AT L-1000 DMS – flying case with a nose cone and 4 fins

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manixFan said:
The amount of force on the fins at those speeds is significant. And any misalignment will greatly increase that stress. That stress will be transferred to directly to the motor case, which was never intended to be a structural component. Fins that are adhered directly to the case will add force/stress that was never part of the design intent of that disposable case. It's likely that at those speeds, and with any non-zero angle of attack, the amount of force/stress transferred to the case will be very significant, and could certainly cause the case to fail.

Or maybe not!


Tony
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The tensile strength of the epoxy fillets (4ksi) is so far below the material strength of the case (30+KSI) the fins will always shear off first.

I have successfully (and unsuccessfully) JB welded aluminum fins to aluminum cases at M2.2

I have always suspected cases (especially high pressure motors in thin wall cases) occasionally can expand enough to crack the brittle JB weld fillet. How much do composite cases expand?

I'd do it again...
 
Landru said:
The tensile strength of the epoxy fillets (4ksi) is so far below the material strength of the case (30+KSI) the fins will always shear off first.

I have successfully (and unsuccessfully) JB welded aluminum fins to aluminum cases at M2.2

I have always suspected cases (especially high pressure motors in thin wall cases) occasionally can expand enough to crack the brittle JB weld fillet. How much do composite cases expand?

I'd do it again...
Click to expand...
I added a PS to my post that I did not mean the case would actually fail, but the fin bond might. The surface of the case may not support the stress placed on it by the fins, which of course is completely different than acting as a pressure vessel that transfers force to the rocket longitudinally. I suspect the failure would be the bond failing at the surface.

I’ve gone to Mach 2.4 multiple times with just surface mount fins on carbon fiber. I consider my building skills to be just average, so it’s not hard with the right materials. (CTI L935 in a CF Mongoose or clone.) It could be bonding fins to a composite motor case is no different. It will be interesting to find out.


Tony

PS: my point really is the strength requirements of a pressure vessel do not necessarily lend themselves to use as a structural support - a composite overwrapped vessel may hold tremendous pressure, but that does not mean its outer surface is capable of holding a structural element.
 
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manixFan said:
I added a PS to my post that I did not mean the case would actually fail, but the fin bond might. The surface of the case may not support the stress placed on it by the fins, which of course is completely different than acting as a pressure vessel that transfers force to the rocket longitudinally. I suspect the failure would be the bond failing at the surface.

I’ve gone to Mach 2.4 multiple times with just surface mount fins on carbon fiber. I consider my building skills to be just average, so it’s not hard with the right materials. (CTI L935 in a CF Mongoose or clone.) It could be bonding fins to a composite motor case is no different. It will be interesting to find out.


Tony

PS: my point really is the strength requirements of a pressure vessel do not necessarily lend themselves to use as a structural support - a composite overwrapped vessel may hold tremendous pressure, but that does not mean its outer surface is capable of holding a structural element.
Click to expand...

Case "flex" (swelling? expansion?) under thrust sounds entirely plausible. As an extension, fillet strain as a result of that flex is also entirely plausible. One step further, fillet failure is therefore plausible.

What about gluing the fins to a fiberglass or CF collar of sorts and creating a composite fin can and then gluing that fin can to the composite motor case? Would that mitigate the risk of failure?

Asking for other ideas and opinions here.
 
Cameron Anderson said:
Do it.

Lots of opinions here of varying levels of support and informed opinion. But until you try (and succeed or fail), no one will know.

Do it.
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Agreed. I bought the parts, not backing out anytime soon. :)

To Tony's point though, I enthusiastically agree that fin misalignment will greatly increase stress and likelihood of a shred. If the fins are adhered straight and true on the airframe, that will increase chance of success here.
 
This thread has me thinking. I have some Cotronics getting a little long in the tooth, kinda use it or lose it territory. Might be a good use for that.

I'm enjoying this thread very much, and hoping to witness a successful flight at Airfest next year.
 
If you do fly it, you might consider a JLCR. It would greatly increase the value of the rocket in terms of a loss, but it would help also keep it in the neighborhood, especially if you are using an RDF tracker. I've gone to 25,000 feet at Argonia with just an RDF tracker and it turns out there is a lot of very unfriendly RDF terrain, especially to the south.


