3D Printing Printed CRs and base heating

[JohnCoker]

One issue for the aft CR is heating from the motor. I'm presuming this is due to base drag pulling hot gasses up, but am not certain. This may prevent the use of printed parts at the aft end of a rocket. More investigation is required.

After an AT J270W flight, the aft CR was blacked with soot and there was an irregular section melted. (It didn't go all the way through the 6mm thick ring, but it's certainly concerning.) This was printed in ABS, using the settings determined in my strength testing experiments.



The CRs (like the fins) were printed in red ABS. You can still see the color in the fins, but the aft end is almost obscured with soot.

 

[manixFan]

Could those motor retainers be vectoring some of the exhaust onto the cr? It looks like there is quite a bit of clearance between the bottom of motor and the metal, which could be the problem. Add to that the melted area is where hot gases might be directed if that is happening.

Just a thought based on my previous experiences.


Tony

 

[thzero]

Put a layer of JB Weld over the end of it.

 

[frogglicker]

I have not seen this with my 3-in rockets. However, I flood the exterior of the aft ring with "whatever brand" epoxy (JB Weld is a great idea). I see soot, but no charring to speak of. This definitely demonstrates that a barrier should be used.

 

[Kelly]

I've never seen this on my rockets.
Could it be that this is happening from upward blast deflection at launch? Do you use a slanted or horizontal blast plate?

 

[Rob Campbell]

What kind of nozzle? J275s and J315s come with Medusa Nozzles, which blistered the paint on the aft CR of my Warlock.

 

[atestani]

Could those motor retainers be vectoring some of the exhaust onto the cr?

I thought of this also.

I use a metal plate with the nozzle coming thru to protect the PAHT-CF thrust ring and retain the motor. I have seen the entire plate get black with soot, residue, etc. It looks like this (with a virgin plate):



(A full description of my interchangeable motor mounts is here:
https://www.rocketryforum.com/threads/thrust-plates-when-and-why.177113/post-2548491 )

Here is a 3X cluster plate with baked on residue from sparky motors.


We have angled deflector plates plus I use a block of wood to keep the nozzles away from the plate. I think the cause is as John said: "due to base drag pulling hot gasses up".

 

[thequick]

I’m starting to loose hope for the boat tail I made using Veroblackplus (HDT 45-50C). It was designed to serve a retainer also. . Still plan to test, but I’ll use friction fit as a back up until I understand the temp damage better.

 

[atestani]

I’m starting to loose hope for the boat tail I made using Veroblackplus (HDT 45-50C). It was designed to serve a retainer also. . Still plan to test, but I’ll use friction fit as a back up until I understand the temp damage better.

I may be wrong, but I don't think that your boat tail will survive. PAHT-CF has an HDT of 190C and I don't think it would survive in that configuration either. BP motors have an exhaust temp of around 275C and composite motors are > 1000C. Possibly, if you moved the motor aft so the nozzle was close to the end of the boat tail/retainer, it might work with PAHT-CF or even your filament

 

[JohnCoker]

Yeah, an epoxy coating might work. What I've done in the past is used a 1/16" aluminum blast plate, but I think that complicates the design and goes beyond what's generally available.

 

[OzHybrid]

Thin stainless steel would be the answer for a lightweight tailcone. But that limits your launch to FAR as stainless is not permitted by tripoli.
There's been some 3D printed fin cans in aluminium that looked good and were reasonably priced. Might be an idea to try that printing service for a custom tail cone....

 

[Zyzzyva1000]

I don't think I have ever had a centering ring be affected like that, also I use printed retainers for most everything now and have never had one melt. Most of them have been ABS or ASA, a few CF filled or PC. None of those have significantly charred or melted either (unless moving the tube slightly away from the CR because of the retainer is actually enough to make a difference). I am building a 6 inch fat boy right now with all printed components (other than the tubes) - I will have to see what happens (since I am quite sure base drag will be a significant effect with it.

