aeropack 38mm min diameter retainer placement and fit

[rocketsam2016]

Hi folks, I've got a 38mm aeropack min diameter retainer to use with a 38mm wildman blackhawk. Two questions

1) It won't fit. I can jam it in perhaps a centimeter (i.e. most of it still sticking out) without tools, but no way I trust myself to put epoxy on it and pray I can get it all the way in without it getting stuck or something breaking. Any advice on whether to sand the retainer or the (fiberglass) body tube? I'm a little hesitant to sand off the anodized coating on the retainer, but sanding the full length of the inside of the tube sounds like a royal pain in the butt.

2) I want to place it to handle a 38mm CTI 6xl case. I have the case but don't currently have a 6xl load for it, so I can't measure how much of an offset from the end I need. Has anyone by chance placed one to handle a 38mm 6xl case and could tell me exactly how far in they placed the retainer?

Thanks!
Sam

 

[mpitfield]

I just completed two 38mm MD builds using the Aeropack MD retainer and they are set up for the 6XL using the MC38 adapter. I don't have the rockets with me as I am at work and they are in my shop, but I can measure them this evening and let you know where I placed them.

One thing in regards to epoxying them in place, I wouldn't. I personally like the option of having a removable retainer for servicing. I drill three holes @ 120 degrees around the airframe into the retainer then epoxy a PEM nut on the inside back of the retainer. I use 4-40 1/4" flat top countersunk screws to hold it in place.

Here are some pics of my 54mm MD Aeropack using that technique:



Below is one good reason for servicing, I bent the eye-hook on a hard deployment. I am not sure how based on the max angle that the harness could achieve but none the less it happened and it was an easy swap. Of course you can still remove these when it is glued in place, it's just much more convenient when you can remove, inspect, and service the full assembly. To be honest if it was epoxied in place I am not even sure I would have noticed that I bent it.



As far as being stuck in the tube, one of my 38 MD rockets is a GLR Mariah and they partially slot the tube for the fin attachment. Where the slots are cut, the tube tends to deform a bit, which is normal. As a result I had to use my dremel with a snake and a wheel flap to take down the high points in order to get the retainer in from the aft. If your tube I/D is tight all the way, then that is a bit more challenging. What kind of airframe is it?

 

[rocketsam2016]

One thing in regards to epoxying them in place, I wouldn't. I personally like the option of having a removable retainer for servicing. I drill three holes @ 120 degrees around the airframe into the retainer then epoxy a PEM nut on the inside back of the retainer. I use 4-40 1/4" flat top countersunk screws to hold it in place.

Yeah I was going back and forth but probably will do like you say and use screws. The tight fit makes me all the more inclined to do so so I don't end up with an emergency if it gets stuck with epoxy on it.

As far as being stuck in the tube, one of my 38 MD rockets is a GLR Mariah and they partially slot the tube for the fin attachment. Where the slots are cut, the tube tends to deform a bit, which is normal. As a result I had to use my dremel with a snake and a wheel flap to take down the high points in order to get the retainer in from the aft. If your tube I/D is tight all the way, then that is a bit more challenging. What kind of airframe is it?

No slots or other cuts in the tube. It's tight the whole way and from both ends. I suppose I don't actually know that it's tight the whole way since I can't get it past the entrance, but there are no cuts in it and it doesn't matter which end I try and enter from.

 

[mpitfield]

Assuming you have a dremel with a snake attachment, if you pick up a coarse wheel flap that fits your tube, then I would try that method first. It sounds like it is almost there so it shouldn't take too much to get the ID right. Below is very similar to my wheel flap, 80 Grit x 40mm, and should work well. If it is a bit tight and if it binds then you can remove a flap every other 5th or so flap, all the way around and that should allow it to move.

https://www.amazon.ca/Wolfcraft-203...=2025&creative=165953&creativeASIN=B01L4P0W3Y

 

[blackjack2564]

Why not just friction fit motors and save weight, time & stress put on airframe by drilling mounting holes in it?

