So How tight is it supposed to be?

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PGerringer

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So, I'm in the home stretch of building the Aerotech Mirage (pictures are coming). The motor mount is in, the fins are in, etc. Now is the time to worry about one problem that I found along the way.

You know how you dry fit all the pieces before you assemble the rocket just to make sure everything is OK? Well, as I was dry fitting the Mirage stuff I noticed that the coupler that holds the body tubes together was very tight. Let me explain... The Mirage has three body tubes joined by two couplers. The top coupler is epoxied to both top and middle tubes. The bottom coupler is only epoxied to the upper tube. The rocket breaks apart at the bottom coupler. Anyway, as I was fitting the bottom tube to the middle tube, the coupler was extremely tight. I had to even wrestle it out of the bottom tube and it was only in about an inch. So, I switched couplers. Immediately I noticed that the top and middle tubes accepted the couplers readily. It appeared the bottom tube was having problems. The new coupler fit the bottom tube but was still very tight.

OK, that is the problem now here is the question. Just how tight is too tight? I'm wondering if the ejection charge is even going to be able to separate the rocket.

What are some techniques that one would use to "loosen" up a tight coupler? Any ideas? I'm thinking about sanding the first layer of "paper" from the coupler. Help...
 
It sounds like the new coupler was extremely tight, which is a bad thing. Here is what I do when testing couplers:

Couple the rocket together, then lift the rocket off the ground. If the coupler seperates, it is too loose. Now, shake the model horizontally and if you get slight seperation it is a good fit. The key to couplers working correctly is a SMOOTH fit that is still tight enough to prevent premature seperation. Also, you will want to drill a 1/8" hole somewhere along your body tube in the coupled area for air pressure equalization.

As far as sanding goes, I have sanded couplers before for a good fit, but these were all phenolic coupler tubes. I have never used a paper coupler, so I am hoping someone else can step in. Sanding the paper may do more damage than good and, if I were to guess, they will tell you to either get a sandable coupler in phenolic that fits or peel off the first layer of paper. Masking tape is used a lot in the field to tighten up couplers and nosecones that are too loose and I imagine you could do the same once it is peeled. If you are concerned about strength loss after adjusting, you could lay some glass cloth on the inside of the coupler.

We Need more input required from those experienced with paper couplers!

Carl
 
> We Need more input required from those experienced with paper couplers!

You're right, paper doesn't really sand at all. It gets fuzzy and not easier to slide into the tube... I guess using an xacto knife to get the top layer or so off is much better... It may work of you soaked the coupler in CA or laminating epoxy, though...

Oliver
 
I have had very good luck fiber glassing the inside of tube couplers to make them stronger. LOC's tube couplers are weak, so I always glass the inside. I have not worked with AeroTech’s TC's, but the LOC TC's are paper. See my project page: https://www.mbrocketry.com/JohnnyBravo/part_1.htm. The fifth paragraph down showing pictures 5 and 6 show how I did it. I have done it several times without the plastic bag, just using the balloon. This works much better. I did use an air compressor to blow the balloons up. But doing this, you would be able to peel a layer or two from the outside. You my want to inspect the lower body tube for kinks. Hope this helps.
 
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