How to address a gap between airframe and switch band?

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soopirV

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Hi all- I'm building an Adventurer 3 from Rocketry Warehouse, and noticed that either I goofed when I glued on the switch band, or the part wasn't quite true, and I'd like to fix it. It's hard to see in pictures perhaps, but it appears the switch band isn't glued on perpendicularly to the axis of the coupler- when the coupler is installed in the booster and payload, there is a gap up to about 1/16" on one side, but it touches on the other. The gap isn't quite opposite on the other tube as one might expect if it's just crooked in one axis, so maybe I really beefed this up and got it crooked in two?
Questions I have:
1) Is it worth worrying about? It's visible, and I'm not happy about it, but I don't think it'll cause much problem except at extremely high airspeed.
2) If it is, how can I fix it? I would love to mill it down and true it up, but lack the skills and tools to do so. I'm afraid that if I just start filing, I'll make it worse (i tend to do that).

IMG_1082.jpgIMG_1081.jpg
 
I use a thin emery board to sand the contacting spots (on both tubes at the same time) until the gaps are acceptable.
 
I just dealt with this on my 54mm rocket. The best way to explain it from the onset is that my goal was to fill the void you have identified with epoxy, and I did it like this:

Step 1: Cover OD of switchband and mating tube with masking tape. Apply some tape to the coupler about 1/2" to 3/4" away from the switchband-to-coupler joint. Adjust the fit of the coupler so that the mating tube is a nice snug fit with as little play on the coupler as possible.

Step 2: Lightly grease the mating tube ID and end face (or thickness, however you want to think of it) with vaseline...we're talking just enough to coat but not so much that it will mix with epoxy.

Step 3: Mix some epoxy and thicken it, or use straight Rocket Poxy. Cake the epoxy into the switchband-to-coupler joint so that you have a nice thick bead all the way around

Step 4: Push the mating tube onto the coupler and ultimately into the epoxy and as hard up against the switch band as possible. The epoxy should extrude out where it's flush but also fill in where there was a gap.

Step 5: A few choices here:

Step 5A: Stand the assembly on the coupler and wait for it to dry. Remove mating tube and clean off grease / remove masking tape. Cut and sand the epoxy until it's flush with switch band to your approval.

Step 5B: Using a putty knife, scrape away the excess epoxy after it has had a few minutes to cure slightly and is still somewhat pliable.

That's how you fill that gap with epoxy. You can either put epoxy around the entire circumference or only at the site of the gap, up to you.
 
You could just leave it as it not likely going to cause you any issues.

That being said, before sanding anything you need to mark the two tubes so you know how they line up. Then mark where they are touching and hit the high spot with a belt sander. G10/ 12 is tough stuff that takes forever to sand by hand. Check frequently for fit as you do not want to go too far in correcting the issue and make it worse.
 
You could scribe it, and then use the method/ tool of your choice to get to your line. No guessing on how much to remove where and you will have a perfect fit.
 
Before doing any modifications, rotate the coupler and tube around and see if you can minimize the gap in that fashion. I've had this happen on a few of my builds and find that usually neither the tube or the vent band are actually cut perfectly square. With luck, they were on either side of the same cut and you'll be able to reduce the gap to near zero with the proper alignment.
 
a small amount of course valve lapping compound works amazingly fast and gives precision fit. Takes very little- a few dots around edge will do.
 
apply pressure and spin it round and round, back and forth, etc. The tubes will sand each other flush.

Learned that trick from wildman's directions on the mini eagle claw. Works wonders.
 
Is there anyway you can take the difference out by trimming off the end of the coupler inside the bod tube to allow them to come together?

That would bug me too so I"m kinda curious to hear what everyone says
 
patelldp said:
I just dealt with this on my 54mm rocket. The best way to explain it from the onset is that my goal was to fill the void you have identified with epoxy, and I did it like this:

Step 1: Cover OD of switchband and mating tube with masking tape. Apply some tape to the coupler about 1/2" to 3/4" away from the switchband-to-coupler joint. Adjust the fit of the coupler so that the mating tube is a nice snug fit with as little play on the coupler as possible.

Step 2: Lightly grease the mating tube ID and end face (or thickness, however you want to think of it) with vaseline...we're talking just enough to coat but not so much that it will mix with epoxy.

Step 3: Mix some epoxy and thicken it, or use straight Rocket Poxy. Cake the epoxy into the switchband-to-coupler joint so that you have a nice thick bead all the way around

Step 4: Push the mating tube onto the coupler and ultimately into the epoxy and as hard up against the switch band as possible. The epoxy should extrude out where it's flush but also fill in where there was a gap.

Step 5: A few choices here:

Step 5A: Stand the assembly on the coupler and wait for it to dry. Remove mating tube and clean off grease / remove masking tape. Cut and sand the epoxy until it's flush with switch band to your approval.

Step 5B: Using a putty knife, scrape away the excess epoxy after it has had a few minutes to cure slightly and is still somewhat pliable.

That's how you fill that gap with epoxy. You can either put epoxy around the entire circumference or only at the site of the gap, up to you.
Click to expand...

That's a very brave method! I understand how you did it, but am convinced I'd find a way to fluff it up and epoxy the whole shebang together! I took the luddites' way out and simply marked the gap start and end with pencil, and then began to file away material from the switch band between the two marks (on the opposite "high" side). Many rechecks and gradually expanding the range of the filing while decreasing the depth did a reasonably good job of getting better mating between the surfaces. It's not perfect, but it's definitely smaller than the 1/8" gap I had before!
Chucking it in a lathe or a mill woulda been awesome!

The valve lapping compound is a good idea- may try that to hone it down even further.
I tried the twist and self-sand method, and saw white dust forming, but I wasn't sure if the material was coming from the mating tube surfaces or from their faces, so I didn't push it. I confirmed the air frame was square cut by placing a piece of glass on it and confirming it met flush all around.

Thanks for all the help, if I can, I'll post a new pic showing the results. Point is, it's doable without sophisticated power tools like lathes and mills...
 
LOL, I just finished doing the same thing on the lower section of the Solar Flare. Mark the high spots, file a bit, check and repeat. Got it close enough for my needs by this method. Not that it was really needed since it needs to be a really large gap to be noticeable on a 10.5 ft long rocket!
 
terryg said:
LOL, I just finished doing the same thing on the lower section of the Solar Flare. Mark the high spots, file a bit, check and repeat. Got it close enough for my needs by this method. Not that it was really needed since it needs to be a really large gap to be noticeable on a 10.5 ft long rocket!
Click to expand...

I was standing right next to it and didn't notice at the feb launch! Thinking it'll be ready to fly come desert heat?
 
It will be ready to fly by Desert Heat, at least for a daylight flight. I am not sure the lighting will be done for a night flight as there is a lot to set up and I am still waiting on LED shipments from China to arrive. It is a bit challenging working on it and avoiding hitting the ceiling when I do so. If it gets much heavier as I finish it up it might have to fly on "K". It is a hard design to sim out accurately so I might be able to get by with a J500G (gotta have color or sparks for a night flight).
 
Last edited:

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