Insanity Plea: A sub minimum diameter M2245 project.

Advertise with us
Wildman Rocketry
Onebadhawk
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
The proper way to prep aluminum for adhesive bonding is to phosphoric acid anodize (no-seal) and use a thin (almost see-through) primer designed for bonding and bond to that. An alternative is to use an adhesive promoting alodine. Another lesser alternative is to sulfuric acid anodize without a seal and the same primer. Adhesive bonding to raw aluminum doesn't work that well as you found out.

Additionally, the design of the fins at the base should have been tapered as high a ratio (shallow) to a knife edge. The stress is all built up at the edges and the weak bond made it unzip.

So for rocketeers, taper from about .020" to desired thickness at fin, say .12", while conforming to shape of rocket, and alodine and bond. Anodize is better but cannot do in garage.
 
Last edited:
One thing you may want to consider is the internal stress/strain of the material.

If the stock you purchased was cast or rolled up into a giant roll, before being forced flat. Once you heat up the material, it may have a tendency to return to its original state. With the leading edge being thin, that would be the natural place for it to return to its "original state".
 
Cool attempt Chris. Id say your epoxy did a pretty good job of holding on given the deformation on the aluminum. DP460 is pretty much done by 250F id guess you were over that. If you can get a close up of the adhesive still on the case it might be easier to tell. There is suspiciously little adhesive left on the fins. This could point to a prep issue. An acid etch BR127 prime that was referenced above would be a good route but a grit blast followed by immediate bonding would be just as good if not better. Just dont park it in a hanger in Florida for a few decades. A better adhesive option might be to get your hands on some metal bonding film adhesive. Not sure where a home gamer would get it though. But some have service temperatures well over the DP420/60 epoxy family.
 
flynfrog said:
Cool attempt Chris. Id say your epoxy did a pretty good job of holding on given the deformation on the aluminum. DP460 is pretty much done by 250F id guess you were over that. If you can get a close up of the adhesive still on the case it might be easier to tell. There is suspiciously little adhesive left on the fins. This could point to a prep issue. An acid etch BR127 prime that was referenced above would be a good route but a grit blast followed by immediate bonding would be just as good if not better. Just dont park it in a hanger in Florida for a few decades. A better adhesive option might be to get your hands on some metal bonding film adhesive. Not sure where a home gamer would get it though. But some have service temperatures well over the DP420/60 epoxy family.
Click to expand...
 
Here are some aerospace grade materials that can be found here and there:

Structural Adhesive Primer: 3M EC-3960. Apply thin (see-through)

Structural Adhesive (350F): EA-9394. Paste adhesive that can be used for thicker joints. Typically less than .040" but thicker is possible and still develop near full strength.

If the joint failure is adhesive (as described above - at the bondline) it is a surface issue. All properly bonded joints should fail cohesively (internal to the adhesive).

Aluminum oxidizes in seconds in air. Sol-gel is another prep method for aluminum. The above primer works well with anodizes because it is thin and gets into pores. Long-chain adhesives do not get into the pores as well.

If the aluminum temp is also an issue the next upgrade is 2219-T6, -T8. Probably not needed.

Figure out some test for your chosen bonding method to ensure the adhesive fails cohesively and you are golden - say bond a bar with .5 X 1.0 overlap and pry off.
 
Chris,
If your going to be doing some testing, I have an extra quart of EA-9396 (thinner then EA 9394). Post cure is IIRC 5 days at 77 degrees and working temp up to 350.
Your welcome to it if you want it.

RocketScience, your thoughts on EA 9396?

Tony
 
tfish said:
Chris,
If your going to be doing some testing, I have an extra quart of EA-9396 (thinner then EA 9394). Post cure is IIRC 5 days at 77 degrees and working temp up to 350.
Your welcome to it if you want it.

RocketScience, your thoughts on EA 9396?

Tony
Click to expand...

For what Rocketeers do, EA-9394 and EA-9396 are fairly comparable. EA-9394 may have the better strength for thicker bonds and more usable as it flows less. EA-9396 is a bit stronger at thinner bond line thicknesses. Should be essentially the same overall in regards to performance provided have good adhesion. Each has roughly same strength in optimum .005" to .020" bond line thickness. A simple trick to control bond line thickness is use fishing line and clamp - locate in areas away from peak stress and trim after cure.

