what an aero heating failure would look like, or whether the fins would rip off first. I always guessed the leading edges would fold over first.

The PET plastic softens about ~80C, and is actually less stiff than PLA, but much tougher.

The previous record was a 38mm flown by me to M1.3 without any fin damage.

I use printed plastic fins and fin cans, but not for extreme flights that go to Mach 2. It is very evident that you had heating at the tips, but plastic by itself isn't stiff and strong enough to withstand the fin flutter at that speed. Crazy that it re-hardened to show the flutter.
While Additive Aerospace makes good stuff, I wouldn't recommend it for extreme stuff. Smaller rockets with G or H motors are OK.

The fact this can stayed intact despite folding over the leading edge shows the can is plenty stiff and strong. There is a lot of headroom to go faster if not for the temperature limits of the plastic. These types of cans are often flown with J motors and this is the ONLY 'failure' I've ever heard of.

 

[plugger]

Looking at that photo I suspect this is solely aerodynamic heating and not fin flutter. Every time I've approached or exceeded Mach 2 in a flight with rattle can paint on the airframe the tip of the NC and the leading edges of the fins are what took the brunt of aerodynamic heating. For example, here's a photo from my L3 that hit ~Mach 1.9.

My guess is that aerodynamic heating softened your leading edges and then the aerodynamic forces incurred from traveling that fast then warped them before they cooled and 'rehardened'.

 

[Rocket501]

The fact this can stayed intact despite folding over the leading edge shows the can is plenty stiff and strong. There is a lot of headroom to go faster if not for the temperature limits of the plastic. These types of cans are often flown with J motors and this is the ONLY 'failure' I've ever heard of.

Huh, my school has a printer that can print plastics with a temperature limits of 145C (Markforged Onyx filament). I wonder how well a fin can printed with that material would stand up to mach 2 flight. At our prices, it looks like it would run me roughly $40 or so in materials if it uses roughly 80cm^3 of filament but it should stand up to these conditions even better.

Would you consider selling the files for this fin can so we could print it ourselves? I'm sure there's plans online or we could model one ourselves, but I'd be happy to pay $10-15 to not have the hassle of having to do that.

 

[ghostfather]

Huh, my school has a printer that can print plastics with a temperature limits of 145C (Markforged Onyx filament). I wonder how well a fin can printed with that material would stand up to mach 2 flight. At our prices, it looks like it would run me roughly $40 or so in materials if it uses roughly 80cm^3 of filament but it should stand up to these conditions even better.

Markforged Onyx filament is based on nylon with chopped carbon fiber. Nylon has a higher melting point than PET, so it should be slightly better. I'm not sure of the contribution of the carbon fiber, though the thermal conductivity of the fin can would be better.
The thermoplastics used in 3D printing are the weakest link, and I don't think the addition of carbon fiber in the printed matrix will affect that much. A layer of fiber carbon or fiberglass saturated by epoxy would be better, as the epoxy fails at a higher temperature than most printed thermoplastics.

 

[Landru]

Huh, my school has a printer that can print plastics with a temperature limits of 145C (Markforged Onyx filament). I wonder how well a fin can printed with that material would stand up to mach 2 flight. At our prices, it looks like it would run me roughly $40 or so in materials if it uses roughly 80cm^3 of filament but it should stand up to these conditions even better.

Would you consider selling the files for this fin can so we could print it ourselves? I'm sure there's plans online or we could model one ourselves, but I'd be happy to pay $10-15 to not have the hassle of having to do that.

I'm thinking about it. I don't sell very many so it may be an option. Half the sauce is getting the print settings dialed for a good surface finish and lightweight finished product with good bonding...

 

[Rocket501]

I'm thinking about it. I don't sell very many so it may be an option. Half the sauce is getting the print settings dialed for a good surface finish and lightweight finished product with good bonding...

Fortunately at school we have people who are very experienced at getting these machines dialed in for good results, but for individual hobbyists getting it professionally printed is likely the best option. However, some people do really like tinkering with their machines to get the best possible results, so this whole printing fincans themselves things may appeal to them as well.

 

[thzero]

There are plenty of examples on thingieverse... or learn CAD.

 

[Rocket501]

It's all a question of what your time is worth and how many failed prints it would take. Sure, I could model my own (and very well may end up doing so), but I'm often willing to pay a little if it saves me a few hours of work and a number of test prints on my personal printer. It's all just a matter of cost versus convenience vs quality.

And, I find it would be an interesting scientific experiment to see, other than different machines and filament, how the same design would fare under reasonably similar flight conditions while changing the design would be a bit more of an apples to oranges comparison. But you do have valid points as well.

 

[smstachwick]

Forget the Speed of Balsa, that’s the Speed of 3D Printing

 

[thzero]

It's all a question of what your time is worth and how many failed prints it would take. Sure, I could model my own (and very well may end up doing so), but I'm often willing to pay a little if it saves me a few hours of work and a number of test prints on my personal printer. It's all just a matter of cost versus convenience vs quality.

Its not like its rocket science.

 

[ghostfather]

It took me a couple of hours to duplicate Landru's fin can a few years back in Inventor or Fusion360, and export it as STL. Don't think I even saved it, as a couple of hours isn't much time.
I spend way more time printing and getting things right with the filament speeds and temps. Rocket501 will certainly need that time with Nylon/CF filaments like the Markforged Onyx filament.

@Rocket501
Part of 3D printing is learning to do the work, as there is always something you want to make that isn't out there to buy, and you have to make yourself. Especially if you get good with the 3D modeling software, you will always find work, or be very marketable as an engineer for prototyping stuff.
Say that you design a rocket with larger or smaller fins, or a different shape. It won't be out there to buy or download, just get good at making it yourself.