Trying to Not Cut Creased Body Tube to Fix It

[brockrwood]

I have a crease in a BT-20 body tube that is following a body tube spiral.

I am desperately trying to avoid cutting the body tube to fix the crease.

I want to completely ream out a spent 18 mm engine and uses the engine casing as a tube reinforcer.

Here is the plan:

1. Spread some wood glue on the inside of the tube at the creased spot using a chopstick. I will come in from the rear.

2. Insert the spent engine from the front (because there is a tube coupler and engine block in the way in the back).

3. Push the spent engine all the way down the tube (15 inches) until it fully enters the creased area with the glue. (Luckily, the shock cord mount is actually aft of the place where the crease is.)

4. Let glue dry.

Will this actually work? Would it be easier to just cut the tube above the crease? If so, should I actually cut out the creased section of tube or just leave it?

One reason I don’t want to cut the tube is because the rocket is fully primed and ready for the first coat of paint.

 

[Steve Shannon]

I would suggest a piece of coupler tubing instead of a reamed out motor.
Wood glue is almost guaranteed to seize while inserting the reinforcement. Lots of rockets have been sacrificed because of that. Epoxy works much better. Then you could sand the crease smooth and reprime.
Good luck.

 

[BEC]

I have poked tiny holes with a very thin modeling pin in the glassine layer, then, while pulling on the tube to pull it straight, dripped thin CA into the holes to let it run in and reinforce a damaged area like this. The is generally a field repair or a repair of an otherwise well-flown model, not something done during a build....but I have a couple of many-times-flown models whose life was extended considerably by this repair technique. It isn't pretty, but it works for both LPR and MPR models.

This is done on models where sliding a coupler in to reinforce the damaged area is not possible because of the motor mount below and the shock cord mount and/or baffle above the damaged spot.

I agree with Steve that a coupler would certainly work as well if not better....and be much lighter for what you're doing. But do check the fit and sand to make it an easy slip fit if using wood glue to install the reinforcement or you'll never get it to where you want it to be before the glue grabs. Alleene's Tacky Glue is a little more forgiving in that situation, but even it will grab if the fit is snug. As Steve suggests, epoxy would also be less likely to grab/seize the reinforcement before you get it where you want it.

 

[Hobie1dog]

I would suggest a piece of coupler tubing instead of a reamed out motor.
Wood glue is almost guaranteed to seize while inserting the reinforcement. Lots of rockets have been sacrificed because of that. Epoxy works much better. Then you could sand the crease smooth and reprime.
Good luck.

This guy knows rockets. Pay attention to him.

 

[prfesser]

On Steve's reply, which is spot-on: if you don't have a piece of coupler tube a piece of body tube, slit down the side, may work. As Steve said, use epoxy. The cut side of the BT should be opposite the damaged area.

If you have a kit that has the little disposable yellow tube used to push an engine block into place....you have a coupler.

 

[Off Grid Gecko]

would agree on epoxy over pva. While I have had luck reinforcing tubing with couplers using wood glue, it's very unpredictable. Even if you work very quickly, once it is thin that stuff can flash set at any moment and you will be left angry and disheartened.
I think the motor tube is a fine idea actually, as that weakened framing with a coupler support is not doing much except hopefully getting it a tad stronger than brand new, but you will need to recheck your COG after the repair either way, and estimate it before the repair if you go with the motor tube.

The rocket I'm going to be doing a lot of flying on (assuming I don't lose it) this year to test DD and locators is actually an estes kit that is reinforce nearly the entire length of the tube forward of the front centering ring for the MMT with couplers. Originally the kit came with balsa fins and was rated for D/E motors (mean machine) but I'm confident I could put her up on an H with all the extra modification and she'd take it just fine.
If this is a rocket you like to fly and you worry that spot could see trouble again, then I would just go with the hollow motor tube as long as it doesn't unbalance the model too much, specially if you don't have couplers on hand.

 

[brockrwood]

I would suggest a piece of coupler tubing instead of a reamed out motor

Sadly, only the very short Estes tube coupler is in my parts bin.

 

[Off Grid Gecko]

Looks like you might be able to get away with half a motor tube??? Just don't forget to thread it on the kevlar before you put it in, haha.

 

[Cape Byron]

If you have a kit that has the little disposable yellow tube used to push an engine block into place....you have a coupler.

I hoard those for just this purpose.

 

[MarkB.]

I'm going to recommend the other direction: cut it. I've tried several times over the years to "repair" a crease like yours. Number one it will always be a weak spot, even with a coupler underneath and number two, it's very hard to hide under paint as you end up sanding so much, the glassine needs to be replaced with some kind of clear coat like polyurethane or acrylic before repriming.

The fact that it's in primer means it's the perfect time to do this. The location is perfect for a graft. Order a coupler and a piece of tubing of suitable length from your favorite vendor and replace. Glue one end of the coupler in first, let it dry and then do the other side. The glue will ooze out just a little filing the joint. A little sandable primer and you're ready to top coat.

 

[GlenP]

Cover it up with a transition to a larger dia body. It won’t show at all if it is the through tube, inside another body tube! All that heavy stuff at the aft end of that small dia long tube is asking a lot, and after you install that coupler, it will crease again just forward of where you installed the coupler next time you are handling it.

 

[Donnager]

Either insert a stiffening tube, or do this.....

Score the outside of the "wrinkle" with something like 24-30 grit paper until you knock the top off of the wrinkle.

Wick some thin CA in the high spot, carefully, while holding the airframe straight (clamped). You should be able to see where the CA wicks into the tube. You may get enough stiffness with your interior gluing that you'd be OK. It's low power, so I'm not sure I'd stress too much over it if the tube feels rigid.

You can also install a couple of "strakes" that stiffen, straighten, and perhaps embellish the current design.....

 

[Back_at_it]

If this were mine I would not use the motor case. I would go with a full length coupler slid down from the top of the rocket. I would have the coupler slid all the way down so that it butts against the top of the motor mount. You will want it recessed into the body tube for enough that you can fit the nose cone.

As for gluing it in place. You have a couple of options. I personally would slide the coupler in dry and get it in place where you want it. Make sure the nose cone fits. Then you can take super thin CA and drip a few drops in from the top and let it run between the coupler and the body tube. If you feel the need you could also put a couple of tiny holes with you hobby knife or pin and let the thin CA soak into the holes.

I built a BT20 sized kit some years back and when it landed it bent the body tube much like what you have. The method described above is how I fixed it and it went on to fly for many more years.

 

[KenECoyote]

Sadly, only the very short Estes tube coupler is in my parts bin.

Just use a short section of regular bt-20 tubing and cut a strip about 1/16" lengthwise down, test fit, try again if too small or big. once the size is good, you can use tape on the inside to hold it together while you insert it with epoxy. I've done this before and it worked like a charm. Some reco a spiral cut for strength, but in this size I don't think it is necessary. Good luck!