I went back through some of the older posts and saw that the parts were anodized but the anodize was sanded away. The best option is an un-sealed anodize (don't fill the pores in the anodize), adhesive-primer, and bond with say .005" fishing line to maintain thickness. The bonding should happen within 24 hours of prime. There are special tapes for protecting the surface and maintaining an active surface for bonding. In general avoid anything that says silcone when bonding. The preferred anodize is phosphoric (ASTM D3933-98) but others should suffice.

The rocket was a beautiful build.
 
Last edited:
RocketScience do you know of a good source for adhesive primer as an individual without having to spend a couple hundred $? I did a similarish project to this last year with alum fins bonded directly to a carbon air-frame but ended up going the beogel method because i couldnt find any primer without having to buy several gallons.
 
I sent a note to one of my local composites suppliers here in Seattle to see if he can get smaller quart kits and how much they cost. We'll see what he comes back with. No guarantees.
 
I've been reading this thread and trying to absorb everyone's comments. I agree that it seems like the prep of the fins is suspect. The epoxy is still bonded to the case. I don't think the failure was heat related. The Bonderite I used is 10-20% phosphoric acid. I did not use any primer. I used lye to strip the anodizing off the case and then used the Bonderite. That seems to have worked.

The fin roots were tapered from the fin to the edge. The edge was only about .020". I'm not sure that I could make them any thinner with any consistency.

I think that the failure started with the thin fin curling, then the NC coupler failed. The NC does have more heat damage on one side than the other.



RocketScience said:
The proper way to prep aluminum for adhesive bonding is to phosphoric acid anodize (no-seal) and use a thin (almost see-through) primer designed for bonding and bond to that. An alternative is to use an adhesive promoting alodine. Another lesser alternative is to sulfuric acid anodize without a seal and the same primer. Adhesive bonding to raw aluminum doesn't work that well as you found out.

Additionally, the design of the fins at the base should have been tapered as high a ratio (shallow) to a knife edge. The stress is all built up at the edges and the weak bond made it unzip.

So for rocketeers, taper from about .020" to desired thickness at fin, say .12", while conforming to shape of rocket, and alodine and bond. Anodize is better but cannot do in garage.
Click to expand...
 
I think I have a quart of that too. I'm pretty sure you gave some to me. ;^)

tfish said:
Chris,
If your going to be doing some testing, I have an extra quart of EA-9396 (thinner then EA 9394). Post cure is IIRC 5 days at 77 degrees and working temp up to 350.
Your welcome to it if you want it.

RocketScience, your thoughts on EA 9396?

Tony
Click to expand...
 
The fins were not anodized. The case was but I removed it with a lye solution. Interestingly the epoxy seems to have stayed bonded to the case. There is almost zero epoxy still bonded to the fins.

RocketScience said:
I went back through some of the older posts and saw that the parts were anodized but the anodize was sanded away. The best option is an un-sealed anodize (don't fill the pores in the anodize), adhesive-primer, and bond with say .005" fishing line to maintain thickness. The bonding should happen within 24 hours of prime. There are special tapes for protecting the surface and maintaining an active surface for bonding. In general avoid anything that says silcone when bonding. The preferred anodize is phosphoric (ASTM D3933-98) but others should suffice.

The rocket was a beautiful build.
Click to expand...
 
I'd like to thank all of you who have contributed to this build and postmortem thread. It's definitely been a learning experience.
 
You’ve bravely pushed the boundaries further than what most call amateur rockets.
 
ChrisAttebery said:
The fins were not anodized. The case was but I removed it with a lye solution. Interestingly the epoxy seems to have stayed bonded to the case. There is almost zero epoxy still bonded to the fins.
Click to expand...

I know you already know this, but even though the fins weren’t anodized, bare aluminum forms a thin layer of oxidation immediately after being exposed. If you didn’t treat the glue surface of the fins, that could explain the glue failures.
 
