Any reason NOT to use cardboard couplers on a fiberglass tube?
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codysmith
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Well that's the thing... I don't see this needing much strength on the coupler... It's such a small rocket I don't think it would matter... or does it?
Eat, sleep, and Fly
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A rocket is only as strong as it's weakest part. If you are comfortable using a cardboard coupler, then I would encourage you to build the entire rocket out of cardboard, for weight savings. If you feel that you need fiberglass for strength, then you should build the whole rocket from fiberglass.
You know first question that needs to be answered is what is the couplers purpose? It could be beefing up a section, avionics bay, etc.
Anything that has to do with beefing up an area I would resort to something stronger other than cardboard.
Alexander Solis
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Mariah 54 - CTI RedLightning- I-100 - 6,345 Feet
Anything that has to do with beefing up an area I would resort to something stronger other than cardboard.
Alexander Solis
TRA - Level 1
Mariah 54 - CTI RedLightning- I-100 - 6,345 Feet
grouch
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Glassing the inside or out? If you glass the outside the coupler won't fit inside your body tube. If you glass the inside, you have a weak point on the rocket again. The paper can ding and create a point of failure. Why don't you just use a fiberglass coupler? Cost isn't an issue at around $8 a crack.
The spirals wouldn't show 
... if that's the main reason for 'glass ...
If I were to build a partially fiberglass rocket, I'd use a cardboard extension tube -- coupled with a fiberglass coupler for accuracy and stiffness. e.g. a Formula 54 and 30" of BT70H.
... if that's the main reason for 'glass ...If I were to build a partially fiberglass rocket, I'd use a cardboard extension tube -- coupled with a fiberglass coupler for accuracy and stiffness. e.g. a Formula 54 and 30" of BT70H.
JDcluster
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The coupler is a possible failure point, especially when attempting to break mach.
You want the joint snug enough that there is no wiggle..... or it could lead to tearing the rocket to pieces.
If the rocket is not perfectly vertical ( at 90 degrees) there is some angle of attach or side force which wants to cause the rocket to bend or bow.
JD
You want the joint snug enough that there is no wiggle..... or it could lead to tearing the rocket to pieces.
If the rocket is not perfectly vertical ( at 90 degrees) there is some angle of attach or side force which wants to cause the rocket to bend or bow.
JD
jimzcatz
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jimzcatz
Boss, Carolina Rocket Mafia
The one time I tried this, it was with 7.5 airframe. First flight the coupler sheared at the joint, looked like it was cut with a razor. coupler was a repair, not an avbay.
chadrog
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Remember that the "standard" sizes are not precisely defined so if you mix and match couplers from difference sources, they may not may not be a good fit (regardless of the material from which they're made).
cbrarick
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We have a winner! The sizes are different! You might get a cardboard coupler, but then you're gonna have to modify it significantly to make it work correctly. It's either gonna be loose and sloppy (read gonna break when you put significant g's to it) or so darn tight that you sand away most of it to make it work (read it's still gonna break). Save your time and effort, get all your stuff the same
codysmith
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Okay.... well than what would you suggest for a homemade tube... I'll just get a fiberglass coupler...
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38mm is probably too small, but you can make couplers to fit custom tubes by laminating fiberglass inside the tube.
Cut a piece of body tube a little longer than the desired coupler length. Line the inside with 2mil Mylar or thin non-porous release fabric. Then wet out fiberglass cloth and roll it around a dowel to insert it into the tube and carefully unwrap it around the inside. Then line that with porous release fabric and breather and use a balloon or low vacuum to press the fabric against the inside of the tube. Once the fiberglass cures, you can trim the coupler to length.
I have used this technique several times to reinforce the IMO overly-thin Loc/Precision couplers. I compare the result with a balloon and a vacuum bag on my Just Married rocket page.
codysmith
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Okay... I like that solution, but I agree it is too small for a LOC cpupler... I'll just have to get fiberglass38mm is probably too small, but you can make couplers to fit custom tubes by laminating fiberglass inside the tube.
Cut a piece of body tube a little longer than the desired coupler length. Line the inside with 2mil Mylar or thin non-porous release fabric. Then wet out fiberglass cloth and roll it around a dowel to insert it into the tube and carefully unwrap it around the inside. Then line that with porous release fabric and breather and use a balloon or low vacuum to press the fabric against the inside of the tube. Once the fiberglass cures, you can trim the coupler to length.
I have used this technique several times to reinforce the IMO overly-thin Loc/Precision couplers. I compare the result with a balloon and a vacuum bag on my Just Married rocket page.
Looks like you made a good decision, but for posterity's sake, here is something no one has mentioned...
If your drogue deployment succeeds but your main deployment fails, then you have a heavy FG rocket falling to the ground on a drogue. If it lands on the CB coupler, it most likely will destroy it. So to me, the coupler should be able to handle a ground hit relative to the weight of the rocket.
If your drogue deployment succeeds but your main deployment fails, then you have a heavy FG rocket falling to the ground on a drogue. If it lands on the CB coupler, it most likely will destroy it. So to me, the coupler should be able to handle a ground hit relative to the weight of the rocket.
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