This will test my metal....

SilicdyneTRF
Onebadhawk
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Jim, not sure how much gap we are talking. Have you considered getting some thin carbon or glass, and doing a layup on the coupler? The you could sand it to fit? Not sure if it would work, but it is a thought that came to mind.
 
markkoelsch said:
Jim, not sure how much gap we are talking. Have you considered getting some thin carbon or glass, and doing a layup on the coupler? The you could sand it to fit? Not sure if it would work, but it is a thought that came to mind.
Click to expand...

Roughly speaking, the 4-foot tube can wiggle up and down about 3/4 of an inch at the worst point. If I have my math correct, that means that the gap at that point is about 0.02 inches (0.0375 on the OD of the coupler versus the ID of the tube). The gap is less at the 90 degree points (just big enough to fit a paper-thin piece of my mylar).

I tried a second pilot test with a little thinner epoxy. The results were not much better. I just finished setting up a third test where I have injected the epoxy through a hole in the airframe at about the middle of the coupler. This may work, but I won't know until tomorrow when I take it apart. I think with the tube being a little out of round, getting something sanded to fit would be very difficult.

Jim
 
It is a bit spendy but if you are doing some serious HPR builds it may be worth having in your stash.

LOCTITE 9309

It is a great structural adhesive and it is commonly used in the manufacture and repair of rotor blades. If properly mixed, it reaches full cure in 24 hrs at room temperature and the cure may be accelerated with heat Look around and you may be able to find smaller quantities ( we use pre-measured blister packs for small jobs ).
 
I have some thin titanium sheet you could use as a shim. I can bring it to the next Hearne launch if it will help.
 
Screaminhelo said:
It is a bit spendy but if you are doing some serious HPR builds it may be worth having in your stash.

LOCTITE 9309

It is a great structural adhesive and it is commonly used in the manufacture and repair of rotor blades. If properly mixed, it reaches full cure in 24 hrs at room temperature and the cure may be accelerated with heat Look around and you may be able to find smaller quantities ( we use pre-measured blister packs for small jobs ).
Click to expand...

Pretty impressive except the service temperature is 180 degrees F. If staying within that realm, should be o.k. If not might have to go with even the pricier high temp stuff. Kurt
 
I used the epoxy injection approach. I got about 75% of the area between the air frame and coupler coated with epoxy. It doesn't look great, but the fit is very good. About a 1/4" wiggle now. I can't repeat the process because the air frame won't fit over the mylar (the fit is too exact now). But, I think the fit is about as good as it can be now. Thanks for the ideas and discussion.

Jim
 
Sometimes people need hard love.

For the sake of being the contrarian, please get back to me after your mission fails so that I can say "I told you so".

I really hope your bandaid approach works. Jim. We have all seen your astounding achievements and high level of flight success. Please don't let this minor issue be the detriment and failure of a most grandiose flight. Please invest in a new airframe. How difficult can it be to assure an exact fit with an aluminum tube, despite your already finished modifications for altimiters, electronics, and fancy robotics super computers? Suck it up, start a Go-Fund Me or something, and do it right. Your staged flight masterpieces deserve nothing less. Consider it a testament to your legacy of perfection!

You have gone this far. Why throw caution to the wind?
 
CORZERO said:
Sometimes people need hard love.

For the sake of being the contrarian, please get back to me after your mission fails so that I can say "I told you so".

I really hope your bandaid approach works. Jim. We have all seen your astounding achievements and high level of flight success. Please don't let this minor issue be the detriment and failure of a most grandiose flight. Please invest in a new airframe. How difficult can it be to assure an exact fit with an aluminum tube, despite your already finished modifications for altimiters, electronics, and fancy robotics super computers? Suck it up, start a Go-Fund Me or something, and do it right. Your staged flight masterpieces deserve nothing less. Consider it a testament to your legacy of perfection!

You have gone this far. Why throw caution to the wind?
Click to expand...

Having no experience with aluminum rockets, I don't really know how hard it is to get an exact fit. I would be willing to bet, though, that the current tolerance is better than most 6" rockets out there. I am quite happy with the fix, and if you want to be able to tell me "I told you so", then perhaps I could suggest a dozen or so things that are now more likely to cause a problem than this.

I'm actually more worried about the fit being too good, and affecting the apogee separation of the booster. The coupler is metal and is connected to the motor tube, which is metal. Although there are several things I can do with this design to minimize the actual metal-to-metal contact, I suspect the coupler will warm up at some point. This issue will get more consideration over the next few weeks.

Jim
 
It seems then now that "wobble" is the least of your concerns. As far as heat tolerance and material expansion is concerned, remember, this is hobby rocketry. I'm not so sure your concern for variables including heat are necessary. Make sure the forward facing camera aft of the stages sees a "wobble-free" ascent and an adequate separation charge taking care of the disconnection of the sections is sufficient and you will remain king of the staging endeavor.
 
JimJarvis50 said:
Having no experience with aluminum rockets, I don't really know how hard it is to get an exact fit.
I'm actually more worried about the fit being too good, and affecting the apogee separation of the booster. The coupler is metal and is connected to the motor tube, which is metal.