Been flying my BH38 that way since beginning.


Just screw/glue a 3/16 eyebolt in CTI closure for shock-cord attachment.

 

[rocketsam2016]

Why not just friction fit motors and save weight, time & stress put on airframe by drilling mounting holes in it?

Been flying my BH38 that way since beginning.

View attachment 329565
Just screw/glue a 3/16 eyebolt in CTI closure for shock-cord attachment.

Friction fit not allowed at my club. Easy answer

 

[A5tr0 An0n]

Friction fit not allowed at my club. Easy answer

I was going to make the same suggestion blackjack said... I am shocked that your club does not allow friction fit. On another note, if you don't have a flap wheel or bead blaster, I would suggest turning down the retainer and being done with it. It would be a lot easier in my opinion.

 

[rocketsam2016]

I was going to make the same suggestion blackjack said... I am shocked that your club does not allow friction fit. On another note, if you don't have a flap wheel or bead blaster, I would suggest turning down the retainer and being done with it. It would be a lot easier in my opinion.

It's a big turf farm. Farmer really doesn't want metal motor cases in his equipment. I think there may have been incidents in the past?

Anyway flap wheel sounds useful, probably will go buy one

 

[BDB]

Sam, you fly with MMMSC, right? I asked Ken about this very thing last year when I was contemplating a MD build. (My question is on the MMMSC FB page.) Since the motor would be tied to the harness, which would be tied to the chute, it would be pretty hard for it to separate from the rocket. Even if the friction fit came loose, the whole thing would still be tied together. The consensus was that CJ’s method counts as “positive motor retention.”


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[markkoelsch]

Friction fit not allowed at my club. Easy answer

So then use a wrap of aluminum tape over the rear of the airframe and motor. Viola- positive retention.


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[Buckeye]

So then use a wrap of aluminum tape over the rear of the airframe and motor. Viola- positive retention.


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Agreed. This wrap of tape is doing most of the work, anyway.

 

[Andrew_ASC]

Friction fit not allowed at my club. Easy answer

Cotronics 4700 and Oven Cure the aerospace grade epoxy from tube wall to casing four hours at 250F, then follow directions and ramp up heat as needed. It takes a 600F thermal loading and still has 11,100 psi tensile strength. That bond survived Mach 1.5 and 168g's on a SEDS rocket at Hellfire 22. We didn't need no stinkin' retainer bud. Downside is if that rocket fails structurally like the SEDS project did, you aren't ever getting the casing out again! Gotta love 1299N-P on min diameter multistage, just call that fuel shred a rocket in can. Split second K class thrust curve, and straight imploded the hobby grade CF body tubes. Literally told epoxy companies on a phone we needed a good glue because lives were on the line, and our option was Cotronics 4700. Didn't know thermal loads on casing so didn't skimp on epoxy. Pick highest thermal loading by Tg value you can get. Hundred bucks a pint, but worth it for the thermal ratings and the strength. The Cf tube did not implode where cotronics bonded casing to wall if that tells you anything for what its worth.

 

[mpitfield]

Cotronics 4700 and Oven Cure the aerospace grade epoxy from tube wall to casing four hours at 250F, then follow directions and ramp up heat as needed. It takes a 600F thermal loading and still has 11,100 psi tensile strength. That bond survived Mach 1.5 and 168g's on a SEDS rocket at Hellfire 22. We didn't need no stinkin' retainer bud. Downside is if that rocket fails structurally like the SEDS project did, you aren't ever getting the casing out again! Gotta love 1299N-P on min diameter multistage, just call that fuel shred a rocket in can. Split second K class thrust curve, and straight imploded the hobby grade CF body tubes. Literally told epoxy companies on a phone we needed a good glue because lives were on the line, and our option was Cotronics 4700. Didn't know thermal loads on casing so didn't skimp on epoxy. Pick highest thermal loading by Tg value you can get. Hundred bucks a pint, but worth it for the thermal ratings and the strength. The Cf tube did not implode where cotronics bonded casing to wall if that tells you anything for what its worth.