Tony
 
I hope the following will add to this thread ...I flew a sub min dia single use (Research) motor rocket at Hamster Dance in 2019. The motor was a K400 Swamp Gas The rocket simmed to just under 15K at mach 1.6 I'd add more info but my kevlar shock cord parted ways and my booster section came in hot.
Yes, Fred blew the fins off all 6 of his rockets... https://www.rocketryforum.com/threads/hamster-dance.154106/ I'm not sure what Fred used for a motor case, fin material, fin shape, epoxy or alignment method...which all affects fin longevity


hamster ready.jpg

now some useful info,,hopefully. My 54mm K400 was built from spent motor parts. FC and Noz -CTI. The motor case was made from a section of spent AT DMS K1000...an interesting fact that the AT DMS K1000 and possibly other or all of the 54mm line is made with the thicker FG motor cases...it does not use any liner. This in my opinion makes for a better motor choice for this application..It's probably stronger and stays cooler before, during and after it's burn.

For my fin attachment I went with just fillets made with Loctite EA E-120HP. I'm not sure about the physics involved.. how close to mach 3 stress ...compares to a lawn dart from 15K..

20201014_115205.jpg



secret DMS info https://www.rocketmotorparts.com/index.aspx?pageid=7452840

secret source for AT parts.. https://www.rocketmotorparts.com/54mm_Fiberglass_Motor_Casings/cat1577810_2775714.aspx

Tony
 
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cerving said:
You're gonna have to use a tower with this one. I can tell you from personal experience that fly-away rail guides do not like high-thrust motors.... I still have numerous pieces in a box.
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At least you got pieces...I used a fly away guide on a L1000 and it vanished David Blain style.
 
I also had one break with a 38mm Madcow rocket on a G64... I would love to see somebody make a fly-away rail guide made out of something other than plastic.
 
Our hamsters don't have a motor case.
We use a filament winder and wind on the casting tube full of propellant.
 
As much as I would like not to have to make a tower, I'll use some G10 plates, 8020 Inc. 1010 rails, and 90 degree angle mounts. Another project to do over the winter.
 
rfjustin said:
As much as I would like not to have to make a tower, I'll use some G10 plates, 8020 Inc. 1010 rails, and 90 degree angle mounts. Another project to do over the winter.
Click to expand...

I will be going to Airfest next year with a tower you are more than welcome to use. My only thing is I want to launch my rocket first just in case something should happen like a cato and bend my rails. It will handle 3 or 4 fins. I'll be launching a 4 inch MD out of it and with any luck, It will see the dreaded 5800! Maybe, you could help me assemble it. I posted some pics of it somewhere, maybe in the show me your towers thread. The Covid better be gone by then!
 
HyperSonic said:
I will be going to Airfest next year with a tower you are more than welcome to use. My only thing is I want to launch my rocket first just in case something should happen like a cato and bend my rails. It will handle 3 or 4 fins. I'll be launching a 4 inch MD out of it and with any luck, It will see the dreaded 5800! Maybe, you could help me assemble it. I posted some pics of it somewhere, maybe in the show me your towers thread. The Covid better be gone by then!
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I appreciate it, thank you. I may make/use my own though for the same exact concern. I feel much better about trashing my own gear than someone else's.

Cheers, hope to see you there with your project next year!
 
I'm glad to see someone documenting a build like this. I'm thinking about doing something similar with an H13 next year.
 
rfjustin said:
As much as I would like not to have to make a tower, I'll use some G10 plates, 8020 Inc. 1010 rails, and 90 degree angle mounts. Another project to do over the winter.
Click to expand...
You don't have to be fancy, just make a dedicated tower for your rocket. A couple pieces of plywood, conduit, and plugs to fit the conduit. This is a tower for a min dia O project in 1992.Cb110twr1.jpgcb110twr2.jpgVideo coverage is in Point 39 Productions LDRS XI video.
 
Well, now that Midwest Power is behind us, I can slowly get back to the builds I want to complete over the winter.

Working on the motor retention and mapping out G10 sled(s). I decided to double down on this project, will be making two. One with glass fins, one with carbon fins (and beefier construction of the fin can). I'll be left with a project that makes me nervous, and another that makes my really nervous. :)Resized_20201111_085731_6590.jpegResized_20201111_090122_9503.jpeg
 
rfjustin said:
I'll be left with a project that makes me nervous, and another that makes my really nervous. :)
Click to expand...

If we didn't get those nervous butter flies...would we still be doing this?

How 'straight' are those forward closures? I'm guessing not very. Each one made for and indexed to fit each motor?

I think I have figured out my coupler for the 98mm DMS. I hope to have it turned and welded up in a few days.

Tony
 
I think my only recommendation would be to space the fins up from the aft end of the tube. I think there's a cutaway view of this motor available from RCS / Aerotech, you could do something like "nozzle length + 0.5" to get the root/fillet away from the area of maximum heat transfer through the case.

Maybe it matters, maybe it doesn't. I don't see much extreme about this otherwise.
 

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