Under-delayed my powerful motor. Reinforcing a body tube.

[WizardOfBoz]

So I bought a pretty powerful (CTI 38mm I566-Vmax) motor for the last launch of my Super DX3. Bought the motor reload at the launch site. It was a really impressive launch, but because I did not have OR to do the sims, I significantly underestimated how much delay was needed. Needed about 10 seconds, used 6 seconds. OR estimates deployment velocity at 141 fps.

I got away relatively unscathed. A slight weakening in the middle of the payload body tube. I think it whacked something on the bottom part of the rocket. I'd feel comfortable launching this with low H motors, but if I use it for an L2 cert flight I want to reinforce the thing.

To do this, I either could just replace the tube (I have a tube that is almost as long as the original, or I could use a whole coupler or part of a c coupler as a patch. I have both cardboard and bluetube couplers. I could just put glue in the tube and slide a coupler in, or cut out half of the of the coupler and just patch 180° of the tube. I suspect a whole coupler will be better. And given that the tube is cardboard I see no reason to use bluetube. Except that the cardboard coupler fits pretty tight. I suspect that epoxy will be better. Not for strength, but to ensure that the cardboard tube doesn't freeze up before its all the way in.

Any comments or advice?

 

[techrat]

To do this, I either could just replace the tube (I have a tube that is almost as long as the original, or I could use a whole coupler or part of a c coupler as a patch. I have both cardboard and bluetube couplers. I could just put glue in the tube and slide a coupler in, or cut out half of the of the coupler and just patch 180° of the tube. I suspect a whole coupler will be better. And given that the tube is cardboard I see no reason to use bluetube. Except that the cardboard coupler fits pretty tight. I suspect that epoxy will be better. Not for strength, but to ensure that the cardboard tube doesn't freeze up before its all the way in.

OK, I just recently built a 3" LOC "Athena" -- but the box from eRockets.biz got a bit mangled in transit, and the tube was slightly compromised. I built my own 3" coupler from a pringles can greased up the inside of the body tube with a lot of Titebond (not II or III, just regular titebond), and slid it down into the body tube. I'm fairly convinced it's strong enough to survive an H motor. But yeah, you need to use a lot of glue and work fast to avoid it freezing up on you. If it doesn't hold, I'll know about it pretty quickly. I had been thinking about fiberglassing where the problem was, but then I'd have to fiber the entire tube and that's just adding too much weight. So, I'm living with the Pringles Can. Hope to paint and fly before the end of this year, so we'll see how she holds up.

 

[WizardOfBoz]

Hilariousm, ingenious and expedient (and thrifty, if you like Pringles!). But you should have used the carbon fiber Pringles can!

I'm pretty sure that the cardboard tube will work. Good point about ensuring enough glue is there to lube the thing. I actually think that titebond might bind cardboard to cardboard as strongly as or more strongly than epoxy. Interestingly, here it might be LESS messy to use (less) epoxy than having to use (more) titebond.

I plan on having a 4 inch wide plank, marked to the right depth, to push the coupler in quickly. It's a 4 inch tube, so I will be able to wipe out all the excess glue quickly with paper towels.

As a benefit, it adds weight to the rocket in the forward section. Increase stability and a lower flight. For me a lower flight is better.

 

[Art Upton]

I would use slow epoxy with the tube on it's side, not up and down. Use a good stiff dowel to put the epoxy at the correct depth of the tube and spread it around that area circumference.

You could epoxy a plastic card , like a cut part of a credit card to the dowel to act as a spreader to keep from making a mess inside the tube.

Is this inside "splint" going to be in the path of chute being ejected ?

 

[WizardOfBoz]

The payload tube will eventually have a shock cord connecting the Avbay to the nosecose. I think that I'll screw the tube to the avbay and "shoot the chute" out of the top. It's a 4" tube thats about 19" inches long, so I can probably put a simple laundry shelf in. The coupler might even hold the shelf in place. So I don't think that the chute will be snagged by the coupler. Least I hope not.

 

[WizardOfBoz]

Put the coupler in with West 205. Pushed it in place with a bluetube coupler. Seemed to work great. Thanks, all. Next step is to repaint the place that was damaged.

I actually spread the epoxy with a gloved hand. After the coupler was in, I wiped out excess with a paper towel, then a paper towel soaked with IPA. Next step is to figure out a package shelf....

 

[WizardOfBoz]

Rubbed some dirt on it and it buffed right out.

And by "buffing" I mean adding an internal coupler, then sanding and wet sanding the tube with 320 paper and 400, 600, 1000, and 2000 grit sanding sponges.
And "dirt" was epoxy for the coupler, then SEM white primer, then 3M Acryl White filler, then more primer, then Krylon white gloss..

So I have the structure fixed and paint almost finished.

I have one more coat of white to add. Then I'm adding an orange belly band on the rocket, and black fins. Final color step will be black squares to make the orange belly band a checkerboard. Last, decals and clear coat.

One last observation. When I'm flying powerful (around I and above, some H's) motors, I'm ALWAYS going to sim the flight first to ensure that I don't deploy 5 seconds early. I got lucky (and to some extent was prepared) here. I had a long (25') kevlar rip cord (thanks to Dennis Kingley for suggesting a long one). And the 42" Spherachute held together pretty well. And putting CA glue on all the tube ends may have helped.

 

[WizardOfBoz]

Here's where I am now. I have a 5 x 4 checkerboard planned. I have 3 of the 5. Here you go (Nike Smoke and Eggtimer EZ-DD in background):

 

[WizardOfBoz]

Finished the main painting.