You went through all that for a Mach 1.5 flight then suggest that a retainer is not needed, like it is overkill...lol

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[dhbarr]

I would think 168g's was the predominant constraint in that paragraph.

 

[rocketsam2016]

Thanks folks for all the good advice. Sounds like I can use tape if I want at my club (mmmsc) and I'll strongly consider that going forward, though anchoring the parachute to a screw in 38mm cti liner makes me nervous since it's plastic.

For this rocket I already bought the retainer though and I'm rather stubborn, so I'm going to make it work

Mpitfield - let me know if you get a chance to measure your location for 38mm cti 6xl.

Thanks everyone!

 

[markkoelsch]

Cotronics 4700 and Oven Cure the aerospace grade epoxy from tube wall to casing four hours at 250F, then follow directions and ramp up heat as needed. It takes a 600F thermal loading and still has 11,100 psi tensile strength. That bond survived Mach 1.5 and 168g's on a SEDS rocket at Hellfire 22. We didn't need no stinkin' retainer bud. Downside is if that rocket fails structurally like the SEDS project did, you aren't ever getting the casing out again! Gotta love 1299N-P on min diameter multistage, just call that fuel shred a rocket in can. Split second K class thrust curve, and straight imploded the hobby grade CF body tubes. Literally told epoxy companies on a phone we needed a good glue because lives were on the line, and our option was Cotronics 4700. Didn't know thermal loads on casing so didn't skimp on epoxy. Pick highest thermal loading by Tg value you can get. Hundred bucks a pint, but worth it for the thermal ratings and the strength. The Cf tube did not implode where cotronics bonded casing to wall if that tells you anything for what its worth.

So, you bonded the case to the rocket? That is better than a retainer? That is really some silly stuff right there.


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[Andrew_ASC]

You went through all that for a Mach 1.5 flight then suggest that a retainer is not needed, like it is overkill...lol

You try to attempt past 22,500ft on an L-1 specific impulse. You're darn right its overkill. We even predicted up to Mach 2.2 was possible. We had NACA65A supersonic airfoils in Nytek1200CF not shear off the booster tube. Transition coupler was bonded with cotronics too. If anything failed it was the transition coupler and tube wall thickness. Theory was reduce drag force by sanding new tube size. We wanted 6061T6 in a new tube size but no one could maintain tolerances nor was producing airfoils possible on scale of parts due to endmill bit sizes by CNC. Audible sonic boom observed to 2,500ft. First stage was at Mach 1.5 at 100ft off ground. I did the aero with a ME degree. No one on team wanted to touch structural FEA with a 1,000ft pole. Nor did the graduate students/profs. We ate reality hard on that launch. We didn't want to eat a casing at that speed and it was our first HPR at university. $2,500 is a nice budget to do multistage L-1 and just push the edge. We drove to Utah in 2 days. Learned in five seconds what not to do, then built it cheaper/simpler lower performance.

 

[Andrew_ASC]

I would think 168g's was the predominant constraint in that paragraph.

The sustainer casing was overhung and taking the brunt of that. It just wasn't overhung enough sadly.

 

[mpitfield]

You try to attempt past 22,500ft on an L-1 specific impulse.

As a matter of fact I just did two weeks ago and that was with standard off the shelf thin-walled 54mm carbon fiber tubing from Madcow. That was that rocket's 11th flight and 8th flight over Mach 1.5, actually that flight hit Mach 2.15. BTW I am not really sure what you mean when you say an L-1 specific impulse.

 

[dhbarr]

As a matter of fact I just did two weeks ago and that was with standard off the shelf thin-walled 54mm carbon fiber tubing from Madcow. That was that rocket's 11th flight and 8th flight over Mach 1.5, actually that flight hit Mach 2.15. BTW I am not really sure what you mean when you say an L-1 specific impulse.

I believe they may have been saying not more than I impulse.