Difference of Opinions, Suggestions Needed

[edwinshap1]

I have a conundrum for the community. Bandman444(Bryce), Tominator79(Tom), and all came together to help Bryce design and assemble the fins for his High School's project.

The project is a 3" MD rocket aiming towards mach 3 to measure aerodynamic heating curves and compare against acceleration.

We attached the fins without much issue, and we T-T'd one side fairly well, but there was a hiccup. Before I left, I attached the coupler to the body, but I neglected to make a major note of it, and we could not put the rocket in the curing oven due to time constraints with the vacuum bag supplier. When they applied the vacuum, the coupler slid into the body tube and cured there.

As I understand it, the epoxy that was pushed into the tube is "gummy", which leads me to believe that the bond is not impossibly strong and will not damage the body to break, though I do not want to test my luck with this.

Now, we have two different options on the table that may or may not work.

Options:

(My option): We use a saw blade and carefully cut out a small section of the coupler (length-wise) so we can delaminate the two pieces of tubing.

(Bryce's Option): We carefully cut the 4.5"-5" section of body tube, and remove the tube so the coupler is clear of the tube, and can be used.

Issues:
(My Option):We could saw into the body tube, which would lower strength. We would also need to buy a new coupler tube, probably out of FG because CF is incredibly expensive and the part will need to be rush ordered.

(Bryce's Option):We could confidently saw into the coupler which would lower the coupler's strength and cause a failure.

Notes:
(My Option): Allows for use of the longest motors, but said motors will not be used for this test. Also, new coupler will need to be ordered.

(Bryce's Option): Will not allow for longest motors, but said motors will not be used regardless. No new parts need to be ordered. The cut will likely not be perfect, so finishing will need to be done to square the edge.

These options will only be considered once less invasive options are tried.


Edit: Please post the reasoning behind your answer, it will give us more insight on how to proceed.

[EeebeeE]

How about a 3rd option. if you can control the depth of your cut, cut the BT about a half-inch down and remove it, exposing a half inch of the coupler. That would give you something to grab hold so you can try to pull it out.

[CarVac]

Could you elaborate on the situation? Was this intended to be a zipperless design? How long is the body tube, the intended motor, and the length of the coupler inside of the body tube? What equipment is available for you to use?

Pictures would be nice. Until I find out more, I cannot form an opinion on this issue.

[troj]

Since you're bagging it, I'd say it's pretty safe to say the coupler is either cardboard or phenolic, meaning there's little or no need to try to cut the coupler.

Instead, slip a hobby knife under the corner of the inner wrap, and start peeling it up and out. You'll have to keep working through layers, but you should be able to remove it with little or no damage to the tube.

-Kevin

[dixontj93060]

Instead, slip a hobby knife under the corner of the inner wrap, and start peeling it up and out. You'll have to keep working through layers, but you should be able to remove it with little or no damage to the tube.

I agree. Do it now (the poll is wasting time and allowing it to cure longer).

[Tominator 2]

It's a Filament wound CF coupler.
And I like Bryce's idea.

[edwinshap1]

Sorry, it's a 9" CF coupler in a CF tube. The entire coupler is inside the tube, and it's recessed from the lip by about 1".

[dixontj93060]

Sorry, it's a 9" CF coupler in a CF tube. The entire coupler is inside the tube, and it's recessed from the lip by about 1".

[edwinshap1]

Thats what I said when I found out about it :P

[DAllen]

Yeesh...sounds like it is either time to start over or completely change the design. I assume it's a md design so what are the prospects of cutting it off below the coupler and using the motor itself as the coupler? Or am I not quite understanding the situation?

[troj]

It's likely too late, now that it's been a few hours.

Knowing now that it's CF in CF, the answer (if you have a motor tube long enough) would be to use the motor casing to force it out.

At this point, you'd be better off cutting the airframe off. With the extra time it's now had to set up, trying to cut and peel would be problematic, at best.

When the vaccuum was being pulled, someone should've noticed the coupler getting sucked in (hindsight is a glorious thing).

-Kevin

[bandman444]

We plan to attempt to pound it out tomorrow.

But heres a picture for those to understand.



It sucks, I get it.

I will post tomorrow the results of our pounding...

[New Ocean]

Pounding it out = sledge hammer. Not sure if she will survive that.

[GDJ]

Hydraulic press, Lots of WD-40, and some prayer?

[JimJarvis50]

What kind of epoxy did you use on the coupler?

Jim

[bandman444]

What kind of epoxy did you use on the coupler?

Jim

Aeropoxy.

[north boy]

Here's a suggestion which may be a silly one but if your gona pound it out and risk destroying your good work would a bit (lot of heat ) from a heat gun to the side of the bt to use heat transfer through to the epoxy to try and soften it ?? Just a suggestion. I dont know the science curing affect of epoxy hence if it will even work like other glues !

Good luck anyway

Js

[troj]

Here's a suggestion which may be a silly one but if your gona pound it out and risk destroying your good work would a bit (lot of heat ) from a heat gun to the side of the bt to use heat transfer through to the epoxy to try and soften it ?? Just a suggestion. I dont know the science curing affect of epoxy hence if it will even work like other glues !

Since it's a carbon fiber tube and coupler, if sufficient heat is applied to soften the resin holding the coupler in, it'll likely soften the resin of the tube and coupler, as well.

-Kevin

[Eat, sleep, and Fly]

Here's a suggestion which may be a silly one but if your gona pound it out and risk destroying your good work would a bit (lot of heat ) from a heat gun to the side of the bt to use heat transfer through to the epoxy to try and soften it ?? Just a suggestion. I dont know the science curing affect of epoxy hence if it will even work like other glues !

Good luck anyway

Js

Not with aeropoxy...

[G_T]

I seriously doubt you'll be able to salvage this without damage. As for pounding/pressing it out, you have a contact surface area of about 85 in^2. Even a much weaker than normal bond is going to result in rediculously high shear force. A decent bond is on the order of 2000 lb/in^2 in shear IIRC. Unless your tubes can take end compressive force on the order of 8 tons or more I wouldn't even bother with the attempt. 8 tons force would be required to break an extremely weak bond of only about 100 lb/in^2 which is likely less than what you are dealing with from what you've posted (if I am interpreting it all correctly). A good bond would require 100+ tons but you are unlikely to have that, as the tubes weren't prepped for this undesired joint and not properly wet out.

Can you instead chop below the embedded coupler to produce a fin can, and then just butt that onto a new tube, using the motor casing as a coupler? IOW, what DAllen also suggested?

If you make that Plan B, then you can try one of the other methods that will probably damage the upper section. Just preserve the fin can area from damage, and don't crush it ...

Ouch, you have my sympathies. Good luck!

Gerald

[bandman444]

Thanks guy for your comments.

Of course I (Bryce) chose to do my method.

And....




IT WORKED!!!

Setup the router table with two stops and 1/128th of an inch at a time cranked her up. After about 15 passes it was clear we were just about there, so I made one final cut up the tube, and after a few minutes of prying, was able to pop the top chunk off and all was well. Even better yet was that the resulting shoulder is perfectly perpendicular with the tube.







Now we are all set to go, with a two week launch date and I will see you later!

Thanks again