 

[bschultz32]

Pardners, behold Son of Son of Dubya. (I may shorten the name to Dubya 3.0, to make it easier to say.) Michigan Team-1 is going to give this blast deflector a test drive at our upcoming 04/20/2024 launch at Birch Run. If it performs well, it gets a pretty coat of paint all over.
I highly doubt that JohnCoker would suffer any more centering ring damage if he flew with a Dubya!

 

[OverTheTop]

Maybe stick on a layer of aluminium tape on the aft end? Between the reflectivity and heat spreading it might do a good job. I am talking about the slightly thicker style of tape, not the alfoil-thin stuff, although that may just work too.

 

[bschultz32]

Dubya 3.0 has thick aluminum tape inside at the 60-degree elbows, just as OverTheTop suggests. Applied on top of that is a layer of black caulk, meant to serve as an ablative material. Rumor has it that one of our flights on the 20th will be an M1939W. I figure if Dubya can handle that, she can handle pretty much anything.

 

[wsume99]

Here are two tailcone retainers I've flown. Both are standard PLA and screw onto the bottom of the fincan. The green retainer is for 38mm motors and has flown 6 times - 3x G76G, 2x H250G and 1x J500G. All the heat damage present occurred after the first flight which was a G76G. It has not gotten worse on the 5 other flights. The black retainer is for 29mm motors. I think it has 4 total flights with no apparent degradation. I think the largest motor flown was a H128W. The others were F-G hobbyline reloads.

I have no hesitation flying printed retainers on everything except long burn motors. Others have their own thoughts/experience but for the parts I've designed and printed (out of the worst case material) I've had no issues.

One benefit of the tailcone retainer is it's replaceable so if it gets a.little over heated I'll just trash it and print another. If I had a printed CR at the aft end I was worried about I'd probably just print a thin disc to install over the CR.

 

[cwbullet]

Here are two tailcone retainers I've flown. Both are standard PLA and screw onto the bottom of the fincan. The green retainer is for 38mm motors and has flown 6 times - 3x G76G, 2x H250G and 1x J500G. All the heat damage present occurred after the first flight which was a G76G. It has not gotten worse on the 5 other flights. The black retainer is for 29mm motors. I think it has 4 total flights with no apparent degradation. I think the largest motor flown was a H128W. The others were F-G hobbyline reloads.

I have no hesitation flying printed retainers on everything except long burn motors. Others have their own thoughts/experience but for the parts I've designed and printed (out of the worst case material) I've had no issues.

One benefit of the tailcone retainer is it's replaceable so if it gets a.little over heated I'll just trash it and print another. If I had a printed CR at the aft end I was worried about I'd probably just print a thin disc to install over the CR.

Proof PLA is not tolerant to heat.

 

[wsume99]

Proof PLA is not tolerant to heat.

This thread started with an ABS part that couldn't tolerate the heat produced from motor exhaust. Motor exhaust is going to degrade any of the materials used in an extrusion printer.

 

[thequick]

Here are two tailcone retainers I've flown. Both are standard PLA and screw onto the bottom of the fincan. The green retainer is for 38mm motors and has flown 6 times - 3x G76G, 2x H250G and 1x J500G. All the heat damage present occurred after the first flight which was a G76G. It has not gotten worse on the 5 other flights. The black retainer is for 29mm motors. I think it has 4 total flights with no apparent degradation. I think the largest motor flown was a H128W. The others were F-G hobbyline reloads.

I have no hesitation flying printed retainers on everything except long burn motors. Others have their own thoughts/experience but for the parts I've designed and printed (out of the worst case material) I've had no issues.

One benefit of the tailcone retainer is it's replaceable so if it gets a.little over heated I'll just trash it and print another. If I had a printed CR at the aft end I was worried about I'd probably just print a thin disc to install over the CR.

Thanks for sharing. This is very encouraging!

 

[JohnCoker]

Yes, it's not clear how much the charring affects the strength. Using it for disposable parts sounds reasonable. We'll investigate options for protecting non-disposable parts, such as the aft CR.