Steve Shannon said:
I know you already know this, but even though the fins weren’t anodized, bare aluminum forms a thin layer of oxidation immediately after being exposed. If you didn’t treat the glue surface of the fins, that could explain the glue failures.
Click to expand...

While this is true the very neat curling could only have been formed in clean air. The air behind the rounded side would become turbulent reducing lift down steam greatly. However the other side of the fin would function relatively normal. This would actually cause spin at first until the issue became too great at which point the rocket will enter a high AOA and break apart.

If the glue failed first and the leading edge was stressed from aero force on it, it would just be bent sideways. There’s no way to curl it over neatly without a contunious near zero AOA air acting on it.
 
I was really enjoying reading this thread, if you hadn't noticed all the likes this morning, lol.

You did an incredible amount of work and it looked amazing. Inspiring, really.

Thanks for adding fuel to the fire of passion for rocketry and sorry for the loss of your hard work!

I hope it comes back in another project better and badder!
 
SpaceManMat said:
While this is true the very neat curling could only have been formed in clean air. The air behind the rounded side would become turbulent reducing lift down steam greatly. However the other side of the fin would function relatively normal. This would actually cause spin at first until the issue became too great at which point the rocket will enter a high AOA and break apart.

If the glue failed first and the leading edge was stressed from aero force on it, it would just be bent sideways. There’s no way to curl it over neatly without a contunious near zero AOA air acting on it.
Click to expand...

Right. I’m not saying the glue failed first.
 
ChrisAttebery said:
I've been thinking about that. The fins had ~9.5 square inches of mounting surface. The epoxy was rated for 4500psi. 9.5 x 4,500 = 42,750 lbs of total holding force. The fins weren't that heavy. They weighed ~3.5 oz or .218 pounds each. 42,750 / .218 = 195,428 gees. I'm pretty sure it wasn't spinning enough to produce those kind of G forces. ;)
Click to expand...
Its not just gee force though, the aerodynamic stress caused by the fins going sideways on to the direction of travel could easily have torn them off.
 
I know I'm way late to this (I should visit TRF more often), but really nice work Chris. You've inspired me to break out the change gears and set up for some thread cutting! This opens up design possibilities not present by more traditional snap ring or pin/bolt arrangements.

I've had success with very similar fins with the addition of machine screws from the inside of the motor (tightening the final 2 fin bolt rows is a challenge on a 3.125 ID tube, but doable). At least up to Mach 2.8.... Gonna try again at balls this year, we'll see if they hold at M3.5. I'd like to find a reliable adhesive method to augment this though (JB Weld doesn't work). Perhaps something I can also use to help bond a pour-in liner. Another topic....

It's vexing to not know the exact cause of failure - electronics firing early (yup), fin flutter (maybe), nose cone shoulder fail (maybe), etc. It's more important to me that the thing just works, so on principal I've come around to the view that I'm willing to sacrifice altitude for reliability and recovery. And like you I like CNC machining so want to use this method wherever possible, optimization is not my goal. Yet :)

Jerry O
 
My guess is the coupler or cone failed.

The damage to your fins is easily explained by the cone and shock cord wrapping around the rocket after your coupler failed, as we know it was spinning pretty fast. Aluminum is a great thermal conductor and the leading edge dissipates a lot of heat through the rest of the fin. This seems too early in the flight for a thermal failure of the leading edge.

Did you find your ejection charges, and can you confirm they are intact? Early charge firing would have done this as well.
 

Similar threads

SteelyEyed
Project Aether Breather
2
Replies
34
Views
2K
Aeva
Aeva
Neutronium95
Bare Minimum: M2245 to 50,000 feet
2
Replies
56
Views
6K
pugachu
pugachu
bandman444
Project Sasquatch (24" diameter Madcow Squat)
2
Replies
34
Views
3K
Rschub
Rschub
James Owen
“Risky Business” - Minimum Diameter mach 2 attempt
2
Replies
49
Views
4K
James Owen
James Owen
Kane
Design and Build Thread: ARO-B Two-Stage
Replies
22
Views
2K
Kane
Kane

Latest posts

Back
Top