Jim
Click to expand...

Our aluminum I/S is a near perfect fit. But a "spritz" of WD-40 on the "part" being inserted , made it perfect.

No wiggle & after inserting motor a few twists.......remove and wipe clean. A final spritz, insert remove and let dry.
Now pick up by sustainer and it fall right off.

This worked on our 3in. to 38mm with both metal to metal, where motor is coupler. Also when aluminum insert is bolted into I/S for use where Al coupler [insert] goes into fiberglass air frame.

2 sustainer built: 1 for smaller motor gets "coupled" . 2nd sustainer uses it's motor for coupler.
All this for testing.


Another 3in Aluminum I/S goes into 3in sustainer with 54 hole. WD-40 trick worked miracle on that.

Again for a 4in. to 3in. minimum to minimum....... Aluminum I/S....... where sustainer motor is coupler.

The first [2] 2-stagers separate like boosted darts at motor burn out. Both flown multiple times, but Airfest will tell all, when flown on full stacks

The above fits, are very snug as tolerances are so close, [no perceivable wiggle] but the WD-40 was like adding ball bearings it slicked up the fit so well. Don't know why I didn't think of this one a long time ago...
 
My uneducated opinion:

You do not need "experience" in aluminum metallurgy to have a feel for your stage coupler. If it fits == rock it. You will know, you are experienfesed. <---Not sure what that word means but I'm sure it should make sense.

Let us know. And for god sakes, please take some paictures and post them so's that we simple folks can make some sense of your first world [problem.]
 
JimJarvis50 said:
Having no experience with aluminum rockets, I don't really know how hard it is to get an exact fit. I would be willing to bet, though, that the current tolerance is better than most 6" rockets out there. I am quite happy with the fix, and if you want to be able to tell me "I told you so", then perhaps I could suggest a dozen or so things that are now more likely to cause a problem than this.

I'm actually more worried about the fit being too good, and affecting the apogee separation of the booster. The coupler is metal and is connected to the motor tube, which is metal. Although there are several things I can do with this design to minimize the actual metal-to-metal contact, I suspect the coupler will warm up at some point. This issue will get more consideration over the next few weeks.

Jim
Click to expand...

Any issues with aluminum CTE since pieces are thermally coupled to your motor?
 
I've used thin teflon sheeting with no adhesive for metal-to-metal coupler shimming. Removes wobble and slides off nicely. Keep everything clean. Just metal and nothing else. It actually helps to have more gap so that the teflon can be thicker and hold up better.
 
jsdemar said:
I've used thin teflon sheeting with no adhesive for metal-to-metal coupler shimming. Removes wobble and slides off nicely. Keep everything clean. Just metal and nothing else. It actually helps to have more gap so that the teflon can be thicker and hold up better.
Click to expand...

+1, PEEK and Polyimide are also alternatives for metal to metal interfacing, depending on the requirements.
 
So, here's the design. The coupler section holds in the closure (not shown). The coupler section can be isolated using the amber high-temp kapton tape - just a tiny air gap. The coupler section doesn't actually touch the lower air frame metal-to-metal except for the bolts themselves. The greatest heat transfer might be from the lower to the upper air frames at their interface. Looking at it, I don't think there's any reason to worry about expansion. I did lose a rocket to a motor-tube coupler a few years back, so I just worry about it a little.

Jim

IMG_0900.jpg

IMG_0901.jpg

IMG_0902.jpg

IMG_0903.jpg
 
Jim,

What about using feeler gauges (here's a link to an example...) to find what the gap is...

https://www.amazon.com/dp/B000BYGIR4/?tag=skimlinks_replacement-20

Then use either sheet brass or aluminum of half the gap (again, another link)

https://www.amazon.com/dp/B0006MZOBC/?tag=skimlinks_replacement-20

Cool thing about this is they are both probably available locally.

I used the feeler gauges to find the largest injector gap on 1 stack of my hilbourn fuel injector, then had the machine shop take metal to make the other 7 the same. Could have also added the sheet metal and had them braze it on too to bring them down to the smallest.

IMG_0515.jpg

fm
 
Last edited by a moderator:
If you are dead set on getting it concentric, some Dykem, a shot bag and deadblow hammer and a lot of patience. Ideally, aluminum should not be coupled together unless at least one or both surfaces have been anodized. Otherwise it will grab. More so if the mating surfaces are polished. You want the mating surfaces to be cross hatched to keep them from grabbing. I'd personally cross hatch before anodizing, and then I'd cross hatch the anodize layer too.

Mating close tolerance parts like these is difficult, both in assembly and disassembly and if you get rotation of the parts in flight, they will gall and friction weld together. Sleeving is a great idea in theory, but not as easy to machine in thin wall tubing as people have stated. Only if you have zero run out. Otherwise your wall thickness is compromised.
 
Well, I used the epoxy injection approach to line the inside of the air frame. It's not pretty, but the fit is quite good and smooth. I don't think metal is touching metal. I just need for Stu not to get too upset when he sees that I've slathered epoxy all over the inside of his air frame. Drastic measures were required!

Jim
 

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