Glassing the inside of an Av bay coupler...?

So a few times I've seen / heard of rocketeers glassing the inside of the AV bay coupler using the balloon technique. Is this purely a matter of aesthetic preference? Does it actually improve the strength of the coupler? (My understanding is that an FG wrap does not increase structural strength per se, only protects against wear and tear, landing dings etc, in which case I'm inclined to forego it, althought the PML website advises 'glassing 4" tubes that may undergo Mach transition speeds. I'm hoping this AV bay will do exactly that at some point).

If it helps, here's the back story/reason for my question:
I'm in the process of constructing a new 4" AV bay, and I'm using a length of PML phenolic coupler rather than a LOC stiffy/coupler combination. I've used PML coupler on 3" and 2.1" av bays (and I'm aware its compression strength being phenolic should be considerably greater (..x 5 IIRC?)but, on its own, the 4" PML coupler looks pretty thin for 4" dia vehicle, and I'm considering doing an internal wrap with either FG or CF, but only if one or t'other would add strength to its function of holding the payload and the booster together and protecting the avionics under launch & flight stresses.

Any shared knowledge based on relevant experience welcome!

Thanks,

Tim51 said:

So a few times I've seen / heard of rocketeers glassing the inside of the AV bay coupler using the balloon technique. Is this purely a matter of aesthetic preference? Does it actually improve the strength of the coupler? (My understanding is that an FG wrap does not increase structural strength per se, only protects against wear and tear, [SNIP ] I'm considering doing an internal wrap with either FG or CF, but only if one or t'other would add strength to its function of holding the payload and the booster together and protecting the avionics under launch & flight stresses.

Click to expand...

Do the interior balloon wrap, use FG instead of CF if you're not absolutely certain you'll never put a radio in there.

On strength, an illustration:
https://www.jcrocket.com/adhesives.shtml

It will help with strength. That said, to really improve the strength you need a few layers. I would suggest glass versus carbon too. Carbon is conductive and blocks RF.


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I have built switches or antennas permanently into the inside of couplers before, could imagine using a layer of fiberglass to cinch it all together or cover up surface irregularities too.

Thank you all for the comments & advice. I wasn't intending to put any transmitters in the Av bay but will probably will go with a few layers of FG to avoid the carbon/RF opacity issue if I change my mind in future.
Cheers!

I fiberglassed the inside of an av-bay a few years ago for a 2.6" rocket. I believe I used 3 layers of glass and it's very stiff. It definitely helped make it stronger. One thing I didn't consider when i did this is that you will need to sand down the smaller diameter bulkplate on each end of your av-bay to make them fit. Do the glassing, sanding, and bulkplate test fitting BEFORE you glue the smaller diameter bulkplate to the larger one, otherwise it's a lot more difficult to sand down and get a good fit. I didn't do this for mine, so the fit is a little loose and i get a little more black powder residue in the av-bay than on my other rockets.

Scoops said:

I fiberglassed the inside of an av-bay a few years ago for a 2.6" rocket. I believe I used 3 layers of glass and it's very stiff. It definitely helped make it stronger. One thing I didn't consider when i did this is that you will need to sand down the smaller diameter bulkplate on each end of your av-bay to make them fit. Do the glassing, sanding, and bulkplate test fitting BEFORE you glue the smaller diameter bulkplate to the larger one, otherwise it's a lot more difficult to sand down and get a good fit. I didn't do this for mine, so the fit is a little loose and i get a little more black powder residue in the av-bay than on my other rockets.

Click to expand...

Thanks - good points. I'll post some pics when it's finished.

I find that using 2 pieces of fiberglass per layer is simple easy way to do.

Get a cheap wood handle "chip" brush & cut the handle short, to fit inside coupler. [ can't seem to upload pics today] https://www.homedepot.com/p/2-in-Chip-Brush-1500-2/100626098

Cut 2 pieces of glass, so they over lap by 1inch or so on each side when fit in coupler.

Brush epoxy inside.

Roll glass on dowel, PVC pipe etc. Insert into coupler and unroll into epoxy.
You should be able to move into wanted position, as epoxy will not soak through glass...yet.
After positioning, then coat the cloth with epoxy.

Roll coupler so other half is now in down position & do same.

Wait till epoxy has cured awhile and continue to add total number of layers.

For doing this, I get the heavy 10oz glass from Auto section at Wally world or any big box store in "Bondo" shelf. A large 4ft x 4ft section is only 4-5 bucks and perfect for this.
When finished on inside, [after all has cured]. Thin a bit of epoxy with alcohol [just a couple drops] and coat the out side. If will slide in/out MUCH better.

I find this method much easier than the balloon method on 4in & under couplers.

Don't forget when cutting to fit, leave enough room on each end for Av-bay caps to fit, or you will be sanding!:smile:

blackjack2564 said:

I find that using 2 pieces of fiberglass per layer is simple easy way to do.

Get a cheap wood handle "chip" brush & cut the handle short, to fit inside coupler. [ can't seem to upload pics today] https://www.homedepot.com/p/2-in-Chip-Brush-1500-2/100626098

Cut 2 pieces of glass, so they over lap by 1inch or so on each side when fit in coupler.

Brush epoxy inside.

Roll glass on dowel, PVC pipe etc. Insert into coupler and unroll into epoxy.
You should be able to move into wanted position, as epoxy will not soak through glass...yet.
After positioning, then coat the cloth with epoxy.

Roll coupler so other half is now in down position & do same.

Wait till epoxy has cured awhile and continue to add total number of layers.

For doing this, I get the heavy 10oz glass from Auto section at Wally world or any big box store in "Bondo" shelf. A large 4ft x 4ft section is only 4-5 bucks and perfect for this.
When finished on inside, [after all has cured]. Thin a bit of epoxy with alcohol [just a couple drops] and coat the out side. If will slide in/out MUCH better.

I find this method much easier than the balloon method on 4in & under couplers.

Don't forget when cutting to fit, leave enough room on each end for Av-bay caps to fit, or you will be sanding!:smile:

Click to expand...

Thankyou sir! Very helpful hints. I'm also having problems uploading pics, but suffice to say all went together ok. I'd anticipated the change in internal diameter and made the stepped AV Bay caps so that they were a little slack before the FG went on, so they're snug now. I'd sorta arrived at the PVC pipe method through 'extensively meditating' on the problem before attempting it .. smoothed out with gloved fingers and then used the balloon method with the FG while it all cured...That got me a beautiful smooth interior, albeit with 'HAPPY BIRTHDAY!' now reverse printed on the inside... :lol:

I compared two techniques:
jcrocket.com/justmarried.shtml#couplers

JohnCoker said:

I compared two techniques:
jcrocket.com/justmarried.shtml#couplers

Click to expand...

And thank you too sir - I imagine like many HPR rocketeers your site's been a great source of info for me and I've visited it numerous times, but I hadn't previously